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α-Halogenation of Aldehydes and Ketones - α-proton of aldehydes and ketones can be replaced with a -Cl, -Br, or -I (-X) with Cl2 , Br2 , or I2 , (X2) respectively. The reaction proceeds through an enol. O H.
Michael reactions of enolates are thermodynamically driven — Stabilizing either the enolate donor or the Michael acceptor (or both, as above) with an additional electron-withdrawing group favors conjugate addition over direct addition.
Summary Sheet 2: Enols and Enolates. Structural Features of the Carbonyl Group: Effects on acidity of alkyl groups. Effect on reactivity of alkenes: Likewise, the presence of a carbonyl group activates alkenes toward nucleophilic attack: The carbonyl is an electron withdrawing π system with low-lying π* orbitals.
Enol is key intermediate X. Two stages: first stage is conversion of aldehyde or ketone to the corresponding enol; is rate-determining second stage is reaction of enol with halogen; is faster than the first stage. examine second stage first. OH.
Chapter 22 (Enolate Chemistry) Reaction Summary. PROTON as ELECTROPHILE. O OH. base, ROH. 1. Ph Ph. -Base-catalyzed keto-enol equilibrium -know mech (either direction) -know impact of substituents on enol concentration. 2. Ph. O. * base, ROH. Ph. CH3 H. optically active racemic. -Racemization of α-chiral optically active carbonyls -Mech.
¥ Product composition determined by relative rates of competing proton-abstraction reactions ¥ Deprotonation is rapid, quantitative, and irreversible. ¥ Favors less substituted enolate
III.D Reactions of Enolates • enolates are ambident nucleophiles and can react at either oxygen or carbon terminus. • SOFT electrophiles ( e.g. most carbon electrophiles) tend to react at carbon (soft centre).
