Organic Chemistry · Aldehydes and Ketones: Nucleophilic Addition Reactions
Conjugate Nucleophilic Addition to α,β‑Unsaturated Aldehydes and Ketones
On this page 9 sections
In 30 seconds
An α,β-unsaturated carbonyl — an enone — contains a C=C conjugated with a C=O. The conjugation creates two electrophilic sites: the carbonyl carbon (site of ordinary 1,2-addition) and the β-carbon of the double bond (site of 1,4- or conjugate addition). Which site a nucleophile attacks depends on the nucleophile and the conditions.
In conjugate addition, the nucleophile attacks the β-carbon, the double-bond electrons shift to make an enolate Carbanion α- to a carbonyl, with negative charge on oxygen Full entry → at the α-carbon, and protonation of that enolate (through the enol tautomer) gives a saturated carbonyl — a "Michael adduct." The net result is that the nucleophile adds across the C=C in a 1,4 fashion:
Nu- + Cβ=Cα-C=O ⟶ Nu–Cβ-CαH–C=O
When the nucleophile is an enolate, the process is the Michael addition Conjugate addition of an enolate to an enone Full entry → and the product is a 1,5-dicarbonyl; combined with an intramolecular aldol reaction (the Robinson annulation Michael addition + intramolecular aldol + dehydration Full entry →), it builds six-membered rings such as the steroid skeleton. The same chemistry operates in biology: cells use conjugate addition of the thiol glutathione to detoxify electrophilic enones.
Why this matters
Conjugate addition is the reaction that builds carbon skeletons. The Michael addition couples an enolate with an enone to give a 1,5-dicarbonyl — a versatile building block for rings via aldol chemistry. The Robinson annulation (Michael addition followed by intramolecular aldol) is a classic route to fused cyclohexenones and was central to the steroid total syntheses of the mid-20th century. In medicinal chemistry, α,β-unsaturated carbonyls are deliberate electrophiles (Michael acceptors) in drugs and toxins, and conjugate addition explains both their reactivity and their detoxification by glutathione.
For exams, this topic tests your ability to predict which carbon gets attacked: a Grignard reagent (1,2), a Gilman cuprate (1,4), a thiol (1,4), or an enolate (1,4) — you must place the nucleophile on the β-carbon or the carbonyl carbon accordingly. Getting 1,2 versus 1,4 right is the single most important skill in this section.
The college version
Core Concepts
Why the β-carbon is electrophilic: resonance
The enone's C=O π electrons can move onto oxygen, shifting the double bond through the conjugated system:
Cβ=Cα-C+=O- ⟷ -Cβ-Cα=C-O-
The resonance forms place partial positive charge on both the carbonyl carbon and the β-carbon, so a nucleophile can react at either site. The β-carbon carries less positive character than the carbonyl carbon, which is why only certain nucleophiles — soft, polarizable ones, or those whose additions are reversible — take the conjugate (1,4) pathway.
1,2 versus 1,4: how to predict the product
Rules that hold on exams:
- Hard, strong nucleophiles — Grignard reagents (RMgX), organolithiums (RLi), and simple hydrides (LiAlH₄, often NaBH₄) — add 1,2 (directly to the carbonyl carbon), giving allylic alcohols.
- Soft, polarizable nucleophiles — thiols/thiolates (RS⁻), amines, cyanide, enolates, and organocuprates (Gilman reagents, R₂CuLi) — add 1,4, giving saturated carbonyls.
- Reversibility matters: when addition is reversible (weak nucleophiles), the more stable 1,4-adduct dominates; irreversible attack by hard nucleophiles is trapped at 1,2.
The classic demonstration: CH₃Li adds 1,2 to cyclohex-2-enone, but (CH₃)₂CuLi adds 1,4. Copper "softens" the carbon nucleophile.
Mechanism of conjugate addition
For an anionic nucleophile (Nu⁻):
- Attack on the β-carbon. The nucleophile bonds to Cβ; the Cβ=Cα π bond shifts to become a Cα=C bond, pushing electrons onto the carbonyl oxygen — an enolate with negative charge on oxygen.
- Protonation. A proton source (water, alcohol, or acid in the workup) protonates the enolate oxygen; the immediate product is an enol.
- tautomerization Rearrangement of enol to keto form Full entry →. The enol converts to the more stable carbonyl, placing the new hydrogen on the α-carbon.
