Dimethylbenzene isomers

Analytical Properties Separation of dimethylbenzene isomers, dihalobenzene isomers (Cl, Br), haloketone benzene isomers, dimethoxybenzene isomers Reference 14... 

Xylenes for solvent purposes consist of a mixture of three dimethylbenzene isomers, ortho-, meta- and para-xylene, and ethylbenzene. The physical properties quoted are for a typical mixture and the only property that is significantly altered by the ratio of the isomers is the flash point of the mixture. This can be significant in the UK and other countries where legislation primarily aimed at the safe storage of petrol regulates the storage and handling of hydrocarbons with flash points of less than 73 °F (22.8 °C) by the Abel method. 

The infrared specfra of the three xylene (dimethylbenzene) isomers, and an additional aromatic hydrocarbon, are given below. Assign the spectra to the isomers and suggest a potential structure for the remaining unknown substance. 

The corresponding dimethylbenzene isomers, also shown in figure 3.20, are then... 

Ring nitration of a dimethylbenzene (a xylene) results in the formation of only one dimethylnitrobenzene. Which dimethylbenzene isomer was the reactant ... 

X CH3 from CgHjo (A) leaves CgH4, a phenyl fragment Compound A must be a dimethylbenzene isomer. Consequently, B must be a bromo(dimethyl) benzene isomer. The question for both is Which isomer To find the answers, it is useful to write down aU the possible options. For A, there are three isomers ortho-, meta-, andpara-dimethylbenzene (xylene Section 15-1). 

If several groups are attached to the benzene ring, their names as well as their relative positions should be indicated. For example, dimethylbenzene or xylene, CgH (CH,)2, has three geometric isomers, with prefixes ortho-, meta-, and para-, indicating the relative positions of the two methyl groups. 

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). 

It is often possible in these cases to predict the correct isomer. In many cases, the groups already on the ring reinforce each other. Thus, 1,3-dimethylbenzene is substituted at the 4 position (ortho to one group and para to the other), but not at the 5 position (meta to both). Likewise the incoming group in p-chlorobenzoic acid goes to the position ortho to the chloro and meta to the carboxyl group. 

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 ... 

Problem 10.28 (a) Draw two Kekule structures for l,2-dimethylbenzene(o-xylene), b) Why are these structures not isomers What are they (c) Give the carbonyl products formed on ozonolysis. M... 

Dimethylbenzene or Xylene> CeH4(CH3)3, mw 106.16. Three isomers ate known ... 

The classical argument concerting the equivalence of the positions on the benzene ring is based on the existence of three (ortho, meta. para) isomers of xylene (dimethylbenzene). How many isomers ore there of dimelhylhorazine ... 

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. 

Which has more isomers, (a) dimethylbenzene or (b) dimethylcyclohexene Explain. 

The three isomers of dimethylbenzene are commonly named ort/zo-xylene, meta-xylene, and para-xylene. These three isomers are difficult to distinguish using proton NMR, but they are instantly identifiable using 13C NMR. 

As in isomers of other hydrocarbons, the physical and chemical properties of the structural isomers of aromatic hydrocarbons differ. The physical properties of the three positional isomers of dimethylbenzene (xylene) are given in Table 1. 

Reaction conditions can also influence the orientation of substitution. An example is the reaction of toluene with chloromethane in the presence of aluminium chloride. At room temperature, a mixture of 1,2- and 1,4-dimethylbenzenes results, but at 80 °C the product is mainly 1,3-dimethylbenzene. In fact, heating either of the 1,2- or 1,4-isomers in the presence of aluminium chloride/hydrochloric acid results in rearrangement to the more stable 1,3-dimethylbenzene. 

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