12.3 Carbonyl Compounds I: Aldehydes and Ketones, Nucleophilic Addition, and Oxidation/Reduction

12.3 Carbonyl Compounds I: Aldehydes and Ketones, Nucleophilic Addition, and Oxidation/Reduction

Quick Answer: Carbonyl groups ($C=O$) possess a polar double bond with a strongly electrophilic $sp^2$ carbon. Aldehydes are more reactive than ketones toward nucleophilic addition due to reduced steric hindrance and fewer electron-donating alkyl groups. Nucleophilic addition can proceed under basic control (direct attack) or acid catalysis (carbonyl protonation). Primary amines yield imines, while secondary amines yield enamines. The Wittig reaction couples carbonyls with phosphorus ylides to form alkenes driven by the formation of triphenylphosphine oxide ($Ph_3P=O$). Carbonyl deoxygenation is achieved under basic conditions using the Wolff-Kishner reduction ($NH_2NH_2/KOH$) or acidic conditions using the Clemmensen reduction ($Zn(Hg)/HCl$). Baeyer-Villiger oxidation converts ketones to esters with retention of stereochemistry at the migrating group ($3^\circ > 2^\circ \approx \text{Ph} > 1^\circ > \text{Me}$).

---

Structure, Polarity, and Reactivity of the Carbonyl Group

The carbonyl group consists of a carbon atom $sp^2$-hybridized in a trigonal planar geometry ($120^\circ$ bond angles) bonded to an oxygen atom through one $\sigma$-bond and one $\pi$-bond.

Polarization and Resonance

Because oxygen is substantially more electronegative ($\chi = 3.44$) than carbon ($\chi = 2.55$), the $\pi$-electron cloud is strongly displaced toward oxygen. This creates a permanent molecular dipole moment of $\mu \approx 2.3 - 2.8\text{ D}$ and can be described by two primary resonance contributors:

Carbonyl Resonance:
   R1                R1
     \   ..            \   ..:-
      C = O:   <==>     C+ - O
     /                 /
   R2                R2
  [Major Contributor]  [Minor contributor: dipolar form]

The minor resonance contributor imparts substantial carbocationic character ($\delta^+$) to the carbonyl carbon, rendering it an exceptional electrophilic target for nucleophiles, while the carbonyl oxygen behaves as a weakly basic, nucleophilic center ($\delta^-$).

Reactivity Hierarchy: Aldehydes vs. Ketones

Aldehydes are universally more reactive toward nucleophilic addition than ketones, both kinetically (faster rates) and thermodynamically (more favorable equilibrium constants):

$\text{Formaldehyde } (HCHO) > \text{Aliphatic Aldehydes } (RCHO) > \text{Aromatic Aldehydes } (ArCHO) > \text{Ketones } (RCOR) > \text{Diaryl Ketones } (ArCOAr)$

This reactivity difference arises from two distinct phenomena:

1. Steric Factors: In the starting carbonyl compound, the bond angles are $120^\circ$. Upon nucleophilic attack, the central carbon rehybridizes to a tetrahedral $sp^3$ intermediate with $109.5^\circ$ bond angles, compressing attached groups closer together. Aldehydes possess one small hydrogen atom ($r_{vdw} \approx 1.2\text{ \AA}$), whereas ketones have two bulky alkyl groups. The resulting non-bonded steric congestion in the tetrahedral intermediate is far greater for ketones, raising $\Delta G^\ddagger$.
2. Electronic Factors: Alkyl groups are weakly electron-donating via inductive effects and hyperconjugation. A ketone has two alkyl groups donating electron density into the carbonyl carbon, partially neutralizing its partial positive charge and stabilizing the ground state. An aldehyde has only one alkyl donor, leaving its carbon more electron-deficient and reactive. In aromatic carbonyls (e.g., benzaldehyde), $+M$ resonance donation from the phenyl $\pi$-system further disperses positive charge into the aromatic ring, dampening reactivity.

