CHAPI'ER II

RESULTS AND DISCUSSIONS

The present work can broadly be divided into two

parts, the first dealing with the effects of electrolytes

on the activity coefficient of·the initial and transition

states as·a function of the identity and concentration of

the electrolyte for the hydrolysis of 1-(p-chlorophenyl)

ethyl hydrogen succinate and the other consisting of the

hydrolysis of 1-phenylethyl hydrogen succinate and its

para substituents (cH3

, c2H

5, CH

3o, F, Cl and Br) and

the solvent chosen for the study is water.

Water is unique among liquids because of the

possibility of tetrahedral coordination with four nearest

neighbours. This built in tendency towards a three

dimensional structured conditions has long been

recognised and has been repeatedly emphasised in dis­

cussions of water structure (40-45). The unusual

properties resulting from the structural nature of water

are reflected in the aqueous solutions of ions and the

different but quite unusual properties of aqueous solutions

of weakly polar molecules such as alkyl halides. Yet

inspite of the recognised importance of some reactions in

aqueous media, until recently but a limited number of

kinetic studies on the hydrolysis of simple halides and

similar compounds had been made in water. Water is a

notoriously poor solvent for most.organic compounds of

interest to kineticist. Mixed solvents, in which water

17

was frequently one component, were more convenient to use

and at the same time provided a graded series of solvolytic

media in which to explore the relation between rates,

mechanism and effects of changing solvent properties.

Various attempts have been made to treat these solvent

effects in a more quantitative manner. These attempts

succeeded within the limitations of the model and the

questionable value of dielectric constant. Such treatments

had some success where ion solvent interactions were large

but were recognised to have limited analytical value.

Laidler (46) has reviewed briefly the problem of inter­

preting kinetic data, and while he recognised the potential

value of such derived functions as the enthalpy and entropy

of activation, he concludes that free energy considerations

provide the soundest basis for an electrostatic model. An

alternative was provided by the correlation equations of

Grunwald and Winstein (47). Many factors favoured water

as a solvolytic medium and it was known to be a very good

ionising medium and hence solvation effects in innogenic

reactions were expected to be large and more easily determined.

18

Salt effect studies can provide considerable information

of theoretical importance as to the complex interactions

of ions and neutral molecules and as to the unique nature

of water as a solvent.

SALT EFFECTS ON THE KINETICS OF HYDROLYSIS OF 1-E-CHLORO­

PHENYL ETHYL HYDROGEN SUCCINATE IN WATER

The addition of any solute to water is likely to be

accompanied by changes in one or several of the parameters

which have been utilized as indicators of the degree of

water structure and it is tempting to correlate these changes

with other effects of the salt on the solution. These

structural changes will certainly be included in a complete

theory of salting out. The insertion of a nonpolar molecule

into water may be divided into, first, the pulling apart of

water molecule to make a hole into which the solute can fit

and second, the insertion of the solute into the hole. In

the case of a nonpolar solute which does not interact

strongly with water the greater part of the free-energy

requirements for this process may be ultimately ascribed

to the decrease in the mutual interaction of water molecule

in the first step. This provides the principal reason for

the low solubility of organic molecules in water when the

solute is not sufficiently polar to provide a compensating,

19

favourable interaction with water in the second step. The

addition of any material which will increase the average

cohensive energy of interaction among the water molecule

will make it more difficult to separate the water molecule

and dissolve the solute where as addition of a substance,

such as an alcohol, which will decrease the average mutual

attraction of the water molecules will facilitate these

process. Most salts increase the average strength of

mutual interaction and cohensive energy-density of an

aqueous solution. The salting out of a nonpolar solute

may be regarded qs simply a squeezing out caused by the

electrostriction and increased average strength of the

mutual interaction of the solvent molecule in the presence

of salt. The strong directional interaction of water mole­

cules with each other makes it certain that the mutual

orientation of water molecules in liquid water is far

from random, and many of the properties of water can be

described by a model in which the orienting effects of water

molecules on each other are considered in terms of a kind

of structure, perhaps a number of 'flickering clusters' of

water molecules (41,45,48). Such oriented regions of

hydrogen bonded molecules will have some of the properties

of one or another form of ice, although it is unlikely that

they represent regions with the structure of ordinary ice.

The introduction of an ion with its surrounding electric

field, will change this state of affairs, although the

ways in which the changes take place are not simple. A

small highly charged ion will orient the immediately

adjacent water molecules into a firm hydration sphere,

which will remain with the ion for an appreciable time

20

and will have properties which are quite different from

those of the bulk water molecules. These water molecule

constitute the immediate hydration shell of the ion and

define the hydrated radius and hydration number although

these quantities vary over a wide range depending on the

method used for these estimation (49,51). These molecules

may be regarded as more structured than the bulk molecules

of liquid water. Outside this firm hydration shell or

immediately adjacent to the surface of larger ions the

electrical field will not be strong enough to bind water

molecules in a hydration shell, but will be strong enough

to compete effectively with the dipole - dipole forces

which provide some orientation and structure to the mole­

cules of liquid water. There are at least two types of

structure forming ions, small ions with a high charge density

that orient surrounding water molecules by a strong electro­

static interaction and very large ions which may orient

surrounding water molecules because their electric field

is too weak to have a significant influence on the solvent.

For this reason a number of the parameters which measure

water structure show maximal values for the smallest and

largest ions, surrounding a minimum for the structure

breaking ions of intermediate size. Some of the most

visualized paramters which are thought to be measures of

water structure are viscosity, entropy, rate of self

diffusion and dielectric relaxation time of water.

21

Simple salts such as the alkali halide cause a

decrease in dielectric relaxation time of water (52). This

is one of the most straight forward lines of evidence for

the structure breaking effect of such salts in water. The

decreased relaxation time reflects the increased freedom of

movement of water molecules in the presence of these ions.

However, in presence of small highly charged ion there will

be some water molecules in the first layer of hydration shell

which are so strongly oriented by the intense electric field

of the ion that they are not free to reorient themselves in

·an-applied electric field at all. These water molecules

which may be designated as irrotationally bound will cause

a decrease in the observed dielectric constant of the solu­

tion (52,49). Because of this structure forming tendency,

such small ion cause a smaller decrease in dielectric

relaxation time than medium sized ions. The fact that the

dielectric relaxation times passes through a minimum as the

22

salt concentration is increased and eventually reach a

value higher than that for pure water. As the salt con-

centration is increased, there is a decrease in the number

of interstiti�l water molecules which have an increased

freedom and which account for the decreased dielectric

relaxation time. Eventually the salt concentration reaches

a point at which all the water molecules are in the

immediate viscinity of ions and have their freedom of move-

ment restricted by the field of ions (53). If the size of

the ion is increased sufficiently the charge will be

effectively shielded from water and the strength of the

interaction of the ion with water molecule will become

even less than that of another water molecule. For such

ions the electrostriction and even the structure breaking

effect associated with most ions disappear and the

behaviour of the ions approaches that of a nonpolar solute.

