(a) Illustrate the following name reactions :
(i) Cannizzaro’s reaction
(ii)Clemmensen reduction

(b) How would you obtain the following :
(i) But-2-enal from ethanal
(ii)Butanoic acid from butanol
(iii)Benzoic acid from ethylbenzene

OR

(a) Give chemical tests to distinguish between the following: [5]
(i) Benzoic acid and ethyl benzoate
(ii)Benzaldehyde and acetophenone

(b) Complete each synthesis by giving missing reagents or products in the following :

(a)

(i) Cannizzaro’s reaction is observed for aldehydes having no α-hydrogen in them. These aldehydes react with strong concentrated alkalis like NaOH to form an alcohol having the same number of carbon atoms as the parent aldehyde and a salt carboxylic acid with the same number of carbon atoms as the parent aldehyde (cation of the salt is same as that of the cation of alkali).

Since 2 moles of aldehyde is taken, one gets reduced to an alcohol and the other gets oxidised to a carboxylic acid. The carboxylic acid then reacts with the alkali to form the respective salt of the carboxylic acid.

An example of Cannizaro’s reaction:

Formaldehyde is an aldehyde with one carbon atom. When it reacts with conc. KOH, the alkali, first formic acid (acid with one carbon atom) is produced, which on further reaction with KOH, produces formic acid. Along with that, alcohol with one carbon atom, methanol is also produced.

Mechanism of the reaction :

(ii) Clemmensen reduction is a chemical reaction described as a reduction of ketones (or aldehydes) which has the group, to alkanes using zinc amalgam and concentrated hydrochloric acid. It means that group is converted to that is reduction has taken place. This reaction is carried out in the presence of Zinc amalgam and dilute HCl.

One example for this reaction is the conversion of propanone to propane.

(b)

(i) Ethanal is converted to but-2-enal by a process called aldol condensation.

First aldol of ethanol is done by treating it with NaOH, that is we get 3-hydroxybutanal which on condensing gives but-2-enal.

(ii) To convert butanol to butanoic acid can be done by simple oxidation process using oxidising agents like acidified potassium permanganate or potassium dichromate solution.

(iii) First, ethyl benzene is oxidised in presence of potassium permanganate and KOH to get potassium benzoate. Potassium benzoate is then acifdified in presence of dilute HCl to get benzoic acid.

OR

(a)

(i) Benzoic acid is a carboxylic acid whereas ethyl benzoate is not. So, benzoic acid will respond to all the tests which an acid does.

When benzoic acid is treated with sodium bicarbonate solution, a brisk effervescence is observed due to the evolution of CO₂ gas. This is not observed in case of benzoic acid.

(ii) Acetophenone has a keto methyl group present in it. Due to this, it responds to the Iodoform test. In presence of iodine and sodium hydroxide, acetophenone produces a yellow precipitate of iodoform.

Benzaldehyde does not have the keto-methyl group, so it does not respond to iodoform test, so no precipitate is observed.

(b)

(i)

SOCl₂ adds –Cl group and the 2 –OH groups in the dicarboxylic acid are replaced by 2 –Cl groups.

(ii)

In NH₂CONHNH₂, one –NH₂ group donates its lone pairs to form the product in such a way that the other –NH₂ group can interact with the reactant molecule so that the loss of a water molecule is facilitated thereby forming a double bonded compound as the product.

(iii)

First, the reaction is hydrolysis of the reactant following anti-Markonikoff’s rule as there is B₂H₆ and peroxide to produce a 1° alcohol which otherwise would have been 2° if Markonikoff’s rule was followed.

Next step is partial oxidation using PCC to produce an aldehyde which would have been a carboxylic acid if complete oxidation was done.