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An aldehyde.
-R is the group attached to the aldehyde group.

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.

Contents

Nomenclature

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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][2][3]

  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.)

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).

Specialty methods

Reaction name Substrate Comment
Ozonolysis alkene ozonolysis of non-fully-substituted alkenes yield aldehydes upon reductive work-up.
Organic reduction ester Reduction of an ester with diisobutylaluminium hydride (DIBAL-H) or sodium aluminium hydride
Rosenmund reaction acid chloride or using lithium tri-t-butoxyaluminium hydride (LiAlH(O-t-C4H9)3).
Wittig reaction ketone reagent methoxymethylenetriphenylphosphine in a modified Wittig reaction.
Formylation reactions nucleophilic arenes various reactions for example the Vilsmeier-Haack reaction
Nef reaction Nitro compound
Zincke reaction pyridines Zincke aldehydes form in a variation
Stephen aldehyde synthesis nitriles reagents tin(II) chloride and hydrochloric acid.
Meyers synthesis oxazine oxazine hydrolysis
McFadyen-Stevens reaction hydrazide is a base-catalyzed thermal decomposition of acylsulfonylhydrazides

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).[4]

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.[7]

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.

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:

RCHO + Nu- → RCH(Nu)HO-
RCH(Nu)O- + H+ → RCH(Nu)OH

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, R-C(H)(OH)(OH). These diols are stable when strong electron withdrawing groups are present, as in chloral hydrate. The mechanism of formation is identical to hemiacetal formation.

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 to yield an imine. This reaction is catalyzed by acid. Hydroxylamine (NH2OH) can also add to the carbonyl group. After the elimination of water, this will result in an oxime. An ammonia derivative of the form H2NNR2 such as hydrazine (H2NNH2) or 2,4-dinitrophenylhydrazine can also be the nucleophile and after the elimination of water, this will result in the formation of a hydrazone. This forms the basis of a test for aldehydes and ketones.

Carbon nucleophiles

The cyano group in HCN can add to the carbonyl group to form cyanohydrins, R-C(H)(OH)(CN). In the Grignard reaction, a Grignard reagent adds to the group, eventually yielding an alcohol with a substituted group from the Grignard reagent. Related reactions are the Barbier reaction and the Nozaki-Hiyama-Kishi reaction. In the aldol reaction, the metal enolates of ketones, esters, amides, and carboxylic acids will add to aldehydes to form β-hydroxycarbonyl compounds (aldols). Acid or base-catalyzed dehydration will then lead to α,β-unsaturated carbonyl compounds. The combination of these two steps is known as the aldol condensation. The Prins reaction occurs when a nucleophilic alkene or alkyne reacts with an aldehyde as electrophile. The product of the Prins reaction varies with reaction conditions and substrates employed.

More complex reactions

Reaction name Product Comment
Wolff-Kishner reduction alkane If an aldehyde is converted to a simple hydrazone (RCH=NHNH2) and this is heated with a base such as KOH, the terminal carbon is fully reduced to a methyl group. The Wolff-Kishner reaction may be performed as a one-pot reaction, giving the overall conversion RCH=O → RCH3.
Pinacol coupling reaction diol with reducing agents such as magnesium
Wittig reaction alkene reagent an ylide
Takai reaction alkene diorganochromium reagent
Corey-Fuchs reactions alkyne phosphine-dibromomethylene reagent
Ohira-Bestmann reaction alkyne reagent dimethyl (diazomethyl)phosphonate
Johnson-Corey-Chaykovsky reaction epoxide reagent a sulfonium ylide
Oxo Diels Alder reaction pyran Aldehydes can, typically in the presence of suitable catalysts, serve as partners in cycloaddition reactions. The aldehyde serves as the dienophile component, giving a pyran or related compound.
Hydroacylation ketone In hydroacylation an aldehyde is added over an alkene to form a ketone.

Examples of aldehydes

Related compounds

Other kinds of organic compounds containing carbonyl groups include

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.

Wikibooks

Up to date as of January 23, 2010

From Wikibooks, the open-content textbooks collection

The aldehyde is a functional group underneath the general group known as carbonyls. An aldehyde is a compound which contains a terminal carbon that is bound to any generic “R” group, doubly bonded to an oxygen, and single-bonded to a hydrogen. The “R” group is normally a simple aliphatic chain. When the “R” group is hydrogen, the resulting aldehyde is the simplest one, and is known as formaldehyde.

The carbon doubly bound to the oxygen is labeled as the alpha-carbon. Thus, the hydrogen bound to that carbon is the alpha-hydrogen. Because of the resonance stabilization from the carbonyl oxygen, the hydrogen is more acidic and has a pKa around 17. Of course, this pKa varies on the “R” group that the carbon is bound to.

Nomenclature

Examples of IUPAC nomenclature for aldehydes.

When naming aldehydes use the IUPAC rules. For some aldehydes, it is much easier to use their common name.

IUPAC rules:
1. The carbonyl carbon is designated as carbon #1.
2. The longest alkyl chain including the carbonyl carbon is designated and named according to normal alkyl rules.
3. The suffix for an aldehyde is –al. The “e” in the alkane chain is replaced with “-al” to designate that the molecule is an aldehyde.
4. Exception: If the carbonyl carbon of an aldehyde is part of a ring, the compound is now a cycloalkane carbaldehyde. The carbon of the ring to which the aldehyde is attached is now carbon #1.
5. An aldehyde takes higher priority than an alcohol group.
6. If an aldehyde is a substituent because there are multiple carbonyl groups present, the carbonyl with the longer alkyl chain predominates and then presence of the other carbonyl group is designated by –oxo.

Several Common Names:

File:37.gif
Benzaldehyde.

In some cases, the aldehyde can be named by removing the suffix and replacing it with aldehyde. For example:
HCHO is named formaldehyde.
CH3CHO is named acetaldehyde.
C6H5CHO is named benzaldehyde.

Reactivity

Like all the carbonyl compounds, the carbonyl carbon in the aldehyde is positively charged due to the electronegative atom. Thus, the carbonyl carbon is an electrophile.

Nucleophilic Attack: Because of the polarity of the carbonyl carbon, nucleophilic attack is a typical reaction aldehydes undergo. Thus, the carbon is subject to nucleophilic attack from other excellent nucleophiles such as cyanide and hydroxide ions. In nucleophilic attack, the lone pair of electrons on the nucleophile attacks the positively charged carbon, which moves one of the two bonds shared between the carbon and oxygen onto the oxygen. Now the oxygen bears a full negative charge. This oxygen can be worked up with acid resulting in the alcohol.

Reducing and Oxidizing Aldehydes
1. Aldehydes can be reduced to primary alcohols by using reducing agents such as sodium borohydrate and lithium aluminum hydride. For example, ethanol is reduced to ethanol by using one of those two reducing agents.
2. Aldehydes can also be oxidized to carboxylic acids using reducing acids such as permanganate or nitric oxide.


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