VKC Notes · Stereochemistry · Civil Services P-II · IFoS P-II · Geo-Scientist P-III
2.10 Stereochemistry and Conformational Analysis: R/S, E/Z, Fischer and Newman Projections, Cyclohexane
★★★ 2021–2025: 16 parts, 128 marks where stereochemistry decides the answer (Civil Services 8 parts, 70 marks; IFoS 6 parts, 45 marks; Geo-Scientist 2 parts, 13 marks). It is rarely asked alone; it decides the product in elimination, substitution and addition questions.
Syllabus lines. Civil Services P-II and IFoS P-II, reaction mechanisms: “… intermediate trapping, stereochemistry …”; stereochemistry is also named under substitution (SN1, SN2, SNi), elimination and addition. Used in: addition 2.4, methods 2.7, rearrangements 2.6, reagents 11.1.
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2.10.1 The map of isomers ★★★
| Term | Meaning | Example |
|---|---|---|
| Constitutional isomers | same formula, different connectivity | butan-1-ol, butan-2-ol |
| Stereoisomers | same connectivity, different 3-D arrangement | below |
| · Configurational | interconvert only by breaking bonds | R/S, E/Z, cis/trans rings |
| · Conformational | interconvert by rotation about σ bonds | anti and gauche butane, chair flip |
| Enantiomers | non-superimposable mirror images; identical except for plane-polarised light and chiral reagents | (R)- and (S)-butan-2-ol |
| Diastereomers | stereoisomers that are not mirror images; different physical properties | (2R,3R) and (2R,3S); cis/trans-but-2-ene |
| Meso | has stereocentres but an internal mirror plane (or centre): achiral, optically inactive | meso-tartaric acid |
- Chiral = not superimposable on its mirror image; the usual cause is a stereocentre (sp³ C with four different groups). Test: no mirror plane and no centre of symmetry. n stereocentres give at most 2ⁿ stereoisomers, fewer when a meso form exists (tartaric acid: 3).SN
2.10.2 R and S: the Cahn–Ingold–Prelog rules ★★★
- Rank the four atoms on the stereocentre by atomic number (I > Br > Cl > S > F > O > N > C > H; isotopes: D > H).
- Tie: go outwards and compare the sets of atoms attached, at the first point of difference (–CH₂OH (O,H,H) > –CH(CH₃)₂ (C,C,H)).
- Multiple bonds: count the doubled or tripled atom again: –CHO = (O,O,H) > –CH₂OH (O,H,H); –CH=CH₂ = (C,C,H).
- View with the lowest-priority group pointing away: 1 → 2 → 3 clockwise = R, anticlockwise = S. If the lowest group points towards you, read the turn and reverse it. Swapping any two groups inverts the descriptor.SN
Draw it in 60 s: butan-2-ol: OH up, ethyl left, CH3 hashed; number 1, 2, 3; H towards you, so reverse. The pent-4-enal: OH hashed at C-2, Cl hashed at C-3; write (R) at each.
- Naming with descriptors: (2R,3R)-3-chloro-2-hydroxypent-4-enal: parent pentanal (CHO = C-1), C=C at 4, substituents alphabetical, locant then descriptor in brackets before the name CSE 2023 Q1(b)(i) 5m.SN
2.10.3 Fischer projections, D/L, erythro/threo ★★
- Main chain vertical, most oxidised carbon at the top; horizontal bonds come towards you, vertical bonds go away. Allowed: rotate the whole drawing by 180° in the plane. Not allowed: 90° rotation or lifting it off the page (each inverts). One swap of two groups inverts; two swaps restore.SN
- R/S from a Fischer: if H (lowest) is on a vertical bond, read 1→2→3 directly; if on a horizontal bond, read and reverse.SN
- D/L (sugars, amino acids): OH or NH₂ on the stereocentre farthest from C-1 on the right = D. D-glyceraldehyde is (R). D/L is not related to (+)/(−) or to R/S in general.
- Erythro / threo (two adjacent stereocentres): like groups on the same side of the Fischer = erythro; opposite = threo. Meso-2,3-dibromobutane is the erythro form.
Draw it in 60 s: cross with CHO top, CH2OH bottom, OH right: D. Three crosses with COOH top and bottom: OH right/left, left/right (mirror images), right/right with a dashed mirror line (meso).
2.10.4 Two stereocentres; stereospecific additions ★★★
- 2,3-Dibromobutane: (2R,3R) and (2S,3S) are enantiomers; (2R,3S) = (2S,3R) is meso. Each chiral form is a diastereomer of the meso form.SN
- Stereospecific: stereoisomeric starting materials give stereoisomerically different products (E2, SN2, Br₂ addition). Stereoselective: one starting material gives one stereoisomer of several possible in excess (Sharpless epoxidation, hydride attack on a hindered ketone).SN
- Anti addition of Br₂ (bridged bromonium ion, opened from the back by Br⁻): trans-but-2-ene → meso, cis-but-2-ene → (±). Syn addition (OsO₄, cold dilute KMnO₄, via a cyclic ester): cis → meso diol, trans → (±) diol CSE 2024 Q3(a)(ii) 5m Geo 2021 Q8(d) 8m. Mechanism of bromination: 2.4.SN
Draw it in 60 s: two alkenes; Br2 (anti) and OsO4 (syn) arrows; products as Fischer crosses with CH3 top and bottom: a dashed mirror line for meso, a pair of crosses for (±). Write the rule line.
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