Fundamentals of organic chemistry

Introduction

BREAKING OF A COVALENT BOND

Breaking of a covalent bond between two atoms can take place mainly in two alternative ways, viz. homolytic and heterolytic

fissions depending upon the relative electronegativity of the two concerned atoms and medium of reaction.

(i) Homolytic fission

takes place when the two atoms (say A and B) are usually of similar electronegativity

(ii) Heterolytic fission

take place when the two atoms (A and B) are of different electronegativities. It may again take place in two different ways.

(a)

When A is more electronegative than B

(b)

When B is more electronegative than A

It is important to note that homolytic fission requires much less energy (e.g., 67.2 kcal/mole for C Br bond in H3C CH2 Br

into H3C ) than the heterolytic fission (e.g., 183 kcal/mole for C Br bond in CH3 CH2 Br into CH3

CH2+and Br

-)

[1] Types of reactions

(i) Electrophilic addition reactions

Theses occur reaction in alkene and alkyne only where primary attack of electrophile takes place due to thick

in between two carbon atoms

polarity in alkene and alkyne is created by + I and I effect of different groups than attacking reagent add with opposite charge.

(II) reaction is not an example of electrophilic addition reaction because no addition occurs across the double bond while reaction

(I) is an example of electrophilic addition reaction where addition occurs.

Due to dense

and delocalisation of electrons aromatic compounds undergo substitution reaction rather than addition reaction.

Nucleophilic addition reaction

(ii) Nucleophilic addition reaction

These type of reaction are given by carbonyl compounds (aldehydes & ketones) where primary attack of nucleophile takes place

due to more stability of intermediate species. >C = O always exist in polar form where carbon with +ve charge while oxygen

contains ve charge.

(iii) Nucleophilic substitution reaction (SN type)

These type of reactions occurs in by alkyl halides, aryl halides, acids and their derivatives where attack of nucleophile takes place

on

and halogen comes out as leaving group.

(iv) Electrophilic substitution reaction (SE type)

Given by Aromatic compounds and their derivatives where high

density is present with delocalisation of electrons so the compound prefers substitution rather than addition.

Weaker the base better the leaving group, due to which RCOCl is more reactive than RCONH

2

, this property of leaving group is known as (FUGISITY)

(v) Elimination Reaction (Mechanism expressed in end of this chapter) (a) Decarboxylation by soda lime [ NaOH + CaO] [Only applicable on acids & their

derivatives ] removal

of CO2

gas means COOH is replaced by -H. Here CaO is porous responsible to absorb CO2gas.

Methods to prepare higher to lower alkanes and vice versa

NaOH + CaO has no selective decarboxylation. So all the COOH are removed in molecule.

(b) Dehydroyhalogenation -

H X is removed using alcoholic KOH or strong bases as reagent.

In this case CH3CH = CHCH3

is the major product. According to Saytzeff's rule more the alkyl groups are attached to the

doubly bonded carbon atoms, more is the stability of alkene. The hydrogen is taken out from

the neighbouring carbon atom or the

, by strong bases like alkoxides of metal.

(c) Dehalogenation

removal of halogen (- X2) by Zn dust.

V

Inductive effect

[2] Inductive effect

Induction of charge due to less or more electronegative element is known as inductive effect. It occurs till four carbon atom and

maximum at first carbon atom due to closeness impact.

Application of inductive effect

(a) Acidic strength of Carboxylic acid

As the 'S' character increases electronegativity of atom also increases so that electro negativity order is sp >sp2> sp

3

.

(b) Basic strength of amines

II pd. elements are more basic than III pd. elements because in II pd elements vacant d- orbitals are absent.

H2O > H2S and NH3 > PH3 due to vacant d orbitals

H2O < NH3

For same period elements consider electronegativity order. Less electro negative element is more basic.

R3N is less basic than R2NH in H2

O because of the steric hinderance and solvation effect caused by the three bulky R groups.

(c) Reactivity of Carbonyl Compounds

HCHO > CH3CHO > CH3COCH3

carbonyl compounds give nucleophilic addition reactions where primary attack of nucleophile takes place.

As the size of alkyl group increases stearic hinderance comes into play thus reactivity decreases.

Reactivity of alkyl halides

(d) Reactivity of alkyl halides

It can be deduced by reacting substance with Ag salt (AgNO3).

On the basis of bond dissociation energy RI is maximum reactive due to largest size of I atom and minimum bond dissociation

energy.

