SN2 with inversion (Walden) template

Hydroxide attacks (R)-2-bromobutane opposite the C–Br bond; through a five-coordinate transition state the configuration inverts to (S)-butan-2-ol.

Mechanism step by step

Backside attack turns the stereocentre inside out: (R)-2-bromobutane gives (S)-butan-2-ol

1. Backside attack – O–C forms as C–Br breaks: one step, inversion

Key points

• Rate = k[R–Br][OH⁻]: both species are in the single, bimolecular step.
• The nucleophile enters opposite the leaving group, so the other three groups flip like an umbrella in the wind.
• Inversion is geometric; R/S may or may not change. Here OH takes the place of Br in the priority order, so R becomes S.
• An optically pure halide gives an optically pure product; SN1 through a planar cation gives a largely racemised one.

Drawn with 24 atoms, 4 arrows, 5 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 15 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020, ch. 10 Textbook
• Walden, Ber. Dtsch. Chem. Ges. 1896, 29, 133 Original paper
• Hughes, Juliusburger, Masterman, Topley & Weiss, J. Chem. Soc. 1935, 1525 Original paper