The regiochemistry is unambiguous: the nucleophile ends up on the β-carbon, and a hydrogen lands on the α-carbon.
Michael addition and Robinson annulation
When the nucleophile is an enolate (from a β-dicarbonyl or other activated methylene), the conjugate addition is the Michael addition, and the product is a 1,5-dicarbonyl — the enolate carbon bonds to the enone's β-carbon:
enolate- + enone ⟶ 1,5-dicarbonyl (Michael adduct)
If the enolate and enone are tethered in one molecule, the adduct's enolate can close a ring by intramolecular aldol addition to the remaining carbonyl; dehydration gives a cyclohexenone. That tandem sequence is the Robinson annulation, historically used to assemble the steroid framework.
How It Works / Step-by-Step Process
- Identify the enone. Locate the C=C conjugated to C=O; label the α- (next to carbonyl) and β-carbons (far end).
- Choose the nucleophile. Decide hardness: Grignard/lithium/hydride → 1,2; thiolate/cyanide/amine/enolate/cuprate → 1,4.
- Attack. For 1,4: the nucleophile bonds to Cβ; the enolate forms on oxygen at the carbonyl end.
- Protonate. Water, alcohol, or acid workup protonates the enolate oxygen to give the enol.
- Tautomerize. The enol converts to the keto form; the new hydrogen sits on the α-carbon.
- Verify. The nucleophile is on the β-carbon; the carbonyl is intact — a saturated ketone/aldehyde with the nucleophile two atoms from the carbonyl carbon.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| 1,2-addition product (allylic alcohol) | 1,4-addition product (saturated carbonyl) | In 1,2 the nucleophile bonds to the carbonyl carbon and the C=C survives; in 1,4 the nucleophile bonds to the β-carbon and the C=C becomes a C–C |
| "The nucleophile always adds to the carbonyl" | Always true | For soft nucleophiles (thiols, cuprates, enolates) it does not — it adds to the β-carbon instead |
| Michael addition | Any conjugate addition | Michael addition specifically means an enolate-type carbon nucleophile adding 1,4; thiol or amine additions are conjugate additions but not Michael additions in the narrow sense |
| CH₃Li behaving like (CH₃)₂CuLi | They behave differently | Lithium reagents are hard → 1,2; cuprates are soft → 1,4 |
| Hydrogen "adds to the double bond" in 1,4 addition | Catalytic hydrogenation | Conjugate addition is not hydrogenation; the C=C is consumed but the product is a carbonyl with the nucleophile on the β-carbon |
| Enol is the stable product | Enol tautomerizes to the keto form | The enol is a transient intermediate; the isolated product is the saturated carbonyl |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine a seesaw with three kids: the carbonyl carbon on one end, the α‑carbon in the middle, and the β‑carbon on the other end. A soft, polite guest (a thiol, an enolate, or a copper reagent) walks up and shakes hands with the β‑carbon, and the whole seesaw tips so the oxygen at the far end catches a hydrogen. A pushy guest (a Grignard reagent) ignores the seesaw entirely and grabs the carbonyl carbon directly. Same playground, two different greetings — and the guest's politeness decides which hand gets shaken.
Worked example
Example 1: Thiol addition to methyl vinyl ketone
Predict the product of methanethiol (CH₃SH) adding to methyl vinyl ketone (CH₃COCH=CH₂, SMILES CC(=O)C=C).
Methyl vinyl ketone has the carbonyl at C2 and a terminal CH₂ as the β-carbon. The thiol is a soft nucleophile, so it attacks the β-carbon; protonation at the α-carbon follows through the enol:
CH3COCH=CH2 + CH3SH ⟶ CH3COCH2CH2SCH3
The product, 4-(methylthio)butan-2-one (SMILES CSCCC(C)=O), is a saturated ketone: the SCH₃ group sits on the carbon that was β (now C4), and the carbonyl is untouched. A 1,2 addition would instead have given an allylic alcohol with the double bond intact — the contrast makes the regiochemistry obvious.
Example 2: Cuprate versus organolithium on cyclohex-2-enone
Compare the products of (CH₃)₂CuLi and CH₃Li with cyclohex-2-enone (SMILES O=C1CC=CCC1).
