Nor-

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. The nor-compound can be derived by demethylation or by removal of a methylenegroup, a CH group or a C atom. This includes the elimination of a methylene group in a cyclic parent compound, followed by ring contraction.^[1][2][3]

"Nor" is an abbreviation of normal. Originally, the term was used to denote the completely demethylated form of the parent compound.^[4] Later, the meaning was restricted to the removal of one group. 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.^[5] Nor is written directly in front of the stem name, without a hyphen between, unless there is another prefix after nor (for example α-).

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use of "nor", in naming the unbranched form of a compound within a series of isomers (also referred to as "normal") is obsolete and not allowed in IUPAC names.

History

1867 publication about nor-compounds

Possibly the earliest known use of the prefix "nor" is that by A. Matthiessen and G.C. Foster in 1867 in a publication about the reaction between a strong acid and opianic acid (see picture).
Opianic acid (C₁₀H₁₀O₅) is a compound with two methyl-groups and they called it "dimethyl nor-opianic acid". After reaction with a strong acid a compound was attained with only one methyl (C₉H₈O₅). This partially demethylated opianic acid they called "methyl normal opianic acid". The completely demethylated compound (C₈H₆O₅) was denoted by the term "normal opianic acid", abbreviated as "nor-opianic acid".

Opianic acid demethylation

Similarly Matthiessen and Foster called narcotine, which has three methoxy groups, "trimethyl nor-narcotine". The singular demethylated narcotine was called "dimethyl nor-narcotine", the more demethylated narcotine "methyl nor-narcotine" and the completely demethylated form "normal narcotine" or "nor-narcotine".^[6]

"Since that time the meaning of the prefix has been generalized to denote the replacement of one or more methyl groups by H, or the disappearance of CH₂ from a carbon chain".^[4]

At present, the meaning is restricted to denote the removal of only one group from the parent structure, rather than the completely demethylated form of the parent compound.^[1]

In literature, "nor" is sometimes called the "next lower homologue", although in this context "homologue" is an inexact term. "Nor" only refers to the removal of one carbon atom with the accompanying hydrogen, not the removal of other units. "Nor" compares two related compounds; it does not describe the relation to a homologous series.

False etymology

It is suggested that "nor" is an acronym of German "N ohne Radikal". At first, the British pharmacologist John H. Gaddum followed this theory^[7], but in response to a review of A.M. Woolman^[8], Gaddum retracted his support for this etymology.^[4] Woolman believed that "N ohne Radikal" was a German mnemonic and likely a backronym, rather than the real meaning of the prefix "nor". This can be argued with the fact "that the prefix nor is used for many compounds which contain no nitrogen at all".^[8] Moreover, in the German language both Nitrogen and Radikal are written with a capital, whereas the prefix "nor" is written in lower case letters.

Obsolete use of the term

Originally, "nor" had an ambiguous meaning, as the term "normal" could also refer to the unbranched form in a series of isomers, for example as with alkanes, alkanols and some amino acids.^[9][10][11]

Names of unbranched alkanes and alkanols, like "normal butane" and "normal propyl alcohol", which are obsolete now^[12], have become the prefix n-,however, not "nor".^[13] Other "normal" compounds got the prefix "nor". Older trivial names, like norleucine and norvaline are retained, but the use of the prefix for isomeric compounds was already discouraged in 1955 or earlier.^[9]

Examples

epinephrine		norepinephrine
testosterone		nortestosterone

See also

• Norsteroid

References

1. ^ ^{a b} IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "nor".
2. ^ Preferred IUPAC Names, Provisional Recommendation september 2004 Chapter 1 13.4.4.3 By the prefix ‘nor’, pp.18-19
3. ^ IUPAC-IUB Joint Commission on Biochemical Nomenclature (JCBN): Nor Retinoids Nomenclature of Retinoids, Recommendations 1981; 4.6.
4. ^ ^{a b c} Gaddum JH (June 1956). "The Prefix 'Nor' in Chemical Nomenclature". Nature 177 (1046): 1046. doi:10.1038/1771046b0. 
5. ^ Preferred IUPAC Names Provisional Recommendation, september 2004; Chapter 1, 16.5.3 Italic terms, pp.89-90
6. ^ A. Matthiessen and G.C. Foster: "Researches into the chemical constitution of narcotine and of its products of decomposition"; J. Chem. Soc., 358 (1868). Abstract published in: "Proceedings of the Royal Society of London, Volume 16" pp. 39-41. PDF: [1]
7. ^ Gaddum JH (February 1953). "Book Review: The Extra Pharmacopoeia". Nature 171 (350): 350. doi:10.1038/177350a0. (non-free access)
8. ^ ^{a b} Woolman AM (June 1956). "The Prefix 'Nor' in Chemical Nomenclature". Nature 177 (1046): 1046. doi:10.1038/1771046a0. 
9. ^ ^{a b} System of Nomenclature for TERPENE HYDROCARBONS-Volume 14 American Chemical Society, (1955); Appendix D The Prefix Nor-; doi:10.1021/ba-1955-0014.ch008
10. ^ IUPAC-IUB Joint Commission on Biochemical Nomenclature (JCBN): 'Nor' Amino Acids, Nomenclature and Symbolism for Amino Acids and Peptides; 15.2.3.
11. ^ IUPAC-IUB Commission on Biochemical Nomenclature (JCBN): Abbreviated designation of amino acid derivatives and peptides-Tentative rules The Journal of Biological Chemistry, Vol. 241, No. 11, Issue of June 10, 1966, p. 2492
12. ^ R. Kober and U. Bünzli-Trepp: IUPAC, Systematic Nomenclature for CIPAC Documentation – an Analysis pp. 12-13. Seventh JOINT CIPAC/FAO/WHO Meeting - Symposium (54th CIPAC Meeting and 9th JMPS Meeting); Ljubljana, June 8th 2010
13. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "alkyl groups".