why would acyl halides give higher yields of amides compared to carboxylic acid or ester on reaction with ammo

Question

why would acyl halides give higher yields of amides compared to carboxylic acid or ester on reaction with ammonia

why should excess alkalide be avoided in the FeCl3 test for CH3COOH and benzoic acid?

pls answer as soon as possible...thanks

revision of the question....
why should excess alkali be avoided....benzoic acid???

Answer 1

Acyl chlorides C-COCl's are more unstable and hence more reactive than esters or acids to the incoming nucleophile ammonia:

R-COCl + NH3 --> R-CONH2 + HCl

the catch is you will have to use an excess of ammonia because the HCl generated will tie up the unreacted NH3 and decrease your yield of product is you don't!

NH3 + HCl --> NH4Cl

In the case of the ester or acid you have to eliminate the small molecule of alcohol or water:

R-CO2R2 + NH3 --> R-CONH2 + R2OH
R-COOH + NH3 --> R-CONH2 + H2O

this takes heat and time, and a more difficult reaction.

I'm not sure I follow the second part of your question other than FeCl3 is not stable in alkaline solution forming a brown precipitate of Fe(OH)3.

Answer 2

Acyl Halides

Answer 3

1. It's all about the leaving group in nucleophilic substitution. Cl- is much better than OH- or CH3CH2O-.

2. If you make solutions of FeCl3 alkaline, all you will get is a red-brown precipitate of Fe(OH)3!

Answer 4

Interesting, I was wondering the same thing myself