Questions: Markovnikov's Rule and Regioselectivity in Addition Reactions

Question 1

HBr adds to 2-methylpropene [(CH₃)₂C=CH₂]. Which product forms predominantly, and why?

• A. Br on the terminal carbon (CH₂), because HBr adds so that H goes to the less substituted carbon
• B. Br on the central carbon [(CH₃)₂CBr–CH₃], because the secondary carbocation intermediate is more stable than the primary
• C. Br on the central carbon, because the central carbon has fewer hydrogens and Markovnikov's rule is a fixed law
• D. An equal mixture of both products, because carbocation stability cannot predict regiochemistry

Answer: (B)

Protonation of 2-methylpropene at the terminal carbon produces a tertiary carbocation [(CH₃)₂C⁺–CH₃], which is far more stable than the primary carbocation that would form from protonation at the internal carbon. Bromide attacks the more stable tertiary carbocation, giving the product with Br on the more substituted carbon. Option C states the right product but the wrong reason — the mnemonic 'fewer hydrogens' is a consequence of carbocation stability, not an independent rule. Understanding the mechanism tells you when the rule applies and when it doesn't.

Question 2

HBr is added to propene (CH₃–CH=CH₂) in the presence of peroxides. Where does the bromine end up?

• A. On C-2 (the more substituted carbon), following Markovnikov's rule
• B. On C-1 (the terminal carbon), anti-Markovnikov, because the reaction proceeds through a radical intermediate
• C. On C-1 (the terminal carbon), because peroxides reverse carbocation stability
• D. On C-2, because peroxides do not affect the regiochemistry of electrophilic addition

Answer: (B)

Peroxides initiate a radical chain mechanism. The bromine radical (not Br⁻) adds to the double bond in the first step, generating a carbon radical. The more stable secondary radical forms on C-2, so Br• adds to C-1, leaving the radical on C-2 where H• subsequently adds. The result is anti-Markovnikov. This is NOT a violation of Markovnikov's rule — the rule specifically describes electrophilic additions through carbocation intermediates. Radical additions operate by an entirely different mechanism, so carbocation stability is simply irrelevant.

Question 3

Markovnikov's rule predicts that in HBr addition to propene, bromine adds to C-2 (the more substituted carbon) because this pathway proceeds through a more stable secondary carbocation intermediate.

• T. True
• F. False

Answer: True

This is precisely correct, and stating the mechanistic reason is the key. Protonation at C-1 generates a secondary carbocation at C-2; protonation at C-2 would generate a primary carbocation at C-1. The secondary carbocation is substantially more stable (better hyperconjugation, more inductive stabilization), so the reaction overwhelmingly takes that pathway. Bromide then attacks C-2. Markovnikov's rule is a summary of this carbocation stability argument, not an independent principle.

Question 4

Anti-Markovnikov additions violate Markovnikov's rule because they place hydrogen on the more substituted carbon rather than the less substituted one.

• T. True
• F. False

Answer: False

Markovnikov's rule applies specifically to electrophilic additions that proceed through carbocation intermediates. Anti-Markovnikov reactions (hydroboration-oxidation, radical HBr addition) proceed via different mechanisms — concerted or radical — where carbocation stability is irrelevant. They do not 'violate' the rule; they simply fall outside its scope. This distinction is crucial: the rule is a mechanistic prediction, not a universal law. Understanding when it applies requires knowing the mechanism, not memorizing outcomes.

Question 5

Hydroboration-oxidation and radical HBr addition both give anti-Markovnikov products. What mechanistic feature do they share that explains this, and how does it differ from standard electrophilic HBr addition?

Model answer: Both reactions avoid forming a carbocation intermediate. Hydroboration is concerted — B and H add simultaneously in a four-center transition state, so steric factors (boron preferring the less hindered carbon) govern regiochemistry. Radical addition proceeds through a carbon radical (not a cation) where the more stable radical again forms on the more substituted carbon, placing H there and Br on the terminal carbon. In standard electrophilic addition, a carbocation forms as the intermediate, and its stability (tertiary > secondary > primary) controls which carbon gets the halide.

The key insight is that regioselectivity is controlled by the mechanism, not by any single rule. Markovnikov's rule is a shorthand for carbocation-stability-controlled addition. Whenever the mechanism bypasses carbocation formation — by being concerted (hydroboration) or radical — a different selectivity principle operates. This is why asking 'what mechanism?' is always the first step in predicting regiochemistry.