Aldaric acid
Encyclopedia
Aldaric acids are a group of sugar
acid
s, where the terminal hydroxyl groups of the sugars have been replaced by terminal carboxylic acids, and are characterised by the formula
HOOC-(CHOH)n-COOH.
Aldaric acids are usually synthesized by the oxidation of aldose
s with nitric acid
. In this reaction it is the open-chain (polyhydroxyaldehyde) form of the sugar that reacts.
An aldaric acid is an aldose
in which both the hydroxyl
function of the terminal carbon and the aldehyde
function of the first carbon have been fully oxidized to carboxylic acid
functions. (Oxidation of just the aldehyde yields an aldonic acid
while oxidation of just the terminal hydroxyl group yields an uronic acid
.) Aldaric acids cannot form cyclic hemiacetal
s like unoxidized sugars, but they can sometimes form lactone
s.
Nomenclature of the aldaric acids is based on the sugars from which they are derived; for example, glucose is oxidized to glucaric acid and xylose to xylaric acid.
Unlike their parent sugars, aldaric acids have the same functional group
at both ends of their carbon chain; therefore, two different sugars can yield the same aldaric acid (this can be understood by looking at the Fischer projection
of a sugar upside down—with normal aldoses, this is a different compound due to the aldehyde
function at the top and the hydroxyl
function at the bottom, but with aldaric acids, there is a carboxylic acid
function on both ends, so upside down and right side up do not matter). For example, D-glucaric acid and L-gularic acid are the same compound. A consequence of this is that some aldaric acids are meso forms with no optical activity despite their multiple chiral
centers—this occurs if a sugar and its enantiomer
oxidize to the same aldaric acid. An example is D-mannose
--it has four chiral
centers, but D-mannaric and L-mannaric acids, which have the opposite configuration at each chiral center and therefore would be expected to be enantiomer
s, are actually the same compound; therefore, mannaric acid is an achiral
meso form with no optical activity. Again, this can be understood by taking the Fischer projection of either acid and looking at it upside down—the configuration is now switched at every carbon.
Adipic acid
, HOOC-(CH2)4-COOH, is not an aldaric acid, though it is structurally similar. In fact, six-carbon aldaric acids can be considered tetrahydroxyl derivatives of adipic acid.
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
s, where the terminal hydroxyl groups of the sugars have been replaced by terminal carboxylic acids, and are characterised by the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
HOOC-(CHOH)n-COOH.
Aldaric acids are usually synthesized by the oxidation of aldose
Aldose
An aldose is a monosaccharide that contains only one aldehyde group per molecule. The chemical formula takes the form Cnn. The simplest possible aldose is the diose glycolaldehyde, which only contains two carbon atoms....
s with nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
. In this reaction it is the open-chain (polyhydroxyaldehyde) form of the sugar that reacts.
An aldaric acid is an aldose
Aldose
An aldose is a monosaccharide that contains only one aldehyde group per molecule. The chemical formula takes the form Cnn. The simplest possible aldose is the diose glycolaldehyde, which only contains two carbon atoms....
in which both the hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
function of the terminal carbon and the aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
function of the first carbon have been fully oxidized to carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
functions. (Oxidation of just the aldehyde yields an aldonic acid
Aldonic acid
An aldonic acid is any of a family of sugar acids obtained by oxidation of the aldehyde functional group of an aldose to form a carboxylic acid functional group. Thus, their general chemical formula is HOOC-n-CH2OH...
while oxidation of just the terminal hydroxyl group yields an uronic acid
Uronic acid
thumb|300px|The [[Fischer projection]]s of [[glucose]] and [[glucuronic acid]]. Glucose's terminal carbon's hydroxyl group has been oxidized to a [[carboxylic acid]]....
.) Aldaric acids cannot form cyclic hemiacetal
Hemiacetal
Hemiacetals and hemiketals are compounds that are derived from aldehydes and ketones respectively. The Greek word hèmi means half...
s like unoxidized sugars, but they can sometimes form lactone
Lactone
In chemistry, a lactone is a cyclic ester which can be seen as the condensation product of an alcohol group -OH and a carboxylic acid group -COOH in the same molecule...
s.
Nomenclature of the aldaric acids is based on the sugars from which they are derived; for example, glucose is oxidized to glucaric acid and xylose to xylaric acid.
Unlike their parent sugars, aldaric acids have the same functional group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...
at both ends of their carbon chain; therefore, two different sugars can yield the same aldaric acid (this can be understood by looking at the Fischer projection
Fischer projection
The Fischer projection, devised by Hermann Emil Fischer in 1891, is a two-dimensional representation of a three-dimensional organic molecule by projection. Fischer projections were originally proposed for the depiction of carbohydrates and used by chemists, particularly in organic chemistry and...
of a sugar upside down—with normal aldoses, this is a different compound due to the aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
function at the top and the hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
function at the bottom, but with aldaric acids, there is a carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
function on both ends, so upside down and right side up do not matter). For example, D-glucaric acid and L-gularic acid are the same compound. A consequence of this is that some aldaric acids are meso forms with no optical activity despite their multiple chiral
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....
centers—this occurs if a sugar and its enantiomer
Enantiomer
In chemistry, an enantiomer is one of two stereoisomers that are mirror images of each other that are non-superposable , much as one's left and right hands are the same except for opposite orientation. It can be clearly understood if you try to place your hands one over the other without...
oxidize to the same aldaric acid. An example is D-mannose
Mannose
Mannose is a sugar monomer of the aldohexose series of carbohydrates. Mannose is a C-2 epimer of glucose. It is not part of human metabolism, but is a component of microbial cell walls, and is therefore a target of the immune system and also of antibiotics....
--it has four chiral
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....
centers, but D-mannaric and L-mannaric acids, which have the opposite configuration at each chiral center and therefore would be expected to be enantiomer
Enantiomer
In chemistry, an enantiomer is one of two stereoisomers that are mirror images of each other that are non-superposable , much as one's left and right hands are the same except for opposite orientation. It can be clearly understood if you try to place your hands one over the other without...
s, are actually the same compound; therefore, mannaric acid is an achiral
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....
meso form with no optical activity. Again, this can be understood by taking the Fischer projection of either acid and looking at it upside down—the configuration is now switched at every carbon.
Adipic acid
Adipic acid
Adipic acid is the organic compound with the formula 42. From the industrial perspective, it is the most important dicarboxylic acid: About 2.5 billion kilograms of this white crystalline powder are produced annually, mainly as a precursor for the production of nylon...
, HOOC-(CH2)4-COOH, is not an aldaric acid, though it is structurally similar. In fact, six-carbon aldaric acids can be considered tetrahydroxyl derivatives of adipic acid.