I don't understand why the reaction under acidic conditions must stop at only one halogen. Is it because if we tried to repeat the process, then instead of a hydrogen leaving to form the pi bond Br leaves to form the pi bond since it's a better leaving group?
Also, why in the world do people like to do these reactions using acedic acid as the solvent. I mean, sure it provides acidic conditions but would't it also react to form bromoacetic acid? (Or chloroacetic acid if we're talking about chlorination)
To answer my own (second) question: No it doesn't. To make chloroacetic acid from acetic acid and Cl2 requires a catalyst. The reaction does not proceed spontaneously.
Probably has something to do with bromine withdrawing electron density from oxygen in the ketone functional group that makes its less basic for protonation and subsequent reaction.
The alpha hydrogens are acidic due to the the chemistry of the carbonyl. There is the difference in electronegativity between the oxygen and the carbon and the conjugate base is resonance stabilized (Keto-Enol Tautomerization). Since they they are acidic in basic conditions with excess Br2 the reaction will have no problem removing all the alpha hydrogens. The stronger the base the easier of a time it will have removing the alpha hydrogens. Think back to General Chemistry and how acids and bases work. This doesn't happen in acidic conditions because water is acting as the base. Since the pH of water is 7 it is only acting as a base because it is surrounded by acid (H3O+). Plus the addition of the Br onto the α-position reduces the basicity of the carbonyl so that it no longer retains the proton added in the first step in the acid mechanism. This ends the reaction because it can not longer form the enol.
why can only one alpha hydrogen be removed in acidic condiitons? Why wont it continue on until there are two bromines on that alpha carbon in the second to last example
