UNIT-3: Carbonyl Compounds

1. Structure, Reactivity, and Preparation of Carbonyl Compounds

Structure of Carbonyl Group

The carbonyl group consists of a carbon atom doubly bonded to an oxygen atom (C=O). The carbonyl carbon is sp2 hybridized and forms three sigma (σ) bonds arranged in a planar triangular geometry with bond angles of approximately 120 degrees. The unhybridized p-orbital of carbon overlaps laterally with a p-orbital of oxygen to form a pi (π) bond.

Key Concept: Due to the higher electronegativity of oxygen (3.5) compared to carbon (2.5), the pi electron cloud is strongly pulled toward oxygen. This creates a highly polar group with a partial negative charge on oxygen and a partial positive charge on carbon: ^δ+C=O^δ-.

Reactivity: Aldehydes vs. Ketones

Aldehydes are generally more reactive than ketones toward nucleophilic attack due to two main reasons:

• Inductive Effect: Alkyl groups are electron-donating (+I effect). Ketones have two alkyl groups attached to the carbonyl carbon, which reduce its positive charge density more than the single alkyl group in aldehydes.
• Steric Effect: Ketones possess two bulky alkyl groups around the carbonyl carbon, which hinder the approach of incoming nucleophiles. Aldehydes have at least one small hydrogen atom, offering less steric hindrance.

Preparation of Carbonyl Compounds

Carbonyl compounds can be prepared through multiple synthetic routes:

1. Oxidation of Alcohols: Primary alcohols undergo controlled oxidation using Pyridinium Chlorochromate (PCC) to yield aldehydes. Secondary alcohols oxidize using CrO3 or KMnO4 to give ketones.

   R-CH2OH + PCC -> R-CHO + H2O

   R-CH(OH)-R' + CrO3 -> R-CO-R' + H2O

2. Ozonolysis of Alkenes: Alkenes react with ozone (O3) followed by reductive workup with Zn/H2O to form aldehydes or ketones depending on substitution.

   R-CH=CH-R' + O3 (then Zn/H2O) -> R-CHO + R'-CHO

3. Hydration of Alkynes: Treatment of alkynes with dilute H2SO4 in the presence of HgSO4 yields carbonyl compounds via enol intermediates. Ethyne gives acetaldehyde, while higher alkynes give ketones.

   RC≡CH + H2O (HgSO4 / H2SO4) -> [R-C(OH)=CH2] -> R-CO-CH3

4. Friedel-Crafts Acylation: Aromatic ketones are prepared by reacting benzene or substituted benzenes with acid chlorides in the presence of anhydrous AlCl3.

   Ar-H + R-COCl (Anhydrous AlCl3) -> Ar-CO-R + HCl

2. Nucleophilic Addition Reactions

General Mechanism

The characteristic reaction of carbonyl compounds is nucleophilic addition. Because the carbonyl carbon is electrophilic, a nucleophile (Nu:-) attacks the planar carbon perpendicular to the carbonyl plane, converting the sp2 hybridized carbon into a sp3 hybridized tetrahedral alkoxide intermediate. Subsequent protonation gives the final addition product.

General Mechanism Steps:
Step 1: Nu:- + >C=O -> >C(Nu)-O- (Slow, Rate-determining step)
Step 2: >C(Nu)-O- + H+ -> >C(Nu)-OH (Fast step)

Representative Nucleophilic Addition Reactions

• Addition of Hydrogen Cyanide (HCN): Carbonyl compounds react with HCN in the presence of a base catalyst (to generate CN-) to yield cyanohydrins.

  R-CHO + HCN -> R-CH(OH)CN

• Addition of Sodium Bisulfite (NaHSO3): Aldehydes and methyl ketones react with saturated aqueous NaHSO3 to form crystalline sodium bisulfite adducts. This reaction is used for purification of carbonyl compounds.

  R-CHO + NaHSO3 -> R-CH(OH)SO3Na

• Addition of Grignard Reagents (R'MgX): Nucleophilic addition of organometallic reagents yields alcohols after acid hydrolysis (Formaldehyde gives primary alcohols; other aldehydes give secondary alcohols; ketones give tertiary alcohols).

  R-CHO + R'MgX (then H3O+) -> R-CH(OH)-R'

• Addition of Alcohols (Acetal and Ketal Formation): Reacting aldehydes with one equivalent of alcohol in dry HCl gas forms a hemiacetal, which reacts with a second equivalent to yield an acetal. Ketones react similarly to form ketals.

