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Chapter 15 Aldehydes and Ketones and Chiral Molecules
15.6 Oxidation and Reduction
15.7 Addition Reactions
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Oxidation
Aldehydes are easily oxidized to carboxylic acids.
O O || [O] ||
CH3—C—H CH3—C—OH
Acetaldehyde Acetic acid
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Hydrate formation Hydrate formation is the reason that aldehydes are
oxidized ([O]=-2H=oxidation) to carboxylic acids in aqueous media.
O OH O
|| H2O | [O] ||
RCH ↔ RC – OH → RCOH
|
H
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Tollens’ Test
Tollens’ reagent, which contains Ag+, oxidizes aldehydes, but not ketones.
Ag+ is reduced to metallic Ag, which appears as a “mirror” in the test tube.
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Tollens’ reagent detects an aldehyde Tollens’ reagent (a solution of Ag+ in aqueous NH3) can be
used to detect the presence of the aldehyde group in the unknown sample.
As the oxidation of the aldehyde proceeds, silver metal is deposited on the walls of the reaction flask as a shiny mirrow.
If upon addition of the Tollens’ reagent to the unknown, the test tube becomes silvery, the unknown is an aldehyde.
RCHO + 2Ag(NH3)2
+ +2OH- → RCOO- + NH4+ + 2Ag + 3NH3 + H2O
an Tollens’ salt of a silver
aldehyde reagent carboxylic acid mirrow
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Benedict’s Test
Benedict’s reagent, which contains Cu2+,
reacts with aldehydes that have an adjacent OH group.
When an aldehyde is oxidized to a carboxylic acid, Cu2+ is reduced to give Cu2O(s).
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Test for glucose (Benedict’s test)
CHO
OHH
HHO
OHH
OHH
CH2OH
+ Cu2+
Benedict's (blue)
COOH
OHH
OHH
HHO
OHH
CH2OH
+Cu2O(red)
D-GlucoseD-Gluconic acid
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Addition Reactions Polar molecules can add to the carbonyl groups
of aldehydes and ketones. The negative part of the added molecule bonds
to the positive carbonyl carbon. The positive part of the added molecule bonds
to the negative carbonyl oxygen.
| + - + - | —C=O + X—Y — C—O—X
| Y
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Acetal Formation
Alcohols add to the carbonyl group of aldehydes and ketones.
The addition of two alcohols forms acetals.
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Hemiacetal Formation The addition of one alcohol to an aldehyde or
ketone forms an intermediate called a hemiacetal. Usually, hemiacetals are unstable and difficult to
isolate.
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Cyclic Hemiacetals
Stable hemiacetals form when the C=O group and the —OH are both part of five- or six-atom carbon compounds.
