Naming Butane and Its Branched Isomer
Give the IUPAC names for (a) CH3CH2CH2CH3 and (b) (CH3)2CHCH3, where (b) is a central CH group bonded to three methyl groups.
Given
Structure 1: CH3CH2CH2CH3 (condensed formula)
Structure 2: (CH3)2CHCH3 — a central CH bonded to two CH3 groups and one CH3
IUPAC root prefixes: meth- (1), eth- (2), prop- (3), but- (4).
Find
The IUPAC names of both structures.
Principle
IUPAC alkane names come from the longest continuous carbon chain (meth-, eth-, prop-, but- for 1–4 carbons, plus the -ane suffix). Any carbon branch off that chain is a substituent, named by its own carbon count (methyl for one carbon) with a number showing its position, chosen to give the lowest locants.
Formula
Solution
(a) Find the longest continuous chain in CH3CH2CH2CH3: 4 carbons in a row, so the root is 'but-' and the saturated suffix is '-ane' → butane.
(b) In (CH3)2CHCH3, trace the longest path: the central CH plus two of its neighbors make 3 carbons in a row, so the parent is propane.
The remaining methyl group branches off the middle carbon of that propane chain.
Number the propane chain to give the substituent the lowest number: carbon 2 (numbering from either end gives 2 here).
Combine: 2-methylpropane.
Unit check
Naming is a classification task with no numerical units; the check here is logical: both names account for all 4 carbons (butane = 4-chain; 2-methylpropane = 3-chain + 1 methyl = 4), so no carbon is double-counted or missed.
What this means physically
The straight four-carbon chain is butane, while the branched C4H10 skeleton is named as a substituted propane: 2-methylpropane. Both share the molecular formula C4H10 — they are structural isomers — and the naming system distinguishes them unambiguously.
Students often count the methyl branch as part of the parent chain (getting 'butane' for 2-methylpropane too) or number from the wrong end. The parent chain is the longest continuous carbon run, and substituents get the lowest possible numbers.