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ARTICLE

aldehyde, any of a class oforganic compounds, in which acarbonatom shares adouble bond

with anoxygenatom, asingle bondwith ahydrogenatom, and a single bond with another atom

or group of atoms (designated R in generalchemical formulasand structure diagrams). The

double bond between carbon and oxygen is characteristic of all aldehydes and is known as thecarbonyl group. Many aldehydes have pleasant odours, and in principle, they are derived from

alcoholsby dehydrogenation (removal of hydrogen), from which process came the namealdehyde.

Aldehydes undergo a wide variety ofchemical reactions, includingpolymerization. Their

combination with other types of molecules produces the so-called aldehyde condensationpolymers, which have been used in plastics such as Bakelite and in the laminate tabletop material

Formica. Aldehydes are also useful as solvents andperfumeingredients and as intermediates in

the production ofdyesandpharmaceuticals. Certain aldehydes are involved in physiologicalprocesses. Examples are retinal (vitamin Aaldehyde), important in human vision, and pyridoxal

phosphate, one of the forms ofvitamin B6.Glucoseand other so-called reducing sugars are

aldehydes, as are several natural and synthetichormones.

Structure of aldehydes

Informaldehyde, the simplest aldehyde, the carbonyl group is bonded to two hydrogen atoms. In

all other aldehydes, the carbonyl group is bonded to one hydrogen and one carbon group. In

condensedstructural formulas, the carbonyl group of an aldehyde is commonly represented as

CHO. Using this convention, the formula of formaldehyde is HCHO and that ofacetaldehydeisCH3CHO.

The carbon atoms bonded to the carbonyl group of an aldehyde may be part of saturated orunsaturatedalkyl groups, or they may be alicyclic, aromatic, or heterocyclic rings.
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Nomenclatureof aldehydes

There are two general ways of naming aldehydes. The first method is based on the system usedby the International Union of Pure and Applied Chemistry (IUPAC) and is often referred to as

systematic nomenclature. This method assumes the longest chain of carbon atoms that contains

the carbonyl group as the parentalkane. The aldehyde is shown by changing the suffix -e to -al.

Because the carbonyl group of an aldehyde can only be on the end of the parent chain and,therefore, must be carbon 1, there is no need to use a number to locate it.

In the compound named 4-methylpentanal, the longest carbon chain contains five carbon atoms,and so the parent name is pentane; the suffix -al is added to indicate the presence of thealdehyde group, and the chain is numbered beginning atthe carbonyl group. The methyl group is given the number 4, because it is bonded to the fourth

carbon of the chain.
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The other method of nomenclature for aldehydes, referred to as common nomenclature, is toname them after the common name of the correspondingcarboxylic acid; i.e., the carboxylic acid

with the same structure as the aldehyde except that COOH appears instead of CHO. The acidsare usually given a name ending in -ic acid. Aldehydes are given the same name but with the

suffix -ic acid replaced by -aldehyde. Two examples are formaldehyde and benzaldehyde.

As another example, the common name of CH2=CHCHO, for which the IUPAC name is 2-propenal, isacrolein, a name derived from that ofacrylic acid, the parent carboxylic acid.

Properties of aldehydes

The only structural difference betweenhydrocarbonsand aldehydes is the presence in the latter

of the carbonyl group, and it is this group that is responsible for the differences in properties,

both physical and chemical. The differences arise because the carbonyl group is inherentlypolarthat is, theelectronsthat make up the C=O bond are drawn closer to theoxygenthan to

thecarbon. This gives the oxygen a partial negative charge and the carbon a partial positive

charge. The polarity of a carbonyl group is often represented using the Greek letter delta () toindicate a partial charge (that is, a charge less than one).

The negative end of one polarmoleculeis attracted to the positive end of another polar molecule,

which may be a molecule either of the same substance or of a different substance.
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Physical properties

The polarity of the carbonyl group notably affects the physical properties ofmelting pointand

boiling point, solubility, anddipole moment. Hydrocarbons, compounds consisting of only the

elementshydrogenand carbon, are essentially nonpolar and thus have low melting andboiling

points. The melting and boiling points of carbonyl-containing compounds are considerablyhigher. For example,butane(CH3CH2CH2CH3), propanal (CH3CH2CHO), andacetone

(CH3COCH3) all have the samemolecular weight(58), but the boiling point of the hydrocarbon

butane is 0 C (32 F), while those of propanal and acetone are 49 C (120 F) and 56 C (133F), respectively. The reason for the large difference is that polar molecules have a greater

attraction for each other than do nonpolar molecules, requiring more energyand thus a higher

temperatureto separate them, which must occur if compounds are to melt or boil.

