The number of structural isomers of pentane is
- (a)5
- (b)4
- (c)2
- (d)3
Correct — D, 3. Pentane is C₅H₁₂, and its five carbon atoms can be joined in exactly three distinct skeletons: an unbranched chain of five carbons (n-pentane), a four-carbon chain carrying one methyl branch (2-methylbutane, or isopentane), and a three-carbon chain with two methyl groups on the middle carbon (2,2-dimethylpropane, or neopentane). Nothing else is possible — any further drawing turns out to be one of these three redrawn.
- (a)5 — Five is the number of carbon atoms in pentane, not the number of isomers. Counting isomers means counting distinct carbon skeletons, and only three survive once duplicates are removed.
- (b)4 — Four is what you get by treating a methyl branch on carbon-2 and one on carbon-3 of a butane chain as different compounds. Numbering the chain from the other end shows that so-called 3-methylbutane is simply 2-methylbutane, so it is not a separate isomer.
- (c)2 — Two counts only the straight chain and one branched form, and misses the doubly branched neopentane, 2,2-dimethylpropane.
Structural isomers are compounds with the same molecular formula but different arrangements of atoms, and therefore different physical properties. Among the alkanes, isomerism begins at butane — methane, ethane and propane each have only one possible structure. Butane, C₄H₁₀, has two isomers; pentane has three; hexane has five; and the count climbs steeply after that.
The whole skill is drawing systematically without double counting. Start with the longest chain possible, then shorten it by one carbon and place the removed carbon as a branch at every distinct position, then shorten again. The classic error is treating 2-methylbutane and 3-methylbutane as two different compounds, when the IUPAC rule of numbering from the end that gives the lowest locant collapses them into one. Learning the ladder of counts — one, one, one, two, three, five for methane through hexane — turns this class of question from construction into recall.
- Pentane is C₅H₁₂ and has three structural isomers: n-pentane, 2-methylbutane and 2,2-dimethylpropane.
- Butane, C₄H₁₀, is the first alkane to show isomerism, with two isomers.
- Hexane, C₆H₁₄, has five structural isomers.
- Structural isomers share a molecular formula but differ in the arrangement of atoms, so their boiling points and other properties differ.
- Among the pentanes, the more branched the isomer the lower its boiling point, so neopentane boils lowest and n-pentane highest.
Three distinct skeletons and no fourth, which gives option (d).
- Answering 5 by reading off the number of carbon atoms instead of counting structures.
- Counting 2-methylbutane and 3-methylbutane as two different compounds.
- Missing neopentane, the doubly branched isomer, and stopping at two.
As a straight isomer count for a named alkane, or by giving two structures and asking whether they are isomers of each other.
No directly related past PYQ was found.
- practice — not a real PYQ
The number of structural isomers of butane is
- (a)1
- (b)2
- (c)3
- (d)4
Answer(b) 2 — n-butane and 2-methylpropane (isobutane); butane is the first alkane to show isomerism.
- practice — not a real PYQ
Which one of the following is an isomer of n-pentane?
- (a)2-methylpentane
- (b)2,2-dimethylpropane
- (c)cyclopentane
- (d)2-methylpropane
Answer(b) 2,2-dimethylpropane — it is C₅H₁₂, the same molecular formula as n-pentane; the others are C₆H₁₄, C₅H₁₀ and C₄H₁₀.