Alkane/Haloalkane Nomenclature & Hydrogenation: 04-BS-12, December 2015, Q2

Question 2: Alkane/Haloalkane Nomenclature & Hydrogenation (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) Reading each name back into a structure is just locating the parent chain length from the suffix (heptane=7C, octane=8C, pentane=5C) and hanging each cited substituent off the numbered backbone carbon.

(i) 2,3,5-Trimethyl-4-propylheptane.

Fig. Q2a(i)

Condensed: CH₃CH(CH₃)CH(CH₃)CH(CH₂CH₂CH₃)CH(CH₃)CH₂CH₃ — formula C₁₃H₂₈.

(ii) 4-Isobutyl-2,5-dimethylheptane.

Fig. Q2a(ii)

Condensed: CH₃CH(CH₃)CH₂CH(CH₂CH(CH₃)CH₃)CH(CH₃)CH₂CH₃ — formula C₁₃H₂₈ (a constitutional isomer of part (i) — same formula, different connectivity).

(iii) 3-Chloro-5-iodo-4-methyloctane.

Fig. Q2a(iii)

Condensed: CH₃CH₂CHClCH(CH₃)CHICH₂CH₂CH₃ — formula C₉H₁₈ClI.

(iv) 1-Bromo-2-chlorohexane.

Fig. Q2a(iv)

Condensed: CH₂BrCHClCH₂CH₂CH₂CH₃ — formula C₆H₁₂BrCl.

(v) 2,2,4-Trimethylpentane (iso-octane).

Fig. Q2a(v)

Condensed: CH₃C(CH₃)₂CH₂CH(CH₃)CH₃ — formula C₈H₁₈ (the reference compound defining octane rating 100).

b) Two additions to a π-system.

(i) Vinylcyclopropane + HBr. The two alkene carbons are not equivalent: the internal one is attached directly to the strained three-membered ring. Protonating the terminal CH₂ places the resulting positive charge on the ring-adjacent carbon, where it is stabilised well beyond an ordinary secondary cation — the cyclopropane ring's bent, high-p-character C–C bonds can donate electron density into the adjacent empty p-orbital much like an allylic/benzylic system (the "cyclopropylcarbinyl cation" effect). Markovnikov's rule and this extra ring stabilisation agree, so Br⁻ captures the ring-adjacent carbon:

HBr

→

Fig. Q2b(i) — Markovnikov addition; the cation forms on the ring-adjacent carbon

\mathrm{\text{cyclopropyl-CH{=}CH}_2 + HBr \longrightarrow \boxed{\text{cyclopropyl-CHBr-CH}_3}}

Product: (1-bromoethyl)cyclopropane.

(ii) Benzene + H₂. Pt/C forces the reduction of all three ring π-bonds; aromaticity's extra resonance stabilisation makes this markedly slower than an isolated-alkene hydrogenation, but Pt/C still drives it to completion:

H2 (3 eq.)

→

Pt/C

Fig. Q2b(ii) — forcing catalytic hydrogenation saturates all three ring double bonds

\mathrm{C_6H_6 + 3H_2 \xrightarrow{Pt/C} \boxed{C_6H_{12}}}

Product: cyclohexane.

Part	Result
a(i)–(v)	five structures/condensed formulas above
b(i)	(1-bromoethyl)cyclopropane
b(ii)	cyclohexane

Concept & Theory

Topic. IUPAC substitutive alkane/haloalkane nomenclature; Markovnikov addition with cyclopropylcarbinyl cation stabilisation; catalytic hydrogenation of an arene.

Key relations. Parent-chain length fixes the base name (pentane=5, heptane=7, octane=8 carbons); every named substituent attaches at its cited locant. Cyclopropylcarbinyl cations are stabilised by C–C σ-bond hyperconjugation from the strained ring, roughly comparable to an allylic cation. Hydrogenation consumes exactly one H₂ per π-bond reduced.

Why this works. A substitutive name is a compact encoding of the backbone-plus-branches graph, so reconstructing the structure is purely mechanical bookkeeping. Cyclopropane's ring strain forces its C–C bonds toward more p-character than a normal sp³ bond, letting them overlap with an adjacent empty p-orbital almost as effectively as a genuine π-system.

Common pitfall. Treating vinylcyclopropane like an ordinary terminal alkene and predicting a plain secondary cation/product, missing the extra ring stabilisation that makes the ring-adjacent regiochemistry even more strongly preferred than a textbook Markovnikov case; forgetting benzene's hydrogenation needs three, not one, equivalents of H₂.

Source. McMurry, Organic Chemistry, 9th ed., Ch. 3 (nomenclature), Ch. 7 (Markovnikov addition, cyclopropylcarbinyl stabilisation), Ch. 15 (aromaticity/hydrogenation).

Question 2 of 5: Alkane/Haloalkane Nomenclature & Hydrogenation

Question 2: Alkane/Haloalkane Nomenclature & Hydrogenation (20 marks)