Formaldehyde (HCHO) is agasunder standard conditions, and acetaldehyde (CH3CHO) boils at

about room temperature. Other aldehydes, except those of high molecular weight, areliquids

under ordinary conditions.

Polar molecules do not mix easily with nonpolar ones, because polar molecules attract oneanother and nonpolar ones are unable to squeeze between them. Thus, hydrocarbons are

insoluble inwater, because water molecules are polar. Aldehydes with fewer than about five

carbon atoms are soluble in water; however, above this number, the hydrocarbon portion of theirmolecules makes them insoluble. The solubility of low-molecular-weightcarbonyl compoundsin

water is caused byhydrogen bondsthat form between the oxygen atom of the carbonyl groupand hydrogen atoms of water molecules.

The polarity of molecules can be quantified by a number called a dipole moment. This value isobtained by putting the compound into anelectric fieldand measuring the facility with which its

molecules line up with the field, the negative ends pointing to the positive side of the field and

the positive ends pointing to the negative side. Most hydrocarbons have no or only exceedinglysmall dipole moments, but those of aldehydes are much higher.

Tautomerism

If an aldehyde possesses at least one hydrogen atom on the carbon atom adjacent to the carbonylgroup, called the alpha () carbon, this hydrogen can migrate to the oxygen atom of the carbonyl

group. The double bond then migrates to the -carbon. As a result, a carbonyl compound with an

-hydrogen can exist in two isomeric forms, called tautomers. In theketo form, the hydrogen is

bonded to the -carbon, while in theenolform it is bonded to the carbonyl oxygen with the
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migration of the double bond.

The name enol is derived from the IUPAC designation of it as both analkene(-ene) and an

alcohol(-ol). Keto and enol isomers exist in equilibrium in which both tautomers are present but,in simple cases, the keto form is much more stable than theenol form. Inacetaldehyde, for

example, only about 6 of every 10 million molecules are in the enol form at any given time.

Nevertheless, the equilibrium always exists, and every molecule of acetaldehyde (as well as any

other aldehyde or ketone with an -hydrogen) is converted to the enol form (and back again)

several times per second. This is an importantcharacteristic because a number of reactions of carbonyl compounds take place only through the

enol forms. Certain carbonyl compounds have a much higher percentage of its molecules in the

enol form, however.

Synthesisof aldehydes

Because aldehydes are important building blocks inorganic chemistry, they are used tosynthesize many other compounds, and there are also many ways to prepare them. Oxidation is

among the principal methods.

Primary alcoholscan be oxidized to aldehydes (RCH2OH RCHO, where R is an alkyl oraryl

group). This is generally not easy to do, because most reagents that oxidize primary alcohols to

aldehydes will oxidize the aldehyde further to acarboxylic acid.

To produce aldehydes on an industrial scale, theprimary alcoholcan be passed over hotcopper

(Cu) or copper chromite (Cu[CrO2]2)catalyst, but this method is less useful on a smaller scalesuch as in chemistry laboratories. On a laboratory scale, a number of reagents have been used,

most notably pyridinium chlorochromate, PCC.

A method for reducing carboxylic acids to aldehydes (RCOOH RCHO) in one step would beuseful, but no general technique has been devised for accomplishing this. However,acyl

chlorides, RCOCl can be reduced to aldehydes by several reagents, including lithium tri-tert-

butoxyaluminum hydride, Li+HAl

(OC[CH3]3)3.

A formyl group (CHO) can be put onto anaromatic ringby several methods (ArH ArCHO).In one of the most common of these, called the Reimer-Tiemann reaction,phenols(ArOH) are
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converted to phenolic aldehydes by treatment withchloroformin basic solution. The CHO

group usually goes into the position adjacent to the OH group.

Acetylene, which is an alkyne (a compound containing a carbon-carbontriple bond), reacts with

water, in the presence of mercuric salts to yieldacetaldehyde(CH3CHO).

