Difficulty: Intermediate to Advanced | Prerequisites: Parts 1 and 2 of these notes. Familiarity with equilibrium, protecting groups, and conjugated systems.
This final section of Chapter 17 covers four named reactions and concepts that build on the nucleophilic addition framework from Part 2. Acetal formation is both a reaction and a protecting-group strategy. The Wittig reaction converts carbonyls into alkenes. The Cannizzaro reaction is a disproportionation unique to aldehydes without alpha-hydrogens. Conjugate (1,4-) addition extends the nucleophilic addition concept to alpha,beta-unsaturated carbonyl compounds. These reactions round out the toolkit for manipulating the carbonyl group, and each appears regularly in synthesis problems.
Acetals protect carbonyls by masking them as unreactive ethers. The Wittig reaction replaces C=O with C=C using a phosphorus ylide. The Cannizzaro reaction converts two equivalents of a non-enolisable aldehyde into one carboxylic acid and one alcohol. Conjugate addition puts the nucleophile at the beta-carbon of an enone instead of at the carbonyl carbon.
Hemiacetal
The intermediate formed when one equivalent of alcohol adds to a carbonyl: one OR and one OH on the same carbon. In simple terms, it is the halfway point between the carbonyl and the full acetal.
Acetal
The product formed when two equivalents of alcohol (or a diol) add to a carbonyl under acid catalysis: two OR groups on the same carbon, with water lost. Think of it as the carbonyl fully disguised as an ether. Acetals are stable to base and nucleophiles but cleaved by aqueous acid.
Protecting group
A temporary chemical modification that hides a reactive functional group so you can carry out a reaction elsewhere in the molecule. Acetals are the standard protecting group for ketones and aldehydes.
Thioacetal
The sulfur analogue of an acetal, formed by reaction of a carbonyl with a dithiol (HS–(CH2)n–SH) under acid catalysis. Thioacetals can be desulfurised with Raney nickel (Ni/H2) to give the corresponding CH2 group, providing another route for deoxygenation.
Wittig reaction
Converts a carbonyl (C=O) to an alkene (C=C) using a phosphorus ylide (R2C=PPh3). The driving force is the formation of the very strong P=O bond in triphenylphosphine oxide (Ph3P=O).
Ylide (phosphonium ylide)
A species with adjacent positive and negative charges: the phosphorus bears a formal positive charge and the carbon a formal negative charge. Made by deprotonating a phosphonium salt with a strong base (NaH, BuLi).
Betaine
The initial zwitterionic intermediate in the Wittig reaction, formed when the ylide carbon attacks the carbonyl carbon. It collapses into a four-membered ring (oxaphosphetane), which then fragments to give the alkene and Ph3P=O.
Oxaphosphetane
The four-membered ring intermediate in the Wittig reaction containing carbon, oxygen, and phosphorus. It fragments in a retro-[2+2] process to release the alkene and triphenylphosphine oxide.
Cannizzaro reaction
A base-induced disproportionation of an aldehyde that has no alpha-hydrogens. One molecule of aldehyde is oxidised to a carboxylate, and the other is reduced to an alcohol. The hydride (H:-) is transferred directly between the two aldehyde molecules.
1,2-Addition
Nucleophilic addition directly to the carbonyl carbon. The nucleophile and a proton end up on C1 (carbonyl carbon) and O2 (carbonyl oxygen). This is the "normal" addition covered in Part 2.
1,4-Addition (conjugate addition, Michael addition)
Nucleophilic addition to the beta-carbon of an alpha,beta-unsaturated carbonyl (enone). The nucleophile adds at C1 (the beta-carbon, position 1 in the conjugated system counted from the far end), and the proton ends up on O4 (the carbonyl oxygen, position 4). The final product, after tautomerisation, is a beta-substituted ketone.
Enone (alpha,beta-unsaturated carbonyl)
A carbonyl compound with a C=C double bond conjugated to the C=O. The conjugation extends the electrophilic character to the beta-carbon, making 1,4-addition possible.
An aldehyde or ketone reacts with two equivalents of alcohol (or one equivalent of a diol) under acid catalysis to form an acetal plus water.
