Isomers of C6H14, Cyclohexane Combustion & Propane Chlorination: 04-BS-12, December 2013, Q2

Question 2: Isomers of C6H14, Cyclohexane Combustion & Propane Chlorination (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

a) i) Definition. Constitutional (structural) isomers are compounds that share the same molecular formula but differ in the connectivity of their atoms — which atom is bonded to which — rather than merely in their three-dimensional arrangement in space.

a) ii) All five constitutional isomers of C₆H₁₄. A 6-carbon saturated acyclic skeleton admits exactly five distinct connectivities: the straight chain, two singly-branched chains, and two doubly-branched chains (no sixth arrangement exists).

Fig. Q2a(ii) — all five constitutional isomers of C6H14

Isomer	Condensed 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) Combustion of cyclohexane. Cyclohexane (C₆H₁₂) is a fully saturated ring, so complete combustion in excess O₂ converts every carbon to CO₂ and every hydrogen to H₂O, releasing heat (exothermic):

\mathrm{C_6H_{12} + 9\,O_2 \longrightarrow 6\,CO_2 + 6\,H_2O + \text{heat}}

Balance check: 6 C both sides; 12 H both sides (6×2 in the water); O: left 9×2=18, right 6×2+6×1=18.

c) Mono-chlorination of propane. Free-radical chlorination (Cl₂, light or heat) substitutes one H of propane for one Cl, releasing HCl:

\mathrm{CH_3CH_2CH_3 + Cl_2 \xrightarrow{h\nu} CH_3CH_2CH_2Cl + HCl}

Propane has two chemically distinct sets of hydrogens — six equivalent primary H's (two terminal CH₃ groups) and two secondary H's (the central CH₂) — so the same balanced equation is also satisfied by the secondary product, CH₃CHClCH₃ (2-chloropropane) + HCl; in practice both 1-chloropropane and 2-chloropropane form as a mixture, with the secondary C–H's weaker bond (and the resulting radical's greater stability) giving 2-chloropropane a higher per-hydrogen reaction rate than statistics (6:2) alone would predict.

Part	Answer
(a)(i)	same formula, different connectivity
(a)(ii)	5 isomers (see figure)
(b)	C6H12 + 9O2 → 6CO2 + 6H2O + heat
(c)	C3H8 + Cl2 → C3H7Cl (1- or 2-chloropropane) + HCl

Concept & Theory

Topic. Constitutional isomerism, alkane/cycloalkane combustion, and free-radical halogenation.

Key relations. Complete combustion: C_nH_2n (cycloalkane) + (3n/2)O₂ → nCO₂ + nH₂O. Radical chlorination proceeds via initiation (Cl₂ → 2Cl•), propagation (H-abstraction then Cl-abstraction), and termination.

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; systematically fix the longest chain length first (6, then 5, then 4 carbons) and place all remaining branches on each.

Source. McMurry, Organic Chemistry, 9th ed., Ch. 3 (isomerism, combustion), Ch. 5 (radical halogenation).

Question 2 of 5: Isomers of C6H14, Cyclohexane Combustion & Propane Chlorination

Question 2: Isomers of C6H14, Cyclohexane Combustion & Propane Chlorination (20 marks)