On the catalytic cyclization of aliphatic hydrocarbons....

On the catalytic cyclization of aliphatic hydrocarbons. II

By E. F. G. H erington and E. K. R id e a l , F.R.S.

{Received 24 November 1943)

The products formed by the passage of a num ber of pure hydrocarbons over an arom atizing catalyst have been analysed by a spectroscopic technique. The occurrence of two different types of isomerization has been dem onstrated. One m ay be exemplified by the formation of and m- xylenes from n-octane. This isomerization accompanies ring closure and proceeds smoothly a t 475° C; it entails the formation and rupture of a subsidiary ring.

The apparent discrepancy between the observations of Hoog, Verheus & Zuiderweg (1939), who report th a t 2 .2 .4-trim ethyl pentane does not arom atize, and the observations of Green & Nash (1941), who report th a t a considerable quantity of aromatics is formed, has been shown to arise from the different tem peratures employed. The formation of aromatics from substitu ted pentanes is a general phenom enon; the products from all the isomeric octanes in question have been analysed. The isomerization proceeds through a five-six carbon atom ring mechanism.

In Part I (Herington & Rideal 1945) it was shown that a satisfactory mechanism for the process of catalytic ring closure of aliphatic hydrocarbons on oxide catalysts could be devised on the assumption that the process involved a two-point contact between the catalyst and a pair of carbon atoms in the hydrocarbon, the ring closing by reaction between a carbon atom of the chain which is in the gas phase and one of the adsorbed carbon atoms. By means of this model the dependence of total yield of aromatic hydrocarbon on the structure of the paraffin could be calculated from simple statistical considerations. This treatment, however, did not correctly predict the relative quantities of different aromatic compounds formed when several possibilities for ring closure were present. The good agreement between the calculated and observed total aromatic yield on the one hand and the simultaneous lack of agreement in the distribution of the products on the other, suggested that in certain cases isomerization must occur, a view strengthened by the reported formation of 2 .5-dimethyl phenol from n-octyl alcohol (Komarewsky & Coley 1941).

Green & Nash (1941) have reported the formation of aromatic com pounds by the passage of 2 .2 .4-trimethyl pentane over an aromatizing molybdenum oxide catalyst at 550° C, but Hoog, Verheus & Zuiderweg (1939) failed to obtain aromatization using a chromium oxide catalyst at

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448 E. F. G. Herington and E. K. Rideal

475° C. The apparent discrepancy will be shown to be due to choice of working temperature.

Hoog et al. (1939) suggested that secondary carbon atoms preferentially participate in ring closure. We have found evidence that certain modes of adsorption and reaction are prohibited.

The rules governing prohibition of adsorption, together with the mechanism of the first type of isomerism which is responsible for the formation of m- and p-xylene from w-octane, are examined in the first section of the present paper. In the second section we will deal with the products obtained by the passage over aromatizing catalysts of hydrocarbons which, without isomerization, cannot yield aromatic hydrocarbons.

E xperim ental

Chromium oxide on alumina was employed as catalyst in the form of granules c. 5 mm. diameter, and the hydrocarbon was injected by means of an electrolytic doser; either nitrogen or hydrogen could be used as carrier gas. In general c. 1 c.c. of liquid hydrocarbon was passed over 20 g. of an active catalyst.

The products were collected in a trap immersed in liquid air and the overall conversion to aromatics was determined by refractive index measurements. The value so obtained served as a basis for the choice of dilution of an aliquot portion with spectroscopically pure cyclohexane to make, in general, 1 c.c. of a solution of 1 % aromatic content. By means of a Hilger medium quartz spectrometer the ultra-violet absorption of various thicknesses of solution were compared with synthetic mixtures of known composition, and analysis was thus effected by changing the composition of the synthetic mixture until a match was obtained.

In some cases the presence of unsaturated hydrocarbons interfered with the accurate determination of the composition. In all such cases, except for the products from %-nonane or w-propyl benzene, most of the undesirable material could be removed by shaking the cyclohexane solution for 20 hr. in a sealed tube with a saturated aqueous solution of mercuric acetate. Blank experiments revealed that this treatment did not remove ordinary aromatic hydrocarbons. In this manner, 0-01 c.c. of aromatic hydrocarbon can be analysed.

