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R-0.1.5 Enclosing marks

Parentheses (round brackets, curves), square brackets, and braces (curly brackets) are used in chemical nomenclature to set off parts of a name dealing with specific structural features in order to convey the structure of a compound as clearly as possible.

R- Parentheses

(a) Parentheses are placed around prefixes defining substituted substituents and after the numerical multiplicative prefixes "bis-", "tris-", etc.

Examples to R-

(b) Parentheses are placed around simple substituent prefixes to separate locants of the same type referring to different structural elements, even though only one may be expressed, and to avoid ambiguity (see also R-

Examples to R-

(2-Naphthyl)phenyldiazene (R-

(c) Parentheses may be used simply to add clarity.

Examples to R-

(Thiobenzoic)anhydride (R-

Butane(dithioic) acid

(d) Parentheses are used to isolate the second locant of a double bond when it differs from the first locant by anything other than unity. When locants for a multiple bond differ by only unity, only the first (lower) locant is cited in the name.

Examples to R-

Hex-2-ene (R-3.1.1)

Bicyclo[6.5.1]tetradec-1(13)-ene (R-3.1.1)

(e) Parentheses are used to enclose "added hydrogen" and its locant, stereodescriptors, such as E, Z, R, S, etc., and in descriptors for isotopically substituted compounds.

Examples to R-

Pyridin-2(1H)-one (R-1.3)

(E)-But-2-ene (R-7.1.2)


R- Square brackets

(a) Square brackets enclose descriptors denoting fusion sites in names of fused ring systems, and ring sizes in names constructed according to the von Baeyer system and certain spiro names. They may also enclose ring assembly names when these are followed by a principal group suffix or a suffix describing a parent substituent prefix, and the names of components in certain spiro names.

Examples to R-

Dibenzo[b,e]oxepine (R-

Bicyclo[3.2.1]octane (R-2.4.2)

Dispiro[]heptadecane (R-2.4.3)

[1,2'-Binaphthalene]-2-sulfonic acid (R-5.7.2)

Spiro[cyclopentane-1,1'-indene] (R-2.4.3)

(b) Square brackets enclose locants for structural features of components, such as double bonds in bridges and heteroatoms of component rings in names of fused-ring systems.

Examples to R-

4a,9a-But[2]enoanthracene (R-

4H-[1,3]Oxathiolo[5,4-b]pyrrole (R-

(c) Square brackets enclose substituent prefixes in which parentheses have already been employed.

Example to R-

[2-(Ethoxycarbonyl)ethyl]trimethylammonium bromide


(d) Square brackets are also used to enclose descriptors in isotopically labelled compounds.

Example to R-


(e) Square brackets are employed in formulae to indicate repetition of groups in a chain.

Example to R-

Tricosane (R-2.2.1)

R- Braces may by used to enclose substituent prefixes in which square brackets have already been used.

Examples to R-

When additional enclosing marks are required, the nesting order is curves, square brackets, braces, then curves, square brackets, braces, etc., i.e. {[()]}, {[({[()]})]}, etc.

Example to R-

(a) = 4'-cyanobiphenyl-4-yl

(b) = (4'-cyanobiphenyl-4-yl)oxy

(c) = 5-[(4'-cyanobiphenyl-4-yl)oxy]pentyl

(d) = {5-[(4'-cyanobiphenyl-4-yl)oxy]pentyl}oxy

(e) = ({5-[(4'-cyanobiphenyl-4-yl)oxy]pentyl}oxy)carbonyl

(f) = 1-[({5-[(4'-cyanobiphenyl-4-yl)oxy]pentyl}oxy)carbonyl]ethylene

(g) = 4,4'-{1-[({5-[(4'-cyanobiphenyl-4-yl)oxy]pentyl}oxy)carbonyl]ethylene}dibenzoic acid

R-0.1.6 Italicization
R-0.1.7 Elision and addition of vowels
R-0.1.8 Order of prefixes

This HTML reproduction is as close as possible to the published version [see IUPAC, Commission on Nomenclature of Organic Chemistry. A Guide to IUPAC Nomenclature of Organic Compounds (Recommendations 1993), 1993, Blackwell Scientific publications, Copyright 1993 IUPAC]. If you need to cite these rules please quote this reference as their source.

Published with permission of the IUPAC by Advanced Chemistry Development, Inc.,, +1(416)368-3435 tel, +1(416)368-5596 fax. For comments or suggestions please contact