Studies of the near infrared absorption spectra of water

show that the spectra are influenced characteristically by

temperature changes and dissolved salts (54). The order

of increased structure breaking power deduced from spectral

studies is

Cl-< N03 - <.._ Br- L..._ I- � Cl04-

Na + L._ K+ z... Cs

+ z. ( CH3) 4 N+.

and Li+ <

Before discussing the present data it might be of

23

advantage to give a brief account of salt effects on the

stability of the initial and transition state of hydrolysis.

Taft and coworkers have studied the salt effects on the

hydrolysis of t-butyl chloride in water (24). The speci­

ficity of salt effects on the initial and transition state was

studied for a number of salts. The finding was that for all

electrolytes both 1-1 and 2-1 electrolyte - the total effect

of the salt contained a specific part, the magnitude of

which depended on the identity of the electrolyte. This

was true of both initial and transition state. However

with simple inorganic salts, the specific effect was

essentially the same in both the initial and transition

state, so that they behaved as though they were true

Debye-Huckel electrolytes. On the other hand, with other

electrolytes, particularly those which showrnicelle forma­

tion, the specific effect were different in the initial and

transition state. It was confirmed that the difference of

the total salt effects of simple inorganic electrolytes

between the initial and transition state was nonspecific

in nature.

Kohnstam studied the problem by considering the

effects of salts on the stability of the initial and tran­

sition state for the SN1 hydrolysis of 4-nitro-4'-phenyl­

diphenyl methyl chloride in 70% v/v aqueous acetone (55).

All salts stabilized the transition state, the magnitude of

stabilization depending on the identity of the salt. Regard­

ing the effect on the initial state tetramethyl ammonium-

fluoride, sodium chloride, sodium benzene sulphonate

decreased the activity coefficient while �odium nitrate,

sodium borofluoride, sodium bromide and sodium perchlorate

had the opposite effect. It was suggested that the effect

on the activity coefficient of the initial state could be

understood on the basis of the operation of a salt induced

medium effect which depends on the capacity of the salt to

dry the solvent. It was also suggested that the known

effect of solvent shanges on the stability of the initial

and transition state in this reaction requires that the

increasing values of t1/�0 where t1 and �o are rates in

the presence and absence of the electrolytes, should arise

mainly from an increase in r/r0

and only to a relatively

small extent from a redu6tion in r*/r0+(55). This conclusion,

that the stability of the transition state is often insen-

sitive to the nature of electrolyte as was originally

suggested by Ingold (56) appears to be derived from the

circumstances that the spread in the values of the activity

ratio of the transition state is smaller than that in the

initial state. The study also disclosed that both the

initial and transition state were stabilized by the per-

chlorate and benzene sulphonates. This is ascribed to

25

specific short range interaction with the substrate (57,58).

The conclusion that the initial state is stabilized by

perchlorate not withstanding an increase in the ratio of

the activity coefficient, appears to be based on the idea

that this electrolyte should have given much larger value

for the activity coefficient ratio.

Bockris and Egan had measured the solubility of

benzoic acid in ethanol water mixture both with and without

added sodium chloride (1M) (59). The benzoic acid is salted

out by the electrolyte in pure water but as the organic

cosolvent is gradually added the salting out changes.to a

salting in and with further increase in the concentration

of the organic cosolvent the electrolyte again salts out

the solute. For methanol water system, the salting out

initially decreases with increase in methanol content and

reaches a minimum at about 58% w/w methanol and then

slightly increases. Although a salting in does not occur

here, the pattern of the result is similar to that of

ethanol water system. In dioxan water, the maximum dioxan

concentration investigated was 24.4% w/w. In this region,

the decrease in the salting out was noticed. It is possible

that a further increase in the dioxan concentration would

bring about the existence of a minimum as with the other two

solvent system. Grunwald has measured the solubility of

nap'}halene and 1-naphthoic acid in 50% w/w dioxan water

26

system, without and with added salts (60). For both organic

solutes, there was a well defined salting out order. Of

the simple inorganic salts examined, the following results

are observed. In the.naphthalene system, sodium perchlorate$,

potassium perchlorate, potassium iodide and potassium

bromide showed a salting out effect, their magnitude

decreasing in the order, potassium chloride and sodium

chloride showed a salting in, the magnitude being about

the same. For naphthoic acid, the salting out decreased

in the order Nac104 :;> KCl04 > KI. The salting in effect

increased in the order KBr< NaCl < KCl. The salting

order in aqueous organic solvent is almost exactly the

reverse of the order in water. In discussing the results,

the authors refer to the model proposed by Mcdevit and

Long (61) applicable to aqueous solutions. This model

implies that when a salt is added to an aqueous solution

of the non-electrolyte, the increase in the initial

pressure resulting from ion solvent interaction squeezes

out the non-electrolyte molecule. Since this model cannot

account for the data in dioxan water system, they proposed

an additional· mechanism for the salt induced effect in

aqueous organic solvents which depends on the various

electrolytes to salt out the organic cosolvent. This

reversal of the salting out order was rationalised by the

authors by a theoretical treatment which showed that one

27

of the factors determining the salting out or salting in

of a particular electrolyte is d 1��

x where xis the

ratio of the thermodynamic activities of the components of

the solvent and z is the mole fraction of water in the

solution without taking into account the mass of the solute.

In water, since z is one and therefore, since d log x/dz

occurs in the denominator, the term containing the factor

vanishes. The authors concede that while the change in

solvent composition would also affect the other terms

contributing to the overall salt effect; the variation in

this term is the most important. The importance of this

work lies in their proposal for a general understanding of

the problem of salt effects on solubility in water and in

aqueous organic solvents.

We believe that much of the difficulty associated

in accounting for salt effect can be understood on the model

proposed by Grunwald, particularly with reference to the

effect of the salt on the stability of the initial state.

For a given substrate, a particular electrolyte will pro­

bably show salting out in pure water thus increasing its

activity coefficient. Regarding the identity of solute, the

following observation is relevant. In solvent water Long

and Mcdevit have shown that for a polar non-electrolyte the

salting out increases with the molar volume of the solute (61).

