Which of the following compounds undergoes the most highly regioselective addition of $HBr$?

• A. $\mathrm{CH}{2}_{}=\mathrm{CH}\mathrm{Cl}$ (chloroethene)
• B. $\mathrm{CH}{2}_{}=\mathrm{CH}\mathrm{CN}$ (acrylonitrile)
• C. $\mathrm{CH}{2}_{}=\mathrm{CH}\mathrm{Br}$ (bromoethene)
• D. $\mathrm{CH}{2}_{}=\mathrm{CH}\mathrm{CH}{3}_{}$ (1-butene)

Answer: (B)

Verified step by step guidance

1. Identify the reaction type: The problem involves the addition of HBr to alkenes, which typically proceeds via electrophilic addition, often following Markovnikov's rule where the proton (H⁺) adds to the carbon with more hydrogens, and the bromide (Br⁻) adds to the more substituted carbon.
2. Analyze the substituents on the alkene: Each compound has a different substituent attached to the alkene carbon (Cl, CN, Br, or an alkyl group). These substituents influence the electron density of the double bond and the stability of the carbocation intermediate formed during the reaction.
3. Consider the electronic effects of substituents: Electron-withdrawing groups (like CN) stabilize the carbocation intermediate through resonance or inductive effects, increasing regioselectivity by favoring one pathway over others. Electron-donating groups or halogens have different effects that can reduce or alter regioselectivity.
4. Predict regioselectivity based on carbocation stability: The more stabilized the carbocation intermediate, the more regioselective the addition will be. For example, acrylonitrile (CH₂=CHCN) has a strong electron-withdrawing cyano group that stabilizes the carbocation formed adjacent to it, leading to highly regioselective addition.
5. Summarize the reasoning: Compare all substituents and their effects on carbocation stability and regioselectivity. The compound with the strongest electron-withdrawing group (CN) will undergo the most highly regioselective addition of HBr.