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. Trace the drawn repeat unit bond-by-bond, classifying every carbonyl–heteroatom linkage as an amide (C(=O)–N) or an ester (C(=O)–O), and stop at each such linkage to identify where one monomer residue ends and the next begins — the trap is mis-splitting a diamine/diamino-acid chain into two separate fragments when in fact no amide bond intervenes partway along it.
Reading the backbone left to right: a –C(=O)CH2CH2C(=O)– diacyl unit (from succinic acid) forms an amide with the nitrogen of an α-amino-acid residue bearing an isobutyl side chain (–CH2CH(CH3)2) — this is leucine. Leucine's own carboxyl is esterified (not amidated) to a –CH2CH2– diol bridge (from ethylene glycol), whose other end is esterified back to a second leucine residue, which in turn forms an amide with a second succinic-acid diacyl unit, which forms an amide with a long –(CH2)4– chain leading to a final stereocentre bearing a pendant benzyl ester (–C(=O)OCH2C6H5). Tracing that final stereocentre all the way out (it is not a separate diamine plus a separate ester residue — the chain runs straight from the ring nitrogen through to the same carbon that carries the benzyl ester, with no intervening amide) identifies it as lysine, used "in reverse": both of lysine's amino groups (the α- and ε-NH2) form the two backbone amide bonds shown, while its own α-carboxyl is capped as a pendant benzyl ester rather than polymerised.
| Monomer | Role |
|---|---|
| Succinic acid, HOOC–CH2CH2–COOH | diacid (forms the two backbone amide linkages) |
| Leucine (amino acid) | amide to succinic acid on N; ester to ethylene glycol on its own COOH |
| Ethylene glycol, HOCH2CH2OH | diol (bridges two leucine residues by a diester) |
| Lysine benzyl ester (amino-acid derivative) | both amines form backbone amides; α-COOH capped as a benzyl ester side group |
This design directly explains the drug-release mechanism described in the question: the body's proteases/esterases recognise the naturally occurring L-leucine and L-lysine residues in the backbone (unlike a purely synthetic polyester, which lacks a recognisable peptide-like linkage) and hydrolyse the amide bonds at a controlled, enzyme-mediated rate, exposing the ester linkages to slower hydrolysis in turn — giving the steady, extended release profile the application requires.
