Difficulty: Intermediate to Advanced | Prerequisites: Grignard and Gilman reagent reactivity, resonance structures, HOMO/LUMO concepts, carbonyl addition mechanisms.
When a carbonyl is conjugated with a C=C double bond (an alpha,beta-unsaturated carbonyl), nucleophiles suddenly have two places they can attack: the carbonyl carbon (1,2-addition) or the beta carbon (1,4-addition, also called conjugate addition). Which site reacts depends on whether the nucleophile is "hard" or "soft." This section also covers the Wittig reaction, a completely different way of using a carbonyl: replacing C=O with C=C to make an alkene. Both topics rely on understanding orbital interactions and are staples of exam synthesis problems.
Hard nucleophiles (Grignards, alkyl lithiums) attack the carbonyl carbon of alpha,beta-unsaturated carbonyls (1,2-addition, kinetic product). Soft nucleophiles (Gilman reagents) attack the beta carbon (1,4-addition/conjugate addition, thermodynamic product). The Wittig reaction converts an aldehyde or ketone into an alkene using a phosphorus ylide, driven by the formation of a very stable phosphine oxide precipitate.
Alpha,beta-unsaturated carbonyl (enone)
A carbonyl compound with a C=C double bond conjugated to the C=O. The classic example is pent-3-en-2-one. In simple terms, the C=C and C=O are separated by exactly one single bond, so their pi systems overlap.
1,2-Addition (direct addition)
Nucleophilic addition directly to the carbonyl carbon. The product, after workup, is an allylic alcohol (C=C preserved, new OH). Think of "1,2" as counting the carbonyl carbon (C1) and its oxygen (C2, loosely).
1,4-Addition (conjugate addition)
Nucleophilic addition to the beta carbon (the carbon at the far end of the conjugated system). The initial product is an enolate, which tautomerises to a ketone on workup. The C=O is preserved in the final product. Think of it as the nucleophile landing four atoms away from the oxygen.
Hard nucleophile
A nucleophile with high charge density, small size, and strong electronegativity difference in its bond. Grignard carbanions and alkyl lithiums are hard. They react under kinetic control, hitting the site with the largest LUMO coefficient (the carbonyl carbon).
Soft nucleophile
A nucleophile with low charge density, larger size, and a less polar bond. Gilman reagents (R2CuLi) are soft. They react under thermodynamic control, preferring 1,4-addition because it preserves the strong C=O bond.
Wittig ylide (phosphonium ylide)
A compound of the form R2C=PPh3, where the carbon bears a formal negative charge and the phosphorus a formal positive charge. In simple terms, a carbon nucleophile tethered to a phosphorus atom.
Oxaphosphetane
The four-membered ring intermediate in the Wittig reaction, containing C, O, P, and C. It breaks apart (retro-[2+2]) to give the alkene and triphenylphosphine oxide.
Triphenylphosphine oxide (Ph3P=O)
The byproduct of the Wittig reaction. It is a very stable solid that precipitates out of solution, driving the reaction to completion (thermodynamic driving force).
An alpha,beta-unsaturated carbonyl has two electrophilic sites, revealed by drawing its resonance structures. The carbonyl carbon carries partial positive charge in one resonance form, and the beta carbon carries partial positive charge in another. A nucleophile can therefore attack either site.
1,2-addition means the nucleophile attacks the carbonyl carbon directly. After workup, the product is an allylic alcohol: the C=C double bond is still present, and a new OH group sits on the former carbonyl carbon.
1,4-addition (conjugate addition) means the nucleophile attacks the beta carbon. The initial product is an enolate ion. On acid workup, the enolate undergoes keto-enol tautomerism to give a saturated ketone (the C=C is consumed, but the C=O is preserved).
What dictates the outcome is the nature of the nucleophile.
Grignard reagents (RMgBr) and alkyl lithiums (RLi) are hard nucleophiles: high charge density, small size, very polar C-Metal bond.