---

General Mechanisms of Nucleophilic Addition

Nucleophilic addition to carbonyls operates under two general mechanistic regimes depending on reagent strength:

1. Basic / Strong Nucleophile Pathway

Strongly basic nucleophiles (hydride donors, Grignard reagents, alkoxides, acetylides, cyanide) attack the neutral carbonyl carbon directly:

1. Step 1 (Attack): The nucleophile donates its lone pair into the empty $\pi^*_{C=O}$ LUMO at an angle of $107^\circ$ relative to the $C=O$ bond (the Bürgi-Dunitz trajectory), forming a tetrahedral alkoxide intermediate.
2. Step 2 (Protonation): Subsequent treatment with an acid or protic source protonates the alkoxide to yield the neutral addition product.

2. Acid-Catalyzed / Weak Nucleophile Pathway

Weak, neutral nucleophiles ($H_2O, ROH, RNH_2$) are not electron-rich enough to attack a neutral carbonyl carbon directly. Addition requires an acid catalyst:

1. Step 1 (Protonation): An acid protonates the carbonyl oxygen, forming a resonance-stabilized oxonium ion ($[R_2C=O^+-H \leftrightarrow R_2C^+-OH]$). This full formal positive charge drastically increases the electrophilicity of the carbonyl carbon.
2. Step 2 (Attack): The weak nucleophile attacks the protonated carbonyl carbon.
3. Step 3 (Deprotonation): Loss of a proton restores the acid catalyst and delivers the neutral product.

---

Addition of Oxygen Nucleophiles

1. Hydration (Gem-Diol Formation)

Carbonyl compounds react reversibly with water to form geminal diols (gem-diols / hydrates):

$R_2C=O + H_2O \rightleftharpoons R_2C(OH)_2 \quad K_{eq} = \frac{[gem\text{-diol}]}{[R_2CO][H_2O]}$

The position of equilibrium is governed by the electronic and steric stabilization of the carbonyl group:

• Formaldehyde ($HCHO$): $K_{eq} \approx 2,000$ ($>99.9\%$ exists as gem-diol in water).
• Acetaldehyde ($CH_3CHO$): $K_{eq} \approx 1.0$ (roughly $50\%$ hydrate at equilibrium).
• Acetone ($(CH_3)_2CO$): $K_{eq} \approx 0.002$ ($0.1\%$ hydrate; equilibrium lies $>99.9\%$ toward ketone).
• Electron-Withdrawing Substituents: Strongly electron-withdrawing groups destabilize the partial positive charge on the carbonyl carbon, driving equilibrium almost entirely toward the hydrate. Hexafluoroacetone and chloral ($CCl_3CHO$) form exceptionally stable, isolable crystalline hydrates (e.g., chloral hydrate, $CCl_3CH(OH)_2$, a classic pharmaceutical hypnotic).

2. Acetal and Ketal Formation: Protecting Group Chemistry

Treating an aldehyde or ketone with two equivalents of a monohydric alcohol (or one equivalent of a $1,2$-diol such as ethylene glycol) in the presence of an anhydrous acid catalyst (e.g., dry $HCl$ gas or $p$-toluenesulfonic acid / $TsOH$) generates an acetal (from aldehydes) or ketal (from ketones):

Acetal Formation Pathway (Reversible Acid Catalysis):
  RCHO  +  R'OH   <==[H+]==>   RCH(OH)(OR')   <==[H+, R'OH]==>   RCH(OR')2  +  H2O
  (Carbonyl)                   (Hemiacetal)                      (Acetal)

1. Protonation of carbonyl oxygen $\to$ attack by first $R'OH$ $\to$ deprotonation yields a hemiacetal (a tetrahedral carbon bearing one $-OH$ and one $-OR'$).
2. Protonation of the hemiacetal $-OH$ group converts it into an excellent leaving group ($H_2O$).
3. Expulsion of water assisted by the adjacent alkoxy oxygen lone pair generates a resonance-stabilized oxocarbenium ion ($[RCH=\overset{+}{O}R' \leftrightarrow R\overset{+}{C}H-OR']$).
4. Attack by the second equivalent of $R'OH$ on the oxocarbenium ion followed by deprotonation affords the acetal ($RCH(OR')_2$).

Protecting Group Application

Every individual step of acetal formation is completely reversible. In synthesis:

• Protection: To drive acetal formation to completion, water is continuously removed from the reaction mixture using a Dean-Stark trap (azeotropic distillation with benzene or toluene) or molecular sieves (Le Chatelier's principle).
• Stability: Acetals are exceptionally stable to strong bases, alkoxides, Grignard reagents ($RMgX$), organolithiums ($RLi$), and metal hydrides ($LiAlH_4$) because they lack a leaving group that can be displaced in the absence of acid.
• Deprotection: The original carbonyl compound is regenerated quantitatively by stirring the acetal in an excess of aqueous acid ($H_3O^+$).

---

Addition of Nitrogen Nucleophiles

Nitrogen nucleophiles ($:NH_2-Y$) attack carbonyls to yield addition products that spontaneously undergo elimination of water.

1. Primary Amines: Imine (Schiff Base) Formation

Reaction of an aldehyde or ketone with a primary amine ($R'-NH_2$) produces an imine (also termed a Schiff base):

$R_2C=O + R'-NH_2 \xrightleftharpoons{H^+} R_2C=N-R' + H_2O$

• Mechanism: Nucleophilic attack of the amine on the carbonyl yields an unstable neutral tetrahedral carbinolamine intermediate ($R_2C(OH)NHR'$). Protonation of the hydroxyl group followed by expulsion of water delivers an iminium ion, which loses a proton from nitrogen to yield the imine ($C=N$).
• The pH Optimum ($pH \approx 4.5 - 5.0$): Imine formation exhibits a sharp rate maximum at mildly acidic pH:
  • If the solution is too acidic ($pH < 3$), the basic primary amine is completely protonated to an unreactive ammonium salt ($R'NH_3^+$), halting Step 1.
  • If the solution is too basic ($pH > 6$), the concentration of hydronium ion is insufficient to protonate the carbinolamine $-OH$ group into a leaving group ($H_2O$), halting Step 2.

2. Secondary Amines: Enamine Formation

Reaction with a secondary amine ($R'_2NH$) yields an enamine (alkene + amine):

$R_2CH-C(=O)R + R'_2NH \xrightleftharpoons{H^+} R_2C=C(R)-NR'_2 + H_2O$

Because a secondary amine carbinolamine ($R_2CH-C(OH)(R)NR'_2$) has no remaining proton on the positively charged nitrogen after water elimination, it cannot form a stable $C=N$ double bond. Instead, a weak base abstracts a $\beta$-hydrogen from the adjacent carbon, establishing a carbon-carbon double bond conjugated with the nitrogen lone pair.

Enamines possess a nucleophilic $\beta$-carbon due to resonance ($[R_2C=C(R)-NR'_2 \leftrightarrow R_2\overset{-}{C}-C(R)=\overset{+}{N}R'_2]$) and are widely used in the Stork enamine synthesis to alkylate and acylate ketones under neutral conditions.

3. Diagnostic Carbonyl Derivatives

Substituted hydrazines and hydroxylamines react with carbonyls to yield highly crystalline, stable derivatives used in qualitative organic analysis:

---

Addition of Carbon Nucleophiles

1. Cyanohydrin Formation

Cyanide ion ($CN^-$) adds reversibly to aldehydes and unhindered ketones in the presence of $HCN$ (or $NaCN$ with buffer at $pH \approx 9-10$) to yield cyanohydrins ($R_2C(OH)CN$):

• Synthetic Value: The nitrile group ($-C\equiv N$) can be hydrolyzed by aqueous acid ($H_3O^+, \Delta$) to a carboxylic acid, producing an $\alpha$-hydroxy carboxylic acid (e.g., benzaldehyde $\to$ mandelic acid).
• Alternatively, catalytic hydrogenation ($H_2 / Ni$) or reduction with $LiAlH_4$ reduces the nitrile to a primary amine, yielding a $\beta$-amino alcohol.

2. The Wittig Reaction: Carbonyl-to-Alkene Olefination

The Wittig reaction is a premier carbon-carbon bond-forming method that transforms an aldehyde or ketone into an alkene with complete regiochemical control over double bond position:

$R_2C=O + Ph_3P=CR'_2 \longrightarrow R_2C=CR'_2 + Ph_3P=O$

Preparation of Phosphorus Ylides (Phosphoranes)

1. $S_N2$ Alkylation: Triphenylphosphine ($Ph_3P$), an excellent neutral nucleophile, attacks an unhindered primary (or methyl) alkyl halide to form a phosphonium salt: $Ph_3P: + CH_3CH_2-Br \xrightarrow{S_N2} [Ph_3\overset{+}{P}-CH_2CH_3]Br^-$
2. Deprotonation: The acidic $\alpha$-proton ($pK_a \approx 20-22$) is abstracted by a strong base (typically $n$-butyllithium / $n$-BuLi, sodium hydride / $NaH$, or potassium tert-butoxide) to generate the phosphorus ylide: $[Ph_3\overset{+}{P}-CH_2CH_3]Br^- + n\text{-BuLi} \longrightarrow Ph_3\overset{+}{P}-\overset{-}{C}HCH_3 \longleftrightarrow Ph_3P=CHCH_3 + \text{Butane} + LiBr$

Mechanism and Driving Force

Wittig Reaction Mechanism:
  Ph3P=CHR'   +   R2C=O  -->  [ Oxaphosphetane Ring ]  -->  R2C=CHR'  +  Ph3P=O
                               |   R2C --- O    |
                               |    |      |    |
                               |   R'HC - PPh3  |

• The nucleophilic carbanion of the ylide attacks the electrophilic carbonyl carbon while the carbonyl oxygen binds to phosphorus in a concerted $[2+2]$ cycloaddition, generating a four-membered oxaphosphetane intermediate.
• The oxaphosphetane spontaneously collapses (retro-$[2+2]$ cycloreversion) to yield the alkene and triphenylphosphine oxide ($Ph_3P=O$).
• Thermodynamic Driving Force: The phosphorus-oxygen double bond in $Ph_3P=O$ is extraordinarily strong ($540\text{ kJ/mol}$), driving the reaction irreversibly to completion.

Stereochemical Control

• Unstabilized Ylides (alkyl-substituted, electron-rich): Reaction is under kinetic control and yields predominantly $(Z)$-alkenes (cis).
• Stabilized Ylides (conjugated with electron-withdrawing groups like $-CO_2R, -CN$): Reversible addition allows thermodynamic equilibration to favor $(E)$-alkenes (trans).
• Horner-Wadsworth-Emmons (HWE) Modification: Uses phosphonate esters ($(EtO)_2P(=O)CH_2CO_2R$) instead of triphenylphosphonium salts. Reaction with aldehydes produces $(E)$-$\alpha,\beta$-unsaturated esters exclusively, with water-soluble dialkyl phosphate by-products that separate cleanly from organic products.

---

Reduction of Carbonyls

1. Chemoselective Hydride Reductions

• $NaBH_4$ in Ethanol: Reducibility: Aldehydes $\to 1^\circ$ alcohols; Ketones $\to 2^\circ$ alcohols. Esters, lactones, and carboxylic acids are unaffected.
• $LiAlH_4$ in Anhydrous Ether: Rapidly reduces aldehydes, ketones, esters, lactones, carboxylic acids, and anhydrides to alcohols.

2. Complete Carbonyl Deoxygenation ($-C=O \to -CH_2-$)

Transforming an aldehyde or ketone into a methylene hydrocarbon requires specific reaction conditions tailored to the acid/base sensitivity of the substrate:

Deoxygenation Comparison:
           Clemmensen: Zn(Hg) / conc. HCl, reflux (STRONGLY ACIDIC)
 R-CO-R' ---------------------------------------------------------> R-CH2-R'
           Wolff-Kishner: NH2NH2 / KOH / diethylene glycol, 200°C (STRONGLY BASIC)

---

Oxidation of Carbonyls

1. Qualitative Diagnostic Tests for Aldehydes

Aldehydes have an oxidizable hydrogen atom attached directly to the carbonyl carbon (the aldehydic hydrogen), allowing them to be oxidized under mild conditions that do not affect ketones:

• Tollens' Silver Mirror Test:
  • Reagent: Ammoniacal silver nitrate solution containing the diamminesilver(I) complex ion, $[Ag(NH_3)_2]^+OH^-$.
  • Reaction: Aldehydes reduce silver(I) to metallic silver, which deposits as a brilliant, reflective silver mirror ($Ag^0\downarrow$) on clean glass walls, while the aldehyde is oxidized to a carboxylate salt: $RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \longrightarrow RCOO^- + 2Ag^0\downarrow + 4NH_3 + 2H_2O$
  • Ketone Selectivity: Ketones give a negative test except $\alpha$-hydroxy ketones (e.g., fructose), which isomerize via tautomeric enediol intermediates into aldoses under the mildly alkaline reaction conditions.
• Fehling's and Benedict's Tests:
  • Reagent: Alkaline solutions of copper(II) sulfate complexed with tartrate (Fehling's) or citrate (Benedict's) to prevent precipitation of $Cu(OH)_2$.
  • Reaction: Aliphatic aldehydes reduce deep-blue copper(II) ($Cu^{2+}$) to an insoluble, brick-red precipitate of copper(I) oxide ($Cu_2O\downarrow$): $RCHO + 2Cu^{2+} + 5OH^- \longrightarrow RCOO^- + Cu_2O\downarrow + 3H_2O$
  • Aromatic aldehydes (e.g., benzaldehyde) give negative Fehling's tests because resonance stabilization reduces the carbonyl carbon's susceptibility to single-electron transfer oxidation.

2. The Baeyer-Villiger Oxidation

Treating a ketone with a peroxy acid (such as meta-chloroperoxybenzoic acid / $mCPBA$, trifluoroperoxyacetic acid / $CF_3CO_3H$, or peroxyacetic acid) converts the ketone into an ester (or cyclic ketones into lactones):

Baeyer-Villiger Mechanism:
  R-CO-R'  +  R''CO3H  -->  [ Criegee Tetrahedral Intermediate ]  -->  R-CO-O-R'  +  R''CO2H
                             |    OH         |
                             |     |         |
                             | R - C - O-O-COR'' |
                             |     |         |
                             |    R'         |

1. Nucleophilic addition of the peroxy acid to the ketone carbonyl carbon forms a tetrahedral Criegee intermediate.
2. In the rate-determining step, one of the two alkyl/aryl groups ($R$ or $R'$) undergoes a 1,2-migration to the adjacent peroxy oxygen with simultaneous heterolytic cleavage of the weak peroxy $O-O$ bond and expulsion of the carboxylate leaving group.
3. Stereospecificity: The migration occurs with complete retention of stereochemical configuration at the migrating chiral center.

Migratory Aptitude Hierarchy

When an unsymmetrical ketone is oxidized, migration is highly regioselective, following the ability of the migrating group to stabilize partial positive charge in the bridgehead transition state:

$\text{Tertiary alkyl } (3^\circ) > \text{Cyclohexyl } \approx \text{Secondary alkyl } (2^\circ) \approx \text{Phenyl } > \text{Benzyl } > \text{Primary alkyl } (1^\circ) > \text{Methyl}$

• Example 1: Oxidation of acetophenone ($Ph-CO-CH_3$) with $mCPBA$ yields phenyl acetate ($CH_3CO_2Ph$), not methyl benzoate, because phenyl migrates far faster than methyl.
• Example 2: Oxidation of cyclohexanone with peroxy acid yields $\varepsilon$-caprolactone (a seven-membered cyclic ester), the commercial monomer for polycaprolactone synthesis.