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#1 2008-10-26 06:14:11


Radical bromination of 1-tert-butyl-1,4-dimethylcyclohexane

When a specific stereoisomer of 1-tert-butyl-1,4-dimethylcyclohexane is treated with bromine in the presence of light, two major alkyl bromides products are formed and, one of those is in greater yield than the other.

There are two issues at play in this problem. One is where does the bromination occur and the second is which isomer will be the major isomer?
For the ratio of bromination, you can find a similar problem posted in which the mechanism for the bromination is given followed by a calculation of the selectivity below it. See

From that problem, the following issue arises. Will the bromine attack to form the axial or equatorial isomer? If you look up the A-values, bromine is sterically smaller than a methyl. One might anticipate bromine attack to form this product would be easier. However, since the radical is planar, then which path shall the reaction with Br2 take? If we use the reduction of cyclohexanone as an example of a planar carbon undergoing nucleophilic attack, then the major product gives the product with the axial nucleophile if the nucleophile is small, see … Felkin.gif 


I would predict the product on the left as the major product. This problem requires understanding the reaction mechanism. I suggest using "A Guide to Organic Chemistry Mechanisms" (, Barnes and Noble or ).


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