In the work on the solvolysis of 1-phenylneopentyl chloride

28

in 80% aqueous acetone the enthalpy of activation has been

observed to be 24.9 k.cal/mole (62), while in water, the

enthalpy of activation is 17.8 k.cal/mole (63). In general

the enthalpy of activation of SN1 reactions of alkyl

chloride is increased while going from aqueous organic sol-

vents to water. For example the enthalpy of activation of

t-butyl chloride in 80% aqueous acetone is 22 k.cal/mole

(64), while in water it is 23.8 k.cal/mole (65). In this

particular case the reverse trend observed must be ascribed

to the fact that the larger molar volume of the organic

solute precludes any significant solvent-solute interaction,

thus destabilising the initial state. Grunwald suggests

that in the presence of salts, 'tight' salvation shells

are formed around the ions whose composition differs from

the bulk of the solvent. Because of this, there is a com-

pensating change in the average composition outside the

salvation shell which would lead to increased solubility.

Oakenfull studied the kinetics of hydrolysis of

acetic anhydride in presence of various electrolytes (66).

There are three possibilities for salt effect in the

hydrolysis of acetic anhydride (i) The specific interaction

between the ions and acetic anhydride (ii) Specific effect

resulting from changes in the medium, ie the dielectric

constant and (iii) The structure of water. Most ionsinter­

act strongly with water and modify its structure (67). The

29

viscosity B. Coefficient from Jone-Dole equation seems to

be a useful 'intuitive' measure of the effect of salts on

the structure of water. These B values show that the order

of the structure making effect is Bu4NC1 � MgC12 > LiCl>

Me4NC1 > NaCl > NaBr > NaI ;> RbCl > CsCl with the last

two being structure breaking. There is obviously a rough

correlation and it is tempting to conclude that salting out

occurs when increased water structure makes it more difficult

to insert the solute. It has been pointed out however that

this kind of argument is unsound. One can equally well

argue that the increased water structure caused for ex�ple

by the large tetraalkyl ammonium ions would make it easier

to form a shell of structured water around the solute.

In the water catalysed neutral hydrolysis of �-nitro­

phenyl and p-methoxyphenyl dichloroacetate in water Engbersen

and Engbert studied the effects of t-BuOH, 1l Bu4 NBr, KBr

and Nac104 (67). Except for aqueous Nac104 extrema in AH*

and 6.S are observed as a result of large compensatory

changes in these quantites of activation as a function of

solvent composition. The s-pecific pattern of the .6...H* and 6S*

relationship clearly depends on the nature and concentration

of the additive and serves to indicate the overwhelming

importance of solvation factors. The combined evidence

strongly suggests that effects due to changes in the diffu-

sionally averaged water structure may provide a rationale for

30

understanding these phenomena. Water is the ubiquitous

solvent for fundamental chemical reactions involved in

life processes. There is abundent evidence that chemical

reactivity in aqueous media is profoundly influenced by

the three dimensional hydrogen bonded structure of liquid

water (68). For instance, one of the.consequences of this

structural property is the unique propensity of water mole-•

cules to participate in intermolecular proton transfer

process (69,70), which constitute such an important feature

of enzyme catalysed reaction. Since it has been recognised

that.diffusionally averaged water structure may be either

appreciably decreased or increased compared with pure water··

around the active site of enzyme (71 ,72) it would be of

great interest to investigate the effect of perturbration

of water structure on the rate and energetics of proton

transfer reactions in water.

In an effort to probe into the effects of electrolyte

on the dynamic basicity of water Menninga and Engberts have

studied the kinetic salt effects on the water catalysed hydro-

lysis of two covalent arylsulphonyl methyl perchlorates in

water (73). The finding that the hydrolysis rates are

retarded by cations and enhanced by anions cannot be

explained by extended Debye-Huckel or Bronsted theories for molecule-

molecule reactions. They proposed that the salt effects

31

originate predominantly from electrostatic ion-water inter-

actions and specifically reflect the nature of the dipolar

transition state for deprotonation. The structure breaking

effect of cations and anions is in the increasing order of . ( ) + / 2+ < 2+ < + effectiveness 74,75 nBu4

N '- Mg Ca Me4N �

Li+ ( Na+ < K+ < Cs+ and so4

2-< Cl-< Br-< c104- where

+ 2+ 2+ + . + + nBu4N, Mg , Ca , Me4N, Li and Na are structure

makers. The salt effect on the hydrolysis of p-nitrophenyl­

sulphonylmethyl perchlorates follow the sequence HC104 >­

MgC12

> CaC12

> NaClO 4 )> HCl > NaCl L/""\ Li Cl '-../"\NaBr V'IKBr >

CsCl > Me4NBr > nBu4NBr > Na2so

4 and for the hydrolysis

of £�methyl phenyl sulphonylmethyl perchlorates Nac104 \.../J

NaBr / NaCl> LiCl > Me4NC1 > nBu4NBr. If water structure

effects or salting in and salting out parameters would

dominate the sign and magnitude of the salt effectJCsCl

would cause a stronger rate decrease than LiCl which is

in contradiction with the experiment. The observed salt

effects may be rationalised by assuming that the magnitude

of the salt effect is primarily determined by the charge

type and charge density of the distinct ions. The salt

effect of £-nitro and £-methylphenyl sulphonylmethyl perchlorates

follow the sequence Mg2+> Ca2+> H+ > Li \J, Na\_..,-, K+

> Cs+>

Me4N+

> nBu4

N+

and c104 > Br-> c1·> so4 2- corresponding

with the order of charge density. The results suggest the

32

existence of an electrostatic interaction between the dipolar

transition state and electrostatic field of the ions

operating via polarised water molecules between the ion

and the partially broken C-H bond (73). The same authors

present another paper of the water catalysed hydrolysis of

two covalent arylsulphonyl methyl perchlorate in dioxan

water, t-BuOH-H2

o and CH3CN-H

20 (76).

Hibbert and Long studied detritiation of malono-

nitrile in mixed aqueous organic solvents and in salt

solutions (77). Addition of tetraammonium halides and

of organic solute cause a strong decrease in the values

of the activity for the transition state (i.e.) a negative

free energy of trans£er (salting in) whereas addition of

alkali halide cause a strong increase in the activity or

a salting out of the transition state. Any solute which

increases the stability of water clusters is said to be

structure making, nonpolar molecules and large ion salts

are structure makers. Additives which increase the water

structure lead to an inereased rate of detritiation,the

reverse is true for structure breakers. Addition of a

structure maker such as tetraethyl ammonium bromide should

decrease the required further structure making and hence

cause a rate acceleration.

The kinetics of salt effect on the hydrolysis depends

on various factors (i) the dielectric constant and the

polarity of the medium (ii) the size and shape of the

substrate (iii) the nature and properties of the electro­

lyte (i.e) the structure breaking and making abilities of

the electrolytesand (iv) the charge type and charge

densities of the distinct ions. The results for the

hydrolysis of 1-£-chlorophenylethyl hydrogen succinate in

water with various electrolytes are set out in Table 1 and

2. One significant feature of the result is that the

electrolytes have practically no effect on the activity of

the initial state except at high concentrations of the

electrolyte5as can be evident from the solubility data.

