Structure Elucidation: Regiochemistry of Ketone Enolate Alkylation: 04-BS-12, December 2016, Q9

Question 9: Structure Elucidation: Regiochemistry of Ketone Enolate Alkylation

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.

Given.

Datum	Value
Starting ketone	2-butanone, CH₃COCH₂CH₃ (C₄H₈O)
Reagents	strong base, then CH₃I (one alkylation)
M⁺	86
¹H NMR	6H doublet (~1.0), 3H singlet (~2.1), 1H septet (~2.5)

Find. The structure of the mono-methylated product W.

Approach. 2-Butanone has two different α-carbons (C1, the terminal CH₃, vs. C3, the CH₂ of the ethyl group); methylation could occur at either, giving two isomeric C₅H₁₀O products (both M⁺ = 86, so mass alone cannot distinguish them) — the NMR splitting pattern is what decides.

Both possible alkylation products have the same molecular formula and M⁺. $$\mathrm{C_4H_8O\ (72) + CH_2\ (14,\ net) = C_5H_{10}O\ (86)}$$
2-butanone

kinetic: alkylate C1
→
(terminal CH₃)
thermodynamic: alkylate C3
→
(more substituted)

3-pentanone (from C1-methylation)
3-methyl-2-butanone (from C3-methylation)
Count distinct ¹H environments for each candidate. 3-Pentanone, CH₃CH₂COCH₂CH₃, is symmetric: it has only two distinct proton environments (a 4H quartet near 2.4 ppm for the two equivalent CH₂'s, a 6H triplet near 1.0 ppm for the two equivalent CH₃'s) — this does not match the observed three-signal (6H/3H/1H) spectrum at all.
3-Methyl-2-butanone (methyl isopropyl ketone) matches every signal. $$\mathrm{CH_3\text{-}\underset{3H,\,s}{\underline{CO}}\text{-}\underset{1H,\,sept}{\underline{CH}}(\underset{6H,\,d}{\underline{CH_3}})_2}$$ The acetyl CH₃ (attached to the carbonyl, no neighbouring H's) is an isolated 3H singlet near 2.1 ppm; the two equivalent isopropyl CH₃'s form a 6H doublet near 1.0 ppm (coupled only to the one CH); and the single isopropyl methine, coupled to all 6 of those equivalent protons, appears as a 1H septet near 2.5 ppm. $$3+1+6=10\ \mathrm{H,\ matching\ C_5H_{10}O}.$$ The IR carbonyl band (~1715 cm⁻¹) is consistent with either ketone isomer and does not itself distinguish them — it only confirms the product is still a ketone.

Quantity	Value
Structure of W	3-methyl-2-butanone (methyl isopropyl ketone), CH₃COCH(CH₃)₂
Regiochemistry	alkylation at the more substituted α-carbon (C3), i.e. the thermodynamic enolate
M⁺	86 (C₅H₁₀O)

Check: the exam states only "a strong base" without naming it explicitly (e.g. LDA at −78 °C vs. NaOEt/EtOH at reflux); the NMR data are decisive regardless of which base was intended, and are read here as evidence that alkylation proceeded through the more-substituted (thermodynamic) enolate.

Topic: Kinetic vs. thermodynamic enolate alkylation, distinguished by ¹H NMR symmetry.

Key relations: the kinetic enolate forms fastest by removing the more accessible (less substituted/less hindered) α-H, giving the less-substituted alkene/enolate; the thermodynamic enolate is the more stable, more substituted one, favoured under reversible (equilibrating) deprotonation conditions.

Why this works: mass spectrometry alone cannot distinguish constitutional isomers of identical formula; symmetry-based NMR signal counting (not just chemical shift) is often the fastest way to rule out a symmetric candidate structure outright.

Common pitfall: assuming "strong base" automatically means kinetic (LDA-type) control and picking the less-substituted product without checking the data — here the actual spectrum requires the thermodynamic product, so the data must always be allowed to override a default assumption about mechanism.

Source: McMurry, Organic Chemistry, 9th ed., Ch. 22 (enolate chemistry, kinetic vs. thermodynamic enolates) and Ch. 13 (¹H NMR splitting and symmetry).

Question 9 of 13: Structure Elucidation: Regiochemistry of Ketone Enolate Alkylation

Question 9: Structure Elucidation: Regiochemistry of Ketone Enolate Alkylation