CSIR-NET Organic — The Reaction Mechanisms That Repeat
Organic chemistry in CSIR-NET Chemical Sciences looks enormous until you notice that the same small number of mechanistic families keep reappearing in new clothing. The substrate changes, the reagent changes, the question stem changes — the electron flow does not. This article maps the families that repeat, states the rule that decides each one, and works through the kind of problem that actually appears in Part B and Part C.
Family 1 — 1,2-shifts to an electron-deficient atom
A huge slice of "predict the product" questions is one idea: a group migrates with its bonding pair to an adjacent atom that is short of electrons. Which atom is deficient decides the name of the reaction.
| Migration terminus | Reaction | Net change |
|---|---|---|
| Electron-deficient carbon | Pinacol–pinacolone, Wagner–Meerwein | Diol → ketone; skeletal rearrangement |
| Electron-deficient nitrogen | Beckmann, Hofmann, Curtius, Schmidt, Lossen | C→N migration; C–N bond formed |
| Electron-deficient oxygen | Baeyer–Villiger, Dakin, hydroperoxide rearrangement | C→O migration; ester or phenol formed |
| Carbene carbon | Wolff rearrangement (Arndt–Eistert) | Ring contraction / chain homologation |
Two rules govern every member of this family, and examiners test exactly the difference between them.
Rule B (aptitude decides): when both groups are free to migrate, the order is tert-alkyl > cyclohexyl > sec-alkyl ≈ benzyl ≈ phenyl > prim-alkyl > methyl, because the group best able to stabilise positive charge in the bridged transition state moves.
Worked example 1 — Beckmann rearrangement (Rule A)
Q. Acetophenone oxime, PhC(CH₃)=N–OH, with the OH group anti to the phenyl ring, is treated with concentrated H₂SO₄. Give the product.
Working. Protonation converts –OH into –OH₂⁺, a good leaving group. The group anti-periplanar to it — here the phenyl — migrates from carbon to nitrogen as water leaves, giving a nitrilium ion PhN≡C–CH₃⁺. Water attacks the nitrilium carbon and tautomerisation gives the amide.
Answer: N-phenylacetamide (acetanilide), PhNH–CO–CH₃.
The trap: methyl has the lowest migratory aptitude of all, so a student reciting the aptitude list would still get this one right — but change the oxime geometry so that the methyl is anti, and the product becomes N-methylbenzamide, PhCO–NHCH₃. The Beckmann is stereospecific; geometry beats aptitude every time.
Worked example 2 — Baeyer–Villiger oxidation (Rule B)
Q. Acetophenone (PhCOCH₃) is treated with m-CPBA. Which ester forms?
Working. The peracid adds to the carbonyl carbon to give the Criegee intermediate. Now either phenyl or methyl may migrate to the electron-deficient oxygen as the O–O bond breaks. Nothing constrains the geometry, so aptitude decides: phenyl > methyl.
Answer: phenyl acetate, CH₃CO–O–Ph — not methyl benzoate. The migrating group retains its configuration, which is why an optically active migrating carbon keeps its stereochemistry throughout.
Family 2 — Pericyclic reactions and the selection rules
Pericyclic questions are pure marks if you memorise one table and can count π electrons. Everything follows from the Woodward–Hoffmann rules; photochemical conditions always invert the thermal answer.
| Reaction type | Electron count | Thermal (Δ) | Photochemical (hν) |
|---|---|---|---|
| Electrocyclic | 4n | Conrotatory | Disrotatory |
| Electrocyclic | 4n + 2 | Disrotatory | Conrotatory |
| Cycloaddition | 4n (e.g. [2+2]) | supra–antara | supra–supra |
| Cycloaddition | 4n + 2 (e.g. [4+2]) | supra–supra | supra–antara |
| Sigmatropic [1,3]-H | 4 electrons | antarafacial (geometrically hard) | suprafacial |
| Sigmatropic [1,5]-H | 6 electrons | suprafacial (easy) | antarafacial |
Q. (2E,4Z,6E)-octa-2,4,6-triene is heated. What is the stereochemistry of the cyclohexadiene formed?
Working. A triene ring closure is a 6π electrocyclic reaction, so 4n + 2 with n = 1. Thermal conditions therefore demand disrotatory closure: the two terminal CH(CH₃) groups rotate in opposite senses. Starting from the E,Z,E geometry, disrotation places both methyl groups on the same face.
