Ochem question

[runner222]

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Hey I had a question with alpha-halogenations of ketones. I thought that in acidic conditions, only mono-halogenation would take place and in basic conditions all alpha hydrogens were substituted. In destroyer it keeps showing that every alpha-hydrogen is replaced while in acidic conditions, such as #14.

Can anyone explain this?

 

[hazlewob]

Not totally sure, but I think with xs Cl you get each alpha carbon chlorinated twice. I don't think this is the case with Br, as it is more selective than Cl. I can't remember why exactly but Cl anions are less specific in there reactions with alkanes compared to other halogens. The acidic conditions make it HCl not Cl2.

I hope this is correct, I am just going by a distant memory of an organic lecture.

 

[hausee]

http://en.wikipedia.org/wiki/Ketone_halogenation

In acidic solution, usually only one alpha hydrogen is replaced by a halogen, because each successive halogenation is slower that the first. The halogen decreases the basicity of the carbonyl oxygen, thus making protonation less likely. However, in basic solution successive halogenations are more rapid, because the halogen withdraws electrons by induction and makes remaining hydrogens more acidic.