CBSE Class 12 Chemistry Aldehydes Ketones And Carboxylic Acids Notes Set 05

Revision Notes

Aldehydes and Ketones
Carbonyl group : The functional group \( \text{C=O} \) is called carbonyl group. Organic compounds containing carbonyl group are aldehydes and ketones. The general formulae of these compounds are as follows:

Aldehyde
\( \text{R—C(=O)—H} \)
(where R can be H or any alkyl, aryl or aralkyl group)

Ketone
\( \text{R—C(=O)—R’} \)
(where R and R’ may be same or different alkyl, aryl or aralkyl group)

Methods of preparation of Aldehydes and Ketones

Preparation of Aldehydes

• By oxidation of primary alcohols: Aldehydes can be prepared by the oxidation of primary alcohols.
  \( \text{RCH}_2\text{OH} + \text{[O]} \xrightarrow{\text{K}_2\text{Cr}_2\text{O}_7/\text{H}_2\text{SO}_4 \text{ or } \text{KMnO}_4} \text{R — CHO} + \text{H}_2\text{O} \)
  \( \text{R — CH}_2 \text{ — OH} \xrightarrow{\text{PCC}} \text{R — CHO} \)

• By dehydrogenation of alcohols:
  \( \text{R — CH}_2 \text{ — OH} \xrightarrow{\text{Cu, 573K}} \text{RCHO} + \text{H}_2 \)
• From hydrocarbons:
  • By ozonolysis of alkenes:
    \( \text{R—CH=CH—R'} + \text{O}_3 \rightarrow \text{Ozonide} \xrightarrow{\text{H}_2\text{O, Zn}} \text{R—CHO} + \text{R'—CHO} \)
  • By hydration of alkynes:
    \( \text{CH} \equiv \text{CH} + \text{H}_2\text{O} \xrightarrow{\text{H}_2\text{SO}_4/\text{HgSO}_4, 333K} [\text{CH}_2 = \text{CH(OH)}] \rightarrow \text{CH}_3\text{CHO} \)
• From acyl chloride (Rosenmund Reduction):
  \( \text{R — C(=O) — Cl} + \text{H}_2 \xrightarrow{\text{Pd-BaSO}_4} \text{R — CHO} + \text{HCl} \)
• From nitriles and esters (Stephen reaction):
  \( \text{R — C} \equiv \text{N} + \text{SnCl}_2 + \text{HCl} \rightarrow \text{R — CH = NH} \cdot \text{HCl} \xrightarrow{\text{H}_2\text{O}} \text{RCHO} + \text{NH}_4\text{Cl} \)
  \( \text{R — CN} \xrightarrow{1. \text{AlH(i-Bu)}_2, 2. \text{H}_2\text{O}} \text{R — CHO} \)

Preparation of Benzaldehyde

• By oxidation of toluene:

  • Etard reaction: Toluene reacts with chromyl chloride (\( \text{CrO}_2\text{Cl}_2 \)) in \( \text{CS}_2 \) to form a chromium complex, which on hydrolysis gives benzaldehyde.
  • Using chromic oxide (\( \text{CrO}_3 \)): Toluene or substituted toluene is converted to benzylidene diacetate on treating with chromic oxide in acetic anhydride. The benzylidene diacetate can be hydrolysed to corresponding benzaldehyde.
• By side chain chlorination followed by hydrolysis: Toluene is chlorinated in the presence of light to give benzal chloride which on hydrolysis gives benzaldehyde.
• By Gattermann-Koch reaction: Benzene or its derivative is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride or cuprous chloride to give benzaldehyde.

Preparation of Ketones

• By oxidation of secondary alcohols:
  \( \text{R-CH(OH)-R'} + \text{[O]} \xrightarrow{\text{K}_2\text{Cr}_2\text{O}_7/\text{H}_2\text{SO}_4 \text{ or } \text{CrO}_3} \text{R-C(=O)-R'} + \text{H}_2\text{O} \)

• By dehydrogenation of secondary alcohols:
  \( \text{R-CH(OH)-R'} \xrightarrow{\text{Cu, 573K}} \text{R-C(=O)-R'} + \text{H}_2 \)
• By ozonolysis of alkenes: Ozonolysis of alkenes followed by reaction with \( \text{Zn} \) dust and \( \text{H}_2\text{O} \) gives ketones when the alkene is disubstituted at the double bond.
• By hydration of alkynes (Kucherov's reaction):
  \( \text{CH}_3\text{—C} \equiv \text{CH} \xrightarrow{\text{H}_2\text{SO}_4/\text{HgSO}_4} \text{CH}_3\text{COCH}_3 \)
• From acyl chlorides: Treatment of acyl chlorides with dialkylcadmium, prepared by the reaction of cadmium chloride with Grignard reagent, gives ketones.
• From nitriles: Treating a nitrile with Grignard reagent followed by hydrolysis yields a ketone.
• Preparation of Aromatic ketones:
  • By Friedel-Crafts acylation: Benzene or substituted benzene reacts with acid chloride in the presence of anhydrous aluminium chloride to give the corresponding ketone.

Physical properties of Aldehydes and Ketones

• Most of the aldehydes (except formaldehyde which is a gas) are liquids at room temperature. The lower ketones are colourless liquids and have a pleasant smell.

