(a) Define isomerism. Draw all possible isomers of butane.

(b) "A compound 'X' on combustion gives a yellow flame with lots of smoke." What inference would you draw from this statement?

(c) State the role of alkaline $KMnO_4$ in the reaction involving conversion of an alcohol to corresponding carboxylic acid.

Q: (a) Define isomerism. Draw all possible isomers of butane. (b) "A compound 'X' on combustion gives a yellow flame with lots of smoke." What inference would you draw from this statement? (c) State the role of alkaline $KMnO_4$ in the reaction involving conversion of an alcohol to corresponding carboxylic acid.

(a) **Isomerism**: The phenomenon in which compounds have the identical molecular formula but different structural arrangements of atoms is called structural isomerism. Butane ($C_4H_{10}$) has two structural isomers: 1. **$n$-Butane** (Straight chain): $$CH_3-CH_2-CH_2-CH_3$$ 2. **Isobutane** (Branched chain, also known as 2-methylpropane): $$\begin{array}{cccc} CH_3 & - & CH & - & CH_3 \\ & & | & & \\ & & CH_3 & & \end{array}$$ (b) **Inference for Compound 'X'**: Since compound 'X' burns with a yellow flame and produces a lot of black smoke (soot), it is inferred to be an **unsaturated hydrocarbon** (an alkene or alkyne). Unsaturated hydrocarbons have a higher carbon-to-hydrogen ratio compared to saturated ones. During combustion in air, the limited oxygen is insufficient to completely oxidize all the carbon, leading to unburnt carbon particles that glow yellow and escape as black smoke. (c) **Role of Alkaline $KMnO_4$**: Alkaline potassium permanganate ($KMnO_4$) acts as an **oxidizing agent**. It provides nascent oxygen to oxidize alcohols into their corresponding carboxylic acids (for example, converting ethanol to ethanoic acid).

Carbon and its Compounds

Chains, Branches and Rings

Official Answer

(a) Isomerism: The phenomenon in which compounds have the identical molecular formula but different structural arrangements of atoms is called structural isomerism.

Butane ( $C_4H_{10}$ ) has two structural isomers:

$n$ -Butane (Straight chain):

$CH_3-CH_2-CH_2-CH_3$

Isobutane (Branched chain, also known as 2-methylpropane):

$\begin{array}{cccc} CH_3 & - & CH & - & CH_3 \\ & & | & & \\ & & CH_3 & & \end{array}$

(b) Inference for Compound 'X':

Since compound 'X' burns with a yellow flame and produces a lot of black smoke (soot), it is inferred to be an unsaturated hydrocarbon (an alkene or alkyne). Unsaturated hydrocarbons have a higher carbon-to-hydrogen ratio compared to saturated ones. During combustion in air, the limited oxygen is insufficient to completely oxidize all the carbon, leading to unburnt carbon particles that glow yellow and escape as black smoke.

Alkaline potassium permanganate ( $KMnO_4$ ) acts as an oxidizing agent. It provides nascent oxygen to oxidize alcohols into their corresponding carboxylic acids (for example, converting ethanol to ethanoic acid).

Isomerismsame molecular formuladifferent structuresn-butaneisobutaneunsaturated hydrocarbonincomplete combustionsooty flameoxidizing agentcarboxylic acid

Marking Scheme

1Define isomerism as compounds having the same molecular formula but different structures. (1 mark)
2Draw and label both isomers of butane ( $n$ -butane and isobutane) correctly. (1 mark)
3Identify compound 'X' as an unsaturated hydrocarbon. (1 mark)
4Explain that incomplete combustion of high-carbon unsaturated compounds leads to a yellow, smoky flame. (0.5 marks)
5State that alkaline $KMnO_4$ acts as an oxidizing agent. (1 mark)
6Explain that it oxidizes alcohol to carboxylic acid. (0.5 marks)

Hint

For (a), think of straight and branched skeletons of 4 carbons. For (b), compare the combustion of saturated vs. unsaturated hydrocarbons. For (c), recall what chemical agents add oxygen to a reactant.

Quick Oral Answer

Isomerism is when molecules have the same formula but different structures. Butane has two isomers: n-butane and isobutane. A yellow smoky flame indicates an unsaturated hydrocarbon due to incomplete combustion. Alkaline potassium permanganate acts as an oxidizing agent to convert alcohols to carboxylic acids.

Analysis & Explanation

Part (a): Isomerism arises when the same set of atoms can be linked in different skeletal pathways. For butane ( $C_4H_{10}$ ), we can have a continuous chain of 4 carbon atoms ( $n$ -butane) or a branched chain where 3 carbon atoms form the main chain and 1 carbon atom is a branch on the middle carbon (isobutane).

Part (b): Saturated hydrocarbons (alkanes) generally burn with a clean blue flame because of complete combustion. Unsaturated hydrocarbons (alkenes/alkynes) contain double or triple bonds, giving them a higher carbon content by mass. In air, they undergo incomplete combustion, producing a yellow, sooty flame.

Part (c): Oxidizing agents are substances that are capable of adding oxygen to other substances. Alkaline $KMnO_4$ is a powerful oxidizing agent that easily oxidizes primary alcohols directly to carboxylic acids.

Common Mistakes

1Drawing pentane isomers instead of butane.
2Forgetting to satisfy all four valencies of carbon with hydrogen atoms in the structural drawings.
3Identifying compound 'X' as a saturated hydrocarbon instead of unsaturated.
4Confusing the role of alkaline $KMnO_4$ as a dehydrating agent instead of an oxidizing agent.

Interesting Facts

LPG (Liquefied Petroleum Gas) used in our homes contains butane as its main constituent, which burns with a blue flame because it is a saturated hydrocarbon and burns in a burner designed for sufficient air mixing.

Alkaline $KMnO_4$ is also known as Baeyer's reagent when used to test for unsaturation in organic chemistry!

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2020?

This question carries 5 marks in the CBSE Class 10 Science 2020 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Carbon and its Compounds" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Chains, Branches and Rings" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2020 paper?

This is a Long Answer question from Section C in the CBSE Class 10 Science 2020 paper.