Question

Which of the following will undergo a faster $S_{N}2$?

• A. I
• B. Neither I nor II
• C. Both undergo $S_{N}2$ at the same rate
• D. II

Answer 1

The correct option is D

II

For $S_{N}2$, the ideal substrates are primary or methyl halides (X = Cl, Br, I). Observe the carbons to which the bromine atom is bonded. In I, the secondary carbon to which the bromine atom is bonded is immediately adjacent to a tertiary carbon which is too crowded. In II, the carbon bearing the bromine is primary although it is near a crowded tertiary carbon. But we can argue that it is relatively unhindered sterically. Hence II will undergo $S_{N}2$ at a much greater pace.