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  •  » Identify benzyl methyl maleate or fumarate from the following data.

#1 2010-05-09 13:43:43


Identify benzyl methyl maleate or fumarate from the following data.

Yahoo!Answers in chemistry wrote:

A 2.81g of an optically active diester A (containing only C, H and O atoms) was hydrolysed with 30 mL of 1M NaOH. Following the hydrolysis, the solution required 6 mL of 1M HCl solution to react with excess NaOH only.

The organic products are found to contain methanol, an organic acid B and an alcohol C, C8H10O. Compound C gives a pale yellow precipitate with warm alkaline aqueous iodine.

i) Calculate the MW of the diester A.
ii) Draw structural formulae for the compounds A, B and C, explaining the above observations
iii) Compounds B and C exhibit stereoisomerism. Explain clearly, the type of stereoismerism exhibited by compound B and C, and draw displayed formulae of the isomers.

A has a MW of 234. Of the 30 mmol of NaOH, 6 were in excess, since it was a diester, only half of the 24 were on a molar basis. 2,810 mg/12 mmol = 234 mg/mmol (or g/mol).

Okay, lets add up the pieces, CH3O (31), C8H9O (121), 2 x C=O (56), + unknown = 234. The missing pieces = 26 or 2 x 13 (CH). The alcohol C has 4 degrees of unsaturation, therefore it probably is a benzene ring. That leaves C2H5O to be attached to it (C8H10O - C6H5). However, since it must contain a chirality center, it cannot be CH2CH2OH. That means C is an alpha methyl benzyl alcohol. The stereoisomers will be mirror images of the chirality center. It will give a positive result from the iodoform reaction (the pale yellow precipitate with alkaline iodine).

The two carboxylic acid groups must be separated by two CH-groups. Therefore it must be maleic or fumaric acid (the cis and trans isomers, the two isomer forms).

The structures A, B, and C could be as follows (with only a single isomer drawn of the cis, trans, R, and S isomers).


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