Free radical substitution reaction

These type of reactions are given by alkane, alkene and some aromatic compounds in the presence of sunlight or high temperatu re.

For free radical substitution in alkenes search for the allylic position and substitution takes place at this position because of

intermediate stabilization through conjugation.

In toluene and their derivatives free radical substitution reaction occurs at benzylic position. If there is possibility of substitution at

both allylic and benzylic position than substitution at benzylic position is given preference due to more intermediate stability.

[3] Resonance

It is to be emphasized that the different contributing structures of a molecule differ only in the distribution of electrons and not in

the arrangement of atoms, i.e., the position of atoms remains the same (different from tautomerism). Moreover, each canonical

form must have same number of paired electrons, only their positions may differ. Usually, more are the number of canonical forms

in a compound, more is its resonance energy, so more is the stability.

(i) Difference in stability of various canonical forms of a compound : All the canonical forms of a compound are not

necessarily equally stable. One form may be more stable than the other and hence contributes more to the actual resonance hybrid.

Following are the main factors governing the stability of a canonical structure :

1. Nonpolar structure is more stable than a polar structure.

2. More are the number of covalent bonds in a canonical structure, more is its stability.

3. In polar structures, that structure is more stable in which the negative and positive charges exist on the most electronegative and

the most electropositive atoms respectively.

4. Out of the two canonical structures, the one with completed octet of various atoms (or duplet of hydrogen) is more stable even if

the more electronegative atom has positive charge.

5. A canonical structure having an electron deficient positively charged atom is much less stable.

6. Also a structure in which like charge are on the atoms close to each other, is least stable.

(ii) Rules for writing resonance structures

(a) No real existence : Resonance structures exist only on paper. Resonance structures are useful because they allow us to describe

molecules, radicals and ions for which a single lewis structure is inadequate. We write two or more lewis structures, calling them

resonance structures of resonance contributors. We connect these structures by double headed arrows and we say that

the real molecule, radical or ion is a hybrid of all of them.

(b) In writing resonance structures we are only allowed to move electrons

Position of nuclei of the atoms must remain same in all resonance structures e.g.{

(c) All of the structures must be proper lewis structures

(d) Charge separation should be low since, to separate charge energy is required, therefore, structure in which opposite charges ar e

separated have greater energy and hence are less stable.

(iii) Resonance Energy

The difference in energy between the hybrid and the most stable canonical structure is called as Resonance energy.

Any halogen or electronegative element always prefers a negative charge if there is both option of +ve and ve charge

In the resonating structure of acryl aldehyde we prefers only 'O' with negative charge due to stability.

There is always two types of bonding one is localised and another one is delocalised which is responsible for resonating

(contributing) structures.

Structural isomers are real molecules whose atoms are linked together in different ways to form different skeletons but

resonating structures are not real. They are written whenever one electronic structure cannot adequately represent the actual

structure.

Cross conjugation

(IV) Cross conjugation

Two systems conjugated with each other, a third system such that they are not conjugated with each other represent a cross

conjugated system for example :

Prob. Account for the shorter C O length in an ester compared with an anhydride.

A degree of Cross conjugation exists in the anhydride; this competition for electron decreases the delocalization of each

carbonyl O and gives the C O s bond less double bond character.

In the anion of CH3COOR, crossconjugation with the O of OR competes with delocalization of the ve charge delocalization

from C and making the ester less acidic.

Prob. C6H5CONH2 is stronger base than CH3CONH2. Why?

Due to Cross conjugation in C6H5CONH2 which increases its basic nature.

Aromatic compounds shows comparatively less exothermic enthalpies of combustion and hydrogenation as compare to aliphatic

system. They also show a resistance towards addition reaction and easily undergo substitution (S E) reaction. In aromatic benzene

type rings actually there is partial fixation of bonds. In this all the carbon atoms does not have same bond length and bond angle

due to different bond order.

(V) Condition for aromaticity (one aspect of resonance)

(a) Which obey (4n + 2) Huckel rule

(b) Planarity of molecule (sp2 hybridisation)

(c) Absence of sp3 hybridised atom in that portion where ring current is present.

(d) High resonating energy.

According to (4n + 2) Huckel rule where n is whole number n = 0,1,2,3 ............ ....... aromatic

system like cyclo propenyl cation is smallest aromatic ion. This rule also help in stability, acidic and basic nature of compounds.

Prob. Why pyridine is more basic than pyrrole ?

In pyridine lone pair cannot take part in resonance due to stable 6 system while in case of pyrrole resonance

occur by lone pair of electron, and it satisfy aromatic rule also.

