Aldehydes and ketones are organic compounds containing the carbonyl functional group (C=O). In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom and one alkyl or aryl group. In ketones, the carbonyl carbon is bonded to two alkyl or aryl groups.
Aldehyde: R-CHO (or Ar-CHO)
Ketone: R-CO-R' (or Ar-CO-Ar, R-CO-Ar)
Specific Aldehydes and Ketones
- Formaldehyde (Methanal): HCHO, the simplest aldehyde.
- Acetaldehyde (Ethanal): CH3CHO, a common aliphatic aldehyde.
- Acetone (Propanone): CH3COCH3, the simplest ketone.
- Benzaldehyde: C6H5CHO, an aromatic aldehyde.
Preparation of Aldehydes and Ketones
Aldehydes and ketones can be synthesized through various methods, notably from acid chlorides and nitriles.
From Acid Chlorides
Acid chlorides (acyl chlorides) are highly reactive derivatives of carboxylic acids and serve as versatile starting materials for the synthesis of aldehydes and ketones.
Preparation of Aldehydes (Rosenmund Reduction)
Aldehydes are prepared by the catalytic hydrogenation of acid chlorides. The reaction uses hydrogen gas (H2) in the presence of palladium (Pd) deposited on barium sulfate (BaSO4). The catalyst is intentionally poisoned with sulfur or quinoline to prevent the over-reduction of the aldehyde to a primary alcohol.
R-COCl + H2 --(Pd/BaSO4, S or Quinoline)--> R-CHO + HCl
Observation: This method is specific for preparing aldehydes; ketones cannot be prepared this way. Aromatic acid chlorides also undergo Rosenmund reduction.
Preparation of Ketones
Ketones can be prepared by reacting acid chlorides with dialkylcadmium (R'2Cd) reagents. Dialkylcadmium is a less reactive organometallic reagent compared to Grignard reagents, preventing further addition to the ketone formed.
Step 1: Formation of Dialkylcadmium
2 R'-MgX + CdCl2 → R'2Cd + 2 MgXCl
Step 2: Reaction with Acid Chloride
R-COCl + R'2Cd → R-CO-R' + R'CdCl
Note: Grignard reagents react twice with acid chlorides, first forming a ketone then a tertiary alcohol. Dialkylcadmium, being milder, reacts only once to give the ketone.
From Nitriles
Nitriles (R-C≡N) are versatile compounds that can be converted into aldehydes or ketones depending on the reagents used.
Preparation of Aldehydes (Stephen Reduction)
The Stephen reduction converts nitriles into aldehydes. The nitrile is first reduced to an imine hydrochloride using stannous chloride (SnCl2) and hydrochloric acid (HCl), which is then hydrolyzed by water to yield the aldehyde.
R-C≡N + SnCl2 + HCl → [R-CH=NH2+Cl-] (Imine hydrochloride)
[R-CH=NH2+Cl-] + H2O → R-CHO + NH4Cl
Alternative Method: Diisobutylaluminium hydride (DIBAL-H) can also reduce nitriles to aldehydes by stopping the reduction at the imine stage at low temperatures (-78 °C), followed by acidic hydrolysis.
R-C≡N --(DIBAL-H, -78 °C)--> R-CH=NH --(H2O/H+)--> R-CHO
Preparation of Ketones
Ketones can be prepared by the reaction of nitriles with Grignard reagents (R'-MgX), followed by hydrolysis. The Grignard reagent adds to the nitrile to form an imine salt, which is then hydrolyzed to a ketone.
R-C≡N + R'-MgX → R-C(R')=NMgX (Imine salt)
R-C(R')=NMgX + H2O/H+ → R-CO-R' + Mg(OH)X + NH3
Important: Unlike with acid chlorides, Grignard reagents react only once with nitriles to give ketones, because the imine salt is unreactive towards a second equivalent of Grignard reagent.
Reactions of Aldehydes and Ketones
The carbonyl group (C=O) in aldehydes and ketones is highly reactive due to the polarity of the carbon-oxygen double bond, making the carbonyl carbon electrophilic and susceptible to nucleophilic attack.
Nucleophilic Addition Reactions
These are characteristic reactions where a nucleophile adds across the carbon-oxygen double bond.
Reaction with Hydrogen Cyanide (HCN)
Aldehydes and ketones react with hydrogen cyanide (HCN) to form cyanohydrins. This reaction typically requires a basic catalyst (like KCN) to generate the more nucleophilic cyanide ion (CN-).