The reaction is an equilibrium. To drive it forward (toward the acetal), use excess alcohol and remove water. To reverse it (regenerate the carbonyl), use excess water with acid.
The mechanism proceeds in two stages:
Hemiacetal formation: acid protonates the carbonyl oxygen, the alcohol attacks the electrophilic carbon, then deprotonation gives the hemiacetal. This step can also occur under basic conditions (alkoxide attacks the carbonyl directly).
Hemiacetal to acetal (requires acid): the OH of the hemiacetal is protonated, water leaves to generate an oxocarbenium ion, a second alcohol molecule attacks, and deprotonation gives the acetal. This step does not occur under basic conditions because hydroxide is a poor leaving group without protonation.
Entropy is unfavourable when using two separate alcohol molecules (3 moles of reactant become 2 moles of product plus water). Using a 1,2-diol or 1,3-diol (which forms a cyclic acetal) avoids this penalty and shifts the equilibrium more strongly toward the acetal.
Acetals are stable toward base, nucleophiles (Grignard reagents, hydride reagents), and oxidising agents. They are only cleaved by aqueous acid.
Synthetic application: if a molecule contains both a ketone and an ester, and you want to reduce only the ester with LiAlH4, you first convert the ketone to its acetal (protect it), carry out the reduction, and then remove the acetal with aqueous acid (deprotect).
The general three-step sequence is: protect, react, deprotect.
A dithiol (e.g., 1,3-propanedithiol) reacts with a carbonyl under acid catalysis to form a cyclic thioacetal, by the same mechanism as acetal formation.
The thioacetal can be desulfurised using Raney nickel (Ni with adsorbed H2) in ethanol, replacing both C–S bonds with C–H bonds. The net result is conversion of C=O to CH2.
This is an alternative to the Wolff-Kishner (base) and Clemmensen (acid) reductions for deoxygenation, and it proceeds under neutral conditions.
Converts a C=O to a C=C. The carbonyl is replaced by an alkene, with the new substituents determined by the ylide used.
The phosphorus ylide is prepared in two steps:
Triphenylphosphine (Ph3P:) reacts with an alkyl halide (R–CH2–X) via an SN2 mechanism to form a phosphonium salt (Ph3P+–CH2R, X-).
A strong base (NaH, n-BuLi) removes an acidic proton alpha to phosphorus, generating the ylide (Ph3P=CHR, or more accurately Ph3P+–CHR-).
The ylide attacks the carbonyl carbon, forming a betaine. The betaine cyclises to an oxaphosphetane, which fragments to give the alkene and triphenylphosphine oxide (Ph3P=O).
The reaction is driven by the thermodynamic stability of the P=O bond (~575 kJ/mol), which is one of the strongest single bonds in organic chemistry.
The Wittig is particularly valuable because it places the double bond in a defined position, unlike elimination reactions which can give mixtures of regioisomers.
An aldehyde without alpha-hydrogens (e.g., benzaldehyde, formaldehyde) undergoes disproportionation in the presence of strong base (concentrated NaOH or KOH).
One molecule of aldehyde is oxidised to a carboxylate anion; the other is reduced to an alcohol.
The mechanism: HO- attacks one aldehyde to form a tetrahedral alkoxide. This alkoxide delivers H:- (hydride) to a second aldehyde molecule, reducing it to an alkoxide (which picks up a proton to become the alcohol). The first molecule, having lost its hydrogen, becomes a carboxylate.
The reaction is irreversible, driven by the formation of the stable carboxylate anion.
Biological parallel: NADPH acts as a hydride donor in metabolism in a manner conceptually similar to the hydride transfer in the Cannizzaro reaction. The dihydropyridine ring of NADPH (non-aromatic) delivers H:- and becomes NAD+ (aromatic), gaining stability.
In a simple aldehyde or ketone, the only option is 1,2-addition: the nucleophile attacks the carbonyl carbon.
In an alpha,beta-unsaturated carbonyl (enone), the conjugated system creates a second electrophilic site at the beta-carbon. The nucleophile can attack either at the carbonyl (1,2-addition) or at the beta-carbon (1,4-addition, conjugate addition).
