S N 2 Reactions — Products and Stereochemistry: 04-BS-12, December 2017, Q4

Question 4: S_N2 Reactions — Products and Stereochemistry (20 marks)

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.

Check: part (a)/(c)'s exact wedge/dash stereochemistry is only partly shown in the paper's hand-drawn structures (only the terminal –Cl label and one explicit stereo-H are unambiguous), so the two secondary chiral 2-halobutane substrates are assumed from the most consistent reading, in keeping with the standard four-part "halide+alkoxide/hydroxide/cyanide, indicate stereochemistry" S_N2 problem set.

Every part below is a textbook back-side (S_N2) attack: the nucleophile approaches opposite the leaving group, so any reaction at a genuine stereocentre inverts its configuration; reaction at a non-stereogenic (primary) carbon simply substitutes, with no stereochemical outcome to report.

a) (R)-2-Chlorobutane + sodium methoxide (NaOCH₃).

(R)-2-chlorobutane — Cl on a wedge, stereocentre at C2

S_N2, −Cl⁻

→

(S)-2-methoxybutane — configuration inverted at C2

Methoxide performs a clean back-side attack at the secondary stereocentre, displacing chloride. Because O and Cl are both the top-priority substituent at that carbon (out-ranking the ethyl, methyl and H groups), the physical inversion of spatial arrangement here is also seen as a flip of the CIP descriptor: (R)-2-chlorobutane → (S)-2-methoxybutane.

b) 1-Bromobutane + sodium hydroxide (NaOH).

1-Bromobutane — primary carbon, no stereocentre

S_N2, −Br⁻

→

1-Butanol

The reacting carbon (C1) is primary and bears two identical H substituents, so it is not a stereocentre either before or after the reaction — there is no stereochemical outcome to specify, only the constitutional product 1-butanol. (A primary substrate is also the ideal S_N2 case: minimal steric hindrance to the incoming hydroxide.)

c) (S)-2-Chlorobutane + sodium ethoxide (NaOCH₂CH₃).

(S)-2-chlorobutane

S_N2, −Cl⁻

→

(R)-2-ethoxybutane — configuration inverted at C2

The mirror-image substrate of part (a): back-side attack by ethoxide again inverts the stereocentre, giving (R)-2-ethoxybutane. This pair (a)/(c) is deliberately the two enantiomeric starting materials, so the two products are likewise enantiomers of one another — a useful self-check.

d) trans-1-Bromo-2-methylcyclopentane + sodium cyanide (NaCN).

trans-1-bromo-2-methylcyclopentane (Br and CH&sub3; on opposite faces)

S_N2, −Br⁻

→

cis-2-methylcyclopentane-1-carbonitrile (CN and CH&sub3; now on the SAME face)

Cyanide attacks C1 from the face opposite the departing bromide. Because the C2 methyl group does not move, inversion at C1 flips which face the new substituent occupies relative to the fixed methyl: a trans (opposite-face) starting halide gives a cis (same-face) nitrile product. This is the general rule for any ring S_N2: trans-starting material → cis product and vice versa, because inversion is defined relative to the unchanging substituent next door, not to some fixed lab frame.

Part	Product	Stereochemical outcome
a	2-methoxybutane	inversion, (R)→(S)
b	1-butanol	none (primary carbon, not stereogenic)
c	2-ethoxybutane	inversion, (S)→(R)
d	2-methylcyclopentane-1-carbonitrile	inversion flips trans→cis (ring)

Concept & Theory

Topic. S_N2 kinetics, back-side attack, and the stereochemical consequence of inversion of configuration.

Key relations. S_N2 is a single concerted step: nucleophile attacks 180° opposite the C–LG bond, so the transition state is a trigonal-bipyramidal "umbrella" that inverts the carbon's configuration exactly like an umbrella flipping in the wind. Primary > secondary >> tertiary in S_N2 rate (steric accessibility of the back side).

Why this works. Inversion is a rigorous geometric consequence of the single-step, back-side mechanism — there is no carbocation intermediate (unlike S_N1) through which the stereochemical information could be scrambled or racemised.

Common pitfall. Reporting "inversion" as always flipping the R/S letter — the letter only flips when the incoming and outgoing groups have the same relative CIP priority rank among the four substituents (true for Cl→OR here, but not universally true); always re-derive CIP priority for the actual atoms involved rather than assuming the label flips. For a ring substrate, track the stereochemical outcome via cis/trans relative to a non-reacting ring substituent, since there the R/S label of the reacting carbon depends on the whole-molecule numbering and is easy to mis-assign.

Source. McMurry, Organic Chemistry, Ch. 11 (S_N2 reaction mechanism and stereochemistry).

Question 4 of 13: S N 2 Reactions — Products and Stereochemistry

Question 4: SN2 Reactions — Products and Stereochemistry (20 marks)

Question 4: S_N2 Reactions — Products and Stereochemistry (20 marks)