Advices For Studying Organic Chemistry

2) In solution, terminal alkynes are more acidic than ammonia, however, they are

less acidic than alcohols and are less acidic than water.

3) In the gas phase, the hydroxide ion is a stronger base than the acetylide ion.

i) In solution, smaller ions (e.g., hydroxide ions) are more effectively solvated

than larger ones (e.g., ethynide ions) and thus they are more stable and

therefore less basic.

ii) In the gas phase, large ions are stabilized by polarization of their bonding

electrons, and the bigger a group is the more polarizable it will be and

consequently larger ions are less basic

7.12 REPLACEMENT OF THE ACETYLENEIC HYDROGEN ATOM OF

TERMINAL ALKYNES

1. Sodium alkynides can be prepared by treating terminal alkynes with NaNH 2 in
   liquid ammonia.
   H–C≡C–H + NaNH 2 liq.NH^3 H–C≡C:– Na+ + NH 3

CH 3 C≡C–H + NaNH 2 liq.NH^3 CH 3 C≡C:– Na+ + NH 3

1) The amide ion (ammonia, pKa = 38) is able to completely remove the acetylenic

protons of terminal alkynes (pKa = 25).

2. Sodium alkynides are useful intermediates for the synthesis of other alkynes.

RCC −Na+ + R'CH 2 Br R C C CH 2 R' +NaBr

Sodium alkynide 1° Alkyl halide Mono- or disubstituted acetylene

CH 3 CH 2 CC −Na+ + CH 3 CH 2 Br CH 3 CH 2 CCCH 2 CH 3 +NaBr
3-Hexyne (75%)