Prob. Cyclopropene is non aromatic while cyclo propenyl cation is a aromatic smallest cation. Why?

Prob. Cyclopropene is non aromatic while cyclo prop2ene1one is aromatic compound. Explain why?

Prob. Explain acidic strength order.

Prob. Why SiH3OH more acidic than CH3OH?

Prob. Explain the acidic strength order

Prob. Explain basic strength order.

Prob. Write down the Lewis acid or base nature order

(a) NF3, NCl3, NBr3, NI3, (Lewis base) Ans: NI3 > NBr3 > NCl3 > NF3

(b) BF3, BCl3, BBr3, BI3, (Lewis Acid) Ans: BI3 > BBr3 > BCl3 > BF3

(c) B(OC2H5) and B(C2H5) (Lewis Acid) Ans: B(C2H5)3 > B(OC2H5)3

(d) (CH3)3N and (SiH3)3N. (Lewis base) Ans: (CH3)3N > (SiH3)3N

In this case as the electron system of oxygen is complete in first case so it is more stable but in (II) case this species does not

contain complete octet of 'C' atom.

Prob. Basic nature: Explain basic strength order for CH3NH2>NH2CONH2>CH3CONH2

Ans: CH3NH2 is maximum basic due to absence of resonance.

Prob. Which is more reactive towards AgNO3 and why ?

CH2 = CH - Cl < Cl CH2 - CH = CH2

In vinyl halide resonance occurs which develops double bond characters in C-Cl bond and reduces reactivity.

Stability of Carbocation

Benzylchloride is more reactive than allylchloride due to more stability of intermediate carbocation.

For example CH3 - CH = CH - Cl Resonance

CH3 C(Cl) = CH2 Resonance CH2 = CH - Cl Resonance

As Cl is attached to doubly bonded carbon atom it is unreactive therefore NaNH2 (which is more powerful than alc KOH) is used

for dehydrohalogenation.

Conjugate diene C - C = C - C = C - C are those where alternate double bond are present. 1,3- Butadiene is also conjugated diene

where resonance occurs and shows both 1,2- & 1-4 electrophilic addition reaction due to conjugation.

1,3 Butadiene at low temperatures mainly forms 1,2 product while at high temperatures forms 1,4 product which is based on

concept of thermodynamic control and chemical kinetics (discussed in next chapters).

Q.1. Explain basic character

Q.3. Why guanidene NH2C (CH2)=NH is a strong base (pKb = 1)

Q.4. Which one is more basic in pyridene and piperedene ?

Ans. Piperdene, due to absence of resonance and sp3 'N' atom.

Q.5. Why chlorobenzene is less reactive than benzyl chloride ?

Ans . Due to resonance in chlorobenzene doubly bonded character in between CCl bond.

VI) Resonance in aromatic compounds

Phenol is acidic in nature due to resonance which develops +ve charge on 'O' atom and

releases H in the solution. Due to negative charge at ortho and para position of benzene

nucleus attack of electrophile takes place at o & p positions.

Prob. What is the separation test between following ?

(a) Directive Influences of functional groups.

Ans: As the number of NO2 group increases, positive charge on carbon atom increases so speed of attack of np also increases.

Hyper conjugation or no bond resonance or Baker-Nathan Effect [4] Hyper conjugation or no bond resonance or Baker-Nathan Effect

Stability order of alkenes

Prob. Write down the reactivity order towards electrophilic substitution reaction (good Puzzle)

Toluene is maximum reactive due to maximum hyperconjugation which develop maximum negative charge on the benzene ring

and accelerates attack on benzene nucleus.

In elimination reactions like dehydration and dehydrohalogenation product are formed according to saytzeff's rule.

Effects of hyperconjugation

(a) Bond Length : Like resonance, hyperconjugation also affects bond lengths because during the process the single bond in

compound acquires some double bonded character and vice-versa.

e.g. C C bond length in propene is as compared to in Ehtylene.

(b) Dipole moment : Since hyperconjugation causes these charge developments, it also affects the dipole moment of the molecule.

(c) Stability of carbonium ions is Tertiary > Secondary > Primary

Above order of stability can be explained by hyperconjugation. In general greater the number of hydrogen atoms attached to

, the more hyperconjugative forms are formed and thus greater is the stability of carbonium ions.

(d) Stability of Free radicals : Stability of Free radicals can also be explained as that of carbonium ion

(e) Orientation influence of methyl group : The o,pdirecting influence of the methyl group in methyl benzene is attributed partly

to inductive and partly of hyperconjugation effect.

