An isomer of C5H12 is

Analysis

The question asks for an isomer of $C_5H_{12}$. To solve this, we must understand what isomers are and how to name alkanes according to IUPAC rules.

Concept: Isomerism

Isomers are compounds that have the same molecular formula but different structural arrangements.

• The molecular formula $C_5H_{12}$ represents an alkane with 5 carbon atoms (pentane).
• For a compound to be an isomer of $C_5H_{12}$, it must also have exactly 5 carbon atoms and 12 hydrogen atoms, but arranged in a different structure than the straight-chain pentane.

Step-by-Step Solution

1. Analyze the options by counting the total number of carbon atoms: • A. 2–ethyl butane: "Butane" has 4 carbons in the parent chain. An "ethyl" group adds 2 more carbons. Total = $4 + 2 = 6$ carbons. This is an isomer of hexane ($C_6H_{14}$), not pentane. • B. Butane: This has 4 carbon atoms. It is a different compound ($C_4H_{10}$). • C. 2–methyl butane: "Butane" has 4 carbons in the parent chain. A "methyl" group adds 1 more carbon. Total = $4 + 1 = 5$ carbons. This matches the molecular formula $C_5H_{12}$. • D. 2–methyl propane: "Propane" has 3 carbons in the parent chain. A "methyl" group adds 1 more carbon. Total = $3 + 1 = 4$ carbons. This is an isomer of butane ($C_4H_{10}$).

2. Verify the structure of 2-methyl butane: • The parent chain is butane ($C-C-C-C$). • A methyl group ($CH_3$) is attached to the second carbon. • Total carbons = 5. Total hydrogens = 12. This is a branched isomer of pentane (also known as isopentane).

Conclusion

Since 2-methyl butane contains exactly 5 carbon atoms and follows the alkane general formula $C_nH_{2n+2}$, it is a valid isomer of $C_5H_{12}$.

Exam Tip

When dealing with isomer questions for alkanes, always count the total number of carbons in the name provided. Remember:

• Meth- = 1, Eth- = 2, Prop- = 3, But- = 4, Pent- = 5.
• If the total count doesn't match the molecular formula given in the question, it cannot be an isomer!