Questions: Markovnikov's Rule and Regioselectivity in Addition

Question 1

Adding HCl to 2-methylpropene [(CH₃)₂C=CH₂] gives predominantly one product. What is it, and why?

• A. 1-chloro-2-methylpropane, because HCl always adds Cl to the carbon with the most hydrogen substituents
• B. 2-chloro-2-methylpropane, because the proton adds to the terminal CH₂, generating a tertiary carbocation that is more stable than the alternative primary carbocation
• C. A 50:50 mixture, because both carbons are equally accessible to the electrophile
• D. 1-chloro-2-methylpropane, because primary carbocations are kinetically preferred due to lower steric hindrance

Answer: (B)

When H⁺ adds to the terminal CH₂ of 2-methylpropene, the positive charge lands on the central carbon bearing three methyl groups — a tertiary carbocation. The alternative (H⁺ adding to the internal carbon) gives a primary carbocation. Tertiary carbocations are far more stable due to hyperconjugative electron donation from three alkyl groups. The major product is 2-chloro-2-methylpropane (tert-butyl chloride), with Cl on the more substituted carbon. Option A is the classic mnemonic confusion: the rule is that Cl goes to the MORE substituted carbon, not to the one with more H's.

Question 2

Hydroboration-oxidation of propene gives 1-propanol (OH on C-1), while acid-catalyzed hydration gives 2-propanol (OH on C-2). The best explanation for this difference is:

• A. Hydroboration is less regioselective and gives a random mixture that happens to favor C-1 addition
• B. Water is a stronger nucleophile than borane and therefore prefers the terminal, less hindered carbon
• C. Hydroboration proceeds through a concerted four-center transition state with no carbocation intermediate, so carbocation stability arguments do not apply and the boron adds to the less substituted carbon
• D. The solvent in hydroboration reverses Markovnikov selectivity by stabilizing the primary carbocation

Answer: (C)

This contrast is the key test of whether you understand Markovnikov's rule mechanistically. Acid-catalyzed hydration goes through a carbocation: H⁺ adds to C-1 to generate the more stable secondary carbocation at C-2, then water attacks C-2 giving 2-propanol. Hydroboration is concerted — boron and hydrogen add simultaneously across the double bond with no carbocation formed at all. Since there is no intermediate to stabilize, the regioselectivity is governed by steric effects and orbital geometry, placing boron on the less substituted (terminal) carbon. Anti-Markovnikov outcome is the result of a different mechanism, not a reversal of carbocation stability.

Question 3

Markovnikov's rule predicts that in the addition of HBr to an unsymmetrical alkene, Br adds to the carbon bearing the most hydrogen substituents.

• T. True
• F. False

Answer: False

This is a common mnemonic error. Markovnikov's rule says H adds to the carbon with more hydrogens — equivalently, Br adds to the MORE substituted carbon (the one with fewer H's). The carbon with more H's is the less substituted one; the H going there generates a carbocation at the more substituted (adjacent) carbon. Confusing which group goes where is one of the most common errors in predicting addition products.

Question 4

A resonance-stabilized allylic carbocation can be the preferred intermediate in an electrophilic addition even if it is not the most alkyl-substituted carbocation, and Markovnikov's rule (understood as a carbocation stability argument) correctly predicts this outcome.

• T. True
• F. False

Answer: True

Markovnikov's rule, properly understood, says the electrophile adds to generate the MORE STABLE carbocation — regardless of whether stability comes from alkyl substitution or resonance. An allylic carbocation (stabilized by delocalization over two carbons) or a benzylic carbocation can be lower in energy than a more substituted but non-resonance-stabilized alternative. The carbocation stability framework predicts both situations correctly; the simple mnemonic ('H goes to C with more H's') fails for resonance-stabilized cases.

Question 5

Why is it insufficient to memorize 'hydrogen goes to the carbon with more hydrogens' as a rule for predicting regioselectivity in electrophilic addition? Give an example where this shorthand fails but the carbocation stability argument correctly predicts the product.

Model answer: The mnemonic fails whenever the most stable carbocation is not the most alkyl-substituted one. For example, consider adding HBr to 1,3-butadiene (CH₂=CH-CH=CH₂). H⁺ can add to C-1 to give an allylic carbocation (charge delocalized over C-2 and C-4) or to C-2 to give a vinyl carbocation. The allylic carbocation at C-2/C-4 is far more stable than a vinyl carbocation, so H adds to C-1 despite C-1 having more H's than C-2. The product distribution (1,2-addition vs. 1,4-addition) follows from allylic stability, which the simple mnemonic cannot capture. Similarly, addition to styrene (PhCH=CH₂) gives the benzylic carbocation at the internal carbon regardless of substitution count, because benzylic stabilization dominates.

Understanding the mechanism rather than the mnemonic is what allows predictions in novel cases. The mnemonic is a heuristic that works for simple alkyl-substituted alkenes — where alkyl substitution and the number of H's are perfectly correlated. Once resonance, conjugation, or unusual substitution patterns enter, only the mechanistic understanding holds up.