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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Ak on August 09, 2008, 06:53:08 AM

Title: ketone halogenation
Post by: Ak on August 09, 2008, 06:53:08 AM
my textbook shows examples of ketone halogenation where 1 R' group on the ketone is halogenated.  How do you know which side will be halogenated.  Ive attached an example and i want to do Br2 in acetic acid, but i dont know which R group is more likely to be halogenated.  Thanks
Title: Re: ketone halogenation
Post by: Ak on August 09, 2008, 12:07:22 PM
and also if there is a ketone and an aldehyde on the same molecule, can you selectively protect the aldehyde and make an acetal but no ketal? Thanks


p.s. i was trying to edit my first post to add this but i wasnt able to...i was able to edit yesterday
Title: Re: ketone halogenation
Post by: spirochete on August 10, 2008, 10:31:53 AM
The first step in bromination is acid catalyzed tautomerization, followed by addition of bromine to the pi bond.  I just wrote out two possible tautomerizations for your reaction, and one looks more favorable than the other.

Second question:  I would guess that the answer is yes, because typically one of these groups is more stable, the one that benefits more from hyperconjugation.  I couldn't tell you a specific way to do it though.

Title: Re: ketone halogenation
Post by: Mitch on August 10, 2008, 10:54:53 AM
Like mentioned above, you'll make the enol first and then brominate.

There is a wonderful paper by Kresge & Schepp in Journal of the Chemical Society Chemical Communications (20), 1548 (1989) detailing some kinetic studies of acetophenone (your molecule) and a related analogue that reacts differently. You'll find some good mechanistic data as well as other references to peruse.

Link to paper: http://dx.doi.org/10.1039/c39890001548

PS: People have been editing their posts to yield wrong information once they find class mates are also viewing this website. This is a recent problem, so there is now a 24 hour or so cutoff to editing posts.
Title: Re: ketone halogenation
Post by: Ak on August 10, 2008, 11:21:26 AM
right so stupid of me...all i had to do was go through the mechanism and i would see that the more highly substituted carbon-carbon double bond would form, and the bromine would then add to the pie bond....thanks!!