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In chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
, epimers are diastereomers that differ in configuration of only one stereogenic center. Diastereomers are a class of stereoisomers that are non-superposable, non-mirror images of one another, unlike enantiomers which are non-superposable mirror images of one another.

For example, the sugar
Sugar

Sugar is a class of edible crystalline substances, mainly sucrose, lactose, and fructose. Human taste buds interpret its flavor as sweet. Sugar as a basic food carbohydrate primarily comes from sugar cane and from sugar beet, but also appears in fruit, honey, sorghum, sugar maple , and in many other sources....
s a-glucose and ß-glucose are epimers.






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In chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
, epimers are diastereomers that differ in configuration of only one stereogenic center. Diastereomers are a class of stereoisomers that are non-superposable, non-mirror images of one another, unlike enantiomers which are non-superposable mirror images of one another.

Glucoseab
For example, the sugar
Sugar

Sugar is a class of edible crystalline substances, mainly sucrose, lactose, and fructose. Human taste buds interpret its flavor as sweet. Sugar as a basic food carbohydrate primarily comes from sugar cane and from sugar beet, but also appears in fruit, honey, sorghum, sugar maple , and in many other sources....
s a-glucose and ß-glucose are epimers. In a-glucose, the -OH group on the first (anomeric) carbon is in the direction opposite the methylene group (in the axial
Axial

Axial has different meanings:* In geometry, it means: along the same line as an axis of rotation or centerline: parallel , contrary to radial, perpendicular or tangential...
 position). In ß-glucose, the -OH group is oriented in the same direction as the methylene group (in the equatorial
Equatorial

Equatorial has different meanings:* In geography it relates to the equator of the Earth.* In meteorology, an Equatorial climate is the type of climate experienced near the equator....
 position). These two molecules are both epimers and anomers.

In this case, ß-D-glucopyranose and ß-D-mannopyranose are epimers because they differ only in the stereochemistry at the 2 position. The hydroxyl group in ß-D-glucopyranose is equatorial
Cyclohexane conformation

Cyclohexane conformation is a much studied topic in organic chemistry because of the complex interrelationship between the different conformational isomerisms of cyclohexane and its derivatives....
 (in the "plane" of the ring) while in ß-D-mannopyranose the 2 hydroxyl group is axial
Cyclohexane conformation

Cyclohexane conformation is a much studied topic in organic chemistry because of the complex interrelationship between the different conformational isomerisms of cyclohexane and its derivatives....
 (up from the "plane" of the ring). These two molecules are epimers but not anomers.

In chemical nomenclature, one of the epimeric pairs is given the prefix epi- for example in quinine
Quinine

Quinine is a natural white crystalline alkaloid having antipyretic , antimalarial drug, analgesic , and anti-inflammatory properties and a bitter taste....
 and epi-quinine. When the pairs are enantiomers, the prefix becomes ent-.

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