Constitutional Isomers of C6H14 & Reaction Classification: 04-BS-12, May 2014, Q5

Question 5: Constitutional Isomers of C6H14 & Reaction Classification (20 marks)

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a) All five constitutional isomers of C₆H₁₄. Fixing the longest chain first (6, then 5, then 4 carbons) and placing all remaining branches on each length systematically finds exactly five distinct connectivities (no sixth arrangement exists): the straight chain, two singly-branched pentane-backbone isomers, and two doubly-branched butane-backbone isomers.

Fig. Q5a — all five constitutional isomers of C6H14

Isomer	Condensed structural formula
hexane	CH3CH2CH2CH2CH2CH3
2-methylpentane	CH3CH(CH3)CH2CH2CH3
3-methylpentane	CH3CH2CH(CH3)CH2CH3
2,2-dimethylbutane	CH3C(CH3)2CH2CH3
2,3-dimethylbutane	CH3CH(CH3)CH(CH3)CH3

b) Reaction classification.

(i) CH₃CH=CH₂ + H₂ → CH₃CH₂CH₃: two new atoms (H, H) are added across the double bond and nothing leaves the molecule — this is an addition reaction (catalytic hydrogenation).

(ii) CH₃CH₂CH₂OH $\xrightarrow{H_2SO_4}$ CH₃CH=CH₂ + H₂O: a small molecule (H₂O) is removed from the substrate and a new π bond forms in its place — this is an elimination reaction (acid-catalyzed E1 dehydration).

Part	Classification
(i)	addition (hydrogenation)
(ii)	elimination (acid-catalyzed dehydration)

Concept & Theory

Topic. Constitutional isomerism of alkanes; classifying organic reactions as addition/elimination/substitution.

Key relations. Addition: two groups add across a π bond (bond count decreases by one π). Elimination: a small molecule leaves and a new π bond forms (reverse of addition). Substitution: one group replaces another with no change in saturation.

Why this works. The number of constitutional isomers of an alkane grows combinatorially with chain length (1, 1, 1, 2, 3, 5, 9, … for C1–C7) because branch position and branch length are independent choices once the longest chain is fixed.

Common pitfall. Missing a doubly-branched isomer (2,2- or 2,3-dimethylbutane) by stopping enumeration at the singly-branched pentane skeletons; always fix the longest chain length first, then place all remaining branches.

Source. McMurry, Organic Chemistry, 9th ed., Ch. 3 (isomerism), Ch. 7 (addition/elimination reaction classification).

Question 5 of 5: Constitutional Isomers of C6H14 & Reaction Classification

Question 5: Constitutional Isomers of C6H14 & Reaction Classification (20 marks)