Question about Alpha Substitution of carboxylic acid (HVZ)

[Frah0709]

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After an Alpha Substitution of an Carboxylic Acid (HVZ), if an NH3 is added why does it attack the alpha carbon instead of the carbonyl?


Rxn was an R group attached to ethanoic acid with 1)Br2,PBr3 2) H20; then NH3 added after

 

[chemtopper]

NH3 attacks on better leaving group ...Br - is better leaving group

 

[amy_k]

So we have an HVZ reaction which gives an alpha substituted bromo carboxylic acid. The conditions of HVZ promote the enol intermediate form of the acyl bromide so the final product favours bromination at the alpha position rather than at the carbonyl. The choices for leaving group are then OH- (a terrible leaving group) or Br if NH3 attacks at the alpha position. You might be thinking of the reaction between an acyl bromide (Br attached to carbonyl carbon) and ammonia, which does attack at the carbonyl, but here the Br leaving group is at the alpha carbon after HVZ. This is actually a method for forming alanine amino acid if the R group is methyl.

http://www.organic-chemistry.org/namedreactions/hell-volhard-zelinsky-reaction.shtm

 

[chemtopper]

On carbonyl carbon it is the nucleophilic addition reaction which is more favored due to good leaving group on the carbonyl carbon

 

[deleted577188]

If NH3 attacks the carbonyl carbon, there will be 2 leaving groups that could potentially leave. One is the positively charged NH3 and the other is the negatively charged OH. Over 99% of the time, the NH3 will leave, getting you back to the starting material. If you have an alpha-bromo substituted carbonyl, then now you have a better leaving group (Br-) instead of the NH3.

Note that in real life the HVZ can take upwards of 4-5 days. This reaction is extremely slow, and I know from experience.

 

[chemtopper]

yes I agree ...HVZ is a very slow reaction