Which of the following is least preffered to make higher alkynes fom sodium acetylide?

Options

A. `CH_3CH_2CH_2I`
B. `CH_3CH_3CH_2Br`
C. `CH_3CH_2CH_2Cl`
D. both (2) and (3)

Answer

C

Explanation

`Na^(+)CH-=C^(-):+CH_3CH_2CH_2Cl to HC-=CH+CH_3CH=CH_2+NaCl`
Alkynides are powerful bases but mild nucleophiles. Hence, `E2` eliminations are very competitive with `S_N2` displacements. Relative to `Br^(-)` or `I^(-)`,`Cl^(-)` is a poorer leaving group in `S_N2` reactions. Moreover, on account of higher `-I` affect, Cl is a better acid-strengthening group than `Br` or `I`. Thus, it facilitates the abstractions of `beta` proton from the alkyl halide. Consequently, with `RCl,E2` elimination is the dominant reaction.