04-BS-12 · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 04-BS-12, Organic Chemistry — May 2017. 3 hours, closed-book examination (no textbook aid beyond one double-sided aid sheet); a Casio or Sharp approved calculator is permitted. The paper prints thirteen questions using plain "Question N:" numbering; all thirteen are answered in full below.
Reference texts: McMurry, Organic Chemistry, 9th ed. (functional-group reactivity, amide/β-lactam resonance, stereochemistry and specific rotation, SN1/SN2 and epoxide-opening regiochemistry, cyclohexane conformational analysis, IR/NMR spectroscopy, electrophilic aromatic substitution and multi-step synthesis design, polymer/step-growth chemistry); Atkins, Physical Chemistry, 11th ed. (entropy of intramolecular vs. intermolecular reactions).
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.
Soap cleans because it is amphiphilic: a long nonpolar hydrocarbon tail dissolves into and surrounds oily/greasy dirt, while a polar head group keeps the resulting aggregate (a micelle, dirt trapped in its nonpolar core) soluble in water so it can be rinsed away. Both molecules shown reproduce this same tail/head architecture without using an ionic (carboxylate) head group:
(a) has the same long alkyl tail as soap, but its head group is a polyethylene-glycol (polyether) chain terminating in –OH. Each ether oxygen and the terminal hydroxyl hydrogen-bond extensively with water, giving the head enough hydrophilicity to solubilise the whole tail-in-dirt aggregate — multiple weak H-bonds substitute for soap's single ionic charge.
(b) has the same long alkyl tail attached through an amide linkage to a bis(2-hydroxyethyl)amino head group. The amide carbonyl and the two pendant –OH groups again provide several hydrogen-bond donors/acceptors, making the head water-soluble by dipole/H-bond interactions rather than by carrying a formal charge.
In both cases the nonpolar tail does the same job as soap's tail (solubilise the dirt), and a cluster of polar, hydrogen-bonding functional groups (ether oxygens/OH in (a); amide + diol in (b)) does the job soap's carboxylate ion does — keep the aggregate suspended in the aqueous wash water — without requiring an ionic head, which is exactly why these "nonionic surfactants" work in hard water (no calcium/magnesium carboxylate scum forms, since there is no carboxylate to precipitate).
