All aldehydes have a hydrogen atom attached to the carbonyl group.  Oxygen is the reagent of choice, being "green" and cheap. Aldehydes or ketones render carboxylic acids with the appropriate oxidant. Possibly because of the high reactivity of the formyl group, aldehydes are not common in several of the natural building blocks: amino acids, nucleic acids, lipids. This word can be recognized in the simplest aldehyde, formaldehyde (structure shown at top of article), and in the simplest carboxylic acid, formic acid (structure shown at right). In general, imines (also called Schiff bases) are stable only if at least one R group is an aromatic ring. A mixture of two acids is formed in this case. The double bond between carbon and oxygen is characteristic of all aldehydes and is known as the carbonyl group. Aldehydes can react with water to form hydrates, R−CH(OH)2. At around 360 kJ/mol (85 kcal/mol), the formyl C–H bond is weaker than that of a typical bond between hydrogen and an sp2-hybridized carbon. Another exception is chloral hydrate, Cl3CH(OH)2, formed from chloral, Cl3CHO, and water. From the left: (1) formaldehyde and (2) its trimer 1,3,5-trioxane, (3) acetaldehyde and (4) its enol vinyl alcohol, (5) glucose (pyranose form as α-D-glucopyranose), (6) the flavorant cinnamaldehyde, (7) the visual pigment retinal, and (8) the vitamin pyridoxal. After the elimination of water, this results in an oxime. Chemically, an aldehyde /ˈældɪhaɪd/ is a compound containing a functional group with the structure −CHO, consisting of a carbonyl center (a carbon double-bonded to oxygen) with the carbon atom also bonded to hydrogen and to any generic alkyl or side chain R group,. Such compounds are often called gem-diols (from the Latin word geminus, meaning “twin”). Nucleophiles add readily to the carbonyl group. For this reason, the acidity of the formyl proton is difficult to measure. It is mainly used in the production of resins when combined with urea, melamine, and phenol (e.g., Bakelite). In general, higher aliphatic aldehydes will accumulate a substantial amount of oligomer (mostly trimer) upon long-term storage and must be freshly distilled when a reaction calls for the monomeric starting material. warrant full correctness of all contents. Treatment of an aldehyde with two moles of an alcohol in the presence of an acid catalyst gives an acetal, a compound with two ether (OR) groups on one carbon. For example, in aqueous solution only a tiny fraction of glucose exists as the aldehyde. This reaction is called autoxidation described by the following mechanism. Aldehydes can easily be oxidized to carboxylic acids by several oxidizing agents—even, in many cases, the oxygen in the air (and as a result it is necessary to keep containers of liquid aldehydes tightly sealed)—but this is not often useful, because in most cases the carboxylic acids are more readily available than the corresponding aldehydes. Simple hemiacetals are usually unstable, although cyclic ones such as glucose can be stable. Due to the presence of the H-atom, aldehydes are easily oxidised by even weak oxidising agents like Ag+, Cu2+ Aldehydes, RCHO, can be oxidised to carboxylic acids, RCO 2 H. Ketones are not oxidised under these conditions as they lack the critical H for the elimination to occur (see mechanism below). The method successfully converts benzylic, aliphatic, heterocyclic, and other heteroatom-containing substrates to the corresponding carboxylic acids in aqueous solution at room temperature.