Questions: Electrophilic Addition to Alkenes

Question 1

When HBr adds to propene (CH3CH=CH2), the major product is 2-bromopropane. Which mechanistic explanation is correct?

• A. Bromine is larger and prefers the less hindered primary carbon
• B. The proton adds to the carbon bearing fewer hydrogens, placing Br on the primary carbon
• C. The proton adds to the carbon bearing more hydrogens, generating a more stable secondary carbocation at the internal carbon, which bromide then attacks
• D. HBr always adds anti-Markovnikov when the alkene is unsymmetrical

Answer: (C)

Markovnikov's rule follows from carbocation stability, not from a rule about atom size. H+ adds to the terminal carbon (more H's), generating a secondary carbocation at C2 — more stable than a primary carbocation at C1 because adjacent alkyl groups stabilize positive charge through hyperconjugation and inductive donation. Bromide then attacks the secondary carbocation, giving 2-bromopropane as the major product.

Question 2

Hydroboration–oxidation of 1-methylcyclohexene gives the same alcohol product as acid-catalyzed hydration (H2SO4/H2O) of the same alkene.

• T. True
• F. False

Answer: False

Acid-catalyzed hydration follows Markovnikov's rule and places OH on the more substituted carbon (the tertiary carbon of 1-methylcyclohexene), giving 1-methylcyclohexanol. Hydroboration–oxidation delivers OH to the less substituted carbon (anti-Markovnikov), giving the secondary alcohol at C2. The two methods are complementary precisely because they give constitutional isomers — choosing the right reagent determines which alcohol you make.

Question 3

Why does halogenation of an alkene with Br2 give exclusively anti (trans) addition rather than a mixture of syn and anti products?

Model answer: Br2 reacts with the pi bond to form a cyclic bromonium ion that bridges both carbons and blocks one face of the former double bond. The second bromide ion must attack from the opposite face (backside, SN2-like), forcing anti addition and giving the trans dibromo product.

If addition proceeded through an open secondary carbocation, bromide could attack from either face, producing a mixture of stereoisomers. The bromonium ion locks the geometry by making both carbons part of a three-membered ring — the bridging Br+ shields the top face, so attack is restricted to the bottom. This anti-addition stereochemistry is diagnostic for a halonium ion mechanism and is one of the key pieces of experimental evidence that bromonium ions exist.