Aldehyde and Ketone

Aldehyde and ketone both contain the carbonyl group (>C=O), hence they are collectively called carbonyl compounds. In aldehydes, the carbonyl group is linked to an alkyl group and a hydrogen atom, while in ketones, it is linked with two alkyl groups.

The properties of aldehydes and ketones are due to the presence of the carbonyl group as a functional group.

Nomenclature

Aldehyde (IUPAC name: Alkanal)

Ketone (IUPAC name: Alkanone)

Structural Isomerism in aldehydes and ketones

1. Chain isomerism

Higher aldehyde and ketone show chain isomerism. For example, butanal and 2-methyl propanal are chain isomers.

Similarly, pentan-2-one and 3-methyl butan-2-one are chain isomers.

2. Positional isomerism

Aldehydes don’t show this isomerism because the -CHO group is always present at one end of the chain but higher ketones show this type of isomerism. For example, pentan-2-one and pentan-3-one are positional isomers of each other.

3. Functional isomerism

Aldehydes and ketones are functional isomers to each other. For example:

4. Tautomerism

i. Ethanal and ethenol show tautomerism. This is keto-enol tautomerism.

ii. Propanone and propen-2-ol shows tautomerism.

Questions

1. Give the structure and IUPAC name of
   i. Ethyl n-propyl ketone ii. Isobutyl Isopentyl ketone iii. Ethyl neo-pentyl ketone iv. vinyl acetone v. Acetyl acetone v. Diiso-propyl ketone
2. Write all possible isomers of C₅H₁₀O with IUPAC name.
3. How many isomeric ketones are possible for 2,2-dimethyl propanal?

General methods of preparation of aldehyde and ketone

1. Catalytic dehydrogenation

a. Primary alcohol is dehydrogenated into an aldehyde.

b. Secondary alcohol is dehydrogenated into ketone.

2. Oxidation of alcohol

i. Primary alcohol is oxidized to aldehyde by using mild oxidizing agents like pyridinium chlorochromate (PCC), collin’s reagent (dipyridinium chromium trioxide complex), etc. to prevent further oxidation.

3. Ozonolysis of alkene

Alkene is treated with ozone in presence of CCl₄ to give an ozonide which is decomposed by water and zinc dust to give aldehyde or ketone or both. Ozonolysis of straight-chain alkenes gives aldehyde.

Ozonolysis of branched-chain alkene gives ketone.

4. By reduction of acid chloride

Acid chlorides are heated with hydrogen in the presence of Pd-BaSO₄ to give aldehyde. This reaction is called Rosenmund reduction.

\begin{align*}\underset{acid\ chloride}{R-COCl}+H_{2}&\xrightarrow[quinoline]{Pd-BaSO_{4}}\underset{aldehyde}{RCHO}+HCl\\ \underset{\substack{Ethanoyl\\ chloride}}{CH_{3}COCl} + H_{2}&\xrightarrow[quinoline]{Pd-BaSO_{4}} \underset{Ethanal}{CH_{3}CHO} +HCl \end{align*}

Here, Pd/BaSO₄ is poisoned by sulphur of quinoline to prevent further reduction. Pd/BaSO₄ is called Rosenmund or Lindler catalyst.

Acid chlorides are reacted with dialkyl cadmium to give ether.

5. From geminal dihalide

Geminal dihalide is reacted with aqueous KOH gives aldehydes or ketones.

6. Catalytic hydration of alkynes

Hydration of acetylene with water in presence of H₂SO₄ and HgSO₄ gives acetaldehyde.

Besides acetylene, all alkynes give ketones on hydration.

Physical properties:

1. State, color, odour: Formaldehyde is a gas having pungent odour whereas aldehydes and ketones up to C₁₁ are colorless volatile liquids having pleasant odour. Higher members are colorless solid.

2. Solubility: Lower members up to C₅ are more or less soluble in water because of the formation of hydrogen bonding between carbonyl group and water.

Solubility in water goes on decreasing with an increase in carbon atoms.

Structure and nature of carbonyl group

In the carbonyl group (>C=O), the carbon atom is sp² hybridized. One of the p-orbitals of the carbonyl carbon remains unhybridized. The three sp2 hybrid orbitals of carbon lie in the trigonal plane with an angle of about 120° between them. These three sp² hybrid orbitals form three σ-bonds; one with the half-filled p-orbital of oxygen and the other two with the other atoms attached with this carbon. The unhybridized p-orbital of the carbonyl carbon undergoes sidewise overlapping with the remaining half-filled p-orbital of the oxygen atom to form a π-bond.

Since oxygen is more electronegative than carbon, partial positive charge is developed in carbon atom whereas partial negative charge is developed in oxygen atom, So, carbonyl compounds are highly polar compounds with dipole moments ranging from 2.3-2.7D.

