The IUPAC Rules for Naming Organic Molecules

1 In the Vol. 83 No. 11 November 2006 Journal of Chemical Education1633 Textbooks used for teaching undergraduate courses pro-vide basic Rules and examples for Naming monofunctionalcompounds. However, students of more advanced organicchemistry courses and graduate students dealing with morecomplex Molecules need a set of additional Rules and guide-lines to identify polyfunctional compounds. The followingsummary of general principles may also be useful to instruc-tors. The formation of the systematic name for an organiccompound involves several steps, to be taken as far as theyare applicable in the following order (1 5).Step 1 Determine the principal functional group in the com-pound. When a compound contains more than one groupin Table 1, the principal group is that which has the highestprecedence. This principal group will be cited as a suffix; allother groups are cited as 2 Determine the parent hydrocarbon (principal chain orparent ring system):(a) If the principal group occurs in a chain, the principalchain is selected as(i) the chain containing the functional group of thehighest seniority as the parent.

2 (ii) If more than one such a choice is possible, selectthe chain with the maximum number of multiplebonds.(iii) If (i) and (ii) together are not definitive, thenchoose the chain with maximum length.(iv) If two chains of the same length are possible, choosethe one with maximum number of (butan-1-yl)-4-methylhexanalOHThe parent chain here contains six carbon IUPAC Rules for Naming Organic MoleculesWStanislaw SkoniecznyDepartment of Chemistry, University of Toronto, Toronto, Ontario, M5S 3H6, Canada; Nbelirtin----C- Nelirtinobrac--onaycOH)C(-bla--oxoOHC-ed yhedlabrac--lymrof)C(> Oeno--oxoHO-lo-c-yxordyhHS-loiht--lynafl usdHN-2)enima-(enaza-d)-onima(-lynazaeHN >)enimi-(enaza-d)-onimi(-enedilynazaeC> <Cene-*C- -Ceny-**RO-,R-,X-**-yxokla,-lykla,-olahO N-,rA-2,**-ortin,-lyraN-,ON-3,**-odiza,- osortin N2RS-,**(,-ozaidRlynaflus-) )C(tonseodti(edirdyhtnerapehtfoemanehtni dedulcnimotanobrac.) ,slonehplanoitisopriehtdna,lorhtnanehp-1 ,lorhtna-1,lohthpan-1,lonehpllitseradica circipdna,lomyht,losercsahcus, , , ,'-otpacrem'otderreferpsi'-lynaflus'etub ,enimidnaenima-otevitanretlanasaenaza-sd nemmocerCAPUI ecnerefer,seugolatac, )-onima(enima-esullitserutaretilcifitnei csdna,skoob, *.

3 Elbatpeccaerahcihw,)-onimi(sixiffuseht,e lucelomehtnitneserpsipuorgytiroirprehgih afiytiroirprehgihehtfoxiffusehtybdewollo fsitidna-ne-otdegnahcpuorgytiroirprehgih afi, **.puorgdewollofsitidna-ny-otdegnahcsixi ffuseht, **.puorgytiroirprehgihehtfoxiffusehtybIn the Classroom1634 Journal of Chemical Education Vol. 83 No. 11 November 2006 (b) If the principal functional group occurs in a cyclic sys-tem, that system forms the (octan-1-yl)cyclohexanolOHStep 3 Name the parent structure and the principal group(s).Give the parent the same name as if it were an alkane, butreplace -ane with the suffix characteristic of the functionalgroup of the highest priority (Table 1). Note: Multiple un-saturation in hydrocarbons is indicated by the numerical pre-fixes di-, tri-, etc. In such cases, however, the ending -ane ofthe parent alkane is replaced with -adiene, -atriene, etc. leav-ing the a in the root alkane whenever the first letter of thesuffix is a consonant to make the name easier to we get alkadienes, alkatriens, alkadiynes, 4 Complete the numbering.

4 (a) Number the carbon atoms consecutively from the endof the chain nearer the functional group of the high-est ,5-dimethylhexan-3-one123546O(b)If the same number for the functional group of highest pri-ority is obtained in both directions, the correct IUPAC nameis the one that contains the lowest substituent ,4-dimethylhex-3-ene123546 First substituent has number 2. If we numbered the chainfrom the other side, the first substituent would have num-ber 3.(c) If first substituents occur at an equal distance from eachend of the chain, number from the end nearer a second sub-stituent. If still the same, continue until first point of dif-ference.(E)-2,3,5-trimethylhex-3-ene 123546(d) If numbering from either side gives the same set of locants,number from the end with the substituent that comes firstin alphabetical order.(e) In monosubstituted cycloalkanes and benzene derivatives,the carbon to which the substituent is bonded is alwaysnumber 1. If there is only one substituent, the locant 1 isomitted.

5 However, it is necessary in polysubstituted com-pounds. (f)In polysubstituted cyclic compounds, the starting point andthe direction of numbering are chosen to give lowest lo-cant to the following factors (if present), considered suc-cessively in the order listed below until a decision is reached.(i) The principal functional group named as suffix is alwaysgiven lowest possible number. When the numbering is pre-determined by the nature of the parent hydrocarbon, as inpolycyclic hydrocarbons and heterocyclic compounds, low-est locants are still the rule. Note: When one heteroatom is present in the ring, the lo-cant 1 is attributed to the heteroatom. If two or morekinds of heteroatom occur in the same ring, the number-ing starts from the heteroatom cited first in the followingdescending order: O, S, Se, N, P, As, Sb, Si, Ge, Sn, Pb, B,and Hg. Other heteroatoms get other lowest possiblelocants.

