Download aldehydes and ketones

Spencer L. Seager
Michael R. Slabaugh
www.cengage.com/chemistry/seager
Chapter 14
Aldehydes and Ketones
Jennifer P. Harris
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.
ALDEHYDES AND KETONES (continued)
• Examples:
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:
3-bromobutanal
2-ethylbutanal
COMMON ALDEHYDES
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:
3-methyl-2-pentanone
2-methylcyclohexanone
COMMON KETONES
PHYSICAL PROPERTIES
• The carbonyl group is moderately polar, but it doesn’t have
any hydrogen atoms bonded to the oxygen atom, so it cannot
hydrogen bond between molecules.
PHYSICAL PROPERTIES (continued)
• 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.
PHYSICAL PROPERTIES (continued)
PHYSICAL PROPERTIES (continued)
• The C=O group can hydrogen bond with water molecules
because the oxygen atom in the carbonyl group has a partial
negative charge that attracts the partial positive charge of a
hydrogen atom in the water molecule.
• This makes low molecular weight aldehydes and ketones
water soluble (they have small hydrophobic sections).
PHYSICAL PROPERTIES (continued)
ALDEHYDE AND KETONE REACTIONS
• Recall the oxidation of alcohols to produce aldehydes and
ketones:
ALDEHYDE AND KETONE REACTIONS
(continued)
• The difference in reactivity toward oxidation is the chief
reason why aldehydes and ketones are classified in
separated families.
ALDEHYDE AND KETONE REACTIONS
(continued)
• 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.
ALDEHYDE AND KETONE REACTIONS
(continued)
• In the presence of aldehydes, Tollens’ reagent produces a
silver coating on glass.
ALDEHYDE AND KETONE REACTIONS
(continued)
• In the presence of aldehydes, Benedict’s reagent produces
a red precipitate.
From left to right, three test
tubes containing Benedict’s
reagent, 0.5% glucose solution,
and 2.0% glucose solution.
The addition of Benedict’s
reagent from the first tube
produces colors (due to the
red Cu2O) that indicate the
amount of glucose present.
ALDEHYDE AND KETONE REACTIONS
(continued)
• The addition of H2 in the presence of catalysts.
ALDEHYDE AND KETONE REACTIONS
(continued)
• Examples:
ALDEHYDE AND KETONE REACTIONS
(continued)
• The addition of alcohols to aldehydes produces an unstable
hemiacetal intermediate.
ALDEHYDE AND KETONE REACTIONS
(continued)
• Example:
ALDEHYDE AND KETONE REACTIONS
(continued)
• Hemiacetal general formula:
• Acetal general formula:
ALDEHYDE AND KETONE REACTIONS
(continued)
• Hemiacetal example:
• Acetal example:
ALDEHYDE AND KETONE REACTIONS
(continued)
ALDEHYDE AND KETONE REACTIONS
(continued)
• Protonation of carbonyl oxygen
• Alcohol oxygen bonds to carbonyl oxygen
• Formation of hemiacetal and regeneration of acid catalyst
ALDEHYDE AND KETONE REACTIONS
(continued)
• The addition of alcohol to ketones produces an unstable
hemiketal intermediate.
ALDEHYDE AND KETONE REACTIONS
(continued)
• Example:
ALDEHYDE AND KETONE REACTIONS
(continued)
• Hemiketal general formula:
• Ketal general formula:
ALDEHYDE AND KETONE REACTIONS
(continued)
• Hemiketal example:
• Ketal example:
ALDEHYDE AND KETONE REACTIONS
(continued)
• Cyclic hemiacetals and hemiketals are more stable than
open-chains and are important in carbohydrate chemistry.
ALDEHYDE AND KETONE REACTIONS
(continued)
• Acetals and ketals are stable, but may be converted back to
aldehydes and ketones through acid catalyzed hydrolysis.
Hydrolysis is the cleavage of a bond by reaction with water.
• Acetal hydrolysis:
ALDEHYDE AND KETONE REACTIONS
(continued)
• Ketal hydrolysis:
• Specific Example:
ALDEHYDE AND KETONE REACTION MAP