In 1,4-addition, the nucleophile bonds to the beta-carbon, the electrons shift through the conjugated system, and an enolate forms. Protonation (tautomerisation) converts the enolate to the final beta-substituted ketone.
Whether 1,2- or 1,4-addition dominates depends on the nucleophile:
Hard nucleophiles (Grignard reagents, organolithium reagents, LiAlH4) tend to give 1,2-addition. They react quickly and are controlled by charge.
Soft nucleophiles (amines, cyanide, cuprates R2CuLi) tend to give 1,4-addition. They are more polarisable and are controlled by orbital overlap at the beta-carbon.
Without a conjugated C=O, 1,4-addition cannot occur. A plain cyclohexene, for example, does not react with nucleophiles, because there is no carbonyl to activate the system.
Amines: Both secondary (Et2NH) and primary (CH3NH2) amines add conjugately to enones to give beta-amino ketones.
HCN (Nagata reaction): Et2AlCN in toluene followed by acid workup delivers CN to the beta-position of a cyclohexenone, giving a beta-cyano ketone.
Cuprates (R2CuLi): Lithium dialkylcuprates add selectively in a 1,4-fashion to enones. This is a key distinction from Grignard and organolithium reagents, which add 1,2-. In a classic selectivity problem, CH3MgBr gives 1,2-addition to cyclohex-2-enone (tertiary allylic alcohol), while (CH3)2CuLi gives 1,4-addition (3-methylcyclohexanone).
Acetal formation: R2C=O + 2 R''OH, H+ ⇌ R2C(OR'')2 + H2O
Cyclic acetal: R2C=O + HO(CH2)nOH, H+ ⇌ cyclic acetal + H2O
Thioacetal: R2C=O + HS(CH2)nSH, H+ → cyclic thioacetal
Deoxygenation via thioacetal: thioacetal + Raney Ni, H2, EtOH → R2CH2
Wittig: R2C=O + Ph3P=CHR' → R2C=CHR' + Ph3P=O
Ylide prep: Ph3P + R-CH2-X → Ph3P+-CH2R X-; then NaH → Ph3P=CHR
Cannizzaro: 2 RCHO + conc. NaOH → RCO2- Na+ + RCH2OH
1,4-addition with cuprate: enone + R2CuLi, then H3O+ → beta-substituted ketone
Acetal protecting groups are used routinely in carbohydrate chemistry. Sugars contain multiple hydroxyl groups and carbonyl groups, and selectively masking one carbonyl as an acetal allows chemists to modify other positions in the molecule without disturbing it.
The Wittig reaction is one of the most important methods in pharmaceutical and natural product synthesis for constructing specific alkene geometries. It was used in early syntheses of vitamin A and beta-carotene.
Students often think acetal formation can proceed under basic conditions. The hemiacetal can form under base, but the conversion of hemiacetal to acetal requires acid catalysis. Base cannot protonate the hydroxyl to make water a leaving group.
A frequent mistake is assuming 1,4-addition works on any alkene. It does not. You need the C=C to be conjugated with a C=O. A plain alkene is not electrophilic enough.
Students sometimes mix up which nucleophiles give 1,2- vs 1,4-addition. Grignard and organolithium reagents give 1,2-; cuprates (R2CuLi) give 1,4-. This selectivity difference is heavily tested.
The Cannizzaro reaction is sometimes incorrectly applied to aldehydes with alpha-hydrogens. Those aldehydes would undergo the aldol reaction instead under basic conditions. The Cannizzaro is specifically for aldehydes without alpha-hydrogens.
⚠️ Protecting group strategy is a favourite synthesis problem. Be ready to explain why you protect a ketone as an acetal before reducing an ester with LiAlH4, and how you remove the acetal afterwards.
⚠️ The Wittig reaction is a common synthesis-planning target. Given a target alkene, you should be able to identify the carbonyl and ylide precursors (retrosynthetic disconnection at the double bond).
⚠️ Expect at least one problem asking you to predict whether 1,2- or 1,4-addition occurs, based on the nucleophile used. Cuprate = 1,4-; Grignard or organolithium = 1,2-.
