What is "nor-"

pre. (context organic chemistry English) a compound derived from another by removal of a radical, especially by removal of methyl or methylene

In chemical nomenclature, nor- is a prefix to name a structural analog that can be derived from a parent compound by the removal of one carbon atom along with the accompanying hydrogen atoms. The nor-compound can be derived by removal of a , , or CH group, or of a C atom. The "nor-" prefix also includes the elimination of a methylene bridge in a cyclic parent compound, followed by ring contraction. (The prefix "homo-" which indicates the next higher member in a homologous series, is usually limited to noncyclic carbons).

"Nor" is an abbreviation of normal. Originally, the term was used to denote the completely demethylated form of the parent compound. Later, the meaning was restricted to the removal of one group. Nor is written directly in front of the stem name, without a hyphen between, unless there is another prefix after nor (for example α-). If multiple groups are eliminated the prefix dinor, trinor, tetranor, etcetera is used. The prefix is preceded by the position number (locant) of the carbon atoms that disappear. For example 2,3-dinor. The original numbering of the parent compound is retained. According to IUPAC nomenclature, this prefix is not written with italic letters and unlike nor, when it is a di or higher nor, at the end of the numbers separated by commas, a hyphen is used. As for example 2,3-dinor-6-keto Prostaglandin F1α is produced by beta oxidation of the parent compound 6-keto Prostaglandin F1α. Here, though actually carbon 1 & 2 are lost by oxidation. The new Carbon 1 has now become a CCOH similar to the parent compound, looking as if just carbon 2 & 3 have been removed from the parent compound. "Dinor" does not have to be reduction in adjacent carbons e.g. 5-Acetyl-4,18-dinor-retinoic acid, where 4 referred to a ring carbon and 18 referred to a methyl group on the 5th carbon on the ring.

The alternative use of "nor", in naming the unbranched form of a compound within a series of isomers (also referred to as "normal") is obsolete and not allowed in IUPAC names.