Question

Identify 'A' in the following reaction. A 3,4-diethyl-3.4-dimethylhexane +2NaC

• A. 3-Chloro-3-methylpentane
• B. 2-Chloro-3-methylpentanc
• C. 2-Chloro-2-methylpentane
• D. 3-Chloro-2-methylpentane

Answer 1

Answer: (D)

3‑Chloro‑2‑methylpentane

Answer 2

The key is to “read between the lines” of the skeletal rearrangement that occurs when 3,4‐diethyl‐3,4‐dimethylhexane is treated with a sodium–halide reagent (here, “2NaC” stands in for the reagent which introduces Cl). Under the reaction conditions a C–C bond is cleaved and one of the ethyl groups attached at the quaternary centres is removed. The chlorine then gets attached at the methylene of that removed ethyl fragment. When the fragments are “re‐numbered” as the longest continuous chain (a pentane) the product turns out to be

\textbf{CH}_3\!-\!\textbf{CH(CH}_3\textbf)}\!-\!\textbf{CH(Cl)}\!-\!\textbf{CH}_2\!-\!\textbf{CH}_3,

which is named 3‑chloro‑2‑methylpentane.

Core Minimal Explanation

1. Write the structure of 3,4‐diethyl‐3,4‐dimethylhexane. Notice that C3 and C4 are quaternary (no H’s).

2. Under the reaction conditions the less hindered secondary hydrogens (on the –CH2– of an ethyl substituent) are abstracted and the ethyl group cleaves off from the main chain.

3. Upon subsequent chlorination of that –CH2– group, the longest chain is re‐defined as pentane giving the structure CH3–CH(CH3)–CH(Cl)–CH2–CH3 → 3‑chloro‑2‑methylpentane.