The accuracy obtainable varies with the aromatic under consideration and also with the other components of the mixture. In general, the error increases in the order p-xylene, w-xylene, monosubstituted benzene, and o-xylene, the latter being the most difficult to determine. In the case of



a single aromatic component the error is not large, as is shown by the following example where 19 % by volume of toluene was found in the product from ^-heptane by spectroscopic analysis as compared with 18 % by refractive index measurements. Analysis of a binary mixture consisting of o-xylene and benzene gave, in two independent determinations, the values 22-9, 24-0 % for o-xylene and 10-4, 11-4 % for benzene. In mixtures of more than two components the errors are greater and are more difficult to assess, but for a mixture such as the product from w-octane (table 1) the values are probably of the order 33 ± 3 % for monosubstituted derivative, 33 + 5 % for o-xylene, 27 + 2 % for m-xylene and 7 ± 1 % for p-xylene. In all cases the total aromatic found by spectroscopy agreed well with the observed refractive index.

On the catalytic, cyclization of aliphatic hydrocarbons 449

Section 1

In table 1 are shown the results of passage of various hydrocarbons over an alumina-supported chromium oxide catalyst at 475° C. The samples of w-heptane, w-octane, w-nonane and w-decane-decene mixture were prepared from a Fischer-Tropsch hydrocarbon-synthesis product.

The presence of m- and p-xylene in the product from w-octane shows that isomerism must occur with this hydrocarbon. The only other case of the detection of an unexpected isomer from the starting materials shown in table 1 is that of p-xylene formed from 2 .3-dimethylhexane.

Whilst 3-methyl heptane and w-nonane yield the expected products, the proportions are different from those anticipated from purely statistical considerations. The presence of a small quantity of indene in the product from w-nonane was detected by the formation of the condensation product with benzaldehyde (Thiele 1900). I t is of interest to note that a solution of the benzaldehyde condensation compounds from w-nonane which matches in intensity in the visible a solution of the indene derivative, actually only contains about half as much indene compound when comparison is based upon the ultra-violet spectrum. The product from n-nonane probably contains substituted cyclopentadiene derivatives which condense with benzaldehyde to give yellow compounds of the type shown in figure 1, where Rv R2, R3 or R4 may be alkyl groups.

Passage of w-propyl benzene over an aromatizing catalyst yields a product containing ^-methyl styrene and in addition a quantity of indene, which shows that subsidiary five carbon rings can be formed. Here, as was to be anticipated, the only yellow compound formed with benzaldehyde was the indene condensation product, and compounds of the type shown in figure 1 were absent.




T able 1. P roperties of aliphatic hydrocarbons

( F - C )hydrocarbon Vd x 101 d f product % yield

heptanes: n-heptane 1-3882 68 0-6845 toluene 100

octanes: n-octane 1-3972 72 0-7032 m onosubstitutedbenzene (mainly 33ethyl benzene,bu t contained20 % toluene)

0 -xylene 33m-xylene 27p -xylene 7

3-methyl heptane 1-3987 72 — ethyl benzene 150 -xylene 25p -xylene 60

3-ethyl hexane 1-4072 75 — ethyl benzene 1002 .3 -dimethyl 1-4020 73 0-7125 0 -xylene 90hexane p -xylene 10

2-methyl heptane 1-3955 72 0-6976 m -xylene 100

nonanes: n-nonane 1-4042 71 — n -propyl benzene 25-290 -methyl ethyl

benzene; in a d 75-71dition containedtrace of indene

decanes: n-decane- 1-4172 80 0-7371 naphthalene one of severaldecene m ixture components(38 % olefine)

n-tetradecane, mol. 1-4295 79 — anthracene and two of m anywt. 200 phenanthrene components

of p ro d u c t; m ean mol. w t. = 135

F igure 1. Benzaldehyde condensation product formed with m aterial from n-nonane.

Naphthalene was formed when w-butyl benzene was passed over an aromatizing catalyst, which reconfirms the observation of Moldavski & Kamuscher (1936).

