Locants are numbers or letters that indicate the positions of functional groups in a molecular structure. The numbers or letters are placed immediately before the part of the name that they relate to and are set off with hyphens:
but-1-ene not 1-butene 3,5-decadien-1-yne
The prime symbol (see Single prime symbol) indicates the position of the functional groups in multi-ring compounds:
3'-methylphthalanilic acid
Letters are italicised:
(Z)-2-butene (2E,5Z)-2,5-decadiene
Also use italics for substituted chemical element symbols:
N-methylethanamine S-ethylcysteine
When element symbols are used with a type of reaction to form an adjective or noun, roman type can be used to distinguish this use from the use of element symbols as substituted atoms:
N-acetylated O-substitution
Note: Some organisations and journals do not follow this style rule.
Use italics for cis- and trans- isomer names, and all other positional, stereochemical, configurational, symmetry and descriptive prefixes, whether with the full name or an abbreviation:
9-cis-retinal trans-1,2-dichlorocyclohexane meso-3,4-dichlorohexane o-methylphenol p,p'-DDT
R (rectus/right-handed) and S (sinister/left-handed) stereocentre designations are given in italics and brackets:
(R)-2-bromobutane (S)-2,3-dihydroxypropanol
Use small capitals for D (dexter/on the right) and L (laevus/on the left) optical isomers (however, take care when using this notation as it has been largely superseded by the R, S system):
Ignore italicised elements and locants when capitalising at the beginning of sentences or in headings:
9-cis-Retinal is … o-Methylphenol can be … N-methylethanamine was …