As the sodium chloride concentration increases from Oto

0.05M the rate of reaction also increases. After when the

electrolyte concentration is increased there is a slight

decrease in the rate and then the rate become almost steady

as the salt concentration is increased. Sodium nitrate and

sodium bromide show similar results but the rate increase

is less. For all these electrolytes the changes in the

rate constant on addition of the electrolyte are the addition

properties of the related anions and cations. The results

can be considered to result from the differences in the

ionic strength effect which is independent of the nature of

the electrolytes and the structure breaking or making effect

of the electrolyte. For anions the structure breaking

34

effect increases in the order Cl-< N03 < Br-< c104 and

for cations Li+< Na+ <. K

+ ..( (CH3

)3N (74,75) and this

order agrees with the experimental observation.

For lithium perchlorate the result is slightly

different. The nature of the interaction of perchlorate

has now to be considered. Besides the ionic strength and

structure breaking effect some other effect is also operat-

ing. Attention has been drawn to Grunwald's view that

perchlorate ion complexes with dioxan (57). Waind has

shown that the solubility of ethyl acetate in water is

increased by the addition of perchlorate which is not

generally true of other salts ( 61 ) • For 'i' -butyrolac�tone

the salt effect on solubility decreases in the order NaCl>

KCl >NaBr > NaI > Nac104·;, KI, the last three salts showing

a salting in. It is interesting to note that there appears

to be a relation between the anion size and salt effect.

Similar results have been obtained by Taft (24). All these

suggest a rationale for understanding the peculiar effect

of perchlorate ion on the rate of reaction. The assumption

is that perchlorate ion will interact with an organic so lute,

the greater its size. Possibly a nearly covalent type of,

linkage is involved. This factor is .particularly important

in understanding the effect of perchlorate ion on the

transition state of the reaction.

TABLE 1

EFFECT OF ADDED ELF.cTROLYTE IN THE RATE OF HYDROLYSIS OF 1 -p-CHLOROPHENYLEI'HYL HYDROGEN

SUCCINATE IN WATER

(est er) -� 0. 008M Temp : 1 00 ° C

---------------------------------------------------------------------------------------------------------·

Electro­lyte

0.00)'1 0.005M 0.011'4 0 .025JvJ 0.05M . 0 .1 OJ'<J ------ ------------ ------------- ------------ ------------ ------------

4 4 % age 4 10 !f

1 1 0 �1 va�i- 10 t1at ion

% age 4 % age 4 % age 4 % age vari- 1 O t

1vari- 1 0 t

1 vari- 1 0 !f

1 vari­

�ioo �ioo �ioo �ioo

0. 1 51'<'�

4 % age

1 0 !f1

vari­ation

0. 251YJ

4 % agE1 0 !f

1 vari· ati01

---------------------------------------------------------------------------------------------------------·

NaCl 0.1 70 0.202 18.8

KCl

NaN03 0.1 92 12.9

NaBr

Na2so4 0.122 -28,2 0.127 -25.3

Li2SO,:,:

0.154 -9.4

LiC104 o.1so 5.9

0.1 97 15 .9 0.213 25.3

o.182 7.0

00209 22.9 0.1 97 1 5 .9

0.157 -7.6 0.185 8.8

0.1 26 -2509 O. 11 0 "'.35 • 3

0.1 48 -12.9 0.151 -11 .2

Q.170 0

0.1 95 14.7 0.185 8.8 0.1 93 13.5

o.187 1 0.0 0.195 14.7

0.1 93 13.5 o.185 8.8

0.125 -26.5 0.128 -24 • ..,

0 • 1 5 6 -8 • 2 O • 1 5 3 _q O • 0 O • 1 46 .q 4 • 1

0 • 1 88 1 0 • 6 0 • 1 87 1 0 • 1)

----------------------------------------------------------------------------------------------------------

\>I V1

36

TABLE 2

EFFECT OF ADDED ELECTROLYTES IN THE SOLUBILITY OF 1-(£-CHLORO-

PHENYL)ETI-IYL HYDROGEN SUCCINATE IN WATER AT 35° c

---------------------

-------------------------------------------

OM 1 .607

0.01M 1.587 1 .511 10554 1.643 1 .604

0.025M 1.582 1.543 1.572 1 .631

0.05M 1.550 1.533 1.548 1.606 10545 1.660

0.10M 1.592 10500 1 .623 1 .Lt-66 1.510

0.15M 1.560 1 .514 1.488 1.594 1.443

0.25M 1 .472 1.567 1.493 1.239 1.289

--------------------

--------------------------------------------

37

Bunton has examined the acid hydrolysis of ethyl

and t-butyl acetate in water and finds that for ethyl acetate,

chloride ion has a larger effect than perchlorate ion while

the reverse is true for t-butyl acetate (78). This result

has been ascribed to an interaction between carbonium-ion

like transi tion state and the perchlorate ion. He has also

shown the effect of salts upon the stability of the tri-£­

anisylmethyl cation relative to that of Q-nitroanilinium

ion to be in the order Nac104 ·> LiCl04) MeS03

Na > NaBr >

NaN03

:> LiCl. The implication is that the magnitude of

the interaction will increase not only with the size of

the organic moiety but also with the increase in the

delocalisation of the cationic charge.

Further support comes from the work of Diamond (79).

He has found that for large unhydrated univalent ions, a

tighting of the surrounding water structure is a dominant

feature of their aqueous solution behaviour. As represented

by their activity and osmotic coefficients, this corresponds

to a rise in the coefficient above the Debye-HUckel limiting

law and the increase is greater, the larger the ion. But

if both the cation and anion are such larger hydrophobic ion,

the hydrogen bonded water structure forces them together to

maximise the water - water interaction and minimise the

disturbance to itself. This water structure enforced ion-

38

pairing is very different from the more usual Bjerrum type

of ion pairing. This is the view accepted and exploited l't

by Bu,ton (78). He has accounted for the large decrease

in the activity coefficient of the transition state in

the solvolysis of isobornyl chloride in both aqueous

methanol and aqueous acetone by invoking this type of

ion pairing between the carbonium ion like transition

state and perchlorate ion. If we postulate this type of

ion pairing in water we can account for the slight increase

in rate when the lithium perchlorate concentration is

increased.

