Example 8.4

An organic compound (A) with molecular formula C H O forms an orange-red precipitate with , -DNP reagent and gives yellow precipitate on heating with iodine in the presence of sodium hydroxide. It neither reduces Tollens’ or Fehlings’ reagent, nor does it decolourise bromine water or Baeyer’s reagent. On drastic oxidation with chromic acid, it gives a carboxylic acid (B) having molecular formula C H O . Identify the compounds (A) and (B) and explain the reactions involved.

(A) forms , -DNP derivative. Therefore, it is an aldehyde or a ketone. Since it does not reduce Tollens’ or Fehling reagent, (A) must be a ketone. (A) responds to iodoform test.

Therefore, it should be a methyl ketone. The molecular formula of (A) indicates high degree of unsaturation, yet it does not decolourise bromine water or Baeyer’s reagent. This indicates the presence of unsaturation due to an aromatic ring. Compound (B), being an oxidation product of a ketone should be a carboxylic acid.

The molecular formula of (B) indicates that it should be benzoic acid and compound (A) should, therefore, be a monosubstituted aromatic methyl ketone. The molecular formula of (A) indicates that it should be phenyl methyl ketone (acetophenone). Reactions are as follows: (iii) Oxidation of methyl ketones by haloform reaction : Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom (methyl ketones) are oxidised by sodium hypohalite to sodium salts of corresponding carboxylic acids having one carbon atom less than that of carbonyl compound. The methyl group is converted to haloform.

This oxidation does not affect a carbon-carbon double bond, if present in the molecule. Iodoform reaction with sodium hypoiodite is also used for detection of CH CO group or CH CH(OH) group which produces CH CO group on oxidation. . Reactions due to a-hydrogen Acidity of aaaaa -hydrogens of aldehydes and ketones: The aldehydes and ketones undergo a number of reactions due to the acidic nature of a -hydrogen.

The acidity of a -hydrogen atoms of carbonyl compounds is due to the strong electron withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base. (i) Aldol condensation: Aldehydes and ketones having at least one a -hydrogen undergo a reaction in the presence of dilute alkali as catalyst to form b -hydroxy aldehydes (aldol) or b -hydroxy ketones (ketol), respectively. This is known as Aldol reaction . The name aldol is derived from the names of the two functional groups, aldehyde and alcohol, present in the products.

The aldol and ketol readily lose water to give a , b -unsaturated carbonyl compounds which are aldol condensation products and the reaction is called Aldol condensation . Though ketones give ketols (compounds containing a keto and alcohol groups), the general name aldol condensation still applies to the reactions of ketones due to their similarity with aldehydes. ( ii ) Cross aldol condensation: When aldol condensation is carried out between two different aldehydes and / or ketones, it is called cross aldol condensation . If both of them contain a -hydrogen atoms, it gives a mixture of four products.

This is illustrated below by aldol reaction of a mixture of ethanal and propanal. Ketones can also be used as one component in the cross aldol reactions. . Other reactions ( i ) Cannizzaro reaction: Aldehydes which do not have an a -hydrogen atom, undergo self oxidation and reduction (disproportionation) reaction on heating with concentrated alkali.

In this reaction, one molecule of the aldehyde is reduced to alcohol while another is oxidised to carboxylic acid salt.