CBSE Class 10 Science 2020 · Paper 31/5/2 · Question 25

Carbon and its Compounds 5 marks Medium

Consider the molecular formula of the carbon compounds (a) and (b) given below :

(a) $C_{3} H_{8} O$
(b) $C_{3} H_{6} O_{2}$ (i) Identify the functional groups in (a) and (b) and write their structures. (ii) Are (a) and (b) isomers ? Give reason. (iii) What happens when alkaline $KMnO_{4}$ is added, drop by drop, into a test tube containing warm propanol ? Write the chemical equation for the reaction and state the role of alkaline $KMnO_{4}$ in this reaction.

Answer

Propanol has $- OH$ and propanoic acid has $- COOH$ ; they are not isomers; alkaline $KMnO_{4}$ oxidises propanol, acting as an oxidising agent.

(i) (a)

C_{3} H_{8} O

is propanol (propan-1-ol), an alcohol: functional group

- OH

; structure

CH_{3} - CH_{2} - CH_{2} - OH

. (b)

C_{3} H_{6} O_{2}

is propanoic acid, a carboxylic acid: functional group

- COOH

; structure

CH_{3} - CH_{2} - COOH

. (ii) No. Isomers have the same molecular formula, but (a) and (b) have different molecular formulae. (iii) The purple colour of alkaline

KMnO_{4}

disappears as it oxidises propanol to propanoic acid (once all the propanol is used up, excess

KMnO_{4}

stays purple):

CH_{3} CH_{2} CH_{2} OH + 2 [O] alkaline KMnO_{4}, heat CH_{3} CH_{2} COOH + H_{2} O

. Alkaline

KMnO_{4}

acts as an oxidising agent.

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