Nomenclature of Organic Compounds Nomenclature of Aliphatic/Acyclic compounds Line-Angle Formula (Skeletal Structure or Stick Figure): Very handy for writing organic structures, mostly for cyclic compoounds. Carbon atoms are assumed to be present wherever two lines meet or a line begins or ends. N, O, X, S (heteroatoms) are shown while C, H are not shown. H is shown onto a heteroatom. O O CH3 C NH NH CH OH OH CH CH H3C = CH3 C CH3 H3C CH3 Alkanes Rule: Longest continuous carbon chain bearing maximum number of branches, numbered in the direction in which any alkyl branch is met first (Lowest Set of Locant Rules). Name of alkyl branches written first with alphabetic order followed by Word Root + ane(primary suffix). The first letter of the complex branch is considered for alphabetic ordering. For simple branch the multiplying prefixes di-, tri-, tetra- are not considered for alphabetic ordring. Complex branches are multiplied with the prefix bis, tris, tetrakis, pentakis etc. Locants are separated by commas and no space in between branches and parent alkane. If the set of locants is same from both side, then the alphabetically senior branch gets the lower locant. branches + alk + ane Examples: Lower Set of Locants Rule: CH3 CH3 7 5 2 4 CH CH 8 6 CH2 CH2 1 3 CH2 CH2 4 CH 5 CH H3C CH CH 3 H3C CH CH 3 3 1 6 8 CH3 CH3 CH2 2 CH3 CH2 7 CH3 CH2 CH2 4-ethyl-6,7-dimethyloctane(wrong) 5-ethyl-2,3-dimethyloctane(correct) CH3 2 4 6 8 10 1 CH2 3 CH 5 CH2 7 CH2 9 CH3 9 7 5 3 1 H3C CH CH2 CH2 CH 10 8 6 4 2 CH3 CH3 3,4,9-trimethyldecane (wrong) (not lower set: begininng with 3) 2,7,8-trimethyldecane (correct) (Lower set: beginning with 2) Dr. S.S.Tripathy Nomenclature of Organic Compounds Same Set of Locants: Alphabetic Serniority: CH3 3-methyl-4-ethylhexane (wrong) 5 CH2 1 3 3-ethyl-4-methylhexan (correct) CH2 4 CH 2 CH3 2' 6' 6 CH 4' H3C 3' CH2 1' 5' CH3 Branching Rule: CH3 CH3 2 4 6 2 4 CH 7 CH 1 3 CH2 CH2 1 3 CH2 CH2 5 CH 5 CH H3C CH CH 3 H3C CH CH 3 6 CH3 (wrong) CH3 (correct) CH CH 7 CH3 CH3 CH3 CH3 2,3-dimethyl-5-propan-2-ylheptane(wrong) 5-ethyl-2,3,6-trimethylheptane(correct) The first one violates the branching rule. In 2nd one, the longest chain has largest number branches attached to it. 8' 7' CH3 H C CH CH 8 3 2' 3 2 4 6 CH3 7 CH CH 5 CH 1' 3 CH H 3 C CH CH 2 C CH3 CH3 H C 3 1 CH3 CH3 6-ethyl-2,2,4,5-tetramethyl-3,7-bis(propan-2-yl)octane N.B: propan-2-yl is a complex branch to be discussed below. Line Structure of Alkanes: 1 H3C 2 CH3 CH CH3 7 3 5 CH3 CH 4 CH 6 CH2 8 = CH2 CH CH CH3 CH2 CH3 CH3 3,4-diethyl-2,5,6-trimethyloctane SAQ I. 1: Give the IUPAC names of the following: CH3 CH2 CH3 H3CCHCH2 CH3 CH CH CH CH3 3 CH2 (i) H3C CH (ii) (iii) H3CC CH3 CH3 CH2 CH3 CH3 Dr. S.S.Tripathy Nomenclature of Organic Compounds CH3 H3C CH CH3 3 CH3 CH3 CH CH2 H CCCH H CC CCH (iv) (v) 3 3 (vi) 3 3 H3C CH CH CH3 CH3 CH3 CH2 CH2 CH3 (vii) (viii) SAQ I.2: Give the structural formula of the following compounds. (i)2,3-dimethylpentane (ii)3-methylhexane (iii)2,2,4-trimethylhexane (iv)2-methylpropane SAQ I.3: Some incorrect names are given. Write the structure of alkanes from them. Also give the correct names. (i)3-methylbutane (ii)2-ethylpropane (iii)4,5-dimethylhexane Naming a complex branch: Latest Rule: Numbering made from one terminal to the other giving lower locant to -yl. When locant of -yl is 1, then the suffix ‘ane’ is replaced by ‘yl’ and the loacnt 1 is dropped. For example, propyl, butyl, etc instead of prop-1-yl, but-1-yl. Simple alkyl groups like methyl, ethyl, propyl, butyl in which the locant of -yl is 1 are called simple branches. All other alkyl branches containing subbranches of simple alkyl groups like 2-methylpropyl or having locant of -yl other than 1 like propan-2-yl etc. are considered as complex branches. The first letter of the complex branches is considered for alphabetic numbering which includes di-, tri-etc. Such branches are multiplied by the prefixes bis, tris, tetrakis etc. When locant of ‘yl’ is not 