Absolute configuration: The actual three- dimensional structure of choral molecules. Absolute configurations are specified verbally by the R, S convention and represented on paper by Fischer projections.
Absorption spectrum: A plot of wavelength of incident light versus amount of light absorbed. Organic molecules show absorption spectra in the infrared and ultraviolet regions of the electromagnetic spectrum. By interpreting these spectra, useful structure information about the sample can be obtained.
Acetal: A functional group consisting of two ether-type oxygen atoms bound to the same carbon. Acetals are often used as protecting groups for ketons and aldehyde, since they are stable to basic and nucleophilic regents but can be easily removed by acidic hydrolysis.
Actiral: Having a lack of hondedness. A molecule is actiral if it has a plane of symmetry and is thus superimposable on its mirror image.
Activating group: The difference in energy levels between ground state and transition state. The amount of activation energy required by a reaction determines the rate at which the reaction proceed. The majority of organic reactions have activation energies of 10- 25 Kcal/mol.
Acylium ion: A resonance stabilised carbocation in which the positive charge is located at a carbonyl-group carbon, R-C=O. Acylium ions are strongly electrophilic and are involved as intermediates in Friedel - Crafts acylation reactions.