The LUMO of an alpha,beta-unsaturated carbonyl shows a larger orbital lobe on the carbonyl carbon than on the beta carbon. A hard nucleophile, governed by kinetics, attacks the site with the best orbital overlap: the carbonyl carbon. The product is the 1,2-addition allylic alcohol.
Gilman reagents (R2CuLi) are soft nucleophiles: low charge density, larger atom (Cu), less polar C-Cu bond.
Soft nucleophiles prefer 1,4-addition because it is the thermodynamically favoured product. The C=O bond (~180 kcal/mol) is stronger than the C=C bond (~150 kcal/mol), so preserving the C=O (as happens in 1,4-addition) leads to a lower-energy product.
After 1,4-addition and acid workup, the product is a saturated ketone (the nucleophile is on the beta carbon, the C=O is intact, and the former C=C is now a C-C single bond).
The Wittig reaction converts an aldehyde or ketone into an alkene. It replaces the C=O entirely with a C=C.
The reagent is a Wittig ylide (R2C=PPh3), which has both a nucleophilic carbon (carbanion character) and a phosphorus cation in resonance.
Mechanism (four steps):
Step 1: The ylide carbon (nucleophile) attacks the carbonyl carbon (electrophile) via nucleophilic addition.
Step 2: The resulting betaine (zwitterionic intermediate) cyclises to form the four-membered oxaphosphetane ring.
Step 3: The oxaphosphetane undergoes a retro-[2+2] cycloreversion.
Step 4: The ring breaks to give the alkene product and triphenylphosphine oxide (Ph3P=O), which precipitates as a solid.
The reaction is irreversible because the formation of the very stable P=O bond and the precipitation of Ph3P=O drive it to completion.
Stereochemistry: Simple aldehydes with unstabilised ylides give Z-alkenes as the major product. Ketones typically give mixtures of E and Z isomers.
Ylide preparation is a two-step process:
Step 1: SN2 reaction of triphenylphosphine (PPh3) with an alkyl halide (primary or methyl) to form a phosphonium salt.
Step 2: Deprotonation of the phosphonium salt with a strong base (typically n-BuLi) to generate the ylide.
R-Br + PPh3 → [R-PPh3]⁺ Br⁻ → (BuLi) → R=PPh3 (the ylide)
1,2-addition (hard nucleophile): Enone + EtMgBr → (1. addition, 2. H3O⁺) → Allylic alcohol (OH on former carbonyl carbon, C=C preserved)
1,4-addition (soft nucleophile): Enone + Et2CuLi → (1. conjugate addition, 2. H3O⁺) → Saturated ketone (Et on beta carbon, C=O preserved)
Wittig reaction: PhCHO + Ph3P=CHCH3 → PhCH=CHCH3 (Z-major) + Ph3P=O (solid precipitate), 94% yield, 7:1 Z/E
Ylide preparation: R-Br + PPh3 → (SN2) → [R-PPh3]⁺ Br⁻ → (BuLi, proton transfer) → R=PPh3
The Wittig reaction is one of the most important methods for making alkenes with control over where the double bond forms. It is used extensively in the synthesis of natural products, fragrances, and pharmaceuticals where a specific alkene position is needed. Conjugate addition with Gilman reagents is a standard method for adding alkyl groups to enone systems in steroid synthesis and prostaglandin synthesis.
Students often confuse which nucleophile gives which addition mode. Hard nucleophiles (Grignard, RLi) give 1,2-addition. Soft nucleophiles (Gilman, R2CuLi) give 1,4-addition. The mnemonic: hard hits the harder electrophile (carbonyl carbon, higher charge density).
Students forget that the 1,4-addition product after workup is a saturated ketone, not an enol. The enolate intermediate tautomerises to the more stable keto form.
Students assume the Wittig reaction is reversible. It is not: the formation of Ph3P=O (an extremely stable P=O bond) makes it irreversible.
