Aldehyde



  An aldehyde is an organic compound containing a terminal oxygen atom (chemical formula O=CH-), is called the aldehyde group. The aldehyde group is also called the formyl or methanoyl group.

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 vinyl.)

The aldehyde group is carbon-oxygen bond towards itself, creating an electron deficiency at the carbon atom.

Owing to resonance stabilization of the conjugate base, an α-hydrogen in an aldehyde is more acidic than a hydrogen atom in an alkane, with a typical pKa of 17.

Nomenclature

IUPAC names for aldehydes

  IUPAC prescribes the following nomenclature for aldehydes:[1][2][3]

  1. Acyclic 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 (-COOH) 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. For example:

Other nomenclature

The carbon atom adjacent to a carbonyl group is called the α carbon. Carbon atoms further away from the group may be named β for the carbon atom bonded to the α carbon, γ for the next, and so on. Hydrogen atoms bonded to these carbon atoms are named likewise: an α hydrogen is a hydrogen atom bonded to the α carbon and so on.

A reaction that introduces an aldehyde group is known as a formylation reaction.

Chemistry

Synthesis

There are several methods for preparing aldehydes:

  • Reacting a primary alcohol with an propionaldehyde, and again with pentan-1-ol being oxidized to form pentanal.
CH3CH2CH2OH —→ CH3CH2CHO

Keto-enol tautomerism

Aldehydes can exist in either the keto or Keto-enol tautomerism is catalyzed by either acid or base.

Common reactions

Reduction and oxidation

  • The aldehyde group can be reduced to the group -CH2OH, changing the aldehyde into a primary alcohol.
  • The aldehyde group can be oxidized to the group -COOH, yielding a potassium dichromate.
  • Another oxidation reaction is the silver mirror test. In this test, an aldehyde is treated with 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 causes the Cannizzaro reaction to occur, producing a mixture of alcohol and carboxylic acid.

Nucleophilic addition reactions

In nucleophilic addition reactions a tetrahedral molecular geometry. Together with protonation of the oxygen atom in the carbonyl group (which can take place either before or after addition); this yields a product where the carbon atom in the carbonyl group is bonded to the nucleophile, a hydrogen atom, and a hydroxyl group.

In many cases, a water molecule is removed after the addition takes place; in this case, the reaction is classed as an condensation reaction.

There are various examples of nucleophilic addition reactions.

  • In the glucose can be stable. Acetals are stable, but revert to the aldehyde in the presence of acid.
  • Aldehydes can react with water (under acidic or basic conditions) to form chloral hydrate. The mechanism is identical to hemiacetal formation.
  • In imine. This reaction is catalyzed by acid.
  • The cyano group in cyanohydrins, R-C(H)(OH)(CN).
  • In the Grignard reagent adds to the group, eventually yielding an alcohol with a substituted group from the Grignard reagent.
  • In the aldol condensation.
  • oxime.
  • An hydrazone. This forms the basis of a test for aldehydes and ketones.

More complex reactions

  • 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 via the one-pot reaction, giving the overall conversion RCH=O → RCH3.
  • Reaction of aldehydes with Pinacol coupling reaction.
  • The epoxides.

Examples of aldehydes

Related compounds

Other kinds of organic compounds containing carbonyl groups include

References

This article needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (December 2007)
  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.


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