Carbonyl group: Mechanisms of addition
المؤلف:
University of Missouri System
المصدر:
Organic Chemistry ii
الجزء والصفحة:
.................
8-10-2020
2989
Carbonyl group: Mechanisms of addition
The carbonyl group is a polar functional group that is made up a carbon and oxygen double bonded together. There are two simple classes of the carbonyl group: Aldehydes and ketones. Aldehydes have the carbon atom of the carbonyl group is bound to a hydrogen and ketones have the carbon atom of the carbonyl group is bound to two other carbons. Since the carbonyl group is extremely polar across the carbon-oxygen double bond, this makes it susceptible to addition reactions like the ones that occur in the pi bond of alkenes, especially by nucleophilic and electrophilic attack.
Ionic Addition to Carbonyl Group
As a result of the dipole shown in the resonance structures, polar reagents such as LiAlH4 and NaBH4 (hydride reagents) or R’MgX (Grignard reagent) will reduce the carbonyl groups, and ultimately convert unsaturated aldehydes and ketones into unsaturated alcohols. Since these reagents are extremely basic, their addition reactions are irreversible.
There are, however, addition reactions with less basic nucleophiles such as water, thiols, and amines that are capable of establishing equilibria or reversible reactions. These less basic reagents can react with the carbonyl group via two pathways: nucleophilic addition-protonation and electrophilic addition-protonation.
Addition of strong nucleophiles: Nucleophilic addition-protonation
With strong nucleophiles, direct nucleophilic attack of the electrophilic carbon takes place. As the nucleophile approaches the electrophilic carbon, two valence electrons from the nucleophile form a covalent bond to the carbon. As this occurs, the electron pair from the pi bond transfers completely over to the oxygen which produces the intermediate alkoxide ion. This alkoxide ion, with a negative charge on oxygen is susceptible to protonation from a protic solvent like water or alcohol, giving the final addition reaction.
Direct nucleophilic addition mechanism (for strong nucleophiles)
Acid-catalyzed nucleophilic addition of weak nucleophiles.
Under acidic conditions, protonation of the carbonyl oxygen takes place. Then nucleophilic attack by the nucleophile finishes the addition reaction. This type of reaction works best when the reagent being used is a very mildly basic nucleophile.
Acid-catalyzed addition of weak nucleophiles
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