Keto-enol Tautomerism
α-hydrogen acidity
The α-hydrogen of a ketone or an aldehyde is more acidic than the α-hydrogen of an ester. This is because the loan pair after the proton is pulled off the alpha carbon in the former is easily delocalized on the carbonyl carbon, while the latter is difficult to delocalize.
Keto–enol tautomers
- For most ketones, the enol tautomer is much less stable than the keto tautomer.
- For a β-diketone, the fraction of the enol tautomer in an aqueous solution is stabilized both by intramolecular hydrogen bonding and by conjugation of the carbon–carbon double bond with the second carbonyl group.
- Phenol its enol tautomer is more stable than its keto tautomer because of aromaticity stabization.
keto-enol interconversion
- In the base-catalyzed reaction, the base removes a proton from an α-carbon in the first step and the oxygen is protonated in the second step.
- In the acid-catalyzed reaction, the oxygen is protonated in the first step and the proton is removed from the α-carbon in the second step.
Halogenation of the α-carbon of Ketone
When Br2, Cl2, or I2 is added to a solution of an aldehyde or a ketone, a halogen replaces one or more of the α-hydrogens of the carbonyl compound.
Acid-Catalyzed Halogenation
- The first step is that the acid catalyzes the protonation of the carbonyl oxygen.
- At the end of the reaction the very acidic protonated carbonyl group loses a proton.
- The electron-withdrawing halogen atom decreases the basicity of the carbonyl oxygen.
- Thereby making protonation of the carbonyl oxygen (the first step in the acid-catalyzed reaction) less favorable.
Under acidic conditions, one A-hydrogen is substituted for a halogen.
Base-Promoted Halogenation
- The first step, hydroxide ion removes a proton from the α-carbon, forming an enolate ion.
- At the end of the reaction, hydroxide ion is not regenerated, so OH– is promoting the reaction, not catalyzing it.
- the electron-withdrawing halogen atom increases the acidity of the remaining α-hydrogens.
Under basic conditions,all of the α-hydrogens are substituted for halogens.
[HVZ reaction] Halogenation of a Carboxylic Acid at the α-carbon
Carboxylic acids cannot have substitution reactions at the α-carbon because the base removes the proton from the OH group, not the α-carbon.
Hell-Volhard-Zelinski reaction
- PBr3 converts the carboxylic acid into an acyl bromide.
- Acyl bromide is in equilibrium with its enol.
- Bromination of the enol forms a protonated α-brominated acyl bromide.
- Α-brominated acyl bromide is hydrolyzed to a α-brominated carboxylic acid.