  R-CHO + R'OH (dry HCl) <-> R-CH(OH)(OR') (Hemiacetal)
  R-CH(OH)(OR') + R'OH (dry HCl) <-> R-CH(OR')2 + H2O (Acetal)

3. Nucleophilic Addition-Elimination Reactions with Ammonia Derivatives

General Mechanism

Reagents containing an primary amino group (NH2-Z, where Z = -OH, -NH2, -NHPh, -NHCONH2, etc.) add to carbonyl compounds. The reaction is catalyzed by mild acid and proceeds in two stages: nucleophilic addition of the nitrogen atom to the carbonyl group followed by elimination of a molecule of water to form a compound containing a carbon-nitrogen double bond (>C=N-Z).

General Reaction Equation:
>C=O + H2N-Z <-> [>C(OH)-NH-Z] -> >C=N-Z + H2O

1. Nucleophilic Attack: The unshared electron pair on nitrogen attacks the electrophilic carbonyl carbon to form a tetrahedral intermediate.
2. Proton Transfer: A proton shifts from nitrogen to oxygen, forming an addition intermediate (carbinolamine derivative).
3. Protonation and Elimination: Acid protonates the hydroxyl group, turning it into a good leaving group (H2O). Loss of H2O and subsequent deprotonation gives the imine derivative (>C=N-Z).

Summary of Ammonia Derivatives and Products

Exam Note: The pH for these reactions must be strictly controlled (pH 4-5). If the solution is too acidic, the amine reagent becomes protonated (+NH3-Z) and loses its nucleophilicity. If it is too basic, oxygen protonation cannot occur to facilitate water loss.

4. Mechanisms of Named Reactions and Rearrangements

Aldol Condensation

Aldehydes or ketones containing at least one alpha-hydrogen undergo self-condensation in the presence of dilute alkali (NaOH, KOH, or Ba(OH)2) to form beta-hydroxy aldehydes (aldols) or beta-hydroxy ketones (ketols), which eliminate water upon heating to give alpha,beta-unsaturated carbonyl compounds.

Reaction: 2 CH3-CHO (dilute NaOH) -> CH3-CH(OH)-CH2-CHO (Heat) -> CH3-CH=CH-CHO + H2O

Mechanism:

1. Enolate Formation: Hydroxide ion abstracts an alpha-hydrogen atom from the carbonyl compound to yield a resonance-stabilized enolate anion.
2. Nucleophilic Addition: The enolate ion attacks the carbonyl carbon of a second aldehyde/ketone molecule to form an alkoxide intermediate.
3. Protonation: Alkoxide abstracts a proton from water to form the aldol product and regenerate OH-.
4. Dehydration: Heating causes loss of a water molecule via enolate-mediated E1cB elimination to form the alpha,beta-unsaturated aldehyde or ketone.

Benzoin Condensation

The reaction between two molecules of an aromatic aldehyde (lacking alpha-hydrogens, such as benzaldehyde) catalyzed by cyanide ion (CN-) to form an alpha-hydroxy ketone (Benzoin).

Reaction: 2 C6H5-CHO (KCN / ethanol) -> C6H5-CH(OH)-CO-C6H5

Mechanism:

1. Nucleophilic Attack: Cyanide ion acts as a specific catalyst and attacks the carbonyl carbon of benzaldehyde to form a cyanohydrin intermediate.
2. Umpolung (Polarity Inversion): Cyanide ion's strong electron-withdrawing effect increases the acidity of the hydrogen attached to the former carbonyl carbon, allowing base to remove it and generate a carbanion.
3. Addition: The carbanion attacks the carbonyl carbon of a second molecule of benzaldehyde.
4. Elimination: Proton transfer followed by loss of the cyanide ion (catalyst regeneration) yields benzoin.

Knoevenagel Condensation

The condensation of an aldehyde or ketone with an active methylene compound (e.g., diethyl malonate, ethyl acetoacetate, malononitrile) in the presence of a weak base (such as primary or secondary amines, e.g., piperidine) to give an alpha,beta-unsaturated compound.

Reaction: C6H5-CHO + CH2(CO2Et)2 (Piperidine) -> C6H5-CH=C(CO2Et)2 + H2O

Mechanism:

1. The weak base deprotonates the active methylene compound, generating a stable carbanion.
2. The carbanion performs nucleophilic attack on the carbonyl carbon of the aldehyde/ketone.
3. Protonation of the resulting alkoxide gives a beta-hydroxy compound.
4. Base-catalyzed dehydration yields the alpha,beta-unsaturated product.

Claisen-Schmidt Condensation

A cross-aldol condensation between an aromatic aldehyde (which lacks alpha-hydrogens) and an aliphatic aldehyde or ketone (which possesses alpha-hydrogens) in the presence of a base, yielding an alpha,beta-unsaturated aromatic carbonyl compound.