The result with sodium sulphate and lithium sulphate

appear to be difficult to account for. The results are in

Table 1 and 2. There is negative salt effect for both

sodium and lithium sulphate. The decrease in rate for

sodium sulphate is greater than that of lithium sulphate

which is in agreement with the structure breaking effect

of the anions. Taft observed rate increase for t-butyl

chloride in water (24) for sodium sulphate. Ramaswamy Iyer

studied the effect of lithium sulphate on the hydrolysis of

t-butyl, t-amyl, diethyl methyl, triethyl chloride in 60%

aqµeous acetone (80). At lower concentration, the rate :nc�e�se

increases as the series is ascended, but the reverse order

is noticed at the higher salt concentration. Since sulphate

39

ion is doubly charged, greater salt induced medium effect and

consequently greater stabilisation of the initial state is

expected. This appears to be the dominant effect at the

higher salt concentration because the trend in the values

is closely similar to that with sodium chloride. On the

other hand the results at low salt concentration appear

to need the postulation of a small contribution due to the

ion pairing effect. Menninga and Engberts showed that in.

the hydrolysis of aryl sulphonyl methyl perchlorates in

water sodium sulphate increases the rate of hydrolysis (73).

Sivaramakrishnan observed negative salt effect for the

hydrolysis of acid phthalates of a number of aliphatic

alcohols with sodium and lithium sulphate in water (81).

The results are set out in Table 3 and 4 and the work in

this field is progressing. At this stage we are not in a

position to predict the exact cause of this negative salt

effect but in the hydrolysis of acid esters of dicarboxylic

acid in water there is negative salt effect both for sodium

sulphate and lithium sulphate. Clearly a great deal more

needs to be learnt about salt effects on rate of reaction in

water.

KINETICS OF THE HYDROLYSIS OF SOME ACID SUCCINATES IN WATER

In this section we consider the kinetic data for

the hydrolysis of acid succinates of 1-phenylethyl alcohol

TABLE 3

EFFECT OF LITHIUM SULPHATE IN THE HYDROLYSIS OF THE HYDROGEN PHTHALATESOF ALIPHATIC

ALCOHOLS IN WATER AT 65° C

(Li2so4

)= O.OOM 0.05:M 0.10M 0.20M 0.30M 0.50M -�------ ------------- -------------- ------------- ------------- ---------------

Ester 104k 104k % age vari--1 -1 at ion

n-Amyl hydro-gen phthalate 0.0549 0.0283 -48.5

Sec.Amyl hydro-0. 01 86 -25 • 9gen phthalat e O .0251

n-Butyl hydro-gen phthalate 0.0662 0.0474 -28.4

iso-Butyl hydrogen 0.0551 0.0408 -29.9 phthalate

t-Butylhydrogen 00815 0.545 -33.1 phthalate

104k % age

4 varia- 10 �1

% age vari--1 tion at ion

0.0239 -56.5 0.0187 -65.9

0.0163 -35.1 0.0132 -47.4

O .0423 -36.1 0.023 -57.7

0.0364 -33.9 0.0318 -42.3

0.520 -36.2 0.522 -35.9

104k % age

104k ;lo age

vari- vari--1 at ion -1 at ion

0 .0142 -74.1 0.0108 -80.3

0.0106 -57.8 0.0104 -58.(

0.0216 -67.4 0.0202 -69.5-

0.0235 -57.4 0.0239 -56.6

0.525 -35.6 0.565 -30.7

-----------------------------------------�-------------------------------------------------------

.p,.

C)

TABLE 4

EFFECT OF SODIUM SULPHATE IN THE :HYDROLYSIS OF THE HYDROGEN PHTHALATES OF

ALIPHATIC &b-�fil ALCOHOLS IN WATER AT 65 ° C

(Na2

so4

)= O.OOM 0.05M 0 .1 OM 0.20M 0.30M 0.50M ------- -------------- -------------- ------------- ------------- --------------·-

Ester

n-Amyl hydro­gen phthalate

Sec.Amyl hydrogen phthalte

n-Butylhydrogenphthalate

Iso-butyl hydrogen phthalate

t-Butylhydrogenohthalate

104

k 104

k% age

104k

% age 104

k% age

104k

% age 104k

7b agE vari- vari- vari- vari- vari-

-1 -1 ati.on-1 at ion

-1at ion

-1 at ion -1 at ion

0.0549 0.0242 -55.9 0.0239 -56.5 0.0212 -61 .4 0.0137 -75.1 0.0130 -76.3

0.0251 0.020 -20.3 0.0204 -18.7 0.0129 -48.6 0.0113 -55.0 0.0104 -58.6

0.0662 0.0451 -31.9 0.0448 �32.3 0.0258 -61.0 0.0244 -63.1 · 0.0204 -69.2

0.0551 0.0418 -24.1 0.0364 -33.9 0.0362 -34.5 0.0266 -51.7 0.0218 -60.4

0.815 0.583 -28.5 0.533 -34.6 o.470 -42.3 o.423 -48.1 o.368 -54.s

-------------------------------------------------------------------------------------------------

Ap

42

and its para substituents (CH3

, C2H5, CH3

o, F, Cl aid Br)

in water.

Table 5 contains the first order rate constants

for the hydrolysis of the acid succinate�-

TABLE 5

FIRST ORDER RATE CONSTANTS IFOR THE HYDROLYSIS IN WATER OF

THE ACID SUCCINATES OF 1-PHENYLETHYL ALCOHOL AND ITS PARA­

SUBSTITUTED DERIVATIVES

1. 1-Phenylethyl hydrogen succinate

Temp oc 1 o4!!1 (sec-1 )

85 0.0811

90 0.139

95 0.216

110 1.03

2. 1.:.P-Tolylethyl hydrogen succinate

65 0.319

70 0.532

73 0.774

80 1 .51

85 2.52

90 4.00

3. 1.:,p-ethylphenylethyl hydrogen succinate

75

80

85

90

00807

1.36

2.01

3.45

4. 1-p-meth0xyphenylethyl hydrogen succinate

20

28

33

40

45

o.457

1 .16

2.05

4.47

8.18

5. 1=,p-fluorophenylethyl hydrogen succinate

90

95

100

110

6. 1-p-chlorophenylethyl

90

95

97

1·00

110

115

0.29

0.534

0.819

2.05

hydrogen succinate

0.0781

0.114

0.142

0.170

0.384

0.598

7. 1=:P-bromophenylethyl hydrogen succinate

97

100

110

115

0.106

0.136

0.245

00355

The first point to be noted is that the rate of

44

hydrolysis of the esters follow the Baker and Nathan order

(82). In a careful review, Berliner has listed two possible

factors, the operation of which, either alone or in con-

junction, could explain the Baker-Nathan order without

recourse to hyper•conjugation, although he himself has

expressed his preference for the latter alternative (83).

