Questions: Markovnikov's Rule and Electrophilic Addition Mechanisms

Question 1

HCl is added to 3,3-dimethyl-1-butene (CH₂=CH–C(CH₃)₃). Simple Markovnikov addition predicts 2-chloro-3,3-dimethylbutane as the major product. What actually forms and why?

• A. 2-chloro-3,3-dimethylbutane, via a stable secondary carbocation at C-2
• B. 2-chloro-2,3-dimethylbutane, because the initially formed secondary carbocation rearranges via a 1,2-methyl shift to a more stable tertiary carbocation
• C. 1-chloro-3,3-dimethylbutane via anti-Markovnikov addition due to steric bulk
• D. Both products form equally because Markovnikov's rule does not apply to branched alkenes

Answer: (B)

The initial Markovnikov protonation places H at C-1, generating a secondary carbocation at C-2. A 1,2-methyl shift from the adjacent quaternary C-3 converts it to a more stable tertiary carbocation at C-3, which is then captured by Cl⁻. The hydrogen-counting mnemonic correctly predicts the initial protonation site but cannot anticipate rearrangements — only the mechanistic principle (always proceed toward greater carbocation stability) predicts the actual product.

Question 2

What is the primary reason Markovnikov's rule correctly predicts the regiochemistry of HX addition to unsymmetrical alkenes?

• A. The carbon with more hydrogens has higher electron density and preferentially attracts the proton
• B. The rate-determining step generates a more stable carbocation when H adds to the less substituted carbon, leaving X to bond to the more substituted carbon
• C. Halides preferentially bond to the less sterically hindered (less substituted) carbon
• D. The thermodynamic product is always the more substituted alkyl halide, driving selectivity

Answer: (B)

The reaction is kinetically controlled: the rate-determining step is carbocation formation. The pathway through the more stable (more substituted) carbocation has lower activation energy and therefore predominates. Option A misidentifies the controlling factor — it is carbocation stability, not electron density. Option C gets the outcome backwards. Option D conflates kinetic and thermodynamic control.

Question 3

Markovnikov's rule is fundamentally a statement about carbocation stability; the empirical 'hydrogen-counting' mnemonic works only because the carbon with fewer attached hydrogens is also the more substituted carbon, which better stabilizes a positive charge.

• T. True
• F. False

Answer: True

The modern mechanistic understanding frames Markovnikov's rule entirely in terms of carbocation stability. The 'H goes to the more-H carbon' mnemonic works in simple cases because the more substituted carbon (fewer H's) is better at stabilizing positive charge via hyperconjugation and inductive effects. But the mnemonic fails when rearrangements occur — the mechanistic principle (greatest carbocation stability) always applies.

Question 4

In electrophilic addition of HBr to an alkene, Br adds to the less substituted carbon because it is electronegative and is attracted to the electron-rich, less hindered end of the double bond.

• T. True
• F. False

Answer: False

Br⁻ is a nucleophile that attacks the carbocation formed on the MORE substituted carbon. The statement reverses both the logic and the outcome. Regiochemistry is determined by which protonation pathway generates the more stable carbocation — the nucleophile then attacks that cationic site. Electronegativity of bromine and steric preference for the less hindered carbon are not the controlling factors.

Question 5

A student claims that Markovnikov's rule is just a memorization trick about counting hydrogens. Using the mechanism of HX addition, explain why this view is incomplete and describe a situation where the mnemonic alone gives an incorrect prediction.

Model answer: Markovnikov's rule is grounded in carbocation stability. In the rate-determining step, the proton adds to give the most stable (most substituted) carbocation; the halide then captures it. The mnemonic works only because the more substituted carbon (fewer H's) better stabilizes positive charge. It breaks down when carbocation rearrangements are possible: if the initially formed carbocation can rearrange via a 1,2-hydride or methyl shift to an even more stable carbocation, the product will not match the simple mnemonic prediction. Only the mechanistic principle — follow the path of greatest carbocation stability — correctly handles these cases.

The hydrogen-counting mnemonic is a useful shortcut for simple substrates but provides no mechanistic insight. Students who understand the underlying mechanism can handle rearrangements, ring-expansion products, and unusual substrates; those who only memorized the mnemonic cannot.