Reaction: C6H5-CHO + CH3-CO-C6H5 (NaOH) -> C6H5-CH=CH-CO-C6H5 + H2O (Chalcone)

Mechanism: Base abstracts an alpha-hydrogen from the aliphatic ketone to generate an enolate, which attacks the aromatic aldehyde. Subsequent protonation and dehydration produce the conjugated aromatic unsaturated product.

Perkin Reaction

The condensation of an aromatic aldehyde lacking alpha-hydrogens with an aliphatic acid anhydride containing at least two alpha-hydrogens, in the presence of the sodium or potassium salt of the corresponding acid, to form an alpha,beta-unsaturated carboxylic acid (e.g., cinnamic acid).

Reaction: C6H5-CHO + (CH3CO)2O (CH3COONa, 180 °C) -> C6H5-CH=CH-COOH + CH3COOH

Mechanism:

1. The carboxylate ion (CH3COO-) acts as a weak base and abstracts an alpha-hydrogen from acetic anhydride to form an enolate ion.
2. The enolate attacks the carbonyl group of the aromatic aldehyde forming an alkoxide adduct.
3. Intramolecular acetyl transfer occurs followed by elimination of carboxylate ion to form an unsaturated anhydride.
4. Hydrolysis of the unsaturated anhydride produces cinnamic acid and acetic acid.

Cannizzaro Reaction

Aldehydes that do not contain an alpha-hydrogen atom (such as HCHO, C6H5CHO) undergo self oxidation-reduction (disproportionation) when treated with concentrated alkali (50% NaOH), giving a mixture of a primary alcohol and a salt of a carboxylic acid.

Reaction: 2 C6H5-CHO + Conc. NaOH -> C6H5-CH2OH + C6H5-COONa

Mechanism:

1. Nucleophilic attack of hydroxide ion (OH-) on the carbonyl carbon of one aldehyde molecule forms a tetrahedral dianion/monoanion intermediate.
2. Hydride Transfer (Rate-determining step): The intermediate transfers a hydride ion (H:-) directly to the carbonyl carbon of a second aldehyde molecule.
3. Fast proton transfer yields the alcohol and carboxylate salt.

Wittig Reaction

The reaction of an aldehyde or ketone with a phosphorus ylide (also called a Wittig reagent, R2C=PPh3) to convert the carbonyl C=O group selectively into an alkene (C=C).

Reaction: R2C=O + Ph3P=CR'2 -> R2C=CR'2 + Ph3P=O (Triphenylphosphine oxide)

Mechanism:

1. The nucleophilic carbon of the phosphorus ylide attacks the electrophilic carbonyl carbon to form a dipolar betaine intermediate.
2. The betaine ring-closes to form a four-membered cyclic oxaphosphetane intermediate.
3. The four-membered ring undergoes concerted collapse driven by the formation of an extremely strong phosphorus-oxygen double bond (P=O), giving the alkene and triphenylphosphine oxide.

Beckmann Rearrangement

The acid-catalyzed conversion of an oxime (derived from a ketone) into an N-substituted amide in the presence of acidic reagents such as H2SO4, PCl5, or polyphosphoric acid (PPA).

Reaction: R2C=N-OH (Acid catalyst) -> R-CO-NH-R

Mechanism:

1. Protonation of the oxime hydroxyl group converts -OH into a good leaving group (-OH2+).
2. Migration step: The group located anti to the hydroxyl group migrates to the nitrogen atom simultaneously with the loss of water, forming a nitrilium ion intermediate.
3. Water attacks the nitrilium ion to form an iminol intermediate.
4. Tautomerization of the iminol gives the stable N-substituted amide.

Benzil-Benzilic Acid Rearrangement

The base-catalyzed rearrangement of an alpha-diketone (1,2-diketone such as Benzil) into a salt of an alpha-hydroxy carboxylic acid (Benzilic acid).

Reaction: C6H5-CO-CO-C6H5 + KOH -> (C6H5)2C(OH)-COOK (Benzilate salt)

Mechanism:

1. Hydroxide ion attacks one of the carbonyl carbons to form a tetrahedral intermediate.
2. 1,2-Aryl Shift: The electron pair on oxygen reforms the carbon-oxygen double bond, causing migration of the phenyl group to the adjacent carbonyl carbon.
3. Intramolecular proton transfer from the carboxylic acid group to the alkoxide oxygen produces the stable benzilate anion.

Haloform Reaction

Compounds containing a methyl carbonyl group (CH3-C=O) or compounds capable of being oxidized to it (e.g., ethanol, secondary methyl alcohols) react with halogen (X2 = Cl2, Br2, I2) in excess aqueous base to produce a haloform (CHX3) and a carboxylate ion.

