Absolute configuration - the real three-dimensional structure of a chiral molecule. See also Relative configuration.
Acetals - the derivatives of a carbonyl compound containing two alkoxyl groups attached to the same carbon. The functional group is >C(OR)2
Achiral - exhibiting no properties of «handedness». Achiral molecules have a plane or other elements of symmetry.
Acid anhydrides - derivatives of carboxylic acid that can be considered as a result of dehydration of two molecules of acids.
Acid halides - derivatives of carboxylic acids of the general formula RC(O)Hal.
Acidity constant, Ka - an evaluation of the strength of an acid HA that is considered as the equilibrium constant for the equation HA + H2O ↔ H3O+ + A". The acid strength is usually expressed as pKa, which is -log Ka.
Activated complex - see Transition state.
Activating group - an electron-donating group, for example, a hydroxyl or amino group that increases the reactivity of aromatic rings towards electrophilic substitution reactions.
Activation energy - the difference in energy levels between reactants and transition state. It is energy that determines the rate of a reaction.
Acylation - introduction of an acyl group, RCO-, into a molecule. A hydroxyl or amino group is more often acylated to give an ester, RCOOR', or an amide, RCONH2, respectively.
Aglycone - non-sugar portion of a glycoside.
Alcohols - organic compounds of the general formula ROH in which a hydroxyl group is attached to a saturated carbon.
Aldehydes - organic compounds of the general formula RCH=O.
Alditols - polyhydric alcohols resulting from reduction of the carbonyl group of a sugar.
Aldol condensation - an addition reaction between two molecules (identical or different) of carbonyl compounds resulting in the formation of an aldol.