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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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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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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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PHYSICAL PROPERTIES, cont.• The C=O group can hydrogen bond with water molecules.

• This makes low molecular weight aldehydes and ketones water soluble (they have small hydrophobic sections).

O||

R-C-R

H

HO

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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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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)