The first factor is based on the idea that there are no

essential difference in ground state solvation of the sub-

strate is not necessarily valid. For example, it has been

suggested that the Baker-Nathan order observed in the

methanolysis of £-alkylbenzyl chloride is due to this cause (84).

Arnett's calorimetric measurements have served to establish

the reality of a.ch differences (85)o Brown has also

supported this possibility (86). Winstein and Fainberg (87)

as also Hyne (88) have come to similar conclusions. Heat

capacity of activation values by Robertson and his associates

have been adduced in favour of the above hypothesis (89).

Other workers have also argued in support (90-94). Here

45

also, the view is taken that the results can be accounted

for by C-H hyperconjugative interaction. It is also

assumed that the hyperconjugative effect has both a

polarisation and polarisability component. It will be

recalled that it was this idea which led Hughes, Ingold

and Taher to examine the kinetics of para alkyl diphenyl-

methyl chloride to obtain clear cut evidence for C-H

hyperconjugation ( 95). They argued that a strong'1lectron

demanding reaction, such as the SN1* reaction was most

likely to invoke a sufficiently strong polarizability

hyperconjugative effect.

The most commonly encountered and widely studied

influence of a substituent in the reactions of organic

molecule, next to its polar and steric effect is perhaps

the phenomenon of neighbouring group participation where

the substituent influences a reaction velocity by stabiliz-

ing the transition state or an intermediate by becoming

bonded or partially bonded to the reaction site. More often

such participation leads to rate enhancement and is then

turned as intramolecular catalysis. Ester hydrolysis is

perhaps the one single reaction where this phenomenon has

been more frequently met with. Prototropic groupssuch as

* The case for regarding the BAL1 reaction as an SN1reaction has been strongly put by Bender (106)���Vl. �-

46

the carboxyl and hydroxyl group may act either as general

acids, as general bases or as nucleophiles in intramolecular

reactions. Bender (96) and Jencks (97) intensively treat pH

dependence in intramolecular catalysis.

Gaetjens and Morawetz studied the role of intra-

molecul'ar carboxylate attack on ester group by following

the rate of hydrolysis of substituted phenyl acid succinates

and phenyl acid glutarates (98). The anions of phenyl acid

succinates and glutarates are hydrolysed by a unimolecular

mechanism involving an attack of the neighbouring carboxylate

on the ester function. The reaction is very fast compared

to the acetate ion catalysed hydrolysis of phenyl esters.

They determined rates of reaction of 20 substituted deri-

vatives. The rate was found to be unusually sensitive to

electron withdrawing para substituents. The E_-nitrophenyl

glutcl:l.rates reacted 540 times as fast as the phenyl glutarates.

The observations are interpreted by assuming that the inter­

molecular carboxylate attack on phenyl esters leads to tetra-

hedrally bonded reaction intermediate, while the intramole-

cular reaction involves a direct displacement of the phenoxide

by attacking carboxylate. Tom,:)Maugh and I{uice studied the role

of intramolecular bifunctional catalysis of hydrogen glutarates

and hydrogen succinates in water (99). In all cases, however,

only one functional group is found to participate directly

47

in the hydrolytic reaction. The hydrolysis of 8-quinolyl

hydrogen succinates and 6-quinolyl hydrogen glutarates have

bell shaped pH rate profile. Succinic anhydride is the

intermediate in the hydrolysis of succinic ester and

glutaric anhydride is probably the intermediate in the

hydrolysis of glutarate esters as at the optimum pH. These

reactions are much faster than the hydrolysis of the

corresponding quinolyl acetate. It seems likely that the

decrease in the rate that occurs not from the loss of an

intramolecular acid catalyst but from the change in inductive

and resonance effect. A similar conclusion was reached con-

cerning the hydrolysis of 2-carboxyphenyl succtntate.

There is no existing evidence for concerbed intramolecular

general acid nucleophilic catalysis for hydrolysis of the

esters in water. We fully agree with the f;indings of

Bruice and believe that there is no bifunctional catalysis

in the hydrolysis of hydrogen succinate in water.

The entropy and enthalpy of activation are higher

for the 1-phenylethyl hydrogen phthalate and its para sub-

stituted derivatives which exhibits intramolecular acid

catalysis than for the terephthalate ester¢ where there is

no such catalysis (100). The values are given in Table 6.

One generally accepted procedure for ascertaining whether

a neighbouring group participates in a reaction involving

a carbonium intermediate is to alter the structure of the

48

compound in such a way that for its reaction the free

energy of activation is sufficiently reduced to make the

need for participation negligible. This principle first

stated by Winstein and extensively used by him in his

brilliant researches in the field of neighbouring group

participation assumes that the more stable the carbonium

ion centre becomes, the less demand that centre will make

upon neighbouring groups for additional stabilisation

through participation (101). Intramolecular hydrogen

bonding as indicated in the transition state structure

of the phthalates would diminish the need for solvation of

the developing negative charge on oxygen, and thereby make

the entropy of the transition state greater than it would

be if solvated to the extent required for the terephthal�c

ester. The decrease in solvation of the phthalate tran-

sition state is achieved at the expense of an increase in

enthalpy. Higher enthalpy and entropy are characteristic

of intramolecular and intermolecular catalysis (102). In

general the enthalpy of activation of SN1 reactions of

alkyl chloride is decreased while going from water to

aqueous organic solvents. For example, the enthalpy of

activation for t-butyl chloride in 80% aqueous acetone is

22.0 k.cal/mole (103) while in water it is 23.8 k.cal/mole

(27). If the same trend. follow in the case of ester

TABLE 6

ENTROPIES AND ENTHALPIES OF ACTIVATION OF HYDROGEN PHTHALATE

AND TEREPHTHALATE OF 1-PHENYLETHYL ALCOHOL AND 1-l2,-ALKYL

PHENYLETHYL ALCOHOLS

Solvent - 75% v/v acetone water

Ester

1-Phenylethyl hydrogen phthalate

1 -12-To·lylethyl hydrogen phthalate

1-12-Ethylphenylethyl hydrogen phthalate

1.-12-Methoxyphenylethyl hydrogen phthalate

1-Phenylethyl hydrogen ter�phthalate

1-12-Tolylethyl hydrogen terephthalate

1-12-Ethylphenylethyl hydrogenterephthalate

1-12-methoxyphenylethyl hydrogenterephthalate

DH+

k.cal/mole

28.7

28.6

28.8

25.5

28.1

26.9

26.9

25.4

6s* e.u.