⚠️ The Cannizzaro reaction may appear as a "what happens when" question: benzaldehyde plus concentrated NaOH. Know the two products (sodium benzoate + benzyl alcohol).
⚠️ Thioacetal desulfurisation with Raney nickel is another deoxygenation route. Be able to compare it with Wolff-Kishner (base) and Clemmensen (acid).
True or false: Acetal formation requires acid catalysis for both the hemiacetal and acetal steps.
Fill in the blank: The Wittig reaction converts a C=O into a ____.
True or false: The Cannizzaro reaction works on any aldehyde.
Fill in the blank: Cuprate reagents (R2CuLi) undergo ____-addition to alpha,beta-unsaturated ketones.
True or false: Acetals are cleaved by treatment with NaOH.
Answers: 1) False (hemiacetal can form under acid or base; only the acetal step requires acid). 2) C=C (alkene). 3) False (only on aldehydes without alpha-hydrogens). 4) 1,4- (conjugate). 5) False (acetals are stable to base; they are cleaved by aqueous acid).
Q: A molecule contains both a ketone and an ester. You want to reduce only the ester to an alcohol with LiAlH4. Outline the three-step strategy.
A: 1) Protect the ketone as an acetal by treating with a diol (e.g., ethylene glycol) and acid catalyst. 2) Reduce the ester with LiAlH4 in Et2O, then water workup. The acetal is unaffected because it is stable to nucleophiles and base. 3) Remove the acetal (deprotect) by treating with aqueous acid (H3O+), regenerating the ketone.
Q: What ylide would you use to convert cyclohexanone to methylenecyclohexane (exocyclic C=CH2)?
A: The Wittig reagent Ph3P=CH2 (methylenetriphenylphosphorane), prepared from Ph3P + CH3Br, then deprotonation with NaH. This places a =CH2 where the =O was.
Q: Benzaldehyde is treated with concentrated NaOH. What are the products?
A: Sodium benzoate (PhCOO- Na+) and benzyl alcohol (PhCH2OH). This is the Cannizzaro reaction: one molecule is oxidised, the other reduced.
Q: 2-Cyclohexenone is treated with (CH3)2CuLi followed by H3O+. What is the product?
A: 3-Methylcyclohexanone. The cuprate adds in a 1,4-fashion to the beta-carbon of the enone. After enolate protonation (tautomerisation), the product is the beta-substituted ketone.
Q: How does the thioacetal/Raney nickel deoxygenation differ from the Wolff-Kishner reduction in terms of conditions?
A: The thioacetal route proceeds under essentially neutral conditions (acid for thioacetal formation, then Raney Ni/H2 for desulfurisation). The Wolff-Kishner requires strongly basic conditions (hydrazine + KOH + heat). The Wolff-Kishner would be inappropriate for base-sensitive substrates, and the thioacetal route would be inappropriate for substrates sensitive to thiol or nickel.
Acetal chemistry connects directly to carbohydrate chemistry (Chapter 25), where sugars exist as cyclic hemiacetals and acetals (glycosides). The Wittig reaction is revisited in advanced synthesis courses and links to retrosynthetic analysis strategies (Chapter 23). Conjugate addition sets the stage for the Michael reaction in Chapter 19 (alpha-carbon chemistry and enolate reactions). The Cannizzaro reaction relates to biological redox chemistry, particularly the role of NAD+/NADH as hydride carriers in metabolism.
acetal, hemiacetal, protecting group, ketal, dimethyl acetal, cyclic acetal, 1,3-dioxolane, diol protection, thioacetal, dithiane, Raney nickel desulfurisation, Wittig reaction, phosphorus ylide, phosphonium salt, triphenylphosphine, betaine, oxaphosphetane, Ph3P=O, Cannizzaro reaction, disproportionation, hydride transfer, formaldehyde Cannizzaro, 1,2-addition, 1,4-addition, conjugate addition, Michael addition, enone, alpha-beta-unsaturated carbonyl, cuprate, R2CuLi, Gilman reagent, Nagata reaction, organic chemistry chapter 17