Aldehydes, Ketones and Carboxylic Acids

Notes

1. Carbonyl Compounds

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

The two important classes discussed are:

Aldehydes: Carbonyl carbon is attached to at least one hydrogen atom.

General formula:

R–CHO

Examples:

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

Ketones: Carbonyl carbon is attached to two carbon/alkyl groups.

General formula:

R–CO–R′

Ketones may be:

• Symmetrical: same groups on both sides
• Unsymmetrical: different groups on the two sides

Example: CH₃COCH₃ = Propanone (Acetone).

2. Nomenclature

For aldehydes, the suffix used is:

–al

Examples:

HCHO → MethanalCH₃CHO → EthanalCH₃CH₂CHO → Propanal

For ketones, the suffix is:

–one

Example:

CH₃COCH₃ → Propanone

The PDF also follows functional-group priority while naming compounds containing more than one functional group.

3. Preparation of Aldehydes and Ketones

3.1 From Alcohols — Oxidation

Primary alcohol → Aldehyde

R–CH₂OH →[PCC/PDC] R–CHO

Secondary alcohol → Ketone

R–CH(OH)–R →[PCC/PDC] R–CO–R

3.2 Catalytic Dehydrogenation of Alcohols

Using heated copper:

R–CH₂OH →[Cu/Δ] R–CHO

Primary alcohol gives aldehyde.

R–CH(OH)–R →[Cu/Δ] R–CO–R

Secondary alcohol gives ketone.

4. From Alkenes — Ozonolysis

Alkenes undergo ozonolysis followed by reductive work-up to produce aldehydes and/or ketones.

Reagents shown in the PDF include:

O₃/Zn/H₂O

and reductive work-up alternatives such as DMS.

General idea:

Alkene → Aldehyde + Ketone

depending upon the structure of the alkene.

For example, the PDF shows:

CH₃–CH=C(CH₃)₂ → CH₃CHO + CH₃COCH₃

Ozonolysis is therefore useful both for preparing carbonyl compounds and for determining the structure of an alkene.

5. From Alkynes

5.1 Hydration of Alkynes

Reagents:

Hg²⁺/H⁺/H₂Oor HgSO₄/H₂SO₄/H₂O

The addition initially gives an enol-type intermediate, which gives the corresponding carbonyl compound.

For terminal alkynes:

R–C≡CH → R–CO–CH₃

Example given:

HC≡CH → CH₃CHO

Phenylacetylene gives acetophenone under the hydration conditions shown in the PDF.

5.2 Hydroboration–Oxidation

The PDF gives:

1. BH₃/THF
2. H₂O₂/OH⁻

For a terminal alkyne:

R–C≡CH → R–CH₂–CHO

Example:

CH₃–C≡CH → CH₃CH₂CHO

Thus, hydroboration–oxidation of a terminal alkyne gives an aldehyde in the examples discussed.

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