Answer: cis-5,6-dimethylcyclohexa-1,3-diene. Irradiate the same triene instead and the closure turns conrotatory, giving the trans isomer. One question, two answers — the only difference is Δ versus hν.
The [1,3]-versus-[1,5]-hydride distinction is worth its own line in your notes: a thermal [1,5]-H shift across a cyclopentadiene ring is fast at room temperature, while a thermal [1,3]-H shift is essentially never observed because the required antarafacial pathway cannot be reached in a small system. Alkyl groups, which can migrate with inversion at the migrating carbon, are the exception that examiners love.
Family 3 — Enolate and carbanion chemistry
Aldol, Claisen, Michael, Mannich, Knoevenagel, Robinson annulation and the Stork enamine reaction are one mechanism with four steps: (i) generate the nucleophilic carbon, (ii) add it to an electrophilic carbon, (iii) protonate or eliminate, (iv) sometimes cyclise. What changes is only which electrophile and which base.
Thermodynamic enolate: NaOEt/EtOH, room temperature or above → the more substituted, more highly conjugated enolate.
A Part C question that gives you 2-methylcyclohexanone and asks for the alkylation product is really asking only one thing: did they write LDA at −78 °C, or NaOEt at reflux?
Family 4 — Substitution and elimination
SN1/SN2 and E1/E2 appear in almost every paper, usually merged with stereochemistry. The three deciding factors are substrate class, nucleophile/base strength and solvent polarity/protic character; the fourth, and the one most often missed, is neighbouring group participation. A β-acetoxy, β-sulfide or aryl group can deliver anchimeric assistance, producing net retention of configuration and a rate enhancement of several orders of magnitude — a result that looks like SN1 kinetics with SN2-like cleanliness.
Common mistakes that cost marks
- Quoting migratory aptitude for a Beckmann. Aptitude applies only when geometry does not decide. In the Beckmann and the Curtius, geometry decides.
- Counting σ electrons in a pericyclic reaction. For an electrocyclic ring closure count only the π electrons of the open form; for a sigmatropic shift count the electrons in the cyclic array, including the σ bond that migrates.
- Assuming hν simply "speeds things up". Photochemical conditions change the symmetry of the HOMO and therefore reverse the allowed stereochemistry.
- Writing a carbanion where a concerted step is intended. E2 and the Diels–Alder are single-step; drawing a discrete intermediate is a mechanism error, not a shortcut.
- Ignoring the migrating group's retention. In Beckmann, Baeyer–Villiger, Hofmann and Curtius, the migrating carbon keeps its configuration. A question with a stereocentre on the migrating group is testing exactly that.
How to revise this efficiently
| Family | One-line rule to memorise | Typical NET framing |
|---|---|---|
| 1,2-shift to N or O | Anti-periplanar group migrates, with retention | Predict the product; assign stereochemistry |
| Baeyer–Villiger / Dakin | Better carbocation-stabilising group migrates | Which ester/phenol forms |
| Pericyclic | 4n conrotatory thermally; 4n+2 disrotatory thermally | cis or trans ring product |
| Enolate chemistry | LDA cold = kinetic; alkoxide warm = thermodynamic | Regiochemistry of alkylation |
| Substitution/elimination | Check for neighbouring-group participation first | Retention where inversion is expected |
Work backwards from past papers rather than forwards from a textbook. Take twenty questions, label each one with its family from the table above, and you will find that a handful of rules covers nearly all of them. Then drill only the rules.
Keep the numerical side sharp too. Organic mechanism questions in Part C often end in a calculation — a rate ratio, an activation energy, an equilibrium constant. The ABC Chemistry Calculator Suite has the physical-chemistry tools you need beside your mechanism practice.
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