Cracking reactions are only important with paraffins higher than C8> for example, w-tetradecane (table 1), which agrees with the findings of Hoog et al. (1939). We will now discuss the mechanism by which and



x y le n e are formed from w-octane; it is important to decide whether the isomerism takes place before, during, or after ring closure. All the following hydrocarbons, o- and p-xylene, ethyl benzene, mesitylene, trans-1 .2 and trans-1.4-dimethyl cyclohexane, ethyl cyclohexane and 1 .3 .5-trimethyl cyclohexane, failed to undergo isomerization when passed over an active catalyst, although under these conditions the naphthenes were dehydrogenated and both ethyl benzene and ethyl cyclohexane yielded some styrene. We conclude that the isomerization does not occur after ring closure.

Since the products obtained by aromatizing n-octane and octane- octene mixtures on two different catalysts and at two different temperatures under the conditions shown in table 2 were identical, we must conclude that isomerization takes place during the act of ring closure.

On the catalytic cyclization of aliphatic hydrocarbons 451

T a b l e 2tem p. %

hydrocarbon catalyst °c arom atizationn- octane Cr20 3 on A120 3 475 56

99 10% alumino vanado m olybdate on A120 3

475 20

99 99 99 550 70octane-octene

(50:50)Cr20 3 on A120 3 550 100

In figure 2 are shown the structures involved for the three octanes which can yield ethyl benzene on the simple theory of two-point adsorption of the hydrocarbon prior to ring closure, while in table 3 the values calculated from this theory are compared with the observed values.

T able 3. Observed and calculated products

hydrocarbon products observed calculatedn-octane m onosubstituted benzenes 33 66

o-xylene 33 33-xylene 27 —

p- xylene 7 —3-methyl heptane m onosubstituted benzenes 15 33

m-xylene 25 33p- xylene 60 33

3-ethyl hexane ethyl benzene 100 100

We note that for n -octane both m -and p-xylene have been formed at the expense of ethyl benzene so that either structure I (i) or I (iii), figure 2, undergoes isomerism, but since neither II (iv), II (v), III (i) nor III (ii)



452

yields m-xylene it seems probable that it is structure I (iii) which undergoes isomerization, since this is the only one in which an ethyl group is attached to a carbon atom which is itself undergoing reaction and is at the same time attached to the catalyst. When we consider 3-methyl heptane we shall see that structure II (v), in which.an ethyl group is attached to a chemisorbed carbon atom, also shows anomalous behaviour.

E. F. G. Herington and E. K. Rideal

i ii iii

I. S tructures for n-octane

iv

II .

v vi

Cr Cr Cr CrStructures for 3-methyl heptane

I I I . S tructures for 3-ethyl hexane

F igure 2

Whilst the following argument can best be understood with the aid of solid models, yet the planar diagrams (figure 3) illustrate the mechanism of the isomerization which occurs with w-octane. We have already advanced arguments to support the view that the changes take place concurrently with ring closure; at the moment when carbon atom 1 is linking with carbon atom 6, atom 8 may combine with 2 or 3, yielding a subsidiary five-carbon atom ring in case of Gor a six ring in case of H.




Whilst the transannular ring can be accommodated for cyclohexane, dehydrogenation of the ring to an aromatic structure renders planar the six carbon atoms of the ring under which condition the ring can no longer be bridged by an ethyl group. The subsidiary ring is thus broken (dotted line) to yield m- or p-xylene. In support of this mechanism we may note that Linstead, Michaelis & Thomas (1940) report tha t the dehydrogenation of pinene over platinized charcoal yields p-cymene (see figure 4).

F igube 4. Dehydrogenation of pinene.

