Important Concepts
1. Hydrogens next to the carbonyl group (α-hydrogens) are acidic because of the electron-withdrawing nature of the functional group and because the resulting enolate ion is resonance stabilized.
2. Electrophilic attack on enolates can occur at both the α-carbon and the oxygen. Haloalkanes usually prefer the α-carbon. Protonation of the oxygen leads to enols.
3. Enamines are neutral analogs of enolates. Resonance donation of the nitrogen lone pair imparts nucleophilic character on the remote double bond carbon, which can be alkylated to give iminium cations that hydrolyze to aldehydes and ketones on aqueous work-up.
4. Aldehydes and ketones are in equilibrium with their tautomeric enol forms; the enol – keto conversionis catalyzed by acid or base. This equilibrium allows for facile a-deuteration and stereochemical equilibration.
5. A-Halogenationof carbonyl compounds may be acid or base catalyzed. With acid, the enol is halogenated by attack at the double bond; subsequent renewed enolization is slowed down by the halogen substituent. With base, the enolate is attacked at carbon, and subsequent enolate formation is accelerated by the halogens introduced.
6. Enolates are nucleophilic and reversibly attack the carbonyl carbon of an aldehyde or a ketone in the aldol condensation. They also attack the β-carbon of an α,β-unsaturated carbonyl compound in the Michael addition.
7. α,β-Unsaturated aldehydes and ketones show the normal chemistry of each individual double bond, but the conjugated system may react as a whole, as revealed by the ability of these compounds to undergo acid- and base-mediated 1,4-additions. Cuprates add in 1,4-manner, whereas alkyllithiums normally attack the carbonyl function.
From "Organic Chemistry" Textbook of VOLLHARDT &SCHORE
No comments:
Post a Comment