Q14

3 marksShort AnswerSection A

What are isomers? Draw the structures of two isomers of butane, $C_4H_{10}$ . Why can’t we have isomers of first three members of alkane series?

Q: What are isomers? Draw the structures of two isomers of butane, $C_4H_{10}$. Why can’t we have isomers of first three members of alkane series?

### 1. Definition of Isomers Compounds that have the identical molecular formula but different structural arrangements of atoms are called **structural isomers**. ### 2. Structures of the Two Isomers of Butane ($C_4H_{10}$) Butane has two structural isomers: * **$n$-Butane (Straight-chain skeleton):** $$\begin{array}{cccc} \text{H} & \text{H} & \text{H} & \text{H} \\ | & | & | & | \\ \text{H}-\text{C}-\text{C}-\text{C}-\text{C}-\text{H} \\ | & | & | & | \\ \text{H} & \text{H} & \text{H} & \text{H} \end{array}$$ *Condensed Formula:* $CH_3-CH_2-CH_2-CH_3$ * **Iso-butane / 2-Methylpropane (Branched-chain skeleton):** $$\begin{array}{ccc} \text{H} & \text{H} & \text{H} \\ | & | & | \\ \text{H}-\text{C} & -\text{C}- & \text{C}-\text{H} \\ | & | & | \\ \text{H} & | & \text{H} \\ & \text{C}-\text{H} & \\ & / \ \ \backslash & \\ & \text{H} \ \ \ \text{H} & \end{array}$$ *Condensed Formula:* $CH_3-CH(CH_3)-CH_3$ ### 3. Why the first three alkanes do not have isomers The first three members of the alkane series are **methane ($CH_4$)**, **ethane ($C_2H_6$)**, and **propane ($C_3H_8$)**. - Isomerism requires the possibility of branching the carbon skeleton. - With $1$, $2$, or $3$ carbon atoms, any arrangement of carbon atoms will always result in a continuous straight chain. - No branching is physically possible because bending a chain of three carbons does not create a new structural arrangement; it remains a continuous path of carbons connected in a single line.

Carbon and its Compounds

Isomerism

Official Answer

1. Definition of Isomers

Compounds that have the identical molecular formula but different structural arrangements of atoms are called structural isomers.

2. Structures of the Two Isomers of Butane ( $C_4H_{10}$ )

Butane has two structural isomers:

$n$ -Butane (Straight-chain skeleton):

$\begin{array}{cccc} \text{H} & \text{H} & \text{H} & \text{H} \\ | & | & | & | \\ \text{H}-\text{C}-\text{C}-\text{C}-\text{C}-\text{H} \\ | & | & | & | \\ \text{H} & \text{H} & \text{H} & \text{H} \end{array}$

Condensed Formula: $CH_3-CH_2-CH_2-CH_3$

Iso-butane / 2-Methylpropane (Branched-chain skeleton):

$\begin{array}{ccc} \text{H} & \text{H} & \text{H} \\ | & | & | \\ \text{H}-\text{C} & -\text{C}- & \text{C}-\text{H} \\ | & | & | \\ \text{H} & | & \text{H} \\ & \text{C}-\text{H} & \\ & / \ \ \backslash & \\ & \text{H} \ \ \ \text{H} & \end{array}$

Condensed Formula: $CH_3-CH(CH_3)-CH_3$

3. Why the first three alkanes do not have isomers

The first three members of the alkane series are methane ( $CH_4$ ), ethane ( $C_2H_6$ ), and propane ( $C_3H_8$ ).

Isomerism requires the possibility of branching the carbon skeleton.
With $1$ , $2$ , or $3$ carbon atoms, any arrangement of carbon atoms will always result in a continuous straight chain.
No branching is physically possible because bending a chain of three carbons does not create a new structural arrangement; it remains a continuous path of carbons connected in a single line.

identical molecular formuladifferent structuresstructural isomersn-butaneisobutanebranchingcontinuous chain

Marking Scheme

1Definition of structural isomers (1 mark)
2Drawing correct structures of $n$ -butane and isobutane (1 mark)
3Explanation of why the first three alkanes cannot form branched chains / isomers (1 mark)

Hint

Recall that structural isomers must have the same molecular formula but different connectivity. Try to branch a chain of 3 carbons and see if it changes the continuous path.

Quick Oral Answer

Isomers are compounds with the same molecular formula but different structural formulas. Butane has two isomers: n-butane and isobutane. The first three alkanes do not have isomers because they do not have enough carbon atoms to form a branched chain.

Analysis & Explanation

According to the NCERT text, carbon has a unique ability to form long chains, branched chains, or rings (catenation). For butane ( $C_4H_{10}$ ), we can construct two different carbon skeletons:

A straight chain: $C-C-C-C$
A branched chain: $C-C(C)-C$

When we satisfy the remaining valencies of carbon with hydrogen, we get two distinct molecules with different physical and chemical properties but the same molecular formula ( $C_4H_{10}$ ). For methane ( $CH_4$ ), ethane ( $CH_3-CH_3$ ), and propane ( $CH_3-CH_2-CH_3$ ), no such branching can be constructed because any attempt to branch a 3-carbon chain simply results in the same continuous chain bent at an angle.

Common Mistakes

1Drawing bent chains of propane and calling them isomers. Bending a chain does not change its connectivity.
2Forgetting to satisfy all four valencies of each carbon atom with hydrogen atoms in the structural drawings.

Interesting Facts

Isobutane is commonly used as a refrigerant in household refrigerators and as a propellant in aerosol sprays because it is much more environmentally friendly than chlorofluorocarbons (CFCs).

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2012?

This question carries 3 marks in the CBSE Class 10 Science 2012 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 "Isomerism" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2012 paper?

This is a Short Answer question from Section A in the CBSE Class 10 Science 2012 paper.