R-CHO + HCN → R-CH(OH)-CN (Aldehyde Cyanohydrin)
R-CO-R' + HCN → R-C(R')(OH)-CN (Ketone Cyanohydrin)
Application: Cyanohydrins are useful synthetic intermediates as they contain both a hydroxyl and a nitrile group, which can be further transformed (e.g., hydrolysis of -CN to -COOH).
Reaction with Alcohols (ROH)
Aldehydes and ketones react with alcohols in the presence of an acid catalyst to form hemiacetals/hemiketals and subsequently acetals/ketals.
With Aldehydes:
R-CHO + R'-OH --(H+)--> R-CH(OH)-OR' (Hemiacetal)
R-CH(OH)-OR' + R'-OH --(H+, -H2O)--> R-CH(OR')2 (Acetal)
With Ketones:
R-CO-R'' + R'-OH --(H+)--> R-C(R'')(OH)-OR' (Hemiketal)
R-C(R'')(OH)-OR' + R'-OH --(H+, -H2O)--> R-C(R'')(OR')2 (Ketal)
Important: Acetals and ketals are generally stable to bases but are readily hydrolyzed back to carbonyl compounds in acidic conditions, making them useful as protecting groups for carbonyl functionalities.
Reaction with Sodium Bisulfite (NaHSO3)
Aldehydes and methyl ketones react with concentrated aqueous sodium bisulfite solution to form crystalline bisulfite addition products (α-hydroxysulfonates). This reaction is reversible.
R-CHO + NaHSO3 → R-CH(OH)-SO3Na
CH3-CO-R' + NaHSO3 → CH3-C(R')(OH)-SO3Na
Application: This reaction is used for the separation and purification of aldehydes and methyl ketones from non-reactive organic compounds. The crystalline adduct can be filtered, and the original carbonyl compound regenerated by treatment with dilute acid or base.
Reactions with Ammonia Derivatives (NH2-G)
Aldehydes and ketones react with compounds containing a primary amino group (NH2-G) to form imines or imine derivatives (Schiff bases). This is a nucleophilic addition followed by the elimination of a water molecule, typically catalyzed by a trace amount of acid.
General Reaction:
R-CHO (or R-CO-R') + NH2-G --(H+, -H2O)--> R-CH=N-G (or R-C(R')=N-G)
| Ammonia Derivative (NH2-G) | G Group | Product Name | Example Product (from R-CHO) |
|---|---|---|---|
| Hydroxylamine (NH2OH) | -OH | Oxime | R-CH=N-OH |
| Hydrazine (NH2NH2) | -NH2 | Hydrazone | R-CH=N-NH2 |
| Phenylhydrazine (NH2NHC6H5) | -NHC6H5 | Phenylhydrazone | R-CH=N-NHC6H5 |
| 2,4-Dinitrophenylhydrazine (2,4-DNP) | -NHC6H3(NO2)2 | 2,4-Dinitrophenylhydrazone | R-CH=N-NHC6H3(NO2)2 |
| Semicarbazide (NH2NHCONH2) | -NHCONH2 | Semicarbazone | R-CH=N-NHCONH2 |
Application: These derivatives are often crystalline solids with sharp melting points, historically used for the characterization and identification of unknown aldehydes and ketones.
Named Reactions of Aldehydes and Ketones
Several important named reactions highlight specific reactivities or synthetic utilities of aldehydes and ketones.
Iodoform Test
The iodoform test is a qualitative chemical test for the presence of compounds containing a methyl ketone functional group (-CO-CH3) or an alcohol that can be oxidized to a methyl ketone (e.g., CH3CH(OH)-R). Acetaldehyde (CH3CHO) also gives a positive test.
Positive Result: Formation of a yellow precipitate of iodoform (CHI3) upon warming with iodine (I2) and a base (NaOH or KOH).
R-CO-CH3 + 3I2 + 4NaOH → R-COONa + CHI3↓ (yellow ppt) + 3NaI + 3H2O
CH3CHO + 3I2 + 4NaOH → HCOONa + CHI3↓ (yellow ppt) + 3NaI + 3H2O
Mechanism: The reaction proceeds via the α-halogenation of the methyl group, followed by nucleophilic attack by hydroxide on the carbonyl carbon and displacement of the trihalomethyl anion (-CI3).
Aldol Condensation
The Aldol condensation is a reaction in which two molecules of an aldehyde or ketone (or one of each, known as crossed aldol condensation) react in the presence of a dilute base (or acid) to form a β-hydroxy aldehyde or β-hydroxy ketone (an