In a process called hydroformylation, alkenes can be treated withcarbon monoxide, (CO),hydrogen (H2), and a transitionmetalcatalyst, most commonlycobalt(Co),rhodium(Rh), or

ruthenium(Ru), to give aldehydes. Hydroformylation of propylene, for example, gives a mixture

of butanal and 2-methylpropanal.

Hydroformylation is more important in commercial applications (where it is known as the oxo

process) than in laboratory syntheses. Oxo aldehydes are of little importance themselves as final

products. Rather, they are reduced to alcohols or oxidized to carboxylic acids. Oxo alcohols areused asraw materialsfor the synthesis ofdetergentsandtextilefibres. Oxo carboxylic acids are

converted toestersand used as industrial and laboratory solvents.

Principal reactions of aldehydes

Aldehydes are important starting materials and intermediates in organic synthesis, because theyundergo a wide variety of reactions and are readily available by many synthetic methods. The

reactivity of these compounds arises largely through two features of their structures: thepolarity

of thecarbonyl groupand the acidity of any -hydrogens that are present.

Aldehydes are polar molecules, and many reagents seekatomswith a deficiency of electrons.

Such reagents are callednucleophiles, meaning nucleus-loving. A nucleophile has electrons that

it can share with a positively-charged centre to form a newcovalent bond. Many reactions of

carbonyl compounds begin with an attack of a nucleophile (abbreviated as Nu ) at the carbonatom of a carbonyl group, followed by combination of the now-negatively chargedoxygenwith

a positive hydrogenion.

Under acidic conditions this sequence can be reversed, with the positivehydrogen ionadding to

the carbonyl oxygen first and then the nucleophile attacking the carbonyl carbon. In some cases

the reaction ends with this step, but in many other cases there are one or more subsequent steps,the most common being the loss of water. The newly formed OH group leaves together with a

hydrogen from an adjoining atom. The result is formation of a double bond between the carbon

and the nucleophile. If the nucleophile added to the carbonyl group is asulfuratom, for example,then loss of water gives a C=S bond.
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Because oftautomerism, the carbon atom adjacent to the carbonyl group is also susceptible to

attack if that carbon atom possesses a hydrogen atom (an -hydrogen); many reactions of suchcarbonyl compounds involve replacement of the -hydrogen.

Oxidation-reduction reactions

Aldehydes can be reduced to primaryalcohols(RCHO RCH2OH) with manyreducing agents,the most commonly used beinglithium aluminum hydride(LiAlH4),sodium borohydride

(NaBH4), or hydrogen (H2) in the presence of a transition catalyst such asnickel(Ni),palladium

(Pd),platinum(Pt), orrhodium(Rh).

Although alcohols are the most common reduction products, there are others. The use of

hydrazine hydrate, H2NNH2 H2O, and a base such aspotassium hydroxide, KOH, (the Wolff-Kishner reaction) or zinc-mercury, Zn(Hg), andhydrochloric acid(the Clemmensen reaction)

removes the oxygen entirely and gives a hydrocarbon (RCHO RCH3).

In bimolecular reduction, brought about by an active metal such assodium(Na) ormagnesium

(Mg), two molecules of an aldehyde combine to give (after hydrolysis) a compound with OH

groups on adjacent carbons; e.g., 2RCHO RCH(OH)CH(OH)R.

Oxidation reactions of aldehydes are less important than

reductions. Aldehydes can easily be oxidized tocarboxylic acidsby severaloxidizing agentseven, 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 thecarboxylic acids are more readily available than the corresponding aldehydes.

Aromatic aldehydes (ArCHO), and other aldehydes that lack an -hydrogen, undergo an unusualoxidation-reduction reaction(theCannizzaro reaction) when treated with a strongbasesuch as

sodium hydroxide(NaOH). Half of the aldehyde molecules are oxidized, and the other half are

reduced. The products (after acidification) are a carboxylic acid and a primary alcohol (2RCHO

RCOOH + RCH2OH).

Nucleophilic addition

Aldehydes undergo many different nucleophilic addition reactions. This is because the positive

carbon atom of an aldehyde molecule, which always has one bond attached to the small

hydrogen atom, is susceptible to attack by a nucleophilic reagent.
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Addition of noncarbon nucleophiles

Water adds as anucleophileto a carbonyl group of an aldehyde to give compounds with twoOH

groupsbonded to one carbon atom (R2C=O + H2O R2C[OH]2). Such compounds are often

called gem-diols (from the Latin word geminus, meaning twin).