Moreover, this mechanism is confirmed by the calculation of the relative amounts of m- and p-xylene. Plans of the models of the boat and chair forms of cyclohexane are shown in figure 5. In these diagrams triangles represent carbon atoms, and circles or segments of circles represent hydrogen atoms; attachment of the hydrocarbon to the catalyst surface js by




two adjacent bonds on the cyclohexane ring. In the chair form both faces of the cyclohexane ring are similar, (a) and (6), and both have three hydrogen atoms in the plane and six in the periphery. In the boat form there are four hydrogen atoms on one face (c) and two on the other By means of scale models it can be shown that if one hydrogen atom (1) in (a), ( ) or (c) be replaced by an ethyl group, then these three structures will give w-xylene by the reaction shown in figure 3. For example, if the ethyl group be on carbon 1 (figure 5 (c)), then the link will be with carbon 3. Configuration ( d)will, however, yield p-xylene.

boat form

F igure 5. Diagram m atic representation of solid models of two cyclohexane forms.

If we assume that at the moment of ring closure the probabilities that the chain exists in the boat or chair form are equal, then calling the probability it exists in configuration (a) at this moment p, then the probability that it will be ( b ) will likewise be p, then the total probability it will be (c) plus (d) will be 2p. There are four ways in which it may be (c) and only two in which it may be ( d ), so the probability for (c) is f 2p = fp, and for (d) is f 2p = fp. Hence the final product will result as in table 4.

T a b l e 4

product structure contributing probabilitym -xylene figure 5 (a) V

(b) V(c) tP

to tal -3-pp- xylene figure 5 (d )

i.e. ratio metafpara — 5/1.1P



In table 5 are shown the values calculated on this basis as compared with observed. This mechanism does not affect the arguments for overall conversion previously published (Herington & Rideal 1945).

On the catalytic cyclizationof aliphatic hydrocarbons 455

T a b l e 5. P r o d u c t s fr o m w-o c t a n e

0//oobserved

overall ra te

product calculated observed calculatedm onosubstituted benzene 33 33 46 42o-xylene 33 33 — —

m-xylene 27 28 — —p -xylene 7 6 — —

3-methyl heptane exhibits a different type of anomalous behaviour, for although the product does not contain unexpected aromatics, yet the distribution is very different from the values calculated on the assumption of equal probabilities for the different configurations (table 3). One each of the structures (figure 2) which should yield ethyl benzene and o-xylene appear to be suppressed. Structures II (v) and II (vi) (figure 2) differ from all the other structures in this figure in so far as II (v) contains an ethyl group attached to a carbon atom adjacent to the position of formation of a new link, while at the same time the ethyl group is attached to a carbon atom which is chemisorbed. In structure II (vi) both chemisorbed carbon atoms carry methyl groups. Steric hindrance may be involved in this type of adsorption leading to suppression of these configurations, a view which receives support from similar behaviour of adsorbed chains in the five-six ring isomerization (§ 2, see p. 379).

Cr Cr Cr Cr(«) (

F igure 6. Structures for 3-methyl heptane (a) which yield structures I I (i) and I I (iv) (figure 2) as compared with n-heptane (b).

Structures II (iv) and II (i) in figure 2 originate from adsorption on the same pair of carbon atoms. To obtain the calculated values shown in table 3 it was assumed that each of these structures made the same contribution as each of the other four. This is equivalent to assuming that the probability of formation of links (1) and (2), figure 6 (a), are equal and both have the same value as for the formation of the link in w-heptane, figure 6 (6), i.e. the velocity of ring closure for the mode of adsorption shown




in figure 6 (a) is twice that for w-heptane in figure 6 (b). If, however, the slow step in the ring closure is the time taken for the chain from Cx to C y (figure 6) to obtain the correct configuration, then once Cx is in the correct position, link 1 will probably rapidly follow. If this be the case, then the probability of Gy reacting in figure 6 (a) is not twice that of Gy in figure 6 (6). The value for the probability of Gy reacting in figure 6 (a) as compared with Gy for figure 6 ( b)can be computed from the analytical data and hence the overall reaction rate may be found.

If the probability that the hydrocarbon reacts so as to give ethyl benzene as in figure 6 (a) link 2 be pe, then the probability it reacts to give o-xylene (figure 6 a link 1) will likewise be pe. Let the probabilities of reaction for structures II (ii) and II (iii) (figure 2) be P. Then remembering that structures II (v) and II (vi) do not contribute, we have

Ratio p -xylene/(Ethyl benzene + o-xylene) =

From experiment this ratio = 60/40, i.e. Pe = \ P .