- (CH₃)₂CuLi (soft, 1,4): the methyl adds to the β-carbon, and protonation at the α-carbon gives 3-methylcyclohexanone (SMILES
CC1CCCC(=O)C1) — a saturated ketone with the methyl on the ring carbon that was β. - CH₃Li (hard, 1,2): the methyl adds to the carbonyl carbon, giving 1-methylcyclohex-2-en-1-ol — an allylic alcohol in which the C=C survives.
Same nucleophile "methyl," opposite products, purely because the copper reagent is soft enough to take the conjugate pathway.
Example 3: Yield calculation with dimensional analysis
2.80 g of methyl vinyl ketone (molar mass 70.09 g/mol) is converted by excess methanethiol to 4-(methylthio)butan-2-one (molar mass 118.19 g/mol) in 90% yield. What mass of product is isolated?
Moles of enone:
n = mM = 2.80 g70.09 g mol-1 = 0.0399 mol
Stoichiometry is 1:1 (one nucleophile per enone):
mtheoretical = n × M = 0.0399 mol × 118.19 g mol-1 = 4.72 g
Apply the 90% yield:
mactual = 0.90 × 4.72 g = 4.25 g
Dimensional check: g mol⁻¹ × mol = g in each step. The product mass exceeds the starting mass because the thiol fragment adds ~48 g/mol — a reminder that yield calculations track moles, not grams, of starting material.
Key takeaways
- Enones have two electrophilic sites: carbonyl carbon (1,2) and β-carbon (1,4); resonance puts partial positive charge on both.
- Hard nucleophiles (Grignards, organolithiums, simple hydrides) → 1,2 addition, allylic alcohol product.
- Soft nucleophiles (thiolates, cyanide, amines, enolates, cuprates R₂CuLi) → 1,4 addition, saturated carbonyl product.
- In 1,4 addition the nucleophile ends up on the β-carbon; the α-carbon gains the proton (via enol → keto tautomerization).
- Michael addition = enolate + enone → 1,5-dicarbonyl; Robinson annulation = Michael + intramolecular aldol → cyclohexenone.
- (CH₃)₂CuLi gives 1,4-addition to cyclohex-2-enone; CH₃Li gives 1,2.
- NaBH₄ with CeCl₃ (Luche conditions) reduces enones 1,2 to allylic alcohols; copper-catalyzed hydride conditions give conjugate reduction.
- Biological relevance: glutathione adds to electrophilic enones (detoxification) by the same conjugate mechanism.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Where does the nucleophile end up in a 1,4 addition, and where does the proton end up?
Show answer
The nucleophile ends up on the β-carbon; a proton (via enol → keto tautomerization) ends up on the α-carbon.
Why does (CH₃)₂CuLi add 1,4 to cyclohex-2-enone while CH₃Li adds 1,2?
Show answer
Copper makes the carbon nucleophile soft and polarizable, favoring attack at the β-carbon (1,4); the hard organolithium attacks the carbonyl carbon (1,2).
What is the product of conjugate addition of an enolate to an enone, and what is that reaction called?
Show answer
A 1,5-dicarbonyl; this is the Michael addition.
Which intermediate forms after the nucleophile attacks the β-carbon, and how is it converted to the product?
Show answer
An enolate (negative charge on the carbonyl oxygen). Protonation gives the enol, which tautomerizes to the saturated carbonyl product.
In Example 1, why is the product a saturated ketone rather than an alcohol?
Show answer
Because the addition is 1,4: the thiol attacked the β-carbon, the C=C became a C–C, and tautomerization restored the C=O — no alcohol forms at the carbonyl carbon.
What two reactions combine in the Robinson annulation, and what ring type results?
Show answer
Michael addition (enolate + enone → 1,5-dicarbonyl) followed by intramolecular aldol condensation; a cyclohexenone ring results.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- α,β-unsaturated carbonyl (enone)
- Carbonyl with a C=C between the α- and β-carbons
- 1,2-addition (direct addition)
- Nucleophile attacks the carbonyl carbon
- 1,4-addition (conjugate addition)
- Nucleophile attacks the β-carbon, electrons flow to oxygen
- enolate
- Carbanion α- to a carbonyl, with negative charge on oxygen
- Michael addition
- Conjugate addition of an enolate to an enone
- organocuprate (Gilman reagent)
- R₂CuLi, a soft carbon nucleophile
- Robinson annulation
- Michael addition + intramolecular aldol + dehydration
- tautomerization
- Rearrangement of enol to keto form
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.