Gem-diols are generally not stable enough to be isolated, because they readily decompose backto the starting compounds. An exception to this generalization isformaldehyde, which is almost

completely in the hydrated form when dissolved in water. Another exception ischloral hydrate,

Cl3CH(OH)2, formed from chloral, Cl3CHO, and water. Chloral hydrate has been used

medicinally as a rapidly acting hypnotic and sedative (it is sometimes called knockout drops).

Treatment of an aldehyde with two moles of an alcohol in the presence of anacidcatalyst gives

an acetal, a compound with twoether(OR) groups on one carbon. Reaction occurs in two stages.First is formed a hemiacetal (a half acetal), which corresponds to the addition of one molecule of

alcohol to the carbonyl group of the aldehyde. The intermediate hemiacetals are no more stable

than the corresponding gem-diols. In stage 2, the acid catalyst promotes the replacement of theOH group by an OR group (from a second molecule of alcohol) to give a stable acetal. Acetal

formation is an equilibrium reaction and can be driven to the left or right depending on the

experimental conditions. An excess of the alcohol and removal of water as it is formed drive the

reaction to the right. An excess of water drives

the equilibrium to the left.

Aminesare more powerful nucleophiles than water or alcohols, and they readily react with

aldehydes.Ammonia(NH3) itself is generally useless because the immediate products rapidly

polymerize. However, primary amines, RNH2, add to giveimines(compounds containing a C=Ngroup) formed by loss of water from the initially formed addition product.

In general, imines (also called Schiff bases) are stable only if at least one R group is an aromatic

ring. Otherwise they too polymerize. Sulfur compounds can also be added to aldehydes.

Addition of carbon nucleophiles

A wide variety of carbon nucleophiles add to aldehydes, and such reactions are of prime

importance in synthetic organic chemistry because the product is a combination of twocarbon

skeletons. Organic chemists have been able to assemble almost anycarbon skeleton, no matter

how complicated, by ingenious uses of these reactions. One of the oldest and most important isthe addition ofGrignard reagents(RMgX, where X is ahalogenatom). French chemistVictor

Grignardwon the 1912Nobel Prizein chemistry for the discovery of these reagents and their

reactions.
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Addition of aGrignard reagentto an aldehyde followed by acidification in aqueous acid gives an

alcohol. Addition to formaldehyde gives a primary alcohol. Addition to an aldehyde other than

formaldehyde gives asecondary alcohol.

Another carbon nucleophile is thecyanideion, CN, which reacts with aldehydes to give, after

acidification, cyanohydrins, compounds containing an OH and CN group on the same carbon

atom.

Benzaldehyde cyanohydrin (mandelonitrile) provides an interesting example of a chemicaldefense mechanismin the biological world. This substance is synthesized bymillipedes

(Apheloria corrugata) and stored in special glands. When a millipede is threatened, the

cyanohydrin is secreted from its storage gland and undergoesenzyme-catalyzed dissociation toproducehydrogen cyanide(HCN). The millipede then releases the HCN gas into its surrounding

environment to ward off predators. The quantity of HCN emitted by a single millipede is

sufficient to kill a small mouse. Mandelonitrile is also found in bitteralmondsandpeachpits. Its

function there is unknown.

Other important reactions in this category include the Knoevenagel reaction, in which the carbonnucleophile is anesterwith at least one -hydrogen. In the presence of a strongbase, the ester

loses an -hydrogen to give a negatively charged carbon that then adds to the carbonyl carbon of

an aldehyde. Acidification followed by loss of a water molecule gives an , -unsaturated ester.

Anotheraddition reactioninvolving a carbon nucleophile is theWittig reaction, in which an

aldehyde reacts with a phosphorane (also called aphosphorus ylide), to give a compound

containing a carbon-carbon double bond. The result of a Wittig reaction is the replacement of the

carbonyl oxygen of an aldehyde by the carbon group bonded tophosphorus. The German

chemistGeorg Wittigshared the 1979 Nobel Prize in chemistry for the discovery of this reactionand the development of its use in synthetic organic chemistry.