Reaction: R-CO-CH3 + 3 X2 + 4 OH- -> R-COO- + CHX3 + 3 X- + 3 H2O

Mechanism:

1. Base abstracts an alpha-hydrogen from the methyl group to form an enolate ion, which reacts with X2 to form a monohalo ketone.
2. This process repeats twice more because halogen substitution increases alpha-hydrogen acidity, forming a trihalomethyl ketone (R-CO-CX3).
3. Hydroxide ion attacks the carbonyl carbon of R-CO-CX3. The -CX3 group leaves as a stable carbanion (:CX3-).
4. Proton transfer converts :CX3- to haloform (CHX3) and the acid to carboxylate (R-COO-).

Baeyer-Villiger Oxidation

The oxidation of aldehydes or ketones to esters (or cyclic ketones to lactones) using peroxy acids such as meta-chloroperoxybenzoic acid (mCPBA), trifluoroperacetic acid, or peracetic acid.

Reaction: R-CO-R' + R''COOOH -> R-COO-R' + R''COOH

Mechanism:

1. Nucleophilic addition of the peroxy acid to the carbonyl carbon gives a tetrahedral Criegee intermediate.
2. Migration of one of the alkyl/aryl groups to the adjacent peroxy oxygen atom occurs simultaneously with the cleavage of the weak O-O bond and loss of the carboxylate leaving group.

Migratory Aptitude: Tertiary alkyl > Secondary alkyl > Cyclohexyl > Benzyl > Phenyl > Primary alkyl > Methyl.

5. Alpha-Substitution Reactions

Alpha-substitution reactions occur at the carbon atom adjacent to the carbonyl group (the alpha-position). These reactions proceed through enol or enolate intermediates.

Enols and Enolates

The alpha-hydrogens of carbonyl compounds are acidic (pKa ~ 19-20) because the resulting conjugate base (enolate anion) is stabilized by resonance, delocalizing the negative charge onto the electronegative carbonyl oxygen.

Tautomerism: Keto Form (>CH-C=O) <-> Enol Form (>C=C-OH)

Halogenation of Alpha-Position

• Acid-Catalyzed Halogenation: Carbonyl compounds react with Cl2, Br2, or I2 in acetic acid to introduce a single halogen at the alpha carbon via an enol intermediate. Monohalogenation is selective because the haloketone is less basic and forms an enol less readily.
• Base-Promoted Halogenation: In basic medium, each halogenation step increases alpha-hydrogen acidity, accelerating subsequent halogenations and leading to complete polyhalogenation (as seen in the haloform reaction).

6. Oxidations and Reductions

Reductions of Carbonyl Compounds

Oxidations of Carbonyl Compounds

• Aldehyde Oxidation: Aldehydes are very easily oxidized to carboxylic acids using mild oxidants (Tollens' reagent [Ag(NH3)2]+, Fehling's solution [Cu2+ tartrate]) as well as strong oxidants (KMnO4, K2Cr2O7/H2SO4, HNO3).
• Ketone Oxidation: Ketones are resistant to mild oxidants. Under drastic conditions (hot concentrated HNO3 or KMnO4), ketones undergo C-C bond cleavage to yield carboxylic acid mixtures according to Popoff's Rule (during cleavage of unsymmetrical ketones, the carbonyl group remains preferentially with the smaller alkyl group).

7. Addition Reactions of Unsaturated Carbonyl Compounds (Michael Addition)

Alpha,Beta-Unsaturated Carbonyl Compounds

Alpha,beta-unsaturated carbonyl compounds (e.g., CH2=CH-CHO) possess conjugated C=C and C=O double bonds. Nucleophiles can attack at two positions:

• 1,2-Addition (Direct Addition): Attack directly on the carbonyl carbon (favored by strong nucleophiles like Grignard reagents or organolithiums).
• 1,4-Addition (Conjugate Addition): Attack at the beta-carbon (favored by weaker, resonance-stabilized nucleophiles like enolates and Gilman reagents).

Michael Addition Mechanism

The Michael Addition is the nucleophilic 1,4-conjugate addition of a resonance-stabilized enolate (Michael donor, such as malonic ester, acetoacetic ester, or nitroalkanes) to an alpha,beta-unsaturated carbonyl compound (Michael acceptor, such as methyl vinyl ketone or acrylonitrile) in the presence of a base.

General Reaction:
Michael Donor + Michael Acceptor (Base) -> 1,5-Dicarbonyl Compound

Step-by-Step Mechanism:

1. Deprotonation: Base abstracts an acidic proton from the Michael donor to form a nucleophilic enolate.
2. Conjugate Nucleophilic Attack: The donor enolate attacks the electrophilic beta-carbon of the Michael acceptor, forming an enolate intermediate at the acceptor molecule.
3. Protonation and Tautomerization: Proton transfer gives the enol form, which tautomerizes to the more stable keto form, yielding a 1,5-dicarbonyl product.