alpha halogenation

Problem # 748

Alpha bromination is usually carried out under acidic conditions via the enol intermediate. 

Alpha bromination is uncontrollable under basic conditions, which goes through the enolate intermediate. Let's explore why.



a) Rank each carbonyl below in order of decreasing alpha-proton acidity (1= most acidic). Explain.

b) based on a), why does the reaction below lead to polyhalogenation?

Problem # 741

Like enolates (Q740), enols can also act as nucleophiles and make new bonds at the alpha position.

Let's go through the mechanism for how this happens, using alpha bromination as our example.

In the reaction below, the carbonyl form is in equilibrium with its enol form (Q738), which can then attack molecular bromine.


Complete the structure of the protonated carbonyl, and show the curved arrows that transform it into the final carbonyl product.