16.6 Reduction of Aldehydes and Ketones
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The reduction of a carbonyl group occurs with the addition of
hydrogen across the double bond to produce an –OH group, a
reaction that is the reverse of the oxidation of an alcohol.
Aldehydes are reduced to primary alcohols, and ketones are
reduced to secondary alcohols.
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16.7 Addition of Alcohols: Hemiacetals and
Acetals
The initial product of addition reactions of aldehydes
and ketones with alcohols are known as hemiacetals.
Compounds with both an -OH group and an -OR
group bonded to the same carbon atom.
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• Ethanol forms hemiacetals with acetaldehyde and
acetone.
• Hemiacetals rapidly revert back to aldehydes or
ketones by loss of alcohol and establish an
equilibrium with the aldehyde or ketone.
• When equilibrium is reached, very little hemiacetal
is present.
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• A major exception occurs when the C=O and
-OH functional groups that react are part of the
same molecule. The resulting cyclic hemiacetal is
more stable than a noncyclic hemiacetal.
• Most simple sugars exist mainly in the cyclic
hemiacetal form, as shown below for glucose, rather
than in the open-chain form.
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• If a small amount of acid catalyst is added to the
reaction of an alcohol with an aldehyde or ketone, the
hemiacetal initially formed is converted into an acetal
in a substitution reaction.
• An acetal is a compound that has two -OR groups
bonded to what was once the carbonyl carbon atom.
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The aldehyde or ketone from which an acetal is
formed can be regenerated by reversing the reaction.
Reversal requires an acid catalyst and a large
quantity of water. Hydrolysis: A reaction in which a
bond or bonds are broken and the -H and -OH of
water add to the atoms of the broken bond or bonds.
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Chapter Summary
• The carbonyl group is a C=O. The group is polar, with
a partial (-) charge on O and a partial (+) charge on C.
The O and the two substituents on the carbonyl-group
C atom form a planar triangle.
• The simplest aldehydes and ketones are known by
common names. Aldehydes are named systematically
by replacing the final -e in an alkane name with -al.
• Ketones are named systematically by replacing the
final -e in an alkane name with -one and numbering
starting with 1 at the end nearer the group. The
location of the carbonyl group is indicated by placing
the number of its carbon before the name.
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Chapter Summary Cont.
• Aldehyde and ketone molecules are polar, do not
hydrogen-bond with each other, but can hydrogenbond with water. Small ones are water-soluble.
Aldehydes and ketones are higher boiling than
alkanes but lower boiling than alcohols.
• Aldehydes and ketones are present in many plants,
where they contribute distinctive, pleasant odors.
Such natural aldehydes and ketones are widely used
in perfumes and flavorings.
• Formaldehyde (an irritating and toxic substance) is
used in polymers, is present in smog-laden air, and is
produced biochemically from ingested methanol.
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Chapter Summary Cont.
• Acetone is a widely used solvent and is a by-product
of food breakdown during diabetes and starvation.
Many sugars are aldehydes or ketones.
• Mild oxidizing agents (Tollens’ and Benedict’s
reagents) convert aldehydes to carboxylic acids but
have no effect on ketones.
• With reducing agents, hydride ion adds to the C of
the group in an aldehyde or ketone and hydrogen ion
adds to the O to produce primary or secondary
alcohols, respectively.
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Chapter Summary Cont.
• Aldehydes and ketones establish equilibria with
alcohols to form hemiacetals or acetals.
• Hemiacetals, which have an -OH and an -OR on
what was the carbonyl carbon, result from addition
of one alcohol molecule to the C=O bond.
• The more stable acetals, which have two -OR groups
on what was the carbonyl carbon, form by addition
of a second alcohol molecule to a hemiacetal.
• The aldehyde or ketone can be regenerated from an
acetal by treatment with an acid catalyst and a large
quantity of water, which is an example of a
hydrolysis reaction.
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