Aldehyde

Aldehyde:
An aldehyde is an organic compound containing a formyl group. This functional group consists of a carbonyl centre bonded to hydrogen, O=CH-. This group is called the aldehyde group or formyl group. Compounds containing this group are called aldehydes. Aldehydes are common in organic chemistry. Many fragrances are aldehydes.

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.
1. 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 CH3CH2CH2CHO 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 CH3CH2CH2CHO is named butanal.
2. In other cases, such as when a -CHO group is attached to a ring, the suffix -carbaldehyde may be used. Thus, C6H11CHO 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-.
3. 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, CHOCH2COOH is named 3-oxopropanoic acid.
4. If replacing the aldehyde group with a carboxyl group (-COOH) 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 seems to have arisen from alcohol dehydrogenated. In the past, aldehydes were sometimes named after the corresponding alcohols, for example, vinous aldehyde for acetaldehyde. (Vinous is from Latin vinum = wine (the traditional source of ethanol), cognate with vinyl.
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Structure and bonding:

Owing to resonance stabilization of the conjugate base, an α-hydrogen in an aldehyde is far more acidic with a pKa near 17, than a C-H bond in a typical alkane, with a pKa in the 30's. This acidification is attributed to (i) the electron-withdrawing quality of the formyl center and (ii) the fact that the conjugate base, an enolate anion, delocalizes its negative charge. Related to (i), the aldehyde group is somewhat polar. The -CHO center is non-acidic.
Aldehydes (except formaldehyde) can exist in either the keto or enol tautomers. Keto-enol tautomerism is catalyzed by either acid or base. Usually the enol is the minority tautomer, but it is more reactive.

Physical properties and characterization:

Aldehydes have properties that are diverse and which depend on the remainder of the molecule. Smaller aldehydes are more soluble in water, formaldehyde and acetaldehyde completely so. The volatile aldehydes have pungent odors. Aldehydes degrade in air via the process of autoxidation.
Both of the small industrially important aldehydes, formaldehyde and acetaldehyde, have complicated behavior because of their tendency to oligomerize or polymerize. They also tend to hydrate in the presence of water, forming the geminal diol. These properties are often not appreciated because the oligomers/polymers and the hydrates exist in equilibrium with the parent aldehyde.
Aldehydes are readily 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 centre absorbs near δ9, which is a distinctive part of the spectrum. This signal characteristically shows coupling to any protons on the alpha carbon.

Applications and occurrence:

Important aldehydes and related compounds. From the left: formaldehyde and its trimer, acetaldehyde and its enol, glucose (pyranose form), the flavorant cinnamaldehyde, the visual pigment retinal, and the vitamin pyridoxal.
Aldehydes are important precursors to commercially useful plasticizers and detergents. Millions of tons of aldehydes are produced industrially each year.

Naturally occurring aldehydes:

Traces of many aldehydes are found in essential oils and often contribute to their favorable odors, e.g. cinnamaldehyde and vanillin. 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. Most sugars, however, are derivatives of aldehydes. Thse "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:

There are several methods for preparing aldehydes,[4]" but the dominant technology is hydroformylation.[5] Illustrative is the generation of butyraldehyde by hydroformylation of propene:
H2 + CO + CH3CH=CH2 → CH3CH2CH2CHO
The method is attractive because the carbon-chain length is extended by one atom.

Oxidative routes:
Aldehydes are commonly generated by alcohol oxidation. In industry, formaldehyde is produced on a large scale by oxidation of methanol. Oxygen is the reagent of choice, being "green" and cheap. In the laboratory, more specialized oxidizing agents are used, but chromium(VI) reagents are popular. Oxidation can be achieved by heating the alcohol with an acidified solution of potassium dichromate. In this case, excess dichromate will further oxidize the aldehyde to a carboxylic acid, so either the aldehyde is distilled out as it forms (if volatile) or milder reagents such as PCC are used.[6]
"O" + CH3(CH2)9OH → CH3(CH2)8CHO + H2O
Oxidation of primary alcohols to form aldehydes and can be achieved under milder, chromium-free conditions by employing methods or reagents such as IBX acid, Dess-Martin periodinane, Swern oxidation, TEMPO, or the Oppenauer oxidation.
Another industrially significant oxidation route is the Wacker process whereby ethylene is oxidized to acetaldehyde in the presence of copper and palladium catalysts (acetaldehyde is also produced on a large scale by the hydration of acetylene).

Common reactions:

Aldehydes are highly reactive and participate in many reactions.[4]" From the industrial perspective, important reactions are condensations, e.g. to prepare plasticizers and polyols, and reduction to produce alcohols, especially "oxo-alcohols." From the biological perspective, the key reactions involve addition of nucleophiles to the formyl carbon in the formation of imines (oxidative deamination) and hemiacetals (structures of aldose sugars).

Reduction:

The" formyl group can be readily reduced to a primary alcohol (-CH2OH). 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 carboxylic acid (-COOH). 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, acetic acid and methanol will convert the aldehyde to a methyl ester.
Another oxidation reaction is the basis of the silver mirror test. In this test, an aldehyde is treated with Tollens' reagent, which is prepared by adding a drop of sodium hydroxide solution into silver nitrate solution to give a precipitate of silver(I) oxide, and then adding just enough dilute ammonia solution to redissolve the precipitate in aqueous ammonia to produce [Ag(NH3)2]+ complex. This reagent will convert aldehydes to carboxylic acids without attacking carbon-carbon double-bonds. The name silver mirror test arises because this reaction will produce a precipitate of silver whose presence can be used to test for the presence of an aldehyde.
If the aldehyde can not 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


References:
1. ^ Short Summary of IUPAC Nomenclature of Organic Compounds, web page, University of Wisconsin Colleges, accessed on line August 4, 2007.
2. ^ §R-5.6.1, Aldehydes, thioaldehydes, and their analogues, A Guide to IUPAC Nomenclature of Organic Compounds: recommendations 1993, IUPAC, Commission on Nomenclature of Organic Chemistry, Blackwell Scientific, 1993.
3. ^ §R-5.7.1, Carboxylic acids, A Guide to IUPAC Nomenclature of Organic Compounds: recommendations 1993, IUPAC, Commission on Nomenclature of Organic Chemistry, Blackwell Scientific, 1993.
4. ^ a b c Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, ISBN 0-471-72091-7
5. ^ W." Bertleff, M. Roeper, X. Sava, “Carbonylation” in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH: Weinheim, 2003. doi: 10.1002/14356007.a05_217.pub2
6. ^ R. W. Ratcliffe (1988), "Oxidation with the Chromium Trioxide-Pyridine Complex Prepared in situ: 1-Decanal", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv6p0373; Coll. Vol. 6: 373
7. ^ New methods for the oxidation of aldehydes to carboxylic acids and esters Elias J. Corey, Norman W. Gilman, and B. E. Ganem J. Am. Chem. Soc. 1968; 90(20) pp 5616 - 5617; doi:10.1021/ja01022a059.