Aldehyde
An aldehyde is an organic compound containing a carbonyl group (C=O) in which the carbon atom is bonded to at least one hydrogen atom, giving the characteristic –CHO functional group at the end of a carbon chain. Aldehydes are more reactive than ketones because the carbonyl carbon is less sterically hindered and more electrophilic. They are important industrial chemicals — formaldehyde is used in resins, acetaldehyde in vinegar production, and benzaldehyde gives almonds their characteristic scent.
Key Formula
R-CHO (where R = H or organic group)
LaTeX: R\text{-}CHO
| Symbol | Meaning | Unit |
|---|---|---|
| R | Organic substituent or hydrogen atom | none |
| CHO | Aldehyde functional group (carbonyl carbon bonded to H) | none |
Worked Example
Problem
Acetaldehyde (CH₃CHO, M = 44 g/mol) undergoes complete combustion in excess oxygen. Write the balanced equation and calculate the heat released when 1.00 mol burns, given ΔH°combustion = −1167 kJ/mol.
Solution
Step 1: Balance the combustion equation. CH₃CHO + O₂ → CO₂ + H₂O C: 2, H: 4, O: 1 (reactant) → 2CO₂ + 2H₂O O balance: 1 + x = 4 + 2 → x = 5/2 → multiply by 2: 2 CH₃CHO + 5 O₂ → 4 CO₂ + 4 H₂O Step 2: Apply ΔH°combustion for 1.00 mol. q = n × ΔH°combustion = 1.00 mol × (−1167 kJ/mol)
Answer
q = −1167 kJ (1167 kJ of heat released per mole of acetaldehyde combusted)
Common Aldehydes — Names, Formulas, and Uses
| IUPAC Name | Common Name | Molecular Formula | Boiling Point (°C) | Key Use |
|---|---|---|---|---|
| Methanal | Formaldehyde | HCHO | −19 | Preservative, resins |
| Ethanal | Acetaldehyde | CH₃CHO | 20 | Acetic acid precursor |
| Propanal | Propionaldehyde | C₂H₅CHO | 49 | Flavouring agent |
| Butanal | Butyraldehyde | C₃H₇CHO | 75 | Plasticiser synthesis |
| Benzaldehyde | Benzaldehyde | C₆H₅CHO | 179 | Almond flavouring |
Interactive Tools
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Related Terms
Ketone
A ketone is an organic compound containing a carbonyl group (C=O) flanked on both sides by carbon atoms, represented by the general formula R–CO–R'. Unlike aldehydes, the carbonyl carbon in a ketone carries no hydrogen atom, making ketones less susceptible to oxidation and generally less reactive towards nucleophiles. Ketones are widely found in nature (fructose, steroid hormones) and in industry (acetone as a solvent, cyclohexanone in nylon synthesis).
Addition Reaction
An addition reaction is a chemical reaction in which two or more molecules combine to form a single, larger product with no atoms lost as a by-product. Addition reactions occur most commonly at carbon–carbon multiple bonds (alkenes, alkynes) and at polar carbonyl groups, where a reagent adds across the unsaturation. They are fundamental to industrial synthesis — the hydrogenation of vegetable oils, the production of polymers like polyethylene, and the manufacture of alcohols from alkenes all proceed via addition reactions.
Nucleophile
A nucleophile is a chemical species that donates an electron pair to an electrophile to form a new covalent bond, acting as a Lewis base. Nucleophiles are characterised by the presence of a lone pair of electrons, a negative charge, or a region of high electron density that attacks electron-deficient centres (electrophilic carbons). Strong nucleophiles drive SN2 reactions and nucleophilic addition; weaker nucleophiles favour SN1 pathways — making nucleophilicity a key parameter in predicting organic reaction outcomes.
Coined by German chemist Justus von Liebig in 1835 as a contraction of the Latin phrase "alcohol dehydrogenatus" (dehydrogenated alcohol), because aldehydes can be obtained by oxidising primary alcohols. The –al suffix in IUPAC nomenclature derives from this contraction.