Compounds containing a trimethylsilyl group (SiMe3, where Me is themethyl group, CH3)

and alithium(Li) atom on the same carbon atom react with aldehydes in the so-called Peterson

reaction to give the same products that would be obtained by a corresponding Wittig reaction.

Displacement at the -carbon
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-Halogenation

An -hydrogen of an aldehyde can be replaced by achlorine(Cl),bromine(Br), oriodine(I)

atom when the compound is treated with Cl2, Br2, or I2, respectively, either without a catalyst or

in the presence of an acidic catalyst.

The reaction can easily be stopped after only one halogen atom is added. -Halogenation actuallytakes place on the enol form (see aboveProperties of aldehydes: Tautomerism) of the aldehyde

rather than on the aldehyde itself. The same reaction occurs if a base is added, but then it cannot

be halted until all -halogens attached to the same carbon have been replaced by halogen atoms.

If there are three -hydrogens on the same carbon, the reaction goes one step further, resulting inthe cleavage of an X3C

ion (where X is a halogen) and the formation of the salt of acarboxylic

acid.

This reaction is called the haloform reaction, because X3C ions react with water or another acid

present in the system to produce compounds of the form X3CH, which are called haloforms (e.g.,

CHCl3 is calledchloroform).

Aldolreaction

Another important reaction of a carbon nucleophile with an aldehyde is the aldol reaction (also

calledaldol condensation), which takes place when any aldehyde possessing at least one -

hydrogen is treated with sodium hydroxide or sometimes with another base. The product of an

aldol reaction is a -hydroxyaldehyde.

Uses of aldehydes

Hundreds of individual aldehydes are used by chemists daily to synthesize other compounds, but

they are less important in industrial synthesis (that is, the production of compounds on a scale of

tons). Only one aldehyde,formaldehyde, is used to a significant degree in industry worldwide, as

determined by the number of

tons of the chemical utilized per year.

Formaldehyde
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Formaldehyde (made predominantly by the oxidation ofmethanol) is a gas but isgenerally handled as a 37 percent solution in water, calledformalin. It is used in tanning,

preserving, and embalming and as a germicide,fungicide, andinsecticidefor plants and

vegetables, but its largest application is in the production of certain polymeric materials. The

plasticBakeliteis made by a reaction between formaldehyde andphenol. It is not a linear chainbut has a three dimensional structure. Similar three-dimensionalpolymersare made from

formaldehyde and the compoundsureaandmelamine. These polymers are used not only as

plasticsbut even more importantly as adhesives and coatings.Plywoodconsists of thin sheets ofwood glued together by one of these polymers. In addition to Bakelite, the trade names Formica

and Melmac are used for some of the polymers made from formaldehyde.

Other carbonyl compounds of industrial use

Other aldehydes of industrial significance are mainly used as solvents,perfumes, andflavouring

agents or as intermediates in the manufacture of plastics,dyes, andpharmaceuticals. Certainaldehydes occur naturally in flavouring agents. Among these arebenzaldehyde, which provides

the odour and flavour of fresh almonds; cinnamaldehyde, or oil ofcinnamon; and vanillin, the

main flavouring agent ofvanillabeans.

In addition, certain aldehydes perform essential functions in humans and other living organisms.Examples are thecarbohydrates(includingsugars,starch, andcellulose), which are based on

compounds that possess an aldehyde or ketone group along with hydroxyl groups; the steroid

hormones, many of which, includingprogesterone,testosterone,cortisone, andaldosterone, areketones; and retinal, an aldehyde, which, upon combining with a protein (opsin) in theretinaof

the eye to formrhodopsin, is the main compound involved in the process of vision. Exposure of

rhodopsin to light initiates a cis-trans isomerization in the retinal portion. The resulting changein molecular geometry is responsible for generating anerve impulsethat is sent to thebrainand

perceived as a visual signal (see alsophotoreception).

Jerry MarchWilliam H. BrownARTICLE

Additional Reading

Gabriel Tojo and Marcos Fernndez, Oxidation of Alcohols to Aldehydes and Ketones: A Guideto Current Common Practice (2006), covers the working chemistry of organic synthesis.

