Question

Car-3-ene undergoes catalytic hydrogenation to give only one of two possible stereoisomeric products. The product has the common name cis-carane indicating the methyl group and the cyclopropane ring are cis to each other. Suggest an explanation for this stereochemical outcome. (The other product, which is not formed would be trans-carane.)

Answer 1

Car-3-ene has a double bond, on which hydrogen is added during catalytic hydrogenation reaction. Addtion of hydrogen can take place from two sides, one is from the side of cyclopropane ring and another from the opposite side of cyclopropane ring. Addition from the side of bulky cyclopropane ring is sterically hindered therefore not favorabale. Addition from the side opposite to the bulkier cyclopropane ring is very much favorable resulting in product in which methyl and cycloproane ring are on same sides.

Therefore, addition of hydrogen takes place from the less hindered side of the ring and resulting product is cis-carane.