Download Preparation of Aldehydes and Ketones

17-4
Preparation of Aldehydes and Ketones
Laboratory syntheses of aldehydes and ketones use four common
methods.
Oxidation of alcohols:
Ozonolysis:
Hydration of the carbon-carbon triple bond yields enols that tautomerize to
carbonyl compounds.
In the presence of mercuric ion, addition of water follows Markovnikov’s rule
to furnish ketones.
Friedel-Crafts alkanoylation (acylation):
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Reactivity of the Carbonyl Group: Mechanisms of
Addition
There are three regions of reactivity in aldehydes and ketones.
The carbonyl group undergoes ionic additions.
Polar reagents add to the dipolar carbonyl group according to
Coulomb’s law and the fundamentals of Lewis acid-Lewis base
interactions.
Nucleophiles bond to the carbonyl carbon and electrophiles bond to
the carbonyl oxygen.
NaBH4 and LiAlH4 reduce carbonyl groups but not carbon-carbon
double bonds:
The following reagents are strong bases and their reactions are irreversible.
Less basic nucleophiles, such as water, alcohols, thiols and amines are not strongly
exothermic and establish equilibria that can be pushed in either direction
depending upon reaction conditions.
Addition of strongly basic nucleophiles typically follow the nucleophilic additionprotonation pathway.
Relatively weak nucleophiles are more suitable for the electrophilic protonationaddition mechanism.
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Addition of Water to Form Hydrates
Water hydrates the carbonyl group.
The addition of water to an aldehyde or ketone is catalyzed by either acid or base.
The equilibrium reaction forms geminal diols, also called carbonyl hydrates:
Hydration is reversible.
The equilibrium for the hydration of ketones lies to the left, while that for
formaldehyde and aldehydes bearing electron-withdrawing substituents lies to
the right.
For ordinary aldehydes, the equilibrium lies close to unity.
These results can be explained by examining the resonance structures of the
carbonyl group:
The carbocation-like carbon atom is stabilized by alkyl groups and destabilized by
electron-withdrawing groups, such as CCl=3= and CF3.
The stabilities of the product diols are affected to a lesser extent.
Relative to formaldehyde, hydrations of aldehydes and ketones are progressively
more endothermic, while hydrations of carbonyl compounds containing electronwithdrawing groups are more exothermic.
The thermodynamic effects are paralleled by kinetic reactivity:
Although hydration is favorable in certain cases, it is usually impossible to isolate
carbonyl hydrates as pure substances.
Carbonyl hydrates play a role as intermediates in subsequent chemistry, such as
the oxidation of aldehydes to ketones under aqueous conditions.