-7.5

-2 .. 3

-2.1

-0.2

-16.7

-13 .9

-14.2

-6.8

TABLE 7

ENTHALPIF.s AND-ENTROPIES OF ACTIVATION OF. THE HYDROGEN

SUCCINATES OF 1-PHENYLETHYL ALCOHOL AND ITS PARA­

SUBSTITUTED DERIVATIVES

Solvent - water

1-Phenylethyl hydrogen succinate 27.2 -7

1-E-Tolylethyl hydrogen succinate 24.1 -9

1-E�Ethylphenylethyl hydrogen·succinate 23.4 -11

1-E�Methoxyphenylethyl hydrogen20.9 -8succinate

49

hydrolysis then the enthalpy for the succinic ester

hydrolysis should decrease, when the solvent water is

changed to aqueous organic solvent which is very much less

than that of the phthalic ester. This is another evidence

that there is no neighbouring group participation in the

hydrolysis of hydrogen succinate in water.

Next we can consider the mechanism involved in the

reaction. The mechanism which are plausible are t�o namely

BAL1 and BAC2. Long amplj:fying a suggestion of Taft and

coworkers (104) have proposed the use of entropy as a

criterion of the mechanism of hydrolysis of reactions. The

enthalpy and entropy and free energy are set out in table 8.

TABLE 8

ENTHALPIES, ENTROPIES AND FREE ENERGIES OF ACTIVATION FOR THE

HYDROLYSIS OF 1-PHENYLETHYL HYDROGEN SUCCINATES AND ITS

PARA SUBSTITUTED DERIVATIVES IN WATER

.6H+

L\s+

.6F*-1 -1k.cal.mole e.u. k.cal.mole

1 -Phenylethyl hydrogen succinate 27.2 -7 29.3

1-2-Tolylethyl hyerogensuccinate 24.1 -9 26.8

1-E-Ethylphenylethyl23.4 hydrogen succinate -11 26.7

1-E-Methoxyphenylethylhydrogen sugcinate 20.9 - 8 23.3

_4.6

_4.4

_4.2

.. 4.0

-3.6

-3.2

_3,Q.._ ________ ..._ ________ ..._ ________ ..i_ ________ ..i... ________ -'-________ �

2.70 2,7S 2.as

(1/'r) 103

2.90 2.95 3.00

FIG.1. PLOT OF 1/T VS LOC �I FOR THE HYDROLYSIS OF 1-(P--ALKYLPHENYL) ETHYL HYDROGEN SUCCtNATE IN WATER.

0 I. e,. TOLYL ETHYL HYDROGEN SUCCINATE. /A I -(e.ETHYLP�ENYL) ETHYL HYDROGEN SUCCINATE.

· .4.5

, .4.3

.4.1

-3.9

9 -3.7

-1.S

.. 3.1

-�.9.,._�----.._��"'"""'�-----i.....---------------�------�-------------3.12 '!,16 ),20 3.24 3.28

(1/T) 103

3.32 ),36 3.40

FIG,2 PLOT OF 1/r VS LOG �i FO� THE HYO�OLYSIS OF 1-(e- METHOXYPHENYL) �THYLHYDROCEN SUCCINArE IN WATER.

3.44

.. 5.1

.4.7

.. 4.S

_4.3

�·

_4-1

· -3 .9

-3.7

/ -3.S

-3.3

-3.1--����...1-����-'-����--����--�����---����-

2.so 2.55 2.60 2.65

(1/T) I03

2,70 2.75 2.�o

FIC.3 PLOT OF 1/T VS LOG k_1 FOR THE HYDROLYSIS Of 1- PHENYL AND 1-(P..-HALOCENOPHENYL) ETHYL H YDROGEN SUCCINATE IN WATER.

+ 1-( P... BROMOPHENYL) ETHYL HYDROGEN SUCCINATE.C:l 1-( 2-CHLOROPHENYL:) ETHYL HYDROGEN SUCCINAT E. 0 I- PHENYL ETHYL HYDROGEN SUCClNATE.A 1-(P-- FLOURO�_HENYL) ETHYL HYDROGEN SUCCINATE.

50

1-£-Fluorophenylethyl hydrogen succinate 25.8 -9 28.5

1-£-Chlorophenylethyl hydrogen succinate . 22.2 -22 28.8

1-£-Bromophenylethyl hydrogen succinate 17. 7 -34 27.8

Valuable discussions have been giv�n by Long on

the values of the entropies of activation in organic

reactions (105) and by Bender (106) on the enthalpies of

activation in ester hydrolysis.

An important principle due to Long (105,107) on the

values to be expected for the entropy of activation can be

summarised as follows. When in a bimolecular reaction, two

initial state particles join together to make transition

state particle, the translational and rotational entropies

of two particles become reduced to those of one, there is a

small additional entropy of vibration but not nearly enough

to compensate for the loss of entropy. These changes con­

stitute a negative contribution to the entropy of activation.

There will be other factors also which will affect the magni-

tude of the entropy of activation. But Long proposed that, on

account of the first factor, namely the molecularity, SN2

reactions should generally possess smaller positive or

greater negative entropies of activation than the most nearly

analogous SN1 reactions. Long amplifying a suggestion of

51

Taft and coworkers (104) have proposed the use of entropy

as a criterion of the mechanism of hydrolysis of reactions.

These reactions are usually classified as unimolecular or

bimolecular. In the former case a water molecule does not

participate in the rate determining step, while a water

molecule is usually considered to be bound in the activated

complex in the latter. It seems reasonable that the loss

of trans·1ational and rotational freedom of a water molecule

associated with the bimolecular process should lead to a

lower entropy of activation relative to the unimolecular

case. This prediction is amply borne out by entropies of

activation for unimolecular and bimolecular ester hydrolysis,

typical values of 63 + being O to 10 e.u. for unimolecular

reactions and -15 to -30 e.u. for bimolecular reactions.

Long and Stafford studied the entropies of activation

and mechanism for the acid catalysed hydrolysis of ethylene,

propylene, isobutylene and trimethylene oxides ( 107). The

close similarity in the entropy of activation values strongly

suggests that all of the oxides hydrolyse by the same A-1

mechanism. In a discussion of acid catalysed hydration of

olefins Ta� and coworkers (108) suggested that reaction

by the A-1 mechanism should be characterised by a relatively

more positive entropy of activation than reaction by the

A-2 mechanism since the latter involves a relative increase

52

of constraint on the reaction system in the transition

state due to the orientation and reaction of a specific

water molecule from the solvent. The work of Stimons (109)

on the rates of acid catalysed hydrolysis of t-butyl

esters is a good example. The hydrolysis of t-butyl benzo­

ates and t-butyl formates in acidified 60% aqueous acetone

gives .6S* value of -9.5 e.u. and -23.7 e.u. suggesting that

the former hydrolysis proceeds by an A-1 mechanism and the

latter by an A-2 mechanism.

