Aldehyde

Aldehyde molecules have a central carbon atom that is connected by a double bond to oxygen, a single bond to hydrogen and another single bond to a third substituent, which is carbon or, in the case of formaldehyde, hydrogen. The central carbon is often described as being sp2-hybridized. The aldehyde group is somewhat polar. The bond length is about 120–122 picometers. ==Physical properties and characterization==

Aldehydes have properties that are diverse and that depend on the remainder of the molecule. Smaller aldehydes such as formaldehyde and acetaldehyde are soluble in water, and the volatile aldehydes have pungent odors. Aldehydes can be identified by spectroscopic methods. Using IR spectroscopy, they display a strong νCO band near 1700 cm−1. In their 1H NMR spectra, the formyl hydrogen center absorbs near δH 9.5 to 10, which is a distinctive part of the spectrum. This signal shows the characteristic coupling to any protons on the α carbon with a small coupling constant typically less than 3.0 Hz. The 13C NMR spectra of aldehydes and ketones gives a suppressed (weak) but distinctive signal at δC 190 to 205. ==Applications and occurrence==

: Important aldehydes and related compounds. The aldehyde group (or formyl group) is colored red. 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--glucopyranose), (6) the flavorant cinnamaldehyde, (7) retinal, which forms with opsins photoreceptors, and (8) the vitamin pyridoxal. Naturally occurring aldehydes Traces of many aldehydes are found in essential oils and often contribute to their pleasant odours, including cinnamaldehyde, cilantro, and vanillin. Possibly due to the high reactivity of the formyl group, aldehydes are not commonly found in organic "building block" molecules, such as amino acids, nucleic acids, and lipids. However, most sugars are derivatives of aldehydes. These aldoses exist as hemiacetals, a sort of masked form of the parent aldehyde. For example, in aqueous solution only a tiny fraction of glucose exists as the aldehyde. ==Synthesis== Hydroformylation Of the several methods for preparing aldehydes, one dominant technology is hydroformylation. Hydroformylation is conducted on a very large scale for diverse aldehydes. It involves treatment of the alkene with a mixture of hydrogen gas and carbon monoxide in the presence of a metal catalyst. Illustrative is the generation of butyraldehyde by hydroformylation of propylene: : One complication with this process is the formation of isomers, such as isobutyraldehyde: : Oxidative routes The largest operations involve methanol and ethanol respectively to formaldehyde and acetaldehyde, which are produced on multimillion ton scale annually. Other large scale aldehydes are produced by autoxidation of hydrocarbons: benzaldehyde from toluene, acrolein from propylene, and methacrolein from isobutene. A variety of reagent systems achieve aldehydes under chromium-free conditions. One such are the hypervalent organoiodine compounds (i.e., IBX acid, Dess–Martin periodinane), although these often also oxidize the α position. A Lux-Flood acid will activate various sulfoxides (e.g. the Swern oxidation), and amine oxides convert alkyl halides to aldehydes (e.g., the Ganem oxidation). Sterically-hindered nitroxyls (i.e., TEMPO) can catalyze aldehyde formation with a cheaper oxidant. Alternatively, vicinal diols or their oxidized sequelae (acyloins or α-hydroxy acids) can be oxidized with cleavage to two aldehydes or an aldehyde and carbon dioxide. Specialty methods ==Common reactions== Aldehydes participate in many reactions. But it becomes the dominant tautomer in strong acid or base solutions, and enolized aldehydes undergo nucleophilic attack at the α position. Reduction The formyl group can be readily reduced to a primary alcohol (). Typically this conversion is accomplished by catalytic hydrogenation either directly or by transfer hydrogenation. Stoichiometric reductions are also popular, as can be effected with sodium borohydride. Oxidation The formyl group readily oxidizes to the corresponding carboxyl group (). The preferred oxidant in industry is oxygen or air. In the laboratory, popular oxidizing agents include potassium permanganate, nitric acid, chromium(VI) oxide, and chromic acid. The combination of manganese dioxide, cyanide, acetic acid and methanol will convert the aldehyde to a methyl ester. If the aldehyde cannot form an enolate (e.g., benzaldehyde), addition of strong base induces the Cannizzaro reaction. This reaction results in disproportionation, producing a mixture of alcohol and carboxylic acid. Nucleophilic addition reactions Nucleophiles add readily to the carbonyl group. In the product, the carbonyl carbon becomes sp3-hybridized, being bonded to the nucleophile, and the oxygen center becomes protonated: : : In many cases, a water molecule is removed after the addition takes place; in this case, the reaction is classed as an addition–elimination or addition–condensation reaction. There are many variations of nucleophilic addition reactions. Oxygen nucleophiles In the acetalisation reaction, under acidic or basic conditions, an alcohol adds to the carbonyl group and a proton is transferred to form a hemiacetal. Under acidic conditions, the hemiacetal and the alcohol can further react to form an acetal and water. Simple hemiacetals are usually unstable, although cyclic ones such as glucose can be stable. Acetals are stable, but revert to the aldehyde in the presence of acid. Aldehydes can react with water to form hydrates, . These diols are stable when strong electron withdrawing groups are present, as in chloral hydrate. The mechanism of formation is identical to hemiacetal formation. Another aldehyde molecule can also act as the nucleophile to give polymeric or oligomeric acetals called paraldehydes. Nitrogen nucleophiles In alkylimino-de-oxo-bisubstitution, a primary or secondary amine adds to the carbonyl group and a proton is transferred from the nitrogen to the oxygen atom to create a carbinolamine. In the case of a primary amine, a water molecule can be eliminated from the carbinolamine intermediate to yield an imine or its trimer, a hexahydrotriazine This reaction is catalyzed by acid. Hydroxylamine () can also add to the carbonyl group. After the elimination of water, this results in an oxime. An ammonia derivative of the form such as hydrazine () or 2,4-dinitrophenylhydrazine can also be the nucleophile and after the elimination of water, resulting in the formation of a hydrazone, which are usually orange crystalline solids. This reaction forms the basis of a test for aldehydes and ketones. More complex reactions == Dialdehydes ==

