Isomerization 1,3-dimethylbenzene

The PMBs, when treated with electrophilic reagents, show much higher reaction rates than the five lower molecular weight homologues (benzene, toluene, (9-, m- and -xylene), because the benzene nucleus is highly activated by the attached methyl groups (Table 2). The PMBs have reaction rates for electrophilic substitution ranging from 7.6 times faster (sulfonylation of durene) to ca 607,000 times faster (nuclear chlorination of durene) than benzene. With rare exception, the PMBs react faster than toluene and the three isomeric dimethylbenzenes (xylenes). 

Xylenes (dimethylbenzenes) are an aromatic mixture composed of three isomers (0-, m-, and p-xylene). They are normally obtained from catalytic reforming and cracking units with other Ce, C7, and Cg aromatics. Separating the aromatic mixture from the reformate is done by extraction-distillation and isomerization processes (Chapter 2). 

This is a situation that must rise where the alternative products are mutually interconvertible under the conditions of the reaction, either by direct isomerisation or by reversal of the reaction to form the starting material which then undergoes new attack to yield a more thermodynamically stable isomer. It is important to emphasise that the relative proportions of alternative products formed will be defined by their relative thermodynamic stabilities under the conditions of the reaction, which may possibly differ from those of the isolated molecules. Thus if m-dimethylbenzene is heated at 82° with HF and a catalytic amount of BF3 the proportions of the three isomeric dimethylbenzenes in the product resemble very closely those calculated thermodynamically ... 

Xylol (dimethylbenzene), used as a rubber solvent. Xylene exists in three isomeric forms, commercial xylol being a mixture of all three. Xylyl Mercaptan... 

When there are two or more substituents on a benzene ring, structural isomerization can result. For instance, there are three possible isomers of xylene (dimethylbenzene). The isomers are commonly designated as ortho, meta, and para. A letter (o, m, or p) designates the isomeric position. 

Exercise 22-2 How many isomeric products could each of the dimethylbenzenes give on introduction of a third substituent Name each isomer, using chlorine as the third substituent. 

Further complications arise from the fact that the alkylation reactions sometimes are under equilibrium control rather than kinetic control. Products often isomerize and disproportionate, particularly in the presence of large amounts of catalyst. Thus 1,2- and 1,4-dimethylbenzenes (ortho- and para-xylenes) are converted by large amounts of Friedel-Crafts catalysts into 1,3-dimethyl-benzene (meta-xylene) ... 

Benzene and its simple alkyl derivatives are the building blocks of the aromatic chemical industry and are also important solvents for many reactions and processes. The simplest derivative, toluene(methylbenzene, 1), is the source of a range of nitrotoluenes and is one of the most important industrial solvents. The three isomeric dimethylbenzenes, o-, m- and p-xylene (2-4) are often used as a mixture in industrial solvents. 

Of the three isomeric C-ethylated anilines, there are thermochemical data24 only for the ortho isomer. Equation 2 predicts enthalpies of formation of —29.7 (lq) and 34.4 (g) k.l mol 1. Equation 2 also suggests that the difference between enthalpies of formation of 2-ethylaniline and any of the C,C-dimethylaniIines would be the same as that between ethylbenzene and any of the dimethylbenzenes, namely ca 11-13 kJmol-1. Or said differently, equation 6 is expected to be thermoneutral. The expectation is met for 2,4- and 2,5-dimethylaniline, and nearly so for 2,6-dimethylaniline. 

The INS spectrum of toluene is shown in Fig. 3.31. 1,4-dimethylbenzene (p-xylene) [30,31] has been studied in order to investigate the spectral effects of interacting methyl groups. INS spectra of the other isomeric xylenes have also been investigated [32]. 

C. The Thermochemistry of Sulphonation Reactions, Part 3 The Isomeric Ethyl- and Dimethylbenzenes... 

In 195932, the heats of sulphonation of the isomeric ethylbenzene and the three dimethylbenzenes (xylenes) were reported by Leitman and Pevzner. After correcting for heats of solvation and dilution, these authors found the following exothermicities for ethylbenzene (37), 5.1 0.2 o-xylene (38a), 5.5 0.2 m-xylene (38b), 3.9 0.1 p-xylene (38c), 4.1 0.2 kcal mol-1. No product analysis was reported in this study so that we cannot ascertain the site(s) of sulphonation or even the possibility of rearrangement and/or transalkylation reactions. However, that 37,38a-38c have comparable heats of formation [AHf(lq) = — 2.9, — 5.8, — 6.1, — 5.8 kcal mol - A], and that their heats of sulphonation are comparable, suggests that all of these results are consistent with each other. The heat of formation of any of the solid isomeric ethyl- or dimethylbenzenesulphonic acids are thus ca — 136 kcal mol-1 with an anticipated few kcal mol-1 spread of values33. 

This molecular sieving effect when alternative reaction products may be formed can also dictate which reactions are able to take place within the pores of a zeolite. This is shown diagrammatically in Figure 3.3 for isomerization versus cracking of hydrocarbons in zeolite A, and for preferential formation of 1,4-dimethylbenzene (/ ara-xylene) in ZSM-5. 

The methylation of benzene with methyl chloride proceeds to give predominantly dimethylbenzene (xylenes) and tetramethylbenzene, with about 10% hexam-ethylbenzene. In the propylation of benzene with 1-chloropropane, not only does polyalkylation occur, but there is also a considerable degree of isomerization of the n-propyl group to the isopropyl isomer (Scheme 5.2-6). In the butylation, complete isomerization of the butyl side chain occurs to give only sec-butyl benzenes. 

The hydrogenation of substituted cyclic alkenes is anomalous in many cases in that substantial amounts of rran -addition products are formed, particularly with palladium catalysts. For example, the alkene 19 on hydrogenation over palladium in acetic acid gives mainly frans-decalin (7.12), and the alkene 1,2-dimethylcyclohexene 20 gives variable mixtures of cis- and trans-, 2-dimethylcyclohexane depending on the conditions (7.13). Similarly, in the hydrogenation of the isomeric dimethylbenzenes (xylenes) over platinum oxide, the cis-dimethylcyclohexanes are the main products, but some trans isomer is always produced. 

When one of the two substituents on the ring imparts a special name to the compound, as, for example, toluene, phenol, and aniline, then we name the compound as a derivative of that parent molecule. In this case, the special substituent occupies ring position number 1. The lUPAC system retains the common name xylene for the three isomeric dimethylbenzenes. When neither group imparts a special name, we locate the two substituents and list them in alphabetical order before the ending -benzene. The carbon of the benzene ring with the substituent of lower alphabetical ranking is numbered C-1. 

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