Methylcyclohexanecarboxylic Acid

Caution Because carbon monoxide is evolved, the reaction should be carried out in a good hood. 

Two hundred seventy milliliters (497 g., 4.86 moles) of 96% sulfuric acid (Note 1) is poured into a 1-1. three-necked flask equipped with a paddle stirrer driven by a powerful motor, a dropping funnel with a gas by-pass, and a thermometer that dips into the acid. The reaction mixture is stirred vigorously (Note 2) and maintained at 15-20° by means of a cooling bath as 3 ml. of 98-100% formic acid (Note 3) is added dropwise. Under the same conditions, a solution of 28.5 g. (0.25 mole) of 2-methyl-cyclohexanol (Note 4) in 46 g. (1.00 mole) of 98-100% formic acid is added in the course of 1 hour. The reaction mixture foams during the additions. The mixture, which is a very light cream color, is stirred for 1 hour at 15-20° and then is poured with stirring onto 1 kg. of crushed ice in a 4-1. beaker. The carboxylic acid separates as a white solid. 

The acid is taken up in 200 ml. of hexane (Note 5), the hexane layer is separated, and the aqueous layer is extracted with two 150-ml. portions of hexane. The combined hexane solutions are extracted twice with a mixture of 175 ml. of 1.4N potassium hydroxide solution and 50 g. of crushed ice. The two alkaline solutions are combined and extracted with 100 ml. of hexane to remove traces of neutral oil, and then acidified to pH 2 with 12N hydrochloric acid (about 35 ml.). The liberated carboxylic acid is taken up in 150 ml. of hexane. The aqueous layer is extracted 

Three moles of 99-100% sulfuric acid may be used in place of the 96% sulfuric acid. 

Technical grade 85% formic acid can be substituted for 98-100% formic acid if the decrease in sulfuric acid concentration that would result is compensated for by a suitable increase in the amount of sulfuric acid charged. 

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In addition to the present method,2 1-amino-1-methylcyclo-hexane has been synthesized by the following procedures Ritter reaction, e.g., with 1-methylcyclohexanol (76%, 67%)3i 4 or 1-methylcyclohexene (35%,) 4 5 Hofmann reaction with 1-methyl-cyclohexanecarboxamide (80% as hydrochloride) 6 reduction of 1-methyl-l-nitrocyclohexane (63%) 6 Schmidt reaction with 1-methylcyclohexanecarboxylic acid (42%).6... 

The enthalpy of fomation of two such species has been measured, namely the cyclopropane and cycloheptane derivatives. The difference between the values for these two species, both as solids, is 238.1 kJmol . Is this difference plausible Consider the difference between the enthalpies of formation of the parent cycloalkanes as solids, 194 kJ mol . The ca 44 kJ mol discrepancy between these two differences seems rather large. However, there are idiosyncracies associated with the enthalpies of formation of compounds with three-membered rings and almost nothing is known at all about the thermochemistry of compounds with seven-membered rings. Rather, we merely note that a seemingly well-defined synthesis of cycloheptyl methyl ketone was shown later to result in a mixture of methyl methylcyclohexyl ketones, and superelectrophilic carbonylation of cycloheptane resulted in the same products as methylcyclohexane, namely esters of 1-methylcyclohexanecarboxylic acid. The difference between the enthalpies of formation of the unsubstituted alicyclic hydrocarbons cycloheptane and methylcyclohexane as solids is 33 kJmol . This alternative structural assignment hereby corrects for most of the above 44 kJ mol discrepancy in the enthalpies of formation of the two oximes. More thermochemical measurements are needed, of oximes and cycloheptanes alike. 

Methylcyclohexanecarboxylic acid can be prepared by car-bonation of the Grignard reagent from 1-chloro-l-methylcyclo-hexane3 or by Friedel-Crafts condensation of 1-chloro-l-methyl-cyclohexane with methyl 2-furancarboxylate followed by saponi-... 

Another example is the synthesis of 1-methylcyclohexanecarboxylic acid from crude 2-methylcyclohexanol. ... 

However, in a cyclohexane system we also need to consider the conformational mobility that generates two different chair forms of the ring (see Section 3.3.2). Let us consider 3-methylcyclohexanecarboxylic acid. This has two chiral centres, and thus there are four configurational stereoisomers. These are the enantiomeric forms of the trans and cis isomers. 

We can spot this type of situation by looking for symmetry in the molecule. Both cis- and trans-4-methylcyclohexanecarboxylic acid isomers have a plane of symmetry, and, as we saw for simple tetrahedral carbons (see Section 3.4.1), this symmetry means the molecule is achiral. 

Cyclohexylpropanoic acid 3-Methylcyclohexanecarboxylic acid ( )-4-Methyl-3-hexenoic acid... 

These investigators found that the normal acids occurred in much greater abundance than the branched acids. The cyclic acids isolated by Lochte and coworkers (31) include cyclopentanecarboxylic acid, 2-methylcyclopentanecarboxylic acid, 3-methylcyclopentane-carboxylic acid, cyclopentaneacetic acid, 3-methylcyclopentaneacetic acid, 2,3-dimethyl-cyclopentaneacetic acid, 1,2,2-trimethylcyclopentanecarboxylic acid, cyclohexanecarbox-ylic acid, p-methylcyclohexanecarboxylic acid, and the cis and trans forms of 2,2,6-tri-methylcyclohexanecarboxylic acid. In addition, Lochte and coworkers (31) isolated dimethylmaleic anhydride, which has also been isolated by Nenitzescu and coworkers (42). 

Several catalytic hydrogenations of aromatic rings in compounds containing free carboxyl groups are described (cf. method 4). Low-pressure hydrogenation over platinum oxide catalyst has been used. p-Toluic acid in acetic acid at 60° gives 4-methylcyclohexanecarboxylic acid (S>5%). 

Ans. (a) 5-chloro-2-methylbenzoic acid, (f>) 5-chloro-2-methylcyclohexanecarboxylic acid, (c) 4-chloro-3-methylhexanoic acid. 

PROBLEM 10.10 Show how 4-methylcyclohexyl chloride can be converted to 4-methylcyclohexanecarboxylic acid. 

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