Detailed coverage is provided in Saul Patai (ed.), The Chemistry of the Carbonyl Group, 2 vol.(196670), and The Chemistry of Double-Bonded Functional Groups, 2 vol. (1977, reissued

1989). William P. Jencks, Carbonyl- and Acyl-Group Reactions, in his Catalysis in


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src="http://adserver.adtechus.com/addyn/3.0/5308.1/1388670/0/170/ADTECH;target=_blank;g

rp=359;key=false;kvqsegs=D;kvtopicid=13527;misc=1307114328782"> Chemistry

and Enzymology (1969, reissued 1987), pp. 463554, provides a detailed discussion of the

mechanisms of these reactions.

Jerry MarchLINKS

Related Articles

Aspects of the topic aldehyde are discussed in the following places at Britannica.

structure and properties

carbonyl group (in carbonyl group (chemistry);in chemical compound: Aldehydesand ketones) carboxylic acids (in carboxylic acid (chemical compound): Oxidation;in carboxylic

acid (chemical compound): Derivatives of carboxylic acids;in carboxylic acid (chemical

compound): Reactions)

keto-enol tautomerism (in acidbase reaction (chemistry): Ketoenol tautomerism,acid- and base-catalyzed)

oximes (in oxime (chemical compound)) toxicity (in poison (physiology): Organic compounds) transition elements and compounds (in transition element (chemical element):

Transition-metal catalysts)

Other

The following is a selection of items (artistic styles or groups, constructions, events, fictionalcharacters, organizations, publications) associated with "aldehyde"

acetaldehyde (CH3CHO) (chemical compound) aldehyde (chemical compound) aldehyde (chemical compound) benzaldehyde (C6H5CHO) (chemical compound) carbonyl group (chemistry) citral (C10H16O) (chemical compound) formaldehyde (chemical compound) formalin (chemistry) ketone (chemical compound) organic compound (chemical compound)

LINKS

External Web Sites
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The topic aldehyde is discussed at the following external Web sites.

Fact Monster - AldehydeHow Stuff Works - Science - Aldehyde

Michigan State University - Aldehydes and Ketones

University of Southern Maine - Aldehydes and Ketones

University of Southern Maine - Aldehydes and KetonesHyperphysics - Aldehydes

Citations
http://www.britannica.com/EBchecked/topic/13527/aldehyde/277600/Nomenclature-of-aldehydeshttp://www.britannica.com/EBchecked/topic/13527/aldehyde/277600/Nomenclature-of-aldehydeshttp://www.britannica.com/EBchecked/topic/13527/aldehyde/277600/Nomenclature-of-aldehydeshttp://www.britannica.com/EBchecked/topic/13527/aldehyde/277600/Nomenclature-of-aldehydeshttp://www.britannica.com/EBchecked/topic/13527/aldehyde/277600/Nomenclature-of-aldehydeshttp://www.britannica.com/EBchecked/topic/13527/aldehyde/277600/Nomenclature-of-aldehydeshttp://www.britannica.com/EBchecked/topic/13527/aldehyde/277600/Nomenclature-of-aldehydeshttp://www.britannica.com/EBchecked/topic/13527/aldehyde/277600/Nomenclature-of-aldehydeshttp://www.britannica.com/EBchecked/topic/13527/aldehyde/277600/Nomenclature-of-aldehydeshttp://www.britannica.com/EBchecked/topic/13527/aldehyde/277600/Nomenclature-of-aldehydeshttp://www.britannica.com/EBchecked/topic/13527/aldehyde/277600/Nomenclature-of-aldehydeshttp://www.britannica.com/EBchecked/topic/13527/aldehyde/277600/Nomenclature-of-aldehydeshttp://www.britannica.com/EBchecked/topic/13527/aldehyde/277600/Nomenclature-of-aldehydeshttp://www.britannica.com/EBchecked/topic/13527/aldehyde/277600/Nomenclature-of-aldehydeshttp://www.britannica.com/EBchecked/topic/13527/aldehyde/277600/Nomenclature-of-aldehydeshttp://www.britannica.com/EBchecked/topic/13527/aldehyde/277600/Nomenclature-of-aldehydeshttp://www.britannica.com/EBchecked/topic/13527/aldehyde/277600/Nomenclature-of-aldehydeshttp://www.britannica.com/EBchecked/topic/13527/aldehyde/277600/Nomenclature-of-aldehydes

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