Named Reactions, Part 2 · 10 sections ·
about 10,410 words · CSIR-NET Chemical Sciences, GATE Chemistry & IIT-JAM
How chemists build a C=C (and, at the end, a C≡C) with control over which geometry forms. Two layers on every reaction — a slow, hand-held beginner path and a research-grade advanced/reference path that treats E/Z selectivity, oxaphosphetane and betaine intermediates, stabilised vs non-stabilised ylides, the Schlosser modification and modern asymmetric variants — closing with a “which olefination do I use?” decision table.
Section C.1 of Part 2,
reproduced in full from the book — figures and all. No sign-in, no paywall on this section.
phosphorus ylidePh₃P=CR₂aldehyde/ketone → alkenePh₃P=O by-productylide type sets E/Z
Beginner layer
A phosphorus “ylide” swaps its carbon for the carbonyl’s oxygen: C=O becomes C=C.
In one line: a phosphorus ylide (Ph₃P=CR₂, a carbon carrying a negative charge that is stabilised by an adjacent positively charged phosphorus) attacks the carbonyl carbon of an aldehyde or ketone; the two partners then trade so that the carbonyl oxygen ends up on phosphorus (as very stable triphenylphosphine oxide) and a brand-new C=C double bond forms between the ylide carbon and the former carbonyl carbon.
Prototype Wittig: benzaldehyde + methylenetriphenylphosphorane → styrene + triphenylphosphine oxide. The ylide carbon (=CH₂) becomes the new alkene terminus. RDKit-rendered.
Make the ylide first. An alkyl halide R–CH₂–X is treated with triphenylphosphine (PPh₃) — an SN2 gives a phosphonium salt [Ph₃P–CH₂R]⁺ X⁻. A base (n-BuLi, NaH, NaOEt, or KOtBu) then removes the slightly acidic proton on the carbon next to P⁺, giving the neutral ylide Ph₃P=CHR (equivalently the carbanion Ph₃P⁺–CH⁻R). That carbanion is the nucleophile.
Left → middle: base removes the α-P⁺ proton to unmask the ylide. Right: an ester-stabilised ylide — the extra EWG changes both its reactivity and, crucially, its E/Z outcome (advanced layer). RDKit-rendered.
The four-membered P–C–C–O ring. Its cis vs trans substitution pattern (R and R′ on the same or opposite faces) is set when it forms and is carried faithfully into the alkene by a syn retro-[2+2] cycloreversion (the P–C and C–O bonds break together, while the ring C–C becomes the C=C and the ring P–O becomes the P=O). Hand-built schematic —
Step 0 — make the ylide. PPh₃ + R–CH₂Br → phosphonium salt (SN2); then base pulls the α-proton. Arrow: base lone pair → H; C–H σ → carbanion. Result: Ph₃P⁺–C⁻HR ↔ Ph₃P=CHR (ylide ↔ ylene resonance).
Step 1 — C–C bond forms. The nucleophilic ylide carbon attacks the electrophilic carbonyl carbon. Arrow: ylide carbanion lone pair → carbonyl C; C=O π → O. This gives a betaine (a zwitterion: P⁺ on one carbon, O⁻ on the adjacent one) — or, in the modern view, goes more or less directly to the next species.
Step 2 — ring closure to the oxaphosphetane. The alkoxide oxygen’s lone pair attacks phosphorus (which is happy to expand its valence). Arrow: O⁻ → P. This closes a strained four-membered ring, the 1,2-oxaphosphetane (P–C–C–O). Steps 1–2 together decide the ring’s cis/trans substitution — and therefore the alkene geometry.
Step 3 — retro-[2+2] cycloreversion. The ring fragments in a single syn step: the P–C and C–O bonds break together while the ring C–C becomes the new C=C and the ring P–O becomes the P=O. Arrows (concerted): C–P σ → new C=C π; C–O σ → P=O π. Out comes the alkene and triphenylphosphine oxide (Ph₃P=O). The very strong P=O bond is the thermodynamic engine of the whole reaction: the P–O bond strength in triphenylphosphine oxide is estimated at more than 540 kJ mol⁻¹ (130 kcal mol⁻¹) — one of the strongest bonds formed in organic chemistry, and the reason the last step is irreversible. (Carey, Organic Chemistry, 4th ed., §17.12.)
Predict the Wittig product of cyclohexanone + Ph₃P=CH₂ Easy
Carbonyl electrophile: cyclohexanone (a ketone — Wittig works on ketones too).
Ylide: methylenetriphenylphosphorane, Ph₃P=CH₂ (delivers a =CH₂ unit).
Swap the C=O oxygen for =CH₂: the ring ketone carbon becomes an exocyclic =CH₂.
Product: methylenecyclohexane + Ph₃P=O. No E/Z issue here (terminal =CH₂ is symmetric).
A terminal methylenation: unambiguous, no geometry to worry about. RDKit-rendered.
Retrosynthesis: which aldehyde + which ylide make (E)-PhCH=CHCO₂Et? Medium
Target: ethyl (E)-cinnamate, Ph–CH=CH–CO₂Et.
Cut the C=C. One fragment carries the =CHCO₂Et (an ylide bearing an ester — a stabilised ylide, Ph₃P=CHCO₂Et); the other is the aldehyde, benzaldehyde (PhCHO).
A stabilised ylide is strongly E-selective — so this disconnection cleanly delivers the desired (E)-alkene. (Choosing the ester on the ylide, not the aldehyde, is what makes the geometry come out right.)
Non-stabilised ylides under salt-free conditions give predominantly the Z (cis) alkene — the kinetically favoured cis-oxaphosphetane forms and collapses with retention of that geometry.
Answer: (Z)-1-phenyl-1-butene predominates. (Add the Schlosser modification to override this and get E — see advanced layer.)
⚠ Common mistakes & exam traps
The new C=C forms between the ylide carbon and the former carbonyl carbon — not anywhere else. Count carbons: the ylide carbon becomes half of the double bond.
Ylide type dictates geometry. Non-stabilised → mostly Z; stabilised (ester, nitrile, ketone on the carbanion) → mostly E; semi-stabilised (benzylic, allylic) → poor/variable selectivity. This is the number-one exam point.
The by-product is triphenylphosphine oxide (Ph₃P=O), and its formation is what drives the reaction — not a mistake to omit it.
Wittig does not touch esters, amides, or nitriles as electrophiles — it is selective for aldehydes and (more slowly) ketones. A molecule with both an aldehyde and an ester reacts only at the aldehyde.
Do not confuse the betaine (open zwitterion) with the oxaphosphetane (closed 4-ring). Modern evidence favours the oxaphosphetane as the key on-path intermediate.
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Read the rest of Part 2
The remaining 9 sections of this part —
Horner–Wadsworth–Emmons (HWE) olefination, Still–Gennari modification (Z-selective HWE), Julia & Julia–Kocienski olefination, Peterson olefination — and all nine parts of
Named Reactions are part of ChemVidya Full Access, along with the other books, 55 Study Notes
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