CBSE Class 12th Chemistry Notes for Chapter 12 Aldehydes, Ketones and Carboxylic Acids

                    UNIT 12 ALDEHYDES, KETONES AND CARBOXYLIC ACIDS

 

• Aldehydes, ketones and carboxylic acids are some of the important classes of organic compounds containing carbonyl group.

 

• These are highly polar molecules and therefore, they boil at higher temperatures than the hydrocarbons and weakly polar compounds such as ethers of comparable molecular masses.

 

• The lower members are more soluble in water because they form hydrogen bonds with water. The higher members, because of large size of hydrophobic chain of carbon atoms, are insoluble in water but soluble in common organic solvents.

 

• Aldehydes are prepared by dehydrogenation or controlled oxidation of primary alcohols and controlled or selective reduction of acyl halides. Aromatic aldehydes may also be prepared by the oxidation of:
  • methylbenzene with chromyl chloride or CrO₃ in the presence of acetic anhydride
  • formylation of arenes with carbon monoxide and hydrochloric acid in the presence of anhydrous aluminium chloride
  • cuprous chloride or by hydrolysis of benzal chloride.

 

• Ketones are prepared by oxidation of secondary alcohols and hydration of alkynes. They are also prepared by reaction of acyl chloride with dialkylcadmium.

 

• A good method for the preparation of aromatic ketones is the Friedel-Crafts acylation of aromatic hydrocarbons with acyl chlorides or anhydrides.

 

• Both aldehydes and ketones can be prepared by ozonolysis of alkenes.

 

• Aldehydes and ketones undergo nucleophilic addition reactions onto the carbonyl group with a number of nucleophiles such as HCN, NaHSO₃, alcohols (or diols), ammonia derivatives, and Grignard reagents.

 

 

 

 

• The ∝- hydrogens in aldehydes and ketones are acidic. Therefore, aldehydes and ketones having at least one ∝- hydrogen, undergo Aldol condensation in the presence of a base to give ∝- hydroxyaldehydes (aldol) and ∝- hydroxyketones (ketol), respectively. Aldehydes having no ∝- hydrogen undergo Canizzaro reaction in the presence of concentrated alkali.

 

• Aldehydes and ketones are reduced to alcohols with NaBH₄, LiAlH₄, or by catalytic hydrogenation.

 

• The carbonyl group of aldehydes and ketones can be reduced to a methylene group by Clemmensen reduction of Wolff-Kishner reduction.

 

• Aldehydes are easily oxidized to carboxylic acids by mild oxidizing reagents such as Tollens’ reagent and Fehling’s reagent. These oxidation reactions are used to distinguish aldehydes from ketones.

 

• Carboxylic acids are prepared by the oxidation of primary alcohols, aldehydes and alkenes by hydrolysis of nitriles, and by treatment of Grignard reagents with carbon dioxide.

 

• Aromatic carboxylic acids are also prepared by side-chain oxidation of alkylbenzenes.

 

• Carboxylic acids are considerably more acidic than alcohols and most of simple phenols.

 

• Carboxylic acids are reduced to primary alcohols with LiAlH₄, or with diborane in ether solution and also undergo ∝- halogenation with Cl₂ and Br₂ in the presence of red phosphorus (Hell-Volhard Zelinsky reaction).