University:
Eastern Kentucky University
Course:
CHE 361 | Organic Chemistry I
Academic year:

2020
Views:

499

Pages:

1
Author:

Killian Mata

Organic Chemistry 7: Aldehydes and Ketones II: Enolate General Principles a -carbon: The carbon adjacent to the carbonyl is the a-carbon. The hydrogens attached to the a-carbon are the a -hydrogens. Aldol Condensation Starts with an aldol addition to create an aldol and create a new C-C bond Then it undergoes a dehydration to give a conjugated enone (α,βunsaturated carbonyl) Aldol: Contains both aldehyde and an alcohol. “Ald – ol” a -hydrogens: Relatively acidic and can be removed by a strong base. The e- withdrawing O of the carbonyl weakens the C-H bonds on a-hydrogens. The enolate resulting from deprotonation can be stabilized by resonance with the carbonyl. Ketones: Ketones are less reactive toward nucleophiles because of steric hindrance and a-carbanion de-stabilization. The presence of an additional alkyl group crowds the transition step and increases energy. The alkyl group also donates edensity to the carbanion, making it less stable. Enolate Chemistry Keto / Enol: Aldehydes and ketones exist in both keto form (more common) and enol form (less common). Tautomers: Isomers that can be interconverted by moving a hydrogen and a double bond. Keto / Enol are tautomers. Michael Addition: An enolate attacks an a,b-unsaturated carbonyl, creating a bond. Kinetic Enolate: Favored by fast, irreversible reactions at LOW TEMP, with strong, sterically hindered bases. Thermodynamic Favored by slower, reversible reactions at HIGH TEMP Enolate: with weaker, smaller bases. Enamines: Tautomers of imines. Like enols, enamines are the less common tautomer. Aldol The nucleophile is the enolate formed from the Nucleophile: deprotonation of the a-carbon. Aldol The electrophile is the aldehyde or ketone in the form Electrophile: of the keto tautomer. Dehydration: After the aldol is formed, a dehydration reaction (loss of water molecule) occurs. This results in an a,bunsaturated carbonyl. Retro-Aldol Reverse of aldol reactions. Catalyzed by heat and base. Reactions: Bond between a- and b-carbon is cleaved.