Aldehydes, Ketones and Carboxylic Acids

Notes

Aldehydes, Ketones and Carboxylic Acids

1. Carbonyl Compounds

Organic compounds containing the carbonyl group (>C=O) are called carbonyl compounds.

The two important classes discussed are:

Aldehydes

In aldehydes, the carbonyl carbon is attached to at least one hydrogen atom.

General formula: R–CHO

Examples:

• HCHO — Methanal (Formaldehyde)
• CH₃CHO — Ethanal (Acetaldehyde)
• CH₃CH₂CHO — Propanal

Ketones

In ketones, the carbonyl carbon is attached to two carbon/alkyl groups.

General formula: R–CO–R′

Ketones may be:

• Symmetrical ketones: Same groups are present on both sides of the carbonyl group.
• Unsymmetrical ketones: Different groups are present on the two sides of the carbonyl group.

Example:

CH₃COCH₃ — Propanone (Acetone)

2. Nomenclature of Aldehydes and Ketones

Aldehydes

The IUPAC suffix used for aldehydes is –al.

Examples:

• HCHO → Methanal
• CH₃CHO → Ethanal
• CH₃CH₂CHO → Propanal

Ketones

The IUPAC suffix used for ketones is –one.

Example:

CH₃COCH₃ → Propanone

When more than one functional group is present, functional-group priority is considered during nomenclature.

3. Preparation of Aldehydes and Ketones

3.1 From Alcohols by Oxidation

A primary alcohol gives an aldehyde on controlled oxidation.

R–CH₂OH →[PCC/PDC] R–CHO

A secondary alcohol gives a ketone.

R–CH(OH)–R′ →[PCC/PDC] R–CO–R′

3.2 Catalytic Dehydrogenation of Alcohols

Alcohol vapours on passing over heated copper undergo dehydrogenation.

Primary alcohol:

R–CH₂OH →[Cu/Δ] R–CHO

Secondary alcohol:

R–CH(OH)–R′ →[Cu/Δ] R–CO–R′

4. Preparation from Alkenes – Reductive Ozonolysis

Alkenes undergo ozonolysis followed by reductive work-up to form aldehydes and/or ketones depending upon the structure of the alkene.

Reagent:

O₃/Zn/H₂O

The PDF also shows reductive work-up using DMS.

Example:

CH₃–CH=C(CH₃)₂ → CH₃CHO + CH₃COCH₃

Ozonolysis is useful for the preparation of carbonyl compounds and also for determining the structure of an alkene.

5. Preparation from Alkynes

5.1 Hydration of Alkynes

Alkynes undergo hydration in the presence of:

HgSO₄/H₂SO₄/H₂O

An enol is initially formed, which changes into the corresponding carbonyl compound.

For a terminal alkyne:

R–C≡CH → R–CO–CH₃

Example:

HC≡CH → CH₃CHO