The role of hyperconjugation in o,pdirecting influence of methyl group is evidenced by the part that nitration of p-iso propyl

toluene and p-tert-butyl toluene form the product in which NO2 group is introduced in the ortho position with respect to methyl

group and not to isopropyl or t- butyl group although the latter groups are more electron donating than methyl groups.

i.e., The substitution takes place contrary to inductive effect. Actually this constitutes an example where hyperconjugation over

powers inductive effect.

[5] Electromeric effect

It is the complete transfer of of a multiple bond towards one of the bonded atoms at the demand of an attacking

reagent. The transfer of electrons takes place towards the more electronegative of the two bonded atoms.

For example, when an addition reaction takes place at a carbonyl group (>C = O), the of the double bond are shifted

at O-atom because it is more electronegative than carbon :

In Ethylene (ethene) molecule, shifting of p electrons may take place at any of the doubly bonded C-atoms since both the atoms are

identical :

However, in Propene the shifting of electrons takes place at carbon atom no.1 because of the +I effect of methyl group :

Electromeric effect (shifting of p electrons) is a temporary effect and takes place only at the demand of an attacking reagent during

the course of a chemical reaction. In short it is termed as the E effect.

If the I-effect and E - effect oppose each other then usually the E - effect predominates, i.e.,

Application of electromeric effect : The mechanisms of several organic reactions particularly the addition reactions, are explained

by the help of electromeric effect.

[6] Steric effects

Whenever a chemical reaction between two compounds takes place, directly or indirectly it results in the formation of bond

between the atoms of these two compounds. The bonding atoms of the two reacting compounds are, in fact, the active centres.

These will react (form the bond) with one another only if they come with in the range of each other, i.e., within the attraction of

each other. If we surround one of them with such mass of other atoms or groups that the other reacting atom is unable to force its

way in, the reaction may not at all take place or may take place only slowly. Such a hindrance due to spatial crowding (crowdin g a

space) is called steric hindrance. However, the spatial crowding may not always hinder a reaction, sometimes it may facilitate a

reaction. Therefore, the term steric effect is better than the term steric hindrance.The phenomenon of steric effect was first

identified by Hofmann in 1872. It may have sufficient influence on the physical and chemical properties of a molecule and may be

defined as modification in molecular properties resulting from a spatial crowding of a reacting atom in a molecule. A good

example of steric effect is discussed here:

The tertiary amine with the name Tri methylamine reacts with methyl iodide to form a quaternary salt tetra methyl ammonium

iodide, but if the alkyl groups in tertiary amine are large, it does not react with methyliodide and so, does not form the quaternary

ammonium salt :

Another good example is that of esterification between a carboxylic acid and an alcohol. Bulkier the alkyl group in acid or alcohol,

slower is esterification.

[7] Reactivity of various chemical reactions

1. Electrophilic addition reaction

Alkene is more reactive than alkyne towards addition reaction because of thick in case of alkyne.

There is no addition reaction in case of benzene because resonance occurs in the molecule which stabilises the cloud

2. Nucleophilic addition reaction

As the - I effect or - M effect increases, + ve charge on > C = O group increases so that speed of primary attack of nucleophile on

carbon also increases.

3. Nucleophilic Substitution reaction

Weaker the base better the leaving group and conjugate base of strong acid always weak.

Resonance or Back bonding of electrons in vacant orbital always creates double bond character in between C X due to this,

chemical reactivity decreases.

C6H5ClCH2 = CHCl,CH3CH=CHCl and CH3CCl=CH2 these all are less reactive halide due to resonance.

While C6H5CH2Cl and Cl CH2 CH = CH2 are very reactive due to absence of resonance and high stability of intermediate

carbocation.

As the +ve charge on carbon attached with halide, increases speed of attacking nature of nucleophile also increases.

4. Electrophilic substitution reaction

Chemical reactivity of aromatic compounds is decided by activating or deactivating series of group. Activating groups are mor e

reactive towards electrophilic substitution reaction because they develop ve charge on the benzene ring. These are o + p directing

in nature. While deactivating groups are less reactive because they develop +ve charge on benzene nucleus and these are meta

directing in nature.

Deactivating groups are always less reactive than activating groups.

Mechanism of elimination reactions

[8] Mechanism of elimination reactions

(a) Decarboxylation by sodalime (NaOH + CaO)

acid undergo easily in decarboxylation even by heating due to highly stable intermediate carbanian.

