Aldehyde

An aldehyde is either a Functional Group consisting of a Terminal Carbonyl Group or a compound containing a terminal carbonyl group.

(Where -R represents the carbon chain.)

STRUCTURE

The aldehyde functional group is a carbon atom bonded to a carbonyl group and a hydrogen atom.

α carbon & α hydrogen

An α (alpha) carbon is a carbon adjacent to a carbonyl group. An α hydrogen is a hydrogen atom bonded to the α carbon. The PKa of an α hydrogen is 20.

Carbonyl group

The other molecules containing Carbonyl Group are:

Ketone s

Carboxylic Acid s

NOMENCLATURE

Aldehydes are named by IUPAC Nomenclature by changing the suffix ''-e'' of the parent Alkane to ''-al''.

Aliphatic aldehydes are named as derivatives of their longest alkyl chain. Thus, HCHO is named as a derivative of methane, and CH₃CH₂CH₂CHO is named as a derivative of butane. The suffix -al replaces the -e of the alkane name. Thus, HCHO is named Methanal , more commonly known as Formaldehyde , and CH₃CH₂CH₂CHO is named Butanal .

When a -CHO group is attached to a ring, the suffix -carbaldehyde is used. Thus, C₆H₁₁-CHO is known as cyclohexanecarbaldehyde. The name Benzaldehyde is used as the root for aldehydes derived from Benzene .

PHYSICAL PROPERTIES

The carbonyl group is Polar . Oxygen being more Electronegative , pulls the bond pair towards itself thus creating electron deficiency at the carbon atom.

CHEMISTRY

Preparation

There are several methods for preparing aldehydes:

Reacting a Primary Alcohol with an Oxidizing Agent . In the laboratory this may be achieved by heating the alcohol with a chromium(VI) reagent an acidified solution of Potassium Dichromate , which is Reduced to green Cr³⁺ during the reaction. Excess Dichromate will further oxidise the aldehyde to form a Carboxylic Acid , so either the aldehyde is Distilled out as it forms (if Volatile ), or milder methods such as PCC oxidation, IBX Acid , Dess-Martin Periodinane or Swern Oxidation are used. The equation is shown below with Propan-1-ol being oxidised to form propanal.

:CH₃CH₂CH₂OH —→ CH₃CH₂CHO

:A similar process reacting pentan-1-ol to form pentanal is illustrated below.

:

Reacting an Alkene (if there is a Vinyl ic hydrogen) with Ozone will cause the C=C bond to break yielding an aldehyde upon workup, in a process called Ozonolysis .

Reacting an Ester with Diisobutyl Aluminium Hydride DIBAL-H or Sodium aluminium hydride can cause reduction, yielding an aldehyde.

Reduction of an Acid Chloride using the Rosenmund Reduction or using Lithium tri-t-butoxyaluminium hydride LiAlH(O-t-C₄H₉)₃.

reaction of ketones with Methoxymethylenetriphenylphosphine in a modified Wittig reaction

Common reactions

aldehydes react with Alcohol s and an Acid or Base to a Hemiacetal , the hemiacetal and alcohol and more Acid Catalyst further react to an Acetal and water

Glucose

Treating aldehydes with oxidizing agents such as Potassium Permanganate , Nitric Acid , Chromium(VI) Oxide or Acidified Potassium Dichromate ), will yield a Carboxylic Acid .

Treating aldehydes 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 {Link without Title} ⁺ complex) will convert aldehydes to carboxylic acids without attacking carbon-carbon double-bonds. This reaction is also known as the silver mirror test.

Aldehydes can react with water (under acidic or basic conditions) to form Hydrates , R-C(H)(OH)(OH), although these are only stable when strong Electron Withdrawing Group s are present, as in Chloral Hydrate . The mechanism is identical to Hemiacetal formation.

Aldehydes can react with HCN to form Cyanohydrin s, R-C(H)(OH)(CN).

Treating an aldehyde with a Grignard Reagent can yield an alcohol with a substituted group from the Grignard reagent.

Treating aldehydes with NH₃ derivatives (like NH₂OH , H₂NNH₂ , 2,4-dinitrophenylhydrazine ) will result in the formation of an Oxime or Hydrazone . This is classed as an Addition - Elimination reaction or Addition - Condensation reaction. If the simple hydrazone (RCH=NHNH₂) of an aldehyde is heated with a base such as KOH , the terminal carbon is fully reduced via the Wolff-Kishner Reaction to a Methyl group. The Wolff-Kishner reaction may be performed as a One-pot Reaction , giving the overall conversion RCH=O → RCH₃.

Reaction of aldehydes with Reducing Agent s such as Magnesium gives Diol s in a Pinacol Coupling Reaction

The Wittig Reaction takes aldehydes to Alkene s and the Corey-Fuchs Reaction takes aldehydes to Alkyne s both with a Triphenylphosphine reagent. The Corey-Chaykovsky Reagent is a Sulfonium Ylide which convert aldehydes to Epoxide s.

Nucleophilic Addition

#aldehyde + Nucleophile —→ Tetrahedral Carbonyl Addition Compound

aldehyde + Ammonia or Primary Amine —→ Tetrahedral Carbonyl Addition Compound

--- Tetrahedral Carbonyl Addition Compound + Acid (catalyst) —→ Imine + Water

Keto-enol Tautomerism

Equilibration of keto and enol Tautomers is catalyzed by acid.

Oxidation & Reduction

Aldehydes are oxidized to Carboxylic Acids .

Aldehydes are reduced to Primary Alcohol s.

EXAMPLES OF ALDEHYDES

Methanal ( Formaldehyde )

Ethanal ( Acetaldehyde )

Propanal

Butanal

Pentanal

Glucose

Benzaldehyde

ETYMOLOGY

The word seems to have arisen from "alcohol '''dehyd'''rogenated". In old times, aldehydes were sometimes named after the corresponding Alcohol s, for example "vinous aldehyde" for Acetaldehyde . ( Latin ''vinum'' = "wine", the traditional source of Ethanol ; confer Vinyl .)

SEE ALSO

Ketone

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Aldehyde