8C/

1 2 3 4 5 6 7c — c — c — c — c — c —cF igure 7. 3-methyl heptane.

Consider the adsorption of 3-methyl heptane; adsorption of the pairs of carbon atoms 4- 5, 5-6 and 5-8 (figure 7) will be blocked because of steric factors, but the pairs 1-2, 2- 3, 3-4 and 6-7 can be chemisorbed. Hence,

4 p _|_ 2the probability of total reaction = -— -----= 0-83P. This value is very near

that given in the first paper (Herington & Rideal 1942), so that the value shown there is practically unchanged. The following results (table 6) arethus obtained:

%

T able 6

%overall reaction rates

products observed calculated observed calculatedethyl benzene 15 20 — —

o-xylene 25 20 35 42p -xylene 60 60 — —

If w-nonane reacted in a similar manner to w-octane we should expect to find m- and p-methyl ethyl benzene together with 1.2.3 and 1.2.4- trimethyl benzene in the products, these having been formed by the reactions shown in figure 8.



Actually these are absent from the product. However, for w-nonane new possibilities arise which are not present for w-octane; these are shown in figure 9. If these reactions (figure 9) take place more readily than bridging the ring, then the absence of m- and p-methyl ethyl benzene, 1 .2.3 and 1 .2 .4-trimethyl benzene is explicable. Moreover, the ratio of o-methyl ethyl benzene to w-propyl benzene should be as 3 : 1, since all the structures in figure 9 except (1) will yield -substituted derivatives. By this mechanism the overall rate will be the same as that calculated from the simple theory. Since hydrindene is readily dehydrogenated to indene by passage over the catalyst we must conclude that the isomerization of w-nonane accompanies ring closure.


Cr Cr Cr Cr Cr Cr

Cr Cr Cr CrF igure 8. Possible isomerization of n-nonane by analogy with n-octane.

Section 2

We consider here the mechanism of aromatization of aliphatic hydrocarbons which can only yield aromatics by isomerization. The experimental procedure was identical with that described in § 1. The hydrocarbons examined and the results obtained are given in table 7.

I t will be noted that all these substituted pentanes yield aromatics on passage over an aromatizing catalyst, but they differ from the hexanes and higher hydrocarbons in that the temperature required for reaction is higher. Thus 2.2.4-trimethyl pentane yields only 4 % aromatics at 475° C and 50 % at 550° C, whilst under similar conditions w-octane



yielded 40 and 100 % aromatics respectively. The 3.3 and 2 .2-dimethyl hexane are of interest, because although direct ring closure is possible they cannot yield aromatics without a migration of a methyl group. Here again only 1-5% aromatics are formed at 475° C, and at 550° C, 5 0 %. The small quantites of toluene formed (for exceptions see p. 378) from the hydrocarbons containing eight carbon atoms and the absence of benzene and dimethyl naphthalenes in the product from 2 .3-dimethyl pentane are strong evidence that the mechanism of isomerization does not involve the cracking of the hydrocarbon into two separate fragments which then recombine in a different way.

T able 7


b.p. °C a t ( F - C ) yieldhydrocarbon 7 60 mm. vl° x 104 d f product %

2 . 3-dimethyl pentane — 1-3925 71 0-6949 toluene 1002 . 2 . 3-trirnethyl pentane 110-0 1-4029 73 0-7156 m-xylene 1002 . 2 . 4-trim ethyl pentane 99-4 1-3918 73 0-6916 jp-xylene 1002 . 3 . 3-trim ethyl pentane 114-7 1-4079 74 0-7258 m-xylene 1002 . 3 . 4-trim ethyl pentane 113-8 1-4045 75 0-7182 o-xylene 60

p -xylene 402-methyl-3-ethyl pentane 115-7 1-4046 75 0-7191 ethyl benzene 12-5

toluene 12-5o-xylene 25-0m-xylene 50-0

3-methyl-3-ethyl pentane 118-5 1-4079 73 0-7272 ethyl benzene 24toluene 24o-xylene 48m-xylene 4

3 . 3-dimethyl hexane 111-7 1-4007 73 0-7103 ethyl benzene 10toluene 10o-xylene 40m-xylene 20p -xylene 20

