CHEMISTRY GLOSSARY

hemiketal    →   poluketal

Hemiketals are organic compounds having the structure R₂C(OH)OR (R ≠ H), derived from ketones by formal addition of an alcohol to the carbonyl group. The rings of ketose sugars are hemiketals. This term, once abandoned, has been reinstated as a subclass of hemiacetals.

Schiff base    →   Schiffova baza

Schiff base is a class of compounds derived by the chemical reaction (condensation) of aldehydes or ketones with aromatic amines, for example

They were named after the German chemist Hugo Schiff (1834-1915).

ketal    →   ketal

Ketals are organic compounds formed by addition of an alcohol to a ketone. If one molecule of ketone (RR’CO) reacts with one molecule of alcohol (R"OH) then a hemiketal is formed. The rings of ketose sugars are hemiketals. Further reaction produces a full ketal (RR’C(OR")₂). This term, once abandoned, has been reinstated as a subclass of acetals.

oxo compound    →   okso-spoj

Oxo compounds are organic compounds that contain the karbonyl group, C=O. The term thus embraces aldehydes, carboxylic acids, ketones, amides, and esters.

tautomerism    →   tautomerija

Tautomerism refers to equilibrium between two different structures of the same compound. Usually the tautomers differ in the point of attachment of a hydrogen atom. One of the most common examples of a tautomeric system is the equilibrium between a ketone (keto) and aldehyde (enol).

monosaccharide    →   monosaharid

Monosaccharides are carbohydrates, with the general formula C_n(H₂O)_n, that cannot be decomposed to a simpler carbohydrates by hydrolysis.

Depending on whether the molecule contains an aldehyde group (-CHO) or a ketone group (-CO-) monosaccharide can be a polyhydroxy aldehyde (aldose) or a polyhydroxy ketone (ketose). These aldehyde and ketone groups confer reduction properties on monosaccharides. They are also classified according to the number of carbon atoms they contain: trioses have three carbon atoms, tetroses four, pentoses five, hexoses six, heptoses seven, etc. These two systems of classification are often combined. For example, a six-carbon polyhydroxy aldehyde such as D-glucose is an aldohexose, whereas a six-carbon polyhydroxy ketone such as D-fructose is a ketohexose.

The notations D and L are used to describe the configurations of carbohydrates. In Fischer projections of monosaccharides, the carbonyl group is always placed on top (in the case of aldoses) or as close to the top as possible (in the case of ketoses). If the OH group attached to the bottom-most asymmetric carbon (the carbon that is second from the bottom) is on the right, then the compound is a D-sugar. If the OH group is on the left, then the compound is an L-sugar. Almost all sugars found in nature are D-sugars.

Monosaccharides can exist as either straight-chain or ring-shaped molecules. During the conversion from straight-chain form to cyclic form, the carbon atom containing the carbonyl oxygen, called the anomeric carbon, becomes a chiral center with two possible configurations (anomers), α and β. When the stereochemistry of the first carbon matches the stereochemistry of the last stereogenic center the sugar is the α-anomer when they are opposite the sugar is the β-anomer.