An aldehyde is an organic compound containing a carbonyl group, C equals O, in which the carbonyl carbon is bonded to at least one hydrogen atom. The general structural representation is RCHO, where R can be hydrogen or an organic group. The simplest aldehyde is methanal, commonly called formaldehyde, with the formula HCHO. Ethanal, commonly called acetaldehyde, is CH3CHO. The defining aldehyde functional group is therefore a terminal carbonyl group with a hydrogen attached directly to the carbonyl carbon.
Aldehydes are important intermediates in organic chemistry because the carbonyl carbon is electrophilic and can participate in numerous addition, condensation and oxidation reactions. They occur naturally in flavour compounds, fragrances, biological metabolites and carbohydrates, while industrial aldehydes are used to manufacture resins, plastics, solvents, pharmaceuticals and other chemicals.
The carbonyl group gives aldehydes their characteristic electronic structure. Oxygen is more electronegative than carbon, so the carbon oxygen bond is strongly polarised, with partial positive character at the carbonyl carbon and partial negative character at oxygen. The carbonyl carbon therefore acts as an electrophilic centre and can be attacked by nucleophiles. Aldehydes generally have trigonal planar geometry around the carbonyl carbon because that carbon is approximately sp2 hybridised.
The carbonyl carbon forms a sigma bond with oxygen and a pi bond through overlap of p orbitals. Because an aldehyde has one hydrogen directly attached to the carbonyl carbon, it can be readily oxidised to a carboxylic acid under appropriate conditions. This oxidation behaviour is one of the most useful distinctions between aldehydes and ketones.
Aldehydes are named systematically using the suffix al. The carbonyl carbon of an aldehyde chain is automatically assigned position one, so propanal contains three carbon atoms and an aldehyde group at the end of the chain. Butanal contains four carbon atoms, while pentanal contains five. Cyclic structures or compounds where the aldehyde carbon is attached to a ring may require specialised naming conventions such as carbaldehyde.
Aldehyde groups can also occur alongside other functional groups, producing molecules with more complex nomenclature. In common terminology, several simple aldehydes have traditional names, including formaldehyde for methanal, acetaldehyde for ethanal and benzaldehyde for benzenecarbaldehyde. Recognising the structural pattern RCHO is often more useful than memorising names because the functional group determines many characteristic chemical reactions.
One of the defining reactions of aldehydes is oxidation to carboxylic acids. Methanal can be oxidised to methanoic acid, while ethanal can be oxidised to ethanoic acid. Oxidising agents such as acidified dichromate or permanganate can accomplish this transformation under appropriate laboratory conditions. Aldehydes can also be reduced to primary alcohols, for example ethanal can be reduced to ethanol. Nucleophilic addition reactions are particularly important because the carbonyl carbon is electrophilic.
Aldehydes can react with hydrogen cyanide to form cyanohydrins, with alcohols to form acetals under suitable acidic conditions and with nitrogen nucleophiles to form compounds such as imines or related derivatives. Aldol reactions can form new carbon carbon bonds when suitable aldehydes contain alpha hydrogens. These transformations make aldehydes exceptionally useful synthetic intermediates.
Aldehydes occur widely in nature and contribute to the smell and flavour of many substances. Benzaldehyde contributes an almond like aroma, vanillin contains an aldehyde group and several naturally occurring aldehydes participate in plant signalling and defence chemistry. Aldehyde groups also occur in carbohydrates. The open chain forms of aldoses contain aldehyde groups, although many sugars exist predominantly in cyclic hemiacetal forms in aqueous solution.
Retinal, a derivative of vitamin A, contains an aldehyde group and plays a critical role in vision by participating in the light sensitive molecular system of photoreceptor cells. These examples demonstrate that aldehydes are not simply industrial chemicals but important structural motifs in biological and natural product chemistry. Their reactivity allows biological systems to transform aldehyde containing intermediates efficiently.
Aldehydes must be distinguished from ketones, which also contain carbonyl groups but have two carbon groups attached to the carbonyl carbon. An aldehyde therefore has at least one carbon hydrogen bond at the carbonyl carbon, whereas a ketone does not. This structural difference explains several chemical differences. Aldehydes are generally more readily oxidised than ketones, and many qualitative laboratory tests exploit this distinction.
Tollens reagent can be reduced by suitable aldehydes to produce a silver deposit, while Fehling or Benedict type reagents can be reduced by many aldehydes under appropriate conditions. These tests are not universally specific for every carbonyl compound and their interpretation depends on experimental conditions. Aldehydes can also be more reactive than ketones toward many nucleophilic additions because they are less sterically hindered and have only one electron donating carbon group attached to the carbonyl carbon.
Aldehydes have industrial significance but can also present health and safety concerns. Formaldehyde is widely used in manufacturing resins, engineered wood products, coatings and other materials, although exposure is carefully regulated because it is toxic and carcinogenic to humans. Acetaldehyde is an important chemical intermediate and occurs naturally in biological metabolism and combustion processes. Aldehydes are often volatile and many have strong odours, although odour should not be treated as a reliable measure of concentration or toxicity. Their chemical reactivity means that aldehydes can participate in atmospheric chemistry and indoor air chemistry, particularly when volatile organic compounds undergo oxidation. Understanding aldehydes therefore has relevance to organic synthesis, industrial chemistry, environmental chemistry, toxicology and atmospheric science.
The chemistry of aldehydes illustrates the importance of functional groups in determining molecular behaviour. Changing the group attached to the aldehyde carbonyl can alter boiling point, solubility, odour, reactivity and biological effects while retaining the fundamental aldehyde functionality. Conjugation with carbon carbon double bonds or aromatic rings can change electronic properties and reaction rates because electron density becomes delocalised across the molecular framework. Some aldehydes readily polymerise or undergo condensation reactions, while others are comparatively stable under particular conditions.
Aldehydes can also exist in equilibrium with hydrated, hemiacetal or other forms depending on solvent, pH and molecular structure. Consequently, the simple definition of an aldehyde identifies the core functional group, but a complete understanding requires considering substituents, resonance, steric effects, reaction conditions, equilibria and the chemical environment in which the aldehyde exists.
