notes on nomenclature Groups in Increasing Order of Sequence Rule Preference 1. 2. 3. 4. 5. 6. 7.
s.
W. C. FERNELIUS
University of South Florida Tompo 3 3 6 2 0
KURT LOENlNG
Chemicol Abrtrocts Service Columbu>,Ohio 4 3 2 1 0
ROY M. ADAMS
Geneva College Beaver Falls, Pennsylvania 1 5 0 1 0
The Sequence Rules A previous notel was devoted to the order for listing related constituents in chemical compounds. The order is either alphabetical or according to the position of an element on a continuous line drawn through the long form of the periodic tahle. There is another scheme for ordering attached groups or ligands which was proposed not for purposes of nomenclature directly but for selecting certain descriptors used in names. This scheme is hased on atomic numbers and extent of substitution. It was proposed for determining the chirality or "handedness" of a molecule. Once established, the Sequence Rules2 have been found useful for other purposes. The Sequence Rules are: 1) The order of precedence of groups attached to a common atom is hased on atomic number; the atom of highest atomic number attached shall he of the highest priority. Thus for the compound CBrClFH the order of priority is Br(35), C1(17), F(9), H(1). With the molecule oriented so that the group of lowest priority is away from the side which includes the other groups (I)
I
n
one draws an arrow from the highest priority group in the sequence, Br, to the next lowest C1, to the last, F. According as the path turns to the right or left, the particular unit, in this case a molecule, is assigned the chiral label R or S. For examole (1). the label would he R. For the compound CHB~(OCH;)COOH (11) the sequence is Br(35), OCHn(8). COOH(6). . . H(1). and the chiral svmhol for example (Ii) is S. When isotopes of the same element are involved, the highest mass number takes precedence. 2) If Rule 1 is inadequate to allow selection of group priority, then the states of substitution are examined. The substituent with atoms of higher atomic number shall have higherpriority.
9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 28.
Hydrogen Methyl Ethyl n-Propyl n-Butyl n-Pentyl n-Heryl Isopentyl Isobvtrl Ally1 Neo~entyl 2-Propynyl Benzyl Impropy1 Vinyl aec-Butyl Cyclohexyl l-Propenyl terl-Butyl Ieopropenyl Acetylenyl Phenyl p-Tolyl p-Nitrohenyl m-Tolyl 3.6-Xylyl
27. 28. 29. 30. 31.
32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 80. 51.
m-Nitrophenyl 52. 3.5-Dinitrophenyl 53. l-Propynyl 54. 55. o-Tolyl 2.6-Xylyl 56. Trityl 57. 0-Nitrophenyl 53. 2.4-Dinitrophenyl 59. 60. Formyl Acetyl 61. Benzoyl 62. Carboxyl 63. Methoxycsrbonyl* 84. Ethoxyearbonyla 65. Benzylorycarbonyl~ 66. ferl-B~tor~esrbonyl"67. Amino 68. Ammonio + H 3 N 69. Methylamino 70. Ethylamino 71. 72. Phenylamino Acetylamino 73. Benzoylamino 74. Fknzyloryearbonyl75. amino 76. Dimethylamino
Diethylamino Trimethylammanlo Phenylazo Nitroso Nitro Hydroxy Methory Ethory Benzyloxy Phenory Glyeosyloay Formyloxy Acetory Benzoylor~ Methylsulhylom Methylsulfonylo~y Fluora Mereapto H S Methylthio CHSMethylsulhyl Methylsulfonyl Sulfo HO6-Chloru Bromo Iado
T h e aroups are ROC(+.
For the compound HC(CeH5)(0H)COOH the order of priorities is OH, COOH, CGHJ,H /--,
I11
n
IY
and the chiral symbol for example (III) is R. Since the C atom of OCHJ is of higher order than the H of OH, example (IV) is S. 3) For multiple-bonded atoms, simple classical forms are used. Double and triple bonds are split into two or three bonds, respectively. A X = O group is treated as V (below) where the ( 0 ) and the (C) are duplicate representations of the atoms a t the other end of the double bond. The group -C-CH is treated as VI and the -C=N is treated as VII.
