Aldehydes are a fundamental class of organic compounds containing the functional group –CHO, consisting of a carbonyl group (C=O) bonded to at least one hydrogen atom. Their general formula is \( R-CHO \) where \( R \) can be a hydrogen or an alkyl/aryl group. Aldehydes play a vital role in organic synthesis and are frequently tested in the JEE syllabus, both in theory and numericals.
The carbon atom of the aldehyde group is sp² hybridized and forms three sigma bonds: one with oxygen (double bond, including pi bond) and two with carbon or hydrogen. The planar trigonal structure results in bond angles close to 120°.
General structure: \[ R - C(=O) - H \]
The polarity of the carbonyl group makes aldehydes reactive electrophiles, facilitating nucleophilic addition reactions. The partial positive charge on the carbonyl carbon attracts nucleophiles, while the oxygen carries a partial negative charge.
Aldehydes are named according to IUPAC rules by replacing the –e ending of the parent alkane with –al.
Primary alcohols are oxidized using mild oxidizing agents to give aldehydes. Stronger oxidants may overoxidize to carboxylic acids.
\[ \mathrm{R-CH_2OH \xrightarrow{[O]} R-CHO} \]
Common reagents: PCC (Pyridinium chlorochromate), Dess–Martin periodinane, or controlled use of \( \mathrm{CrO_3} \) or \( \mathrm{KMnO_4} \).
Ozone cleaves double bonds in alkenes to give aldehydes or ketones depending on substitution.
\[ \mathrm{R-CH=CH-R' \xrightarrow{O_3}} R-CHO + R'-CHO \]
An industrial method where alkenes react with CO and H₂ in the presence of catalysts to form aldehydes with one more carbon.
\[ \mathrm{R-CH=CH_2 + CO + H_2 \xrightarrow{Catalyst}} R-CH_2-CH_2-CHO \]
Partial reduction of acid chlorides with reagents like LiAlH(O-t-Bu)₃ gives aldehydes.
Aldehydes undergo nucleophilic addition at the carbonyl carbon due to its electrophilicity. Common nucleophiles include hydride ions, cyanide ions, and Grignard reagents.
Forms cyanohydrins which are important synthetic intermediates.
\[ \mathrm{R-CHO + HCN \rightarrow R-CH(OH)-CN} \]
Gives secondary or tertiary alcohols after hydrolysis.
\[ \mathrm{R-CHO + R'MgX \rightarrow R-CH(OMgX)-R' \xrightarrow{H_2O} R-CH(OH)-R'} \]
Aldehydes are easily oxidized to carboxylic acids by oxidizing agents like Tollens’ reagent, Fehling’s solution, and \( \mathrm{KMnO_4} \).
\[ \mathrm{R-CHO + [O] \rightarrow R-COOH} \]
Aldehydes can be reduced to primary alcohols by catalytic hydrogenation or with reducing agents like NaBH₄ or LiAlH₄.
When aldehydes with alpha hydrogens react under basic conditions, they undergo self-condensation to form β-hydroxy aldehydes (aldols), which can dehydrate to α,β-unsaturated aldehydes.
\[ 2 \mathrm{R-CHO} \xrightarrow{Base} \mathrm{R-CH(OH)-CH_2-CHO} \xrightarrow{\Delta} \mathrm{R-CH=CH-CHO} \]
Aldehydes without alpha hydrogens (e.g., formaldehyde, benzaldehyde) undergo disproportionation in presence of concentrated base giving alcohol and carboxylic acid salts.
\[ 2 \mathrm{R-CHO} + OH^- \rightarrow \mathrm{R-CH_2OH} + \mathrm{R-COO}^- \]
Aldehydes react with alcohols under acidic conditions to form hemiacetals and acetals. Acetals are important as protecting groups.
\[ \mathrm{R-CHO + R'OH \xrightarrow{H^+} R-CH(OR')-OH} \quad \text{(Hemiacetal)} \]
\[ \mathrm{R-CH(OR')-OH + R'OH \xrightarrow{H^+} R-CH(OR')_2 + H_2O} \quad \text{(Acetal)} \]
\[ \mathrm{R-CHO + Nu^- \rightarrow R-CH(ONu)^- \xrightarrow{H^+} R-CH(OH)-Nu} \]
Base abstracts alpha hydrogen forming enolate ion which attacks another aldehyde molecule at carbonyl carbon forming β-hydroxy aldehyde which dehydrates on heating.
Aldehydes are versatile intermediates used in the synthesis of alcohols, acids, polymers, and pharmaceuticals. Their reactivity and transformations form a crucial part of the JEE syllabus, especially in questions involving reaction mechanisms and synthesis pathways.
| Reaction | Reagents/Conditions | Product | Notes |
|---|---|---|---|
| Oxidation of Primary Alcohol | PCC or mild oxidants | Aldehyde | Controlled oxidation |
| Ozonolysis | O₃, Zn/H₂O | Aldehyde + Ketone (depending on alkene) | Cleaves double bonds |
| Hydroformylation | CO + H₂, Catalyst | Aldehyde (one C longer) | Industrial process |
| Aldol Condensation | Base, Heat | α,β-Unsaturated Aldehyde | Self-condensation of aldehydes with α-H |
| Cannizzaro Reaction | Conc. base, no α-H | Alcohol + Carboxylate Salt | Disproportionation |