Chapter 14:
Aldehydes and Ketones
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ALDEHYDES AND KETONES
• The carbonyl group:
• Aldehydes have at least one hydrogen attached to the
carbonyl group.
• Ketones have two carbons attached to the carbonyl group.
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Coached Problem
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NAMING ALDEHYDES
• Find the longest carbon chain that contains the aldehyde
group.
• Change ending of the root hydrocarbon name by dropping
–e and adding –al.
• All other branches and groups are named and located
using standard IUPAC system.
Examples:
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NAMING KETONES
• Find the longest chain that contains C=O.
• Using the root alkane name, drop the –e ending and
change to –one.
• Number the longest carbon chain so the C=O group has
the lowest number.
• Name and number other substituents as before.
Examples:
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Coached Problem
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PHYSICAL PROPERTIES
• The carbonyl group is moderately polar, but it doesn’t have
any hydrogen atoms attached, so it cannot hydrogen bond
between molecules.
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PHYSICAL PROPERTIES, cont.
• Because of the polarity of the C=O group, these groups
can interact, but the attraction is not as strong as hydrogen
bonding.
• This makes the boiling point of aldehydes and ketones
higher than alkanes, but lower than alcohols.
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Coached Problem
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PHYSICAL PROPERTIES, cont.
• The C=O group can hydrogen bond with water molecules.
H
O
||
R-C-R
O
H
• This makes low molecular weight aldehydes and ketones
water soluble (they have small hydrophobic sections).
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ALDEHYDE AND KETONE REACTIONS
• Recall the oxidation of alcohols to produce aldehydes and
ketones:
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REACTIONS, cont.
• The ease with which aldehydes are oxidized allows us to
test for the presence of aldehydes with Tollens’ reagent or
Benedict’s reagent.
• In general, ketones fail to react with these reagents.
From left to right, three test tubes
containing potassium dichromate
(K2Cr2O7), acetone, and benzaldehyde.
After the addition of equal amounts
of K2Cr2O7, the acetone remains
unreacted, whereas the
benzaldehyde is oxidized.
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REACTIONS, cont.
• In the presence of aldehydes, Tollens’ reagent produces a
silver coating on glass.
• In the presence of aldehydes, Benedict’s reagent produces
a red precipitate.
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REACTIONS, cont.
• The addition of H2 in the presence of catalysts.
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Examples:
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REACTIONS, cont.
• The addition of alcohols to aldehydes produces an
unstable hemiacetal intermediate.
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REACTIONS, cont.
• General formulas for:
Hemiacetals
Acetals
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REACTIONS, cont.
• The addition of alcohol to ketones produces an unstable
hemiketal intermediate.
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REACTIONS, cont.
• General formula for:
Hemiketal
Ketal
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REACTIONS, cont.
• Cyclical hemiacetals and hemiketals are more stable than
open chains and are important in carbohydrate chemistry.
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REACTIONS, cont.
• Acetals and ketals are stable, but may be converted back
to aldehydes and ketones through acid catalyzed
hydrolysis. Hydrolysis is the breakage of a bond by
reaction with water.
• Acetal hydrolysis:
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REACTIONS, cont.
• Ketal hydrolysis:
Specific Example:
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Coached Problem
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IMPORTANT ALDEHYDES AND KETONES
• Formaldehyde
Gas at room temperature
Formalin – 37% aqueous solution
Sterilizer
Embalming fluid
Starting material for plastics such as Formica and
Bakelite
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IMPORTANT ALDEHYDES AND KETONES, cont.
• Acetone
(propanone)
Important organic solvent
Used in such things as nail polish remover
Miscible with water
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IMPORTANT ALDEHYDES AND KETONES, cont.
• Progesterone and testosterone (female and male sex
hormones) are ketones.
• Some aldehydes and ketones are very fragrant and are
used in flavorings.
Vanillin (vanilla)
Cinnamaldehyde (cinnamon)
Citral (citrus flavoring)
Camphor (medicinal odor)
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