Two examples of hydrolysis of esters following the

BAL 1 mechanism has been provided by Kohnstam (110). This

relates to the hydrolysis of the :12.-nitrobenzoates of

diphenyl methanol and its para-methoxy derivatives in 70%

aqu$us acetone. The entropy values are respectively -5.9

e.u. and -8 e.u. If we accept the argument that resonance

stabilisation of a carbonium ion increases the entropy of

activation of a reaction involving a carbonium ion as

intermediate - the reason is that resonance results in

delocalisation of the positive charge and reduces the

need for solvation - then we should expect a greater entropy

of activation for the esters of the diphenyl methanols than

for the 1-phenylethyl esters in aqueous acetone. For the

hydrolysis of 1-phenylethyl hydrogen succinates in water

the entropy of activation should be greater than for the

53

hydrolysis of the ester in aqueous organic solvent and is

approximately equal to the diphenyl methanol system.

Two recent examples of hydrolysis of esters follow­

ing BAL1 mechanism has been provided by Hawkins (111).

This relates to the hydrolysis of hydrogen phthalates of

diphenyl methanol and its para methoxy derivative in 20%

v/v dioxan. The entropy value for diphenyl methyl hydrogen

phthalate is -5 e.u. Radhakrishnan Nair studied the

hydrolysis of acid phthalic and acid terephthalic esters

of 1-phenylethyl alcohol and its para-alkyl and para methoxy

derivatives in aqueous acetone proceeding by the BAL1

mechanism. The entropy and enthalpy values are set out

in table 9 for comparison. He found that optically active

monophthalate of 1-phenylethyl and 1 -12.-t-butylphenylethyl

alcohols yielded racemic alcohols on hydrolysis (100).

Before applying these principles to the present

data, it might be well to remember Long's warning that his

criterion must be used with circumspection because within

the same mechanism a great scatter of values of entropies

of activation is found. But we believe that Long's

postulate might be safely and profitably applied in the

present case. First we note that the trend in values within

series is the same for the succinic, phthalic and tere-

phthalic esters which is also shown by other series follow-

ing Baker-Nathan order. The entropy values for pH, p-CH3

,

54

p-C2H5 and P-CH3o are essentially similar to acid phthalicester suggesting that the hydrolysis proceeds by BAL1

mech�ism. For p-Cl and p-Br derivatives the entropy

values are highly negative -22 e.u. and -34 e.u.

respectively which is typical for a bimolecular reaction

i.e. BAC2 mechanism.

The Okomoto-Brown equation has been applied to the

present data. It is known that in system showing Baker-

Nathan order the Hammett eqµation in its original form is

not applicable (112). The plot is linear one for all

except for Cl and Br derivatives, the correlation coeffi-

ient being 0.997. The suggestion is made that the first

five compounds Q-H, Q-CH3, Q-C2H5, Q-CH3o, Q-F show the

behaviour which is characteristic of the BAL1 mechanism

while the other two follow what is generally observed as

the BAc2 mechanism.

The rate ratios k/k8 for the hydrolysis of succinic

ester in water, phthalic ester in aqueous acetone (100),

benzhydryl chloride in 70% aqueous acetone (113) and benzyl

chloride in 50% aqueous acetone (114) are set out in

table 11.

55

TABLE 9

ENTI-IALPIES AND ENTROPIES OF ACTIVATION FOR THE HYDROLYSIS

OF PHTI-IALATE AND TEREPHTI-IALATES OF 1-PHENYLETHYL ALCOHOL,

1-(p-ALKYLPHENYL) ETHYL ALCOHOIS, 1-(p-METHOXYPHENYL)

ETHYL ALCOHOL

Solvent 75% v/v acetone - water

Ester

1 -Phenylethyl hydrogen phtha;iate

1-2-Tolylethyl hydrogen phthalate

1-(2-ethylphenyl)ethyl hydrogen phthalate

1-(2-iso-propylphenyl)ethyl hydrogen phthalate

1-(2-tert-butylphenyl)ethyl hydrogen phthalate

1-(2-methoxyphenyl)ethyl hydrogen phthalate

1-Phenylethyl hydrogent erephthalat e

1-(2-tolylethyl) hydrogen t erephthalat e

1-(2-ethylphenyl)ethyl hydrogen terephthalate

1-(2-tert-butylphenyl)ethyl hydrogen terephthalate

1-(2-methoxyphenyl)ethyl hydrogen terephthalate

* _1-6.S e .u.

k.cal mole

28o7 -7.5

28.6 -2.3

28.8 -2.1

29�1 -1 .4

29.3 -1 .5

25.5 -0.2

28.1 -16. 7

26.9 -13.9

26.9 -13.9

27 .1 -14.4

·25.4 - 6.8

:r: .x• ...;.,.,

« a: �·

0 . 9.

4

3

2

0

_, -0·9

FIG.4 PLOT OF

-0,7 -0-5 -0,3 -0-1 0,1 0-3

� VS LOG �P., R/�ij FOR THE HYDROLYSIS OF HYDROGEN S!:)CC lNATES lN WATER .

TABLE 10

OKOMOTO-BFOWN � VALUES AND LOG k/k FOR THE HYDROLYSIS. 0

OF 1-PHENYLETHYL HYDROGEN SUCCINATF.1 AND ITS PARA SUB-

STITUENTS IN WATER

Substituent log k/k0

OCH3

3.6034 -0.778

CH31 .4594 -0.311

Et 1 .3993 -0.295

H 0 0

F 0.3265 -0.073I

Cl -0.2510 0.11§

Br -0.3314 0.150

56

57

TABLE 11

RATE RATIOS .KJ'kH FOR THE HYDROLYSIS OF ACID SUCCINIC ESTER,H

ACID PHTHALIC ESTER, BENZJDRYL CHLORIDE AND BENZYL CHLORIDE

Acid succinic ester 29 25 2.1 0.56 o.47

Acid phthalic ester 16.4 14.2

Benzhydryl chloride 29.6 22.2 1.9 0.32 0.25

Benzylchloride 9 1.7 0.59 o.47

Reactions 2 and 3 are unimolecular substitution

reaction (100,113) while reaction 4 though mechanistically

marginal is predominantly bimolecular (115). Kohnstam (116)

Robertson (117) and their coworkers have provided adequate

confirmation. The entropy values for benzyl chloride

clearly points to the conclusion that the reaction is

largely SN2. The same conclusion has been reached by

Kohnstam from consideration of the heat capacity data (116,

118). It can be clearly seen that the rate ratios differ

markedly for reactions belonging to the two categories.

From the results we can infer £-H, £-CH3, £-C2H5, £-F

substituents hydrolyse by the SN1 mechanism while £-Cl

and Q-Br by SN2 mechanism.