2 . 2-dimethyl hexane 107-1 1-3940 74 0-6954 m-xylene 100

In view of the great ease with which the reaction cyclohexane ̂ m ethyl cyclopentane proceeds under certain circumstances, a five-six ring isomerization for the mechanism of aromatization of these hydrocarbons seems probable. The general type of reaction can be represented by considering the case of 3-methyl pentane (figure 10). A study of the action of the catalyst on the 2 .3-dimethyl butane and on 2 . 2 .3 .3-tetramethyl butane confirms this view, for the former gives rise to 2 .3-dimethyl butadiene but no benzene, whilst the latter passes through unchanged. No aromatics are formed since no five-six ring isomerization is possible.



Further evidence in support of this mechanism is furnished by the observation that both ethyl cyclopentane and trans-1 .2-dimethyl cyclopentane give toluene when passed over the aromatizing catalyst at 550° C. The same catalyst yielded a product containing 30 % toluene from 2 .3-dimethyl pentane, 20 % from ethyl cyclopentane, and 27 % from trans-1 .2-dimethyl cyclopentane under identical conditions. These figures show that the rate of aromatization from cyclopentane derivatives is not greater but rather smaller than from the substituted pentanes,


t 1\ / 3ao c - c - c - c - * ' * /%

iC -------- C2/ \Cr Cr

^ ^ 3a

/ \: c

\ /F igure 10. Aromatization of 3-methyl pentane.

l a

F igure 11. Designation of bonds in five-six ring isomerism.

suggesting that the slow step in the overall conversion of these substituted pentanes is the rupture of the five ring, and that conditions fpllowing the formation of a five ring from a pentane are advantageous for subsequent fission. The rates of aromatization of all the hydrocarbons cited in table 7 are practically the same, the only step identical in all being the fission of the five ring. Analysis of the products of aromatization permits us to make certain generalizations concerning the five-six ring reaction.

If we consider the pentane chain chemisorbed at atoms Cx and C2 with the ring closing from C5 (figure 11), we find that, in general, the bond y is broken unless C3 carries an ethyl group, or C2 and C3 both carry a methyl group, in which case it is the bond /? which is broken.

Certain rules can be formulated for the subsequent closure to a six ring. Thus if C3 or C4 is a quaternary carbon atom, the link is remade to one

Vol. 184. A. 29



460 E. E. G. Herington and E. K. Rideal

of the carbon atoms attached to this quaternary atom. If there be no quaternary carbon atom, then the links with either of the two groups adjacent to the bond broken are equally likely, as shown in figure 12. Finally, it appears that if ring closure should produce ethyl benzene, half of this actually appears in the product as toluene.

F igure 12. Remaking bonds in case of 2 .3 .4 -trim ethy l pentane.

T able 8 percentages

hydrocarbon product observed calculated2 . 3-dimethyl pentane toluene 100 1002 . 2 .3 -trim ethyl pentane m-xylene 100 1002 . 2 . 4-trim ethyl pentane p- xylene 100 1002 .3 .3 -trim ethyl pentane m-xylene 100 1002 .3 .4 -trim ethyl pentane o-xylene 60 50

p-xylene 40 502-m ethyl-3-ethyl pentane ethyl benzene 12-5 12-5

toluene 12-5 12-5o-xylene 25-0 25-0m-xylene 50-0 50-0

3-m ethyl-3-ethyl pentane ethyl benzene 24 25toluene 24 25o-xylene 48 50p -xylene 4 —

3 .3-dimethyl hexane ethyl benzene 10 10toluene 10 10o-xylene 40 40m-xylene 20 20p-xylene 20 20

2 . 2-dimethyl hexane m-xylene 100 100



In table 8 are shown the observed yields compared with the calculated values obtained by the application of the above rules in conjunction with the same assumptions for the formation of the five ring as were used for the six ring (Herington & Rideal 1945). The rules which determine whether


Cr Cr Cr Cr

F ig ure 13. A rom atization o f 2 .3 -d im eth y l pentane, five-six ring isom erism , bond y broken.

