Ortho-dimethylbenzene

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. 

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. 

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. 

XYLENE. [CAS 1330-20-7], C6H4(CH3)2, formula weight 106.16. There are three xylenes, ortho-, meta-, and para-xylene. Sometimes referred to as dimethylbenzenes. lire xylenes have the following key physical properties ... 

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) ... 

Electrophilic attack at methyl-bearing carbons, particularly in ortho- and para-dimethylbenzenes, would appear quite reasonable because the electron-donating character of the other methyl group should activate the ring by stabilizing the intermediate ion ... 

In alkylated benzenes a correction factor of 0.57 must be applied if two substituents are ortho to each other. For example, AH° of 1,2-dimethylbenzene is calculated as follows ... 

If there is more than one side chain on the benzene ring the name of each radical is preceded by the postion number on the ring. Examples are 1,3-dimethylbenzene and l-ethyl-4-(l-methylethyl)benzene. Position numbers 1,2-, 1,3-, and 1,4- correspond to the designations ortho - (o-), meta - (m-) and para - (p-). 

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. 

Benzene molecules bearing alkyl substituents are called alkylbenzenes. The simplest of these is methylbenzene (common name, toluene), shown in Figure 27-1 Ic. The dimethyl-benzenes are called xylenes. Three different compounds (Table 27-6) have the formula CgH4(CH3)2 (see margin). These three xylenes are constitutional isomers. In naming these (as well as other disubstituted benzenes), we use prefixes ortho- (abbreviated o-), meta- m-), or para- (p-) to refer to relative positions of substituents on the benzene ring. The ortho- prefix refers to two substituents located on adjacent carbon atoms for example, 1,2-dimethylbenzene is o-xylene. The meta- prefix identifies substituents on C atoms 1 and 3, so 1,3-dimethylbenzene is 7w-xylene. The para- prefix refers to substituents on C atoms 1 and 4, so 1,4-dimethylbenzene is p-xylene. 

Conversion of Phenols to Amines. Aniline and some diphenylamine are formed when phenol and NHs solution are heated under pressure in the presence of FeCh, Al(OH)s, or Fe(OH)j. When NH and phenol or ortho-or para-cresols are reacted in the vapor phase over an AljO catalyst, yields of up to 88 per cent of the corresponding amines are obtained. However, these amines are customarily obtained by reducing the parent nitro compound, except in cases where it is difficult to obtain the required nitro isomer. For example, it is considered that the amination of symr xylenol is the best method of preparing sym-xylidine (l-amino-3,5-di-methylbenzene). When sym-xylenol is heated under pressure to 320 C with ammonium chloride, about equal amounts of sj/m-xylidine and sym-dixylylamine (5-imino-bis-l,3-dimethylbenzene) are formed. The ortho-and para-nitrophenols and nitrocresols can be aminat more readily. 2-Nitro-p-cresol [OH(l), N02(2), CHj(4)] and o-nitrophenol have been aminated in aqueous ammonia containing ammonium salts of weak acids to inhibit decomposition. Phosphoric, boric, carbonic, and formic acids were used. In one case it is claimed that 55-65 per cent yields of 2-nitro-p-toluidine (MNPT of commerce) were obtained when 2-nitro-p-cresol, 28 per cent aqueous NHj, and monoammonium phosphate, 1 11.5 0.2 molar ratio, were heated under pressure for 10 hr at 140-150°C and then 5 hr at about 160 C. Earlier workers, employing somewhat similar conditions, claimed excellent yields of MNPT when 1 mole of ammonium formate was used per mole of 2-nitro-p-cresol. ... 

Synonyms/Trade Names 1,2-Dimethylbenzene ortho-Xylene o-Xylol... 

Several tricyclic monomers required for oxidative (electrochemical or chemical) polymerization have been prepared by coupling reactions. Thus, l,4-diiodo-2,5-dimethylbenzene was coupled with 2-thienylmagnesium bromide in the presence of nickel(II) bis(diphenylphosphino)propane dibromide (NidpppBr2). However when methoxy groups were present in the benzene ring ortho to the site of coupling, the decreased electrophilicity necessitated the use of a more nucleophilic thiophene derivative, 2-thienylcopper (Equation (60)). 

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. 

DMS described above [10]. It is important to note that the structural isomers of xylene (meta-, para-, and ortho-) have the same molecular weight, but they differ in position of the second methyl group. Ortho-xylene is 1,2-dimethylbenzene meta-xylene is 1,3-dimethylbenzene and para-xylene is 1,4-dimethylbenzene. At low electric field, they have similar cross section and can only be separated by an ion mobility spectrometer with ultrahigh resolving power. However, owing... 

Materials. Anisole (1), terephthalonitrile (2) m-cyanoanisole (3b), o- and p-dimethylbenzenes (5a and c), 15-crown-5, 18-crown-6, benzonitrile, and KCN, were purchased. CH2CI2 was distilled from CaH2. PEG 200- 1000 (Nakarai Chem. Co) were dried azeotropically by benzene. Ortho- and para-cyanoanisoles (3a and 3c ) were prepared according to the methods of the literatures. 1,3-Pentadiene and acetone were distilled directly before use. 

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). 

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