Integrated SequencePhysics Chemistry Organic Biology

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Chemical Kinetics

Reactions of Aldehydes and Ketones

The rate of alpha halogenation is independent of the concentration (or identity) of the halogen. The rate depends only on the concentration of aldehyde or ketone. What does that tell you? The best conclusion must be that halogen does not participate in the rate determining step of the reaction. What is the rate determining step? The answer is enolization.

Can you think of some 'wrong' answers a test-writer might think up to put with correct ones like these above within an MCAT multiple choice question? Here's one. Maybe they would try to trick you with some half remembered idea from enzyme kinetics, even though there is no enzyme involved, like trying to get you to agree that halogen must be present in greater than saturating concentration. Try not to think like the test-writers too much! It will change your view of human nature.

The Chemical Bond

Nucleophiles and Electrophiles

Reactions of Alkenes

Reactions of Aldehydes and Ketones

The reactive form in alpha halogenation is the enol; the π electrons of the double bond are approached by halogen in the second stage of the reaction. In fact, the double bond in an enol is significantly more nucleophilic than a typical alkene double bond because the enolic oxygen is electron donating by resonance. Electrophlic addition occurs readily, but unlike addition to a typical alkene, a cyclic halonium ion intermediate is not formed.

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