6 1,2-oxathiolane15423OS 1,3,5-triazine123564 NNNIn the Vol. 83 No. 11 November 2006 Journal of Chemical Education1635(ii) Lowest locant for multiple bonds in cyclic compounds. Note: In Naming cycloalkenes, the double bond is locatedbetween C1 and C2, and the 1 is usually omitted in thename. The ring is numbered clockwise or counterclockwiseto give the first substituent the lower number. However,when a higher seniority functional group is present, the car-bon bonded to such a group becomes C1 and then mul-tiple bonds are considered for lowest possible locants.(iii) Lowest locant for a first substituent (or first point of dif-ference). (iv) If the same numbers will result from numbering in eitherdirection, give the lowest locant for that substituent whichis cited first. Step 5 Name alkyl groups, halides, and other substituents anddetermine their position on the chain by the numbering es-tablished by step 6 Assign the stereochemistry to chiral carbon atoms (ste-reocenters) and the double bond(s):(a) Disubstituted alkenes may be named in two ways: (i)using terms cis- and trans- or (ii) using terms (E) or(Z).

7 Note: Use of (E Z) stereodescriptors is , the cis trans terms are still accepted. (b) Tri- and tetra-substituted alkenes may only be named us-ing the (E Z) system.(c) Disubstituted cycloalkanes may be named using the termscis trans or (R S).(d) Chiral carbon atoms are assigned (R) or (S) configuration.(S)-5-methylhexan-3-olHOH( e)When both (R S) and (E Z) stereodescriptors are present,they are placed in parentheses followed by a hyphen; eachstereodescriptor is immediately preceded by the lower or lessprimed locant and they are arranged in numerical order. In the Classroom1636 Journal of Chemical Education Vol. 83 No. 11 November 2006 1. Some examples of IUPAC names for commonly known compoundsStep 7 Compounds with multiple double and triple bonds havethe following suffixes: -adiene, -adiyne, -atriene, -atetraene, andso on. Specify the location of each multiple bond by a locantplaced between -a and -diene, -diyne, -triene, 8 Compounds with both double bonds and triple bondsare called enynes.

8 Start the numbering of compounds withboth double and triple bonds from the end nearer the firstmultiple bond, regardless of type. When a double bond isthe same distance from one end as a triple bond from theother end, assign the double bond the lower number. Step 9 Write the complete name of the compound as a singleword with the correct locants for all substituents, which arelisted in alphabetical order. The stereochemistry is indicatedby placing the appropriate prefix within parentheses followedby a hyphen in front of the prefixes di-, tri-, tetra-, etc. do not alter the alphabeticalordering of substituents.(a) The numbers indicating the locations of substituentsare separated by commas.(b) The letters and numbers are joined by :Trivial (retained):azabenzenepyridineIUPAC:Trivi al (retained):phenylazanebenzenamineaniline IUPAC :Trivial (retained):benzenecarboxylic acidbenzoic acidmethyl benzenecarboxylatemethyl benzoate1-phenylpropan-1-onepropiophenon e2-methylphenylazane2-methylbenzenamineo -toluidinenaphthalen-2-ylazanenaphthalen -2-amine2-naphthylamine1,2-dehydrobenzen ebenzynebenzenolphenolIn the Vol.

9 83 No. 11 November 2006 Journal of Chemical Education1637 Step 10A complex substituent is named by applying the abovesteps just as if the substituent were a compound itself. Webegin numbering at the point of attachment to the parenthydrocarbon and we give this substituent the suffix -yl andput its name in parentheses when the name of the completecompound is : There are many traditional names accepted by IUPACand retained because of their wide use in Organic , biochemi-cal, and polymer nomenclature. A few examples are givenhere: toluene, styrene, phenol, aniline, acetone, acetylene,acetic acid, benzoic acid, and phthalic acid (Figure 1). A listof such compounds can be found on the Web (6).Other names are retained for referring to unsubstitutedcompounds only. Compounds derived from them by substi-tution must be named systematically. For example, the namesbutanoic acid (systematic) and butyric acid (retained) are bothapproved by IUPAC for the unsubstituted acid.

10 However,BrCH2CH2CH2 COOH must be named systematically as 4-bromobutanoic acid. Some other examples of such names re-tained for unsubstituted compounds only include lactic acid,toluidine, anisole, glycerol, propionic acid, isoprene, cumene,pinacol, and many MaterialExamples of names of Organic compounds, seniority ofchains in Naming , seniority of ring systems in Naming , andthe Hantzsch Widman sytem of Naming are available in thisissue of JCE extended Hantzsch Widman system used to name het-erocycles is presented in the Supplemental , R. B.; Powell, W. H. Nomenclature of Organic Compounds,Principles and Practice, 2nd ed.; Oxford University Press: Ox-ford, Hellwinkel, D. Systematic Nomenclature of Organic Chemistry,A Directory to Comprehension and Application of its Basic Prin-ciples; Springer: Berlin, New York, London, Leigh, G. J.; Favre, H. A.; Metanomski, W. V. Principles ofChemical Nomenclature, A Guide to IUPAC Recommendations;Blackwell Science: Oxford, Panico, R.