Doubts in Organic Chemistry

[Abdul Quadeer]

1) My book says that we can distinguish between 2 hydroxy butanoic acid (A) and 3 hydroxy butanoic acid (B) by haloform tests. It says that B gives positive haloform test. I am fine with it.
I don't get why A doesnot give positive test as it is also a sec. alcohol .

2)In the base-catalysed hydrolysis of amides, they undergo nucleophilic addition of OH- and finally form acetate.
In hoffmann degradation, we use OH- (of an alkali) where it extracts a proton from the -NH2 group and does not add to the amide.

Why is the action of same OH- different in two cases?

Any help appreciated.

[DDTea]

1) Regarding the first question: the haloform reaction requires a methyl group to be adjacent to a carbonyl or a secondary alcohol that can be oxidized to a carbonyl. So in alpha-hydroxy butanoic acid, there is no methyl group adjacent to the hydroxyl group. In beta-hydroxy butanoic acid, C-4 is adjacent to the hydroxyl group.

2) I'm speculating a bit here: it sounds like one is the kinetic product (nucleophilic addition) and the other is the thermodynamic product (deprotonation of the amide). I imagine that in practice, the reaction conditions would be tailored to favor one over the other, but that both *could* happen depending on how the hydroxide ion attacks the amide. This is the case with many reactions and is one way that product yields are lowered (competing reactions).

[Abdul Quadeer]

Thanks DDTea!