04-BS-12 · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 04-BS-12, Organic Chemistry — May 2019 sitting (the page-1 header and the running footer, "04-BS-12/May 2019", both give the date). 3 hours, closed-book examination; one Casio/Sharp-approved calculator permitted. NOTES on page 1 state that TEN (10) questions constitute a complete exam paper and only the first 10 as they appear in the answer book are marked, but this sitting prints 13 numbered questions — every question and sub-part below is answered in full.
Reference texts: McMurry, Organic Chemistry, 9th ed. (acid/base theory, functional-group identification, SN1/SN2 mechanisms and stereochemistry, alkyne/acetylide synthesis, electrophilic aromatic substitution, IR/NMR/mass-spectral structure elucidation, named-drug synthesis design, and step-growth polymer chemistry). Every molecular formula, exact mass, and stereochemical (R/S, cis/trans) assignment below.
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.
Approach. Both reagent systems add HBr across the same alkene but by completely different mechanisms — ionic (Markovnikov) vs. radical-chain (anti-Markovnikov) — and only the ionic pathway generates a carbocation intermediate capable of rearranging.
HBr alone (ionic, Markovnikov). Protonation of the terminal alkene carbon places H on the less-substituted (terminal, CH2) carbon and the positive charge on the more substituted internal carbon, giving a secondary carbocation, (CH3)3C–C+H–CH3. This cation sits directly adjacent to a tertiary carbon bearing three methyl groups: a single 1,2-hydride shift converts the secondary cation into a much more stable tertiary cation, (CH3)2C+–CH(CH3)–CH3. Bromide then captures this rearranged, more stable cation, giving 2-bromo-2,3-dimethylbutane as the major product — a skeletal rearrangement is only possible because a discrete, free carbocation exists long enough to reorganise before it is trapped.
HBr + peroxide (radical chain, anti-Markovnikov). Peroxide initiates a radical chain: Br• (generated by O–O homolysis and H-atom abstraction from HBr) adds first, to the terminal, less hindered alkene carbon, because that placement generates the more stable tertiary carbon radical directly — no rearrangement is needed or possible, since a neutral radical does not undergo the cationic 1,2-hydride-shift chemistry available to a carbocation. That tertiary radical then abstracts an H atom from a second equivalent of HBr, regenerating Br• and closing the chain, and delivering the bromine atom to the terminal carbon: 1-bromo-3,3-dimethylbutane, with the original carbon skeleton fully intact.
