are formed by oxidation of primary
*alcohols; further oxidation yields
carboxylic acids. They are reducing
agents and tests for aldehydes include *Fehling’s test and *Tollens
reagent. Aldehydes have certain characteristic addition and condensation
reactions. With sodium hydrogensulphate(IV) they form addition compounds of the type [RCOH(SO 3 )H]
–
Na
+ . Formerly these were known as
bisulphite addition compounds. They
also form addition compounds with
hydrogen cyanide to give *cyanohydrins and with alcohols to give *acetals and undergo condensation
reactions to yield *oximes, *hydrazones, and *semicarbazones. Aldehydes readily polymerize. See also
ketones.
A
• Information about IUPAC nomenclature
17
alicyclic compound
a
R
C
O
H
aldehyde group
Aldehyde
aldohexose See monosaccharide.
aldol See aldol reaction.
aldol reaction A reaction of aldehydes of the type
2RCH 2 CHO ˆ
RCH 2 CH(OH)CHRCHO
where R is a hydrocarbon group. The
resulting compound is a hydroxyaldehyde, i.e. an aldehyde–alcohol or
aldol, containing alcohol (–OH) and
aldehyde (–CHO) groups on adjacent
carbon atoms. The reaction is basecatalysed, the Ürst step being the formation of a carbanion of the type
RHC
–
CHO, which adds to the carbonyl group of the other aldehyde
molecule. For the carbanion to form,
the aldehyde must have a hydrogen
atom on the carbon next to the carbonyl group.
Aldols can be further converted to
other products; in particular, they
are a source of unsaturated aldehydes. For example, the reaction of
ethanal gives 3-hydroxybutenal (acetaldol):
2CH 3 CHO ˆ CH 3 CH(OH)CH 2 CHO
This can be further dehydrated to 2butenal (crotonaldehyde):
CH 3 CH(OH)CH 2 CHO → H 2 O +
CH 3 CH:CHCHO
aldose See monosaccharide.
aldosterone A hormone produced
by the adrenal glands that controls
excretion of sodium by the kidneys
and thereby maintains the balance of
salt and water in the body Ûuids.
aldoximes Compounds formed by
reaction between hydroxylamine and
an aldehyde
RCOH + H 2 NOH → RCH:NOH + H 2 O
If R is an aliphatic group, the aldoxime is generally a liquid or lowmelting solid. If R is aromatic, the
aldoxime is a crystalline solid. Aldoximes have a planar structure and
can exist in two isomeric forms. In
the syn form, the OH group is on the
same side of the double bond as the
H. In the anti form the OH and H are
on opposite sides. Typically, aliphatic
aldehydes give anti aldoximes; aromatic aldehydes give syn aldoximes.
algin (alginic acid) A complex polysaccharide occurring in the cell walls
of the brown algae (Phaeophyta).
Algin strongly absorbs water to form
a viscous gel. It is produced commercially from a variety of species of
Laminaria and from Macrocystis pyrifera
in the form of alginates, which are
used mainly as a stabilizer and texturing agent in the food industry.
alicyclic compound A compound
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*alcohols; further oxidation yields
carboxylic acids. They are reducing
agents and tests for aldehydes include *Fehling’s test and *Tollens
reagent. Aldehydes have certain characteristic addition and condensation
reactions. With sodium hydrogensulphate(IV) they form addition compounds of the type [RCOH(SO 3 )H]
–
Na
+ . Formerly these were known as
bisulphite addition compounds. They
also form addition compounds with
hydrogen cyanide to give *cyanohydrins and with alcohols to give *acetals and undergo condensation
reactions to yield *oximes, *hydrazones, and *semicarbazones. Aldehydes readily polymerize. See also
ketones.
A
• Information about IUPAC nomenclature
17
alicyclic compound
a
R
C
O
H
aldehyde group
Aldehyde
aldohexose See monosaccharide.
aldol See aldol reaction.
aldol reaction A reaction of aldehydes of the type
2RCH 2 CHO ˆ
RCH 2 CH(OH)CHRCHO
where R is a hydrocarbon group. The
resulting compound is a hydroxyaldehyde, i.e. an aldehyde–alcohol or
aldol, containing alcohol (–OH) and
aldehyde (–CHO) groups on adjacent
carbon atoms. The reaction is basecatalysed, the Ürst step being the formation of a carbanion of the type
RHC
–
CHO, which adds to the carbonyl group of the other aldehyde
molecule. For the carbanion to form,
the aldehyde must have a hydrogen
atom on the carbon next to the carbonyl group.
Aldols can be further converted to
other products; in particular, they
are a source of unsaturated aldehydes. For example, the reaction of
ethanal gives 3-hydroxybutenal (acetaldol):
2CH 3 CHO ˆ CH 3 CH(OH)CH 2 CHO
This can be further dehydrated to 2butenal (crotonaldehyde):
CH 3 CH(OH)CH 2 CHO → H 2 O +
CH 3 CH:CHCHO
aldose See monosaccharide.
aldosterone A hormone produced
by the adrenal glands that controls
excretion of sodium by the kidneys
and thereby maintains the balance of
salt and water in the body Ûuids.
aldoximes Compounds formed by
reaction between hydroxylamine and
an aldehyde
RCOH + H 2 NOH → RCH:NOH + H 2 O
If R is an aliphatic group, the aldoxime is generally a liquid or lowmelting solid. If R is aromatic, the
aldoxime is a crystalline solid. Aldoximes have a planar structure and
can exist in two isomeric forms. In
the syn form, the OH group is on the
same side of the double bond as the
H. In the anti form the OH and H are
on opposite sides. Typically, aliphatic
aldehydes give anti aldoximes; aromatic aldehydes give syn aldoximes.
algin (alginic acid) A complex polysaccharide occurring in the cell walls
of the brown algae (Phaeophyta).
Algin strongly absorbs water to form
a viscous gel. It is produced commercially from a variety of species of
Laminaria and from Macrocystis pyrifera
in the form of alginates, which are
used mainly as a stabilizer and texturing agent in the food industry.
alicyclic compound A compound
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