Which of the following alkanes has a straight-chain structure?

Analysis of the Question

The question asks us to identify which of the listed hydrocarbons is a straight-chain alkane. An alkane is a saturated hydrocarbon with the general formula $C_nH_{2n+2}$. A "straight-chain" structure means the carbon atoms are connected in a continuous, unbranched sequence.

Concept Explanation

1. General Formula: Alkanes follow the $C_nH_{2n+2}$ rule.
2. Straight-chain vs. Branched-chain: • Straight-chain: Every carbon atom is connected to at most two other carbon atoms (except for the ends). • Branched-chain: At least one carbon atom is connected to three or four other carbon atoms (e.g., "iso-" compounds or "methyl-" substituted compounds). • Cyclic: Compounds like "cyclo-" have carbon atoms joined in a ring, which is not a straight chain.

Step-by-Step Solution

• Option A: Cyclopentane ($C_5H_{10}$) This is a cyclic alkane. The carbon atoms form a ring, not a straight chain. It also follows the formula $C_nH_{2n}$, which is characteristic of cycloalkanes, not straight-chain alkanes.

• Option B: Isobutane ($C_4H_{10}$) Isobutane is the common name for 2-methylpropane. The prefix "iso-" indicates a branched structure where a methyl group is attached to the second carbon of a propane chain.

• Option C: 2-methylpropane ($C_4H_{10}$) As the name suggests, there is a methyl group ($-CH_3$) branching off the main propane chain. This is a branched-chain alkane.

• Option D: Pentane ($C_5H_{12}$) Pentane consists of five carbon atoms linked in a continuous line: $CH_3-CH_2-CH_2-CH_2-CH_3$. It follows the general formula $C_nH_{2n+2}$ ($C_5H_{2(5)+2} = C_5H_{12}$) and has no branches. Therefore, it is a straight-chain alkane.

Conclusion

Comparing the structures, only Pentane represents a continuous, unbranched chain of carbon atoms.

Exam Tip

When you see names with prefixes like "iso-", "neo-", or numbers like "2-methyl", you are almost certainly looking at a branched structure. Straight-chain alkanes are usually named simply as "n-alkane" (e.g., n-pentane) or just by the parent name (e.g., Pentane).