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I encountered the following compound on Wikipedia:

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Its systematic IUPAC name is:

4,4',5,5',6,6'-hexahydroxy-2,2'-biphenyldicarboxylic acid

What do the prime symbols in the locants 4', 5', 6', and 2' signify?

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2 Answers 2

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Primes, double primes, triple primes, etc., are added to differentiate between the same locant in different parts of the structure. According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), they are used

  • to differentiate the nitrogen atoms of hydrazides, imidamides (amidines), amidrazones, and hydrazidines;

    N'-acetyl-N'-ethyl-N-methylpropanehydrazide

  • with superscripted numerical locants indicating the position of the suffix on the parent structure, to differentiate the nitrogen atoms in other cases, especially when symmetry conditions are not fulfilled;*

    N3-ethyl-N'3-methylhexane-3,3-diamine

  • in multiplicative nomenclature to denote multiplied units and modify locants accordingy;

    2,2',2''-nitrilotri(ethan-1-ol)

  • in spiro-fused compounds, to denote positions in polycyclic systems

    7,7'-spirobi[bicyclo[4.1.0]heptane]

  • in ring assemblies, to number identical ring components;

    1,1'-biphenyl

  • in fusion nomenclature, to identify first and higher attached components, identical attached components, and multiparent names;

    difuro[3,2-b:2',3'-e]pyridine

  • in fullerenes ortho-fused to organic ring or ring systems, to identify positions in the nonfullerene component;

    3'H-cyclopropa[1,9](C60-Ih)[5,6]fullerene

  • in natural product nomenclature, to identify positions in ring(s) fused to a fundamental parent hydride;

    naphtho[2',1':2,3]-5α-androstane

The example that is given in the question is a ring assembly. The numbering of locants corresponds to the numbering of 1,1'-biphenyl, where primes are used to differentiate between the same locant in different phenyl groups. Therefore, the correct systematic name is 4,4',5,5',6,6'-hexahydroxy-[1,1'-biphenyl]-2,2'-dicarboxylic acid.

4,4',5,5',6,6'-hexahydroxy-[1,1'-biphenyl]-2,2'-dicarboxylic acid

* Note that, according to current IUPAC recommendations, primes are no longer used to differentiate the nitrogen atoms of symmetrical geminal diamines, diimines, diamides, di(imidamides), (diamidines), di(hydrazonamides), di(amidrazones), di(hydrazonohydrazides), imidohydrazides, and poylimidopolycarbonic and inorganic oxoacids.

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The locants are "primed" and are used for ring systems not fused together.

For example, this is the locants for biphenyl (courtesy ACD/Labs):

Relevant recommendations (Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H. Pergamon Press, Oxford, 1979. Copyright 1979 IUPAC.):

$52.2$ - If there is a choice in numbering, unprimed numbers are assigned to the system which has the lower-numbered point of attachment.

$52.3$ - If two identical hydrocarbon systems have the same point of attachment and contain substituents at different positions, the locants of these substituents are assigned according to Rule A-$2.2$; for this purpose an unprimed number is considered lower than the same number when primed. Assemblies of primed and unprimed numbers are arranged in ascending numerical order.

The name follows from the usual rules.

More information

For more information about how non-identical ring systems or ring systems with three or more rings are numbered, see Rule A-$53$ and Rule A-$54$.

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