(a) When HBr adds across the double bond of 1,2-dimethylcyclopentene, the product is a mixture of the cis and trans isomers. Show why this addition is not stereospecific.

(b) When 1,2 dimethylcyclopropene undergoes hydroboration oxidation, one diastereomer of the product predominantes. Show why this addition is stereospecific, and predict the stereochemistry of the major product.

This is a two step process and in the first step three member cyclic bromine cation is formed and then this intermediate undergoes eitherS_N1 or S_N2 pathway to give cis- trans isomers.

As in S_N1 pathway ring opening first takes place and then bromine anion attacks the carbocation formed from either below the plane to give the cis isomer. In S_N2 pathway the bromine anion attacks from the plane opposite to that of the ring resulting in the formation of trans isomer.

Anti Markovnikov product is formed when hydrogen attached to the less hindered carbon on alkene and markovnikov produced is formed where the hydrogen is in the more hindered position.

But here the olefin is equally substituted in both side of double bond and so only one diastereomer is obtained.