• Both of these have relatively high b.p. as compared to hydrocarbons of comparable molecular masses due to presence of polar carbonyl group. But they have lower b.p. than alcohols of comparable molecular masses.
• The lower members of aldehydes and ketones (up to four carbon atoms) are soluble in water due to hydrogen bonding.

Chemical properties of Aldehydes and Ketones

Nucleophilic addition reactions:

• Addition of hydrogen cyanide (HCN): Aldehydes and ketones react with hydrogen cyanide (HCN) to yield cyanohydrins.
• Addition to sodium hydrogen sulphite: Aldehydes and ketones react with sodium hydrogen sulphite to form bisulphite addition products.
• Addition of Grignard reagent: Yields alcohols.
• Addition of alcohols: Aldehydes react with one equivalent of monohydric alcohol in the presence of dry hydrogen chloride to yield hemiacetals, which further react with one more molecule of alcohol to give acetals. Ketones react with ethylene glycol to form cyclic products known as ethylene glycol ketals.
• Addition of ammonia and its derivatives: Nucleophiles, such as ammonia and its derivatives \( \text{H}_2\text{N-Z} \) add to the carbonyl group of aldehydes and ketones.

Reduction:

• Reduction to alcohols: Aldehydes and ketones are reduced to primary and secondary alcohols respectively by \( \text{NaBH}_4 \) or \( \text{LiAlH}_4 \) or catalytic hydrogenation.
• Reduction to hydrocarbons:
  • Clemmensen reduction: The carbonyl group of aldehydes and ketones is reduced to \( \text{CH}_2 \) group on treatment with zinc-amalgam and concentrated hydrochloric acid.
  • Wolff-Kishner reduction: The carbonyl group is reduced to \( \text{CH}_2 \) group on treatment with hydrazine followed by heating with sodium or potassium hydroxide in high boiling solvent such as ethylene glycol.

Oxidation:

• Aldehydes are easily oxidised to carboxylic acids on treatment with common oxidising agents or mild oxidising agent like Tollen’s reagent or Fehling’s solution.
• Ketones undergo oxidation under vigorous conditions with cleavage of carbon bond to give a mixture of carboxylic acids having lesser number of carbon atoms than the parent ketone.

Reaction due to \(\alpha\)-hydrogen:

\(\alpha\)-hydrogen in aldehydes and ketones is acidic in nature due to strong electron withdrawing effect of carbonyl group.

• Aldol condensation: Aldehydes and ketones having at least one \(\alpha\)-hydrogen react in presence of dilute alkali to form \(\beta\)-hydroxy aldehydes (aldol) or \(\beta\)-hydroxy ketones (ketol).
• Cross aldol condensation: When two different aldehydes and/or ketones undergo aldol condensation, it is called cross aldol condensation.
• Cannizzaro Reaction: Aldehydes which do not have \(\alpha\)-hydrogen atom, undergo self oxidation and reduction (disproportionation) reaction on heating with concentrated alkali. In this reaction, one molecule of aldehyde is reduced to alcohol while another is oxidised to carboxylic acid salt.

Carboxylic Acids

Nomenclature, Structure and Uses:

• Common names: End with –ic.
• IUPAC names: Replace –e in the corresponding alkane with –oic acid.
• Structure of Carboxyl Group: In carboxylic acids, the bonds to the carboxyl carbon lie in one plane and are separated by about \( 120^\circ \). The carboxylic carbon is less electrophilic than carbonyl carbon because of the resonance structure.

Uses of Carboxylic Acids:

• Methanoic acid in rubber, textile, dyeing, leather industries.
• Ethanoic acid as solvent.
• Higher fatty acids in manufacture of soaps and detergents.

Physical properties of Carboxylic acids:

• Higher boiling points than aldehydes, ketones or alcohols.
• Solubility decreases with increasing number of C atoms.

Chemical properties of Carboxylic acids:

• \( 2\text{RCOOH} + 2\text{Na} \rightarrow 2\text{RCOONa} + \text{H}_2 \)
• Forms corresponding anhydride on heating with mineral acids.
• \( \text{RCOOH} + \text{R’OH} \rightarrow \text{RCOOR’} + \text{H}_2\text{O} \) (Esterification)
• \( \text{RCOOH} + \text{PCl}_5 \rightarrow \text{RCOCl} + \text{POCl}_3 + \text{HCl} \)
• \( \text{CH}_3\text{COOH} + \text{NH}_3 \rightarrow \text{CH}_3\text{COONH}_4 \xrightarrow{\text{Heat}} \text{CH}_3\text{CONH}_2 \)
• Reduction: \( \text{RCOOH} \xrightarrow{\text{B}_2\text{H}_6 / \text{H}_3\text{O}^+} \text{RCH}_2\text{OH} \)
• Decarboxylation: \( \text{RCOONa} \xrightarrow{\text{NaOH & CaO, Heat}} \text{R–H} + \text{Na}_2\text{CO}_3 \)
• HVZ reaction: \( \text{RCH}_2\text{COOH} \xrightarrow{\text{X}_2/\text{Red P, H}_2\text{O}} \text{R–CH(X)–COOH} \) (where X = Cl, Br)