Chemical Properties

Carbonyl compounds commonly give nucleophilic addition reactions. Nucleophiles attack the carbon atom of a carbon-oxygen double bond because carbon has a partial positive charge.
Aldehydes are more reactive than ketones towards the nucleophilic addition reaction due to the following two factors:

i. Inductive effect

Alkyl groups are electron releasing in nature having a +I effect. Ketones contain two alkyl groups whereas aldehydes contain one alkyl group. Hence, carbonyl carbon becomes less electropositive in ketones and causes low reactivity than aldehydes.

ii. Steric effect

Ketones contain two bulky alkyl groups attached with carbonyl carbon. More the number of alkyl groups attached with carbonyl carbon more will be the steric hindrance to nucleophilic attack and less would be the reactivity. Aldehydes give reducing properties that are absent in ketones.

1. Addition with HCN

Aldehydes and ketones are added with HCN to form cyanohydrin.

Note: Hydrogen cyanide or hydrocyanic acid (HCN) is a poisonous compound. It is prepared in situ by the action of sodium cyanide with dilute mineral acid.
The hydrolysis of cyanohydrin either in acidic or basic medium gives α-hydroxy carboxylic acid.
For example,

2. Addition with sodium bisulphite

Aldehydes and ketones are added with sodium bisulphite to give crystalline bisulphite addition products.

3. Action with hydrogen (reduction)

Aldehydes and ketones are easily reduced to primary and secondary alcohols respectively by using reducing agents like H₂/Ni or LiAlH₄, etc.

Action with ammonia derivatives

1. Action with hydroxyl amine: Formation of oxime- Aldehyde and ketones react with hydroxyl amine to give oxime.

2. Action with Hydrazine: Formation of hydrazone- Aldehydes and ketones react with hydrazine to give hydrazone.

3. Action with phenylhydrazine: Formation of phenylhydrazone- Aldehydes and ketones react with phenylhydrazine to give phenylhydrazone.

4. Action with 2,4-dinitrophenyl hydrazine (2,4-DNP):
Formation of 2,4-dinitrophenyl hydrazone [2,4-DNP test]- Aldehydes and ketones react with 2,4-dinitrophenyl hydrazine to form yellow ppt. of 2,4-dinitrophenyl hydrazone.

5. Action with semicarbazide: Formation of semicarbazone: Aldehydes and ketones react with semicarbazide to give crystalline ppt. of semicarbazone.

Oxidation of aldehyde and ketone

1. Oxidation with Tollen’s reagent (silver mirror test): Aldehyde reduces Tollen’s reagent to give metallic silver. Ketones don’t give this test.

Note: Tollen’s reagent is prepared by the reaction between ammonical silver nitrate and dilute NaOH.

\begin{align*} AgNO_{3}+NaOH &\rightarrow AgOH+NaNO_{3}\\ AgOH + 2NH_{4}OH&\rightarrow \underset{\substack{Diamine\ silver\\ hydroxide\\ (Tollen's\ reagent)}}{2[Ag(NH_{3})_{2}]OH} + H_{2}O \end{align*}

2. Oxidation with Fehling’s solution (Fehling’s test): Aldehyde reduces Fehling’s solution to give a red ppt. of cuprous oxide. It is only given by aliphatic aldehydes. It is not given by aromatic aldehydes and ketones.

Note: Fehling’s solution is a mixture of alkaline copper sulphate solution containing sodium potassium tartrate (Rochelle’s salt) in equal volume. It is used to diagnose diabetes in human urine.

Reduction of aldehyde and ketone

1. Clemmensen reduction

Aldehydes and ketones are reduced to alkane by zinc amalgam and conc. HCl.

2. Wolff Kishner reduction

Aldehydes and ketones are reduced to alkane by a mixture of hydrazine and KOH in glycol.

Aldol condensation reaction

Aldehydes and ketones having α-hydrogen react with dilute NaOH undergo condensation to give β-hydroxy aldehyde or β-hydroxy ketone. This reaction is called aldol condensation.

Note: Methanal, benzaldehyde and benzophenone does not undergo aldol condensation as they do not contain α- hydrogen.

Special reactions of methanal

1. Cannizzaro reaction: Aldehyde that does not have α-hydrogen reacts with conc. NaOH to give a mixture of alcohol and a salt of carboxylic acid.

Note: Cannizzaro reaction is an example of auto-redox or disproportional reaction in which an aldehyde is oxidized to its corresponding salt of carboxylic acid and the other is reduced to a corresponding primary alcohol.

2. Action with ammonia: Formaldehyde reacts with ammonia to give hexamethylene tetramine (Urotropine). It is used as a urinary antiseptic drug.

\underset{\substack{formal-\\ dehyde}}{6HCHO} + 4NH_{3}\rightarrow \underset{\substack{Hexamethylene\\ tetramine\\ (Urotropine)}}{(CH_{2})_{6}N_{4}} + 6H_{2}O

3. Action with phenol: Phenol reacts with formaldehyde to form bakelite.

Action with PCl₅

Formalin

Formaldehyde is sold as a 40% aqueous solution under the name formalin.

Uses of formalin

• As antiseptic.
• Preservation of biological specimen.
• In the manufacture of bakelite.