Aldehyde
An aldehyde is an organic compound containing
a
terminal carbonyl group.
This
functional group, which consists of
a
carbon atom which is bonded to a
hydrogen atom and
double-bonded to an
oxygen atom ,
is called the aldehyde group. The aldehyde group
is also called the formyl or methanoyl group.
Encyclopedia
An
aldehyde is an organic compound containing
a
terminal carbonyl group.
This
functional group, which consists of
a
carbon atom which is bonded to a
hydrogen atom and
double-bonded to an
oxygen atom ,
is called the
aldehyde group. The aldehyde group
is also called the
formyl or
methanoyl group.
Nomenclature
IUPAC prescribes the following nomenclature for aldehydes:
- 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.
- 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 another functional group is present which IUPAC rules prescribe must be named as a suffix, the aldehyde group is named with the prefix formyl-. This prefix is preferred to methanoyl-.
- If replacing the aldehyde group with a carboxyl 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.
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.
Physical properties
The aldehyde group is
polar.
Oxygen, being more electronegative, pulls the electrons in the carbon-oxygen bond towards itself, thus 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.
Chemistry
Synthesis
There are several methods for preparing aldehydes:
...
. In the laboratory this may be achieved by heating the alcohol with a chromium reagent an acidified solution of
potassium dichromate, which is
reduced to green Cr
3+ 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 , or milder methods and reagents such as
PCC oxidation,
IBX acid,
Dess-Martin periodinane or
Swern oxidation are used. The reaction is illustrated below with
propan-1-ol being oxidised to form propanal, and again with pentan-1-ol being oxidized to form pentanal.
- CH3CH2CH2OH —→ CH3CH2CHO
Aldehydes can exist in either the keto or
enol tautomers. Tautomerization is catalyzed by either acid or base.
Common reactions
Reduction and oxidation
| R-phrases [i]
...
solution to give a precipitate of silver oxide, and then adding just enough dilute
ammonia solution to redissolve the precipitate in aqueous ammonia to produce [Ag
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.
Nucleophilic addition reactions
A
nucleophile can add to the carbon atom in the carbonyl group, yielding an addition compound where this
carbon atom has tetrahedral geometry. Together with protonation of the oxygen atom in the carbonyl group
, 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 addition-
elimination or addition-
condensation reaction.
There are various examples of nucleophilic addition reactions.
- 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, although these are only stable when strong electron withdrawing groups are present, as in chloral hydrate. The mechanism is identical to hemiacetal formation.
- 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.
- The cyano group in HCN can add to the carbonyl group to form cyanohydrins, R-C.
- In the Grignard reaction, a Grignard reagent adds to the group, eventually yielding an alcohol with a substituted group from the Grignard reagent.
- Hydroxylamine can add to the carbonyl group. After the elimination of water, this will result in an oxime
[i]
[i]
...
.
...
can add to the carbonyl group. After the elimination of water, this will result in the formation of a
hydrazone.
More complex reactions
Examples of aldehydes
Related compounds
Other kinds of organic compounds containing
carbonyl groups include:
