04-BS-12 · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 04-BS-12, Organic Chemistry — December 2014. 3 hours, closed-book examination; any non-communicating (non-programmable) calculator permitted. Answer ALL FIVE problems; each problem is of equal value (20 points), and the lettered sub-parts of a given problem may be treated independently.
Reference texts: McMurry, Organic Chemistry, 9th ed. (functional-group identification, degree of unsaturation, electrophilic addition and Markovnikov's rule, alkane nomenclature, catalytic hydrogenation, Friedel–Crafts acylation mechanism, combustion, carbon classification, benzylic oxidation, stereochemistry/enantiomers).
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.
Condensed: CH3CH(CH3)CH(CH3)CH(CH2CH2CH3)CH(CH3)CH2CH3 — formula C13H28.
(ii) 4-Isobutyl-2,5-dimethylheptane.
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.
Condensed: CH3CH2CHClCH(CH3)CHICH2CH2CH3 — formula C9H18ClI.
(iv) 1-Bromo-2-chlorohexane.
Condensed: CH2BrCHClCH2CH2CH2CH3 — formula C6H12BrCl.
(v) 2,2,4-Trimethylpentane (iso-octane).
Condensed: CH3C(CH3)2CH2CH(CH3)CH3 — formula C8H18 (the reference compound defining octane rating 100).
b) Catalytic hydrogenation. Pt/C (or Pd) adds H2 across a π-bond; an isolated alkene reduces easily at room temperature and pressure, while a full aromatic ring needs three equivalents of H2 and typically more forcing conditions to saturate all three ring double bonds.
(i) Isobutylene + H2.
$$\mathrm{(CH_3)_2C{=}CH_2 + H_2 \xrightarrow{Pt/C} (CH_3)_2CHCH_3}$$
Product: isobutane (2-methylpropane).
(ii) Benzene + H2.
$$\mathrm{C_6H_6 + 3H_2 \xrightarrow{Pt/C} C_6H_{12}}$$
Product: cyclohexane (all three ring double bonds are saturated; the aromatic ring's resonance stabilisation makes this markedly slower than an isolated-alkene hydrogenation, but Pt/C still drives it to completion).
| Part | Product |
|---|---|
| (i) | isobutane (2-methylpropane) |
| (ii) | cyclohexane |