Students sometimes think Wittig reagents react with esters or acid chlorides. They react only with aldehydes and ketones.
⚠️ You will be asked to predict whether a given nucleophile gives 1,2- or 1,4-addition. Know the hard/soft rule cold.
⚠️ The Wittig mechanism (all four steps, including ylide preparation) is a favourite exam question. Be able to draw it from the ylide and the aldehyde through the oxaphosphetane to the products.
⚠️ Stereochemistry of the Wittig: simple aldehydes with unstabilised ylides give mainly Z-alkenes. This is testable.
⚠️ Synthesis problems may require you to identify the correct ylide and aldehyde/ketone needed to produce a target alkene (retrosynthetic disconnection at the C=C).
⚠️ The driving force for the Wittig reaction (precipitation of Ph3P=O) is a commonly asked conceptual question.
True or false: A Grignard reagent undergoes 1,4-addition to an alpha,beta-unsaturated ketone.
Fill in the blank: Gilman reagents are classified as ______ nucleophiles.
True or false: The Wittig reaction produces an alcohol.
Fill in the blank: The driving force for the Wittig reaction is the formation of ______.
True or false: Simple aldehydes with unstabilised Wittig ylides give E-alkenes as the major product.
Answers: 1. False (1,2-addition). 2. Soft. 3. False (it produces an alkene). 4. Triphenylphosphine oxide (Ph3P=O). 5. False (Z-alkenes are major).
Q: An alpha,beta-unsaturated ketone is treated with EtMgBr, then H3O⁺. What is the product and what type of addition occurred?
A: The product is an allylic alcohol (ethyl group on the former carbonyl carbon, C=C preserved, OH present). This is 1,2-addition. Grignards are hard nucleophiles and attack the carbonyl carbon.
Q: The same enone is treated with Et2CuLi, then H3O⁺. What is the product?
A: A saturated ketone with an ethyl group on the beta carbon. This is 1,4-addition (conjugate addition). The C=O bond is preserved. Gilman reagents are soft nucleophiles that favour thermodynamic control.
Q: Draw the Wittig ylide you would need to convert cyclohexanone into methylenecyclohexane (CH2= on the ring).
A: You need Ph3P=CH2 (a methylenetriphenylphosphorane). This is made from CH3Br + PPh3 → phosphonium salt → BuLi → ylide.
Q: Why is the Wittig reaction irreversible?
A: The byproduct Ph3P=O is extremely stable (very strong P=O bond) and precipitates out of solution as a solid. The formation of this precipitate removes it from the equilibrium, driving the reaction forward.
Q: Explain, using LUMO analysis, why a hard nucleophile attacks the carbonyl carbon rather than the beta carbon of an enone.
A: The LUMO of the enone shows a larger orbital lobe (higher coefficient) on the carbonyl carbon than on the beta carbon. A hard nucleophile reacts under kinetic control, seeking the site with the best orbital overlap, which is the carbonyl carbon.
The 1,2- vs 1,4-addition topic ties directly back to Grignard and Gilman reagent reactivity from the previous section. It also connects to the concept of kinetic vs thermodynamic control from general chemistry. The Wittig reaction connects to retrosynthesis strategies in LT6, LT12, and LT18, where you may need to disconnect an alkene and identify the correct aldehyde/ketone + ylide pair.
1,2-addition, 1,4-addition, conjugate addition, direct addition, alpha beta unsaturated carbonyl, enone, hard nucleophile, soft nucleophile, kinetic control, thermodynamic control, LUMO, orbital overlap, Wittig reaction, Wittig ylide, phosphonium ylide, phosphorus ylide, Ph3P=CHR, oxaphosphetane, retro 2+2, triphenylphosphine oxide, Ph3P=O, Z-alkene, E-alkene, alkene synthesis, Grignard 1,2-addition, Gilman conjugate addition, enol tautomerism, CHEM 212, LT8, organic chemistry II, Penn State