bond /? or bond y be broken in figure 11 are related to the conditions which inhibit modes of adsorption such as I I (v) and I I (vi) in figure 2. The sequence of steps leading to the formation of toluene from 2 .3-dimethyl pentane can be depicted as in figure 13. Thus bond y is broken by a mechanism which resembles th a t proposed for double-bond migration in olefines (Twigg 1939, 1941; Twigg & Rideal 1939, 1940). In the present instance the rupture *of the link is effected by the addition of a hydrogen

29-2




atom which has been liberated by the formation of the five-carbon ring. After formation, the five ring partially rotates and is adsorbed following attack by the chemisorbed hydrogen atom. This explains why the speed of aromatization of substituted pentanes is greater than for substituted

/CH3 / A h CH3 Q K _ CH — CH — CH — CH3-

c h .c h 3

c h 2

c h .c h 3

\ CHS.CH CH,

e|cH 2̂ a

CH.CH3 ^

/p V p laCH3.CH r C-CH

H ^\Cr

' c -C H 3\Cr

f \Cr Cr

CH,-CH"

- Ac h 3.c h 2

c h 2 c h 3. c h - ^ — •CH,

\ e> c h 2

A

A V, , CH3-C H 2

's»r n

CH/ «

p i I

*C. CH 3

\

V I I 0— —

/Cr

V i l

\Cr

Cr

CH3.CH — CH2

V \CH2 ,CH 3

\6 P

/Cr

CH.CH3

VF igure 14, Aromatization of 2 .3 .4-trim ethyl pentane, five-six ring

isomerism, bond (3 broken.



cyclopentanes, although the latter would appear to involve fewer steps. The pentanes automatically liberate a hydrogen atom in the correct position for the cracking reaction, while the cyclopentanes do not.

If, however, C3 carries an ethyl group, or C2 and C3 both carry a methyl group, bond /? (figure 11) cannot be adsorbed and it breaks instead. The mechanism in this case is different and proceeds through a radical as shown in figure 14.

Here again a bond is broken by hydrogen attack, but this is followed by adsorption of either the methyl group attached to carbon C*, or by adsorption of the adjacent methylene group. We may note that the adsorption in the last stage, (a) figure 14,*only requires the rotation of a CH3 group, while (6) entails the whole molecule, and experimentally the ratio a/6 is not unity but is as 60/40.

We wish to express our thanks to Professor R. G. W. Norrish for facilities in the use of the ultra-violet spectrometer, to Dr W. C. Price for advice in connexion with analysis by means of the spectra, and to Dr G. B. B. M. Sutherland and Dr E. E. Turner for specimens of pure hydrocarbons.

This work, which forms part of the programme of the Fuel Research Board of the Department of Scientific and Industrial Research, has been carried out in the Department of Colloid Science, Cambridge. The account is printed by permission of the Director of Fuel Research. The illustrations are Crown copyright and are reproduced by permission of the Controller of H.M. Stationery Office.


R eferences

Green, S. W. & Nash, A. W. 1941 Chem. Ind. 60, 801.Herington, E. F. G. & Rideal, E. K. 1945 Proc. Roy. Soc. A, 184, 434.Hoog, H., Verheus, J . & Zuiderweg, F. J . 1939 Trans. Faraday Soc. 35, 993. Komarewsky, V. I. & Coley, J . R. 1941 ^ • Amer. Ghem. Soc. 63, 700.Linstead, R. P., Michaelis, K. O. A. & Thomas, S. L. S. 1940 J . Chem. Soc. p. 1139. Moldavski, B. & Kamuscher, H. 1936 C.R. Acad. Sci. U.R.S.S. 1, 355.Thiele, J . 1900 Ber. dtsch. chem. Ges. 33, 3395.Twigg, G. H. 1939 Trans. Faraday Soc. 35, 934.Twigg, G. H. 1941 Proc. Roy. Soc. A, 178, 106.Twigg, G. H. & Rideal, E. K. 1939 Proc. Roy. Soc. A, 171, 35.Twigg, G. H. & Rideal, E. K. 1940 Trans. Faraday Soc. 36, 533.



On the catalytic cyclization of aliphatic hydrocarbons....

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