(b) Dehalogenation by Zinc dust

1,1,dihalogen (gem) or 1,2dihalogen (Vicinal) derivatives of the alkanes by reaction with zinc dust and methanol produces

alkenes by loss of two halogen atoms (dehalogenation).

If sodium is used (instead of zinc) then 3 hexane is mainly produced from 1,1dibromo propane which is same as wurtz reaction.

NaI in acetone can also be used instead of zinc dust for dehalogenation due to large size of iodine atom.

Addition (to the alkene) and elimination (from the dibromide) of the two bromine atoms are predominately trans so the reaction is

stereoselective.

Elimination reactions are divided into three classes depending on the type of cleavage of two

E 1 (Elimination) reaction

Step 1 : The C L bond is broken heterolytically to form a carbocation (as in SN1 reaction)

Step 2 : Carbocation loses a proton from an adjacent carbon atom to form a in presence of a nucleophile.

Step 1, in which is lost is rate determining step hence we call it E1 (unimolecular elimination) reaction.

If base B is added to the transition state then we call it nucleophilic substitution reaction whether a deprotonation or a substitution

takes place on the transition state 1 depends on their relative rates but remember.

E1 reaction is favoured in compounds in which the leaving group is at a secondary or tertiary position while SN reaction is favoured

in which leaving group is at primary carbon.

E1 CB (Elimination) reaction

Step 1 : Consists of the removal of a proton, H+, by a base, generating a carbanion (2).

Step 2 : Carbanion (2) loses a leaving group to form alkene.

Because step 1 (deprotonation) is fast and reversible, the reaction rate is controlled by how fast the leaving group is lost from the

carbanion (2) (conjugate base). The loss of L from (2) in step (2) is rate determining step and is unimolecular. We call it E1 CB

(unimolecular conjugate base elimination) reaction.

E2 (Elimination) Reaction

In this mechanism, two s bonds are broken and a formation takes place simultaneously.

It is bimolecular since substrate and base are involved in the rate determining step. So we call it E2 (bimolecular elimination)

reaction. Isotopic effect occurs because C H bond breaking take place in a slow process.

E2 process does not proceed through an intermediate carbocation.

(c) Dehydration of Alcohols

Dehydration of alcohol in presence of an acid gives alkene by intermediate carbocation formation so rearrangement also occurs .

H2O is a better leaving group than OH hence carbonium ion is formed easily by elimination of H2O by E1 mechanism. In case of

1 alcohol, loss of H2O forms less stable carbonium ion hence loss of water and deprotonation to form alkene takes place

simultaneously and intermediate carbonium ion is not isolated. This will follow then E2 mechanism.

2 and 3 alcohol by E1 process and 1 alcohol by E2 process.

Alcohols leading to conjugated alkenes are more easily dehydrated than the alcohols leading to nonconjugated alkenes due to

more stable intermediate carbonium so but 1ene2ol is more easily dehydrated than 2butanol.

Similarly

Dehydrohalogenation (removal of HX)

(d) Dehydrohalogenation (removal of HX)

Elimination of HX from alkyl halide is called dehydrohalogenation, takes place in presence of strong bases like alcoholic KOH or

NaNH2 or potassium tertiary butoxide or metal alkoxides.

Dehydrohalogenation can follow go through by following mechanisms.

(a) E1 mechanism (b) E2 mechanism

Reactivity of alkyl halide towards E2 or E1 elimination is the same 3 > 2 > 1.

E1 Dehydrohalogenation

Step 1 : Loss of Cl produces a 2 carbonium ion in rate determining step

We know substituted alkenes are more stable, hence formation of 2butene is preferred to 1butene.

There can be 1, 2 hydride and 1,2methyl shift to attain greater stability of the carbonium ion. This property of carbocation affects

the regiochemistry of E1 elimination.

The formation of the lesssubstituted alkene in an elimination reaction is called as a Hofmann Elimination and that of more

substituted alkene as a Zaitsev Elimination.

E2 Dehydrohalogenation

Loss of proton and leaving group (Br) takes place simultaneously in presence of a base. Thus rate determining step, in which base

and alkyl halide are involved, is bimolecular (E2).

Elimination of can also take place from C1 to give Hofmann product.

If base are large, due to steric hindrance, less substituted alkenes are preferred (Hofmann product). But if base is small, m ore

substituted alkenes are preferred (Zaitsev product).

E2 elimination is stereospecific (Anti)

For transelimination leaving groups H and X must be as far apart as possible in anti relationship.