Sunday, January 20, 2008

AIEEE Chemistry Unit 23D Aldehydes and Ketones

Aldehydes contain carbonyl group C=O as functional group and the carbonyl atom carries at least one H atom.


In ketones, also carbonyl group C=O is the functional group. But the carbonyl carbon does not contain any H atoms, but it is attached to two alkyl or aryl groups.

Nomenclature of Aldehydes and Ketones

Common names are used for the simplest aldehydes and ketones:
formaldehyde, butyraldehyde, benzaldehyde,
acetone, benzophenone, acetophenone

Common names are also used for carbonyl-containing substituent groups,
which are known collectively as acyl groups:
formyl, acetyl, benzoyl

Traditional names are used for a great many aldehydes and ketones which
were recognized as substances long before systems of nomenclature were
cinnamaldehyde, furfural, acrolein

Structure of Aldehydes and Ketones

• The carbonyl carbon of an aldehyde or ketone is sp2-hybridized.
-• The bond angle is close to 120° (trigonal planar).
• The carbon-oxygen double bond consists of:
– A sigma C-O bond
– A pi C=O bond

Properties of Aldehydes and Ketones

Aldehydes and ketones are polar molecules because the C=O bond has a
dipole moment:

• Their polarity makes aldehydes and ketones have higher boiling points than
alkenes of similar molecular weight.
• Aldehydes and ketones are not hydrogen bond donors (they can can’t donate a
proton); therefore, they have lower boiling points than alcohols of similar
molecular weight.
• Aldehydes and ketones are hydrogen bond acceptors; this makes them have
considerable solubilities in water.

Ketones such as acetone are good solvents because they dissolve both aqueous and organic compounds
Acetone is a polar, aprotic solvent.

Reactions of Aldehydes and Ketones

The reactions of aldehydes and ketones can be divided into two main
– Reactions of the carbonyl group

- Reactions involving the alpha-carbon
Carbonyl group reactions fall into three main groups:
– Reactions with acids
– Addition reactions
– Oxidation

Reactions with acids:
– The carbonyl oxygen is weakly basic.
– Both Bronsted and Lewis acids can interact with a lone pair of electrons on
the carbonyl oxygen.

Addition Reactions
– Carbonyl groups in aldehydes and ketones undergo addition reactions.
– This is one of the most important reactions of the carbonyl group.

Addition reactions occur by two different mechanisms:
– Base-catalyzed addition (under basic or neutral conditions)
– Acid-catalyzed addition (under acidic conditions)
• In some cases, we can carry out the same overall reaction using either set of
conditions (acidic or basic).


Carbonyl groups in aldehydes and ketones may be oxidized to form
compounds at the next “oxidation level”, that of carboxylic acids.

• Alcohols are oxidized to aldehydes and ketones
(example: biological oxidation of ethanol to acetaldehyde)
• The carbonyl group may be further oxidized to carboxylic acids

Basicity of Aldehydes and Ketones

Reactions which occur at the carbonyl oxygen xygen of aldehydes and ketones:
– The weakly basic carbonyl oxygen reacts with protons or Lewis acids
– The protonated form of the aldehyde or ketone is resonance-stabilized
– This gives the aldehyde/ketone conjugate acid carbocation character

Protonated aldehydes and ketones can be thought of as alpha-hydroxy carbocations
• When an alkyl group replaces (conceptually) the proton, an alpha-alkoxy
carbocation is formed:

Addition Using Grignard Reagents
• Primary, secondary and tertiary alcohols may be formed in the reactions of
aldehydes or ketones with Grignard reagents.

primary alcohols from formaldehyde
secondary alcohols from aldehydes
tertiary alcohols from ketones
Gatterman reaction?

The Gattermann reaction, named for the German chemist Ludwig Gattermann, in organic chemistry refers to a reaction of hydrocyanic acid with an aromatic compound, in this case benzene, under catalysis of with Friedel-Crafts catalyst (aluminium chloride).The reaction is similar to the Friedel-Crafts reaction.


Reducing R-COCl to an aldehyde?

By catalytic hydrogenation in the presence of palladium (Pd) catalyst supported over barium sulphate The catalytic mixture is poisoned by the addition of a small amount of sulphur or quinoline. This reaction is known as Rosemmund reduction.

Getting an aldehyde from methylbenzene

by oxidation

Getting ketone from alcohols?

By oxidation of secondary alcohols

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