and in tert-butanol to a tertiary carbon atom. The terms sec and tert are considered obsolete and should only be used for unsubstituted sec-butoxy, sec-butyl or tert-butyl groups. There are various spellings such as "sec-butyl", "s-butyl", "sBu" or "bus" which are also considered obsolete. spiro The prefix "spiro" followed by a Von-Baeyer descriptor describes in the nomenclature of organic compounds ring systems linked by only one common atom, the spiro atom. If several spiro atoms are present in the molecule, the prefix "spiro" is provided with a prefix ("dispiro", "trispiro", etc.) corresponding to the number of spiro atoms. Typically "spiro" is set as normal. catena The term catena is used in the inorganic nomenclature to describe linear, chain-like polymers from identical polyatomic units. One example are catenatriphosphazenes. Related compounds in organic chemistry are the catenanes. Stereodescriptors of absolute configurations (R), (S) See: Cahn–Ingold–Prelog priority rules The stereochemical descriptors (R) (from Latin rectus = right) and (S) (from lat. sinister = left) are used to describe the absolute configuration of a stereocenter (usually a chiral carbon atom). For this purpose, all substituents at the stereocentre are prioritized according to the CIP rules and the substituent with the lowest priority ("D") is pointed backwards (away from the viewing direction). The stereocenter is (S) configured if the remaining substituents describe a circle descending in priority ("A" → "B" → "C") to the left. The (R) configuration is assigned to the stereocenter if the direction of rotation is directed to the right. If one molecule contains several stereocenters, a locant must be placed before the descriptor (for example, in (1R, 2S)-2-amino-1-phenylpropan-1-ol, the systematic designation of norephedrine). If all stereocenters are configured the same, the naming of the locants can be omitted in favor of an "all-R" or "(all-S)" spelling. Typographically, (R) and (S) are placed in uppercase and italic; the frequently preceding locants, the enclosing round brackets and the commas, on the other hand, as normal. (r), (s) The descriptors (r) and (s) are used to describe the absolute configuration of pseudoasymmetric (pseudochiral) centers. Pseudoasymmetry occurs when four different substituents are attached to one carbon atom, two of which differ only by their absolute stereochemical configuration. Examples of such are meso compounds such the tropane alkaloids; the parent compound is tropine, whose systematic name is (1R, 3r, 5S)-8-methyl-8-azabicyclo[3.2.1]octane-3-ol. In this structure, the C3 atom—the carbon to which the hydroxyl group is attached—is pseudo-asymmetric; therefore,