A dialdehyde is an organic chemical compound with two aldehyde groups. The nomenclature of dialdehydes have the ending -dial or sometimes -dialdehyde. Short aliphatic dialdehydes are sometimes named after the diacid from which they can be derived. An example is butanedial, which is also called succinaldehyde (from succinic acid). ==Biochemistry==

Some aldehydes are substrates for aldehyde dehydrogenase enzymes which metabolize aldehydes in the body. There are toxicities associated with some aldehydes that are related to neurodegenerative disease, heart disease, and some types of cancer. ==Examples of aldehydes==

• Formaldehyde (methanal) • Acetaldehyde (ethanal) • Propionaldehyde (propanal) • Butyraldehyde (butanal) • Isovaleraldehyde • Benzaldehyde (phenylmethanal) • Cinnamaldehyde • Vanillin • Tolualdehyde • Furfural • Retinaldehyde • Glycolaldehyde ==Examples of dialdehydes==

• Glutaraldehyde • Glyoxal • Malondialdehyde • Phthalaldehyde • Succindialdehyde ==Uses==

Of all aldehydes, formaldehyde is produced on the largest scale, about . It is mainly used in the production of resins when combined with urea, melamine, and phenol (e.g., Bakelite). It is a precursor to methylene diphenyl diisocyanate ("MDI"), a precursor to polyurethanes. The second main aldehyde is butyraldehyde, of which about are prepared by hydroformylation. It is the principal precursor to 2-ethylhexanol, which is used as a plasticizer. Acetaldehyde once was a dominating product, but production levels have declined to less than because it mainly served as a precursor to acetic acid, which is now prepared by carbonylation of methanol. Many other aldehydes find commercial applications, often as precursors to alcohols, the so-called oxo alcohols, which are used in detergents. Some aldehydes are produced only on a small scale (less than 1000 tons per year) and are used as ingredients in flavours and perfumes such as Chanel No. 5. Fresh, green, citrusy and nutty notes in perfumes are often due to aldehydes. These include cinnamaldehyde and its derivatives, citral, and lilial. ==Nomenclature==

IUPAC names for aldehydes The common names for aldehydes do not strictly follow official guidelines, such as those recommended by IUPAC, but these rules are useful. IUPAC prescribes the following nomenclature for aldehydes: • Acyclic aliphatic aldehydes are named as derivatives of the longest carbon chain containing the aldehyde group. Thus, HCHO is named as a derivative of methane, and is named as a derivative of butane. The name is formed by changing the suffix -e of the parent alkane to -al, so that HCHO is named methanal, and is named butanal. • In other cases, such as when a group is attached to a ring, the suffix -carbaldehyde may be used. Thus, is known as cyclohexanecarbaldehyde. If the presence of another functional group demands the use of a suffix, the aldehyde group is named with the prefix formyl-. This prefix is preferred to methanoyl-. • If the compound is a natural product or a carboxylic acid, the prefix oxo- may be used to indicate which carbon atom is part of the aldehyde group; for example, is named 2-oxoethanoic acid. • If replacing the aldehyde group with a carboxyl group () would yield a carboxylic acid with a trivial name, the aldehyde may be named by replacing the suffix -ic acid or -oic acid in this trivial name by -aldehyde. Etymology The word aldehyde was coined by Justus von Liebig as a contraction of the Latin (dehydrogenated alcohol). In the past, aldehydes were sometimes named after the corresponding alcohols, for example, vinous aldehyde for acetaldehyde. (Vinous is from Latin "wine", the traditional source of ethanol, cognate with vinyl.) The term formyl group is derived from the Latin word "ant". This word can be recognized in the simplest aldehyde, formaldehyde, and in the simplest carboxylic acid, formic acid. == See also ==