1, then letter ‘e’ of ‘ane’ is replaced by ‘yl’,with the locant of ‘yl’ prefixing it. For example propan-2-yl etc. 3 2 3 2 1 CH3 CH CH3 CH2 CH2 1 CH 3 propyl propan-2-yl 4 3 2 4 3 2 1 CH3 CH2 CH CH3 CH2 CH2 CH2 1 CH3 butyl butan-2-yl 1 CH CH3 3 CH3 3 1 3 3 2 2 1 2 CH CH CH CH C CH CH3 C 3 2 3 2 CH CH3 2-methylpropyl 3 2-methylpropan-2-yl 2,2-dimethylpropyl Dr. S.S.Tripathy Nomenclature of Organic Compounds Old Rule: The carbon atom of the alkyl branch directly bonded to the principal carbon chain is numbered 1’ to find the longest chain. Ultimately prime over the locant is dropped once it is put insidie parenthesis. CH CH CH 3 3 3 2' 1' CH3 C CH3 CH CH3 CH2 CH 2' 1' 3' 2' 1' CH3 1-methylethyl 1-methylpropyl 1,1-dimethylethyl isopropyl(propan-2-yl) sec-butyl(butan-2-yl) tert-butyl(2-methylpropan-2-yl) CH3 CH3 3' 2' 1' CH3 C CH2 CH3 CH CH2 3' 2' 1' CH3 2- methylpropyl 2,2-dimethylpropyl isobutyl neopentyl 2-methylpropyl and 2,2-dimethylpropyl remain the same for both old and new conventions. N.B: Many text books still use the old convention of naming a complex alkyl branch(1,1-dimethylethyl). The names like isobutyl, isopropyl, sec-butyl, tert-butyl, neopentyl are also used in some texts in IUPAC naming. So the student should not take much critically on this aspect. Remember that a structure can have more than one IUPAC names out of which one is PIN(Preferred IUPAC Name) as per the latest conventions but one name can have a unique structure. Two branches having same initial words: When two groups have same initial words(or letters), the longer branch gets the higher locant. Beween methyl and methylpropyl the former gets a lower locant. 3' 1' 2 4 2' 10 12 3 5 1 6 7 8 2' 9 1' 3' 11 6-methyl-8-(2-methylpropan-2-yl)-5-(2-methylpropyl)dodecane (latest) 8-(1,1-dimethylethyl)-6-methyl-5-(2-methylpropyl)dodecane (old) 8-(tert-butyl)-5-isobutyl-6-methyldodecane (old) If you write according to old system, then for complex branch the prefixes di-, tri- are considered for alphabetic ordering as in the new convention. Using the common names of the alkyl groups, the prefix tert- and sec- are not considered while the prefixes iso and neo are considered for alphabetic ordering. 3' 1' 2' 2 4 10 12 3 5 1 6 7 8 9 11 6,6-bis(2,2-dimethylpropyl)dodecane 2' 1' 3' A complex branch has to be multiplied with the prefixes bis, tris etc. (not di, tri etc.) Dr. S.S.Tripathy Nomenclature of Organic Compounds SHORT REPRESENTATIONS OF SOME COMMON ALKYL GROUPS: The following abbreviations are used for the alkyl groups written against their names. methyl Me isobutyl i-Bu ethyl Et sec-butyl s-Bu n-propyl n-Pr tert-butyl t-Bu isopropyl i-Pr n-butyl n-Bu SAQ I.4: Give the IUPAC names of the following compounds shown in line drawings. (i) (ii) (iii) (iv) (v) (vi) (vii) (viii) (Rule for naming compound (viii): When two or more longest chains containing same number of branches and same set of locants for the branches compete with each other, IUPAC recommends that the longest chain which contains greatest number of carbon atoms in the branches is correct. (ix) (ix) (x) Dr. S.S.Tripathy Nomenclature of Organic Compounds (xi) (xii) (xiii) SAQ I.5: Draw the line structures for the following molecules. (i) 5-(2-methylpropy)dodecane (ii) 4-(propan-2-yl)-4-propylnonane (iii) 5-(butan-2-yl)-5-(propan-2-yl)decane (iv) 4,4-bis-(2-methylpropan-2-yl)heptane (v) 3-ethyl-6-(5-methylhexan-3-yl)-8-(propan-2-yl)undecane Answers to SAQs SAQ I.1 (i) 2,3-dimethylpentane(ii)3-methylpentane 5 CH3 4 CH CH3 3 (iii) H3CC CH3 3,3,4-trimethylpentane(wrong); the set of locants =3,3,4 which is higher 1 2 CH2 CH3 2,3,3-tripmethylpentane(correct) , The set of locants 2,3,3 is lower and is correct.. 1 7 CH3 7 CH3 H3C H3C 2 6 CH 6 CH3 3 CH CH CH2 CH2 1 3 3 5 5 (iv) CH (correct); H C 2 CH (wrong) H3C 4 CH 3 4 CH CH2 CH CH2 CH2 CH3 CH2 3 The second one is violating the branching rule. In the first, there are more number of branches attached to the main chain. Count for yourself to verify.