A multiple number of true single linkages would have precedence over the corresponding multiple bonds
From the above three rules a table of groups in increasing order of sequence rule preference3 may he constructed. 1 Fernelius. W. C.. Loenine, .K. L.. and Adams, R. M., J. CHEM. EDUC., 49,3'33(19723. 2 Cahn, R. S., J. CHEM. EDUC. 41,116-25,503(1964)."Nomenclature of Oreanic Chemistrv." Section E. Fundamental StereoChem . 35, 2863-67(1970). See footnote 19 for chemistry: J - o ~ ~ additional references. 3Any alteratmn to structure, or substitution, etc., may alter the order of preference.
VOlume51. Number 77. November 1974
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There is a great deal more to the sequence rule than the rudimentary presentation given above. However, even this much will enable us to see another application of ordering groups for various purposes. The Octahedral Sequence Rule With only minor modification, the Sequence Rules can be applied to compounds with centers exhibiting coordination numbers greater than 4. The rules necessary for application to octahedral complexes will he examined here. 1) Ordering of ligands by classes: Ligands of higher multiplicity (denticity) precede those of lower. 2) Ordering within each class: A leading ligating atom is chosen for each ligand on the basis of priority and the ligands are ordered as their leading ligating atoms would be ordered by the Sequence Rules. For a linear or branched ligand, the leading ligating atom must be a terminal one. Ligands with terminal ligating atoms take precedence over cyclic ones. 3) Ordering of atoms in a ligand: The ligating atoms of a ligand are ordered according to position, with diminishing precedence from the leading ligating atom along the longest continuous chain of atoms that leads to a terminal ligating atom, and thereafter in the successively situated branches from that chain. The sense of the succession in a cyclic ligand is taken as from the leading ligating atom to that next ligating atom which is preferred by the Sequence Rules, or, in case a choice remains, by following the path preferred by the Sequence Rules. 4) Octahedral numbering: a) The numbers 1and 6 mark trans-positions. b) The numbers 2-5 form a cyclic sequence. C) With the number 1 is associated the ligating-atom which is most preferred by the Octahedral Sequence Rule; or, in case a choice is open, that most preferred ligating atom which allows the least preferred possible to be associated with the number 6; or, if a choice is still open, that which allows more preferred ligating atoms to be associated with lower numbers in the range 2-5. d ) The numbers 2-4 are successively associated with the most preferred of the ligating atoms remaining available to each. Rule 4b does not specify the direction, clockwise or counterclockwise, to assign locant numbers to the median belt of ligands. The numbering schemes below are the mirror images of each other.
WI
I t is easy to show that each of these is an achiral structure since they can he numbered in two completely equivalent ways. For (X) reverse the direction of numbering from clockwise to counterclockwise and the a ligands retain the same numbers; so do the b and c ligands. For (XI) rotate the formula so that the a a t position 3 becomes the a a t position 1 and c at position 6 becomes c at position 3. Then the ordering of the atoms is identical but the previously clockwise direction becomes counterclockwise. For (XII) reversing the a's a t positions 2 and 4 is equivalent to reversing the direction around the median belt. For (XIII) the same order results whether the progress around the belt is clockwise or counterclockwise. For (XIV) rotating the formula so that one B occupies the position formerly occupied by the other B, results in a counterclockwise progress from a - b-c. The following formulas are all chiral.
xv
XVI
X W
M
A chiral structure can then always be represented as one of the enantiomeric forms (VIII) and (IX). The face 1-2-3 is observed from the side remote from the face 4-5-6 (as marked by arrows), and the path 1 2 3 is observed; in (VIII) this path is crosswise (R) and in (IX) it is anticlockwise (S). If, for a given formula, one obtains the same numbering of ligands (the mirror image is identical to the formula reflected) by proceeding both clockwise and counterclockwise around the median belt of ligands the compound is achiral. Consider the following formulas
--
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Journal of Chemical Education
For some of the formulas rearranging the positions occupied by the various ligands does not result in new geometrical species (XXIII, XXVI, XXIX), for others geometrical isomers are possible ((XV) is 15 pairs of enantiomers, (XVI) 4 inactive isomers and a pair of enantiomers, (XVII) 3 inactive isomers and a pair of enantiomers, etc.). 5) Octahedral Chirality Rule. The chiral center of the numbered model is specified as R or S, according as the path of the numbers 1, 2, 3 in order appears right- or lefthanded from the side of the model remote from 4,5,6. According to this rule, all the structures (XV)-(XXIX) are in the R-form. Note: Chirality symbols R and S are used by Chemical Abstracts when given in the original document.
