Home » Ask & Discuss » Chemistry
« Back to Discussion

Organic Chemistry

New kid on the Block

Joined:	3 Nov 2009
Post:	25

3 Nov 2009 08:22:21 IST

0 People liked this

1197

in birch reduction of internal alkynes how is trans radical anion more stable?

Engineering Entrance , Medical Entrance , AIPMT , JEE Main , AIIMS , JEE Advanced , Chemistry , Organic Chemistry

in birch reduction of internal alkynes how is trans radical anion more stable?

Comments (1)

umesh

Forum Expert

Joined: 23 Oct 2009

Posts: 580

3 Nov 2009 09:57:00 IST

Like 0 people liked this

The smallest and most reactive nucleophilic species is probably an electron. Electron addition to a functional group is by definition a reduction, and we noted earlier that alkynes are reduced by solutions of sodium in liquid ammonia to trans-alkenes. To understand how this reduction occurs we first need to identify two distinct reactions of sodium with liquid ammonia (boiling point -78 ºC). In the first, sodium dissolves in the pure liquid to give a deep blue solution consisting of very mobile and loosely bound electrons together with solvated sodium cations (first equation below). For practical purposes, we can consider such solutions to be a source of "free electrons" which may be used as powerful reducing agents. In the second case, ferric salts catalyze the reaction of sodium with ammonia, liberating hydrogen and forming the colorless salt sodium amide (second equation). This is analogous to the reaction of sodium with water to give sodium hydroxide, but since ammonia is 10¹⁸ times weaker an acid than water, the reaction is less violent. The usefulness of this reaction is that sodium amide, NaNH₂, is an exceedingly strong base (18 powers of ten stronger than sodium hydroxide), which may be used to convert very weak acids into their conjugate bases.

Na + NH₃ (liquid, –78 ºC ) ——> Na⁽⁺⁾ + e^(–) (a blue solution)

Na + NH₃ (liquid, –78 ºC ) + Fe ——> H₂ + NaNH₂ (a colorless solution)

Returning to the reducing capability of the blue electron solutions, we can write a plausible mechanism for the reduction of alkynes to trans-alkenes, as shown below. Isolated carbon double-bonds are not reduced by sodium in liquid ammonia, confirming the electronegativity difference between sp and sp² hybridized carbons.

Quick Reply

Reply

	Some HTML allowed.
	Keep your comments above the belt or risk having them deleted.
	Signup for a avatar to have your pictures show up by your comment
	If Members see a thread that violates the Posting Rules, bring it to the attention of the Moderator Team