2 marksShort AnswerSection A

Consider the carbon compounds having following molecular formula:

(i) $\mathrm{C}_3\mathrm{H}_6$ (ii) $\mathrm{C}_3\mathrm{H}_8$ (iii) $\mathrm{C}_4\mathrm{H}_6$ (iv) $\mathrm{C}_6\mathrm{H}_6$ (v) $\mathrm{C}_6\mathrm{H}_{12}$

(a) State the number of double covalent bonds present in $\mathrm{C}_3\mathrm{H}_6$

(b) Write the formula of first member of the homologous series to which the carbon compound $\mathrm{C}_4\mathrm{H}_6$ belongs.

(c) Which one of the above compounds forms ring structure of carbon atoms?

(d) Identify, which of the above compounds, is a member of alkane series.

Q: Consider the carbon compounds having following molecular formula: (i) $\mathrm{C}_3\mathrm{H}_6$ (ii) $\mathrm{C}_3\mathrm{H}_8$ (iii) $\mathrm{C}_4\mathrm{H}_6$ (iv) $\mathrm{C}_6\mathrm{H}_6$ (v) $\mathrm{C}_6\mathrm{H}_{12}$ (a) State the number of double covalent bonds present in $\mathrm{C}_3\mathrm{H}_6$ (b) Write the formula of first member of the homologous series to which the carbon compound $\mathrm{C}_4\mathrm{H}_6$ belongs. (c) Which one of the above compounds forms ring structure of carbon atoms? (d) Identify, which of the above compounds, is a member of alkane series.

**(a)** There is **$1$** double covalent bond present in $\mathrm{C}_3\mathrm{H}_6$ (propene). **(b)** The compound $\mathrm{C}_4\mathrm{H}_6$ (butyne) belongs to the alkyne homologous series (general formula $\mathrm{C}_n\mathrm{H}_{2n-2}$). The first member of this series is **ethyne**, which has the molecular formula **$\mathrm{C}_2\mathrm{H}_2$**. **(c)** Both **$\mathrm{C}_6\mathrm{H}_6$** (benzene) and **$\mathrm{C}_6\mathrm{H}_{12}$** (cyclohexane) form ring structures of carbon atoms. **(d)** **$\mathrm{C}_3\mathrm{H}_8$** (propane) is a member of the alkane series (general formula $\mathrm{C}_n\mathrm{H}_{2n+2}$).

Carbon and its Compounds

Saturated and Unsaturated Carbon Compounds, Chains, Branches and Rings

Official Answer

(a) There is $1$ double covalent bond present in $\mathrm{C}_3\mathrm{H}_6$ (propene).

(b) The compound $\mathrm{C}_4\mathrm{H}_6$ (butyne) belongs to the alkyne homologous series (general formula $\mathrm{C}_n\mathrm{H}_{2n-2}$ ). The first member of this series is ethyne, which has the molecular formula $\mathrm{C}_2\mathrm{H}_2$ .

(c) Both $\mathrm{C}_6\mathrm{H}_6$ (benzene) and $\mathrm{C}_6\mathrm{H}_{12}$ (cyclohexane) form ring structures of carbon atoms.

(d) $\mathrm{C}_3\mathrm{H}_8$ (propane) is a member of the alkane series (general formula $\mathrm{C}_n\mathrm{H}_{2n+2}$ ).

propenedouble covalent bondalkyneethyne

\mathrm{C}_2\mathrm{H}_2

benzenecyclohexanering structurealkanepropane

\mathrm{C}_3\mathrm{H}_8

Marking Scheme

1State 1 double bond for (a) (0.5 marks)
2Identify ethyne / $\mathrm{C}_2\mathrm{H}_2$ as the first member for (b) (0.5 marks)
3Identify $\mathrm{C}_6\mathrm{H}_6$ and/or $\mathrm{C}_6\mathrm{H}_{12}$ as ring structures for (c) (0.5 marks)
4Identify $\mathrm{C}_3\mathrm{H}_8$ as the alkane for (d) (0.5 marks)

Hint

Recall the general formulas: Alkanes = $\mathrm{C}_n\mathrm{H}_{2n+2}$ , Alkenes = $\mathrm{C}_n\mathrm{H}_{2n}$ , Alkynes = $\mathrm{C}_n\mathrm{H}_{2n-2}$ . Remember that the simplest alkyne must contain at least two carbon atoms.

Quick Oral Answer

Q: Why can't a one-carbon alkyne exist? A: An alkyne requires a carbon-carbon triple bond ( $\mathrm{C}\equiv\mathrm{C}$ ), which is only possible if there are at least two carbon atoms. Therefore, the simplest alkyne is ethyne ( $\mathrm{C}_2\mathrm{H}_2$ ).

Analysis & Explanation

This question tests the student's ability to identify and classify organic compounds based on their molecular formulas. By applying the general formulas for alkanes ( $\mathrm{C}_n\mathrm{H}_{2n+2}$ ), alkenes ( $\mathrm{C}_n\mathrm{H}_{2n}$ ), and alkynes ( $\mathrm{C}_n\mathrm{H}_{2n-2}$ ), students can determine the degree of unsaturation and homologous series. It also evaluates their understanding of cyclic hydrocarbons like cyclohexane and benzene.

Common Mistakes

1Writing the first member of the alkyne series as methyne (CH) instead of ethyne ( $\mathrm{C}_2\mathrm{H}_2$ ). A triple bond requires at least two carbon atoms.
2Confusing cyclohexane ( $\mathrm{C}_6\mathrm{H}_{12}$ ) with hexane ( $\mathrm{C}_6\mathrm{H}_{14}$ ) or benzene ( $\mathrm{C}_6\mathrm{H}_6$ ).
3Miscalculating the number of double bonds in propene as more than one.

Interesting Facts

Ethyne is commonly known as acetylene and is widely used in gas welding due to the extremely high temperature of its flame when burnt with oxygen.

Benzene was discovered by Michael Faraday in 1825 and has a unique cyclic structure with alternating double bonds that exhibits resonance stability.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2022?

This question carries 2 marks in the CBSE Class 10 Science 2022 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 "Saturated and Unsaturated Carbon Compounds, Chains, Branches and Rings" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2022 paper?

This is a Short Answer question from Section A in the CBSE Class 10 Science 2022 paper.