preface to fifteenth edition

ORGANIC COMPOUNDS 1.45
group at the first point of difference is the determining factor. For example,
(2Z,5E)-2,5-Heptadienedioic acid
Rule 1. Priority is assigned to atoms on the basis of atomic number. Higher priority is assigned
to atoms of higher atomic number. If two atoms are isotopes of the same element, the atom of higher
mass number has the higher priority. For example, in 2-butene, the carbon atom of each methyl
group receives first priority over the hydrogen atom connected to the same carbon atom. Around the
asymmetric carbon atom in chloroiodomethanesulfonic acid, the priority sequence is I, Cl, S, H. In
1-bromo-1-deuteroethane, the priority sequence is Cl, C, D, H.
Rule 2. When atoms attached directly to a double-bonded carbon have the same priority, the
second atoms are considered and so on, if necessary, working outward once again from the double
bond or chiral center. For example, in 1-chloro-2-methylbutene, in CH 3 the second atoms are
H, H, H and in CH 2 CH 3 they are C, H, H. Since carbon has a higher atomic number than hydrogen,
the ethyl group has the next highest priority after the chlorine atom.
(Z)-1-Chloro-2-methylbutene
(E)-1-Chloro-2-methylbutene
Rule 3. When groups under consideration have double or triple bonds, the multiple-bonded
atom is replaced conceptually by two or three single bonds to that same kind of atom.
A
A
Thus, "A is considered to be equivalent to two A's, or and #A equals A. However, a
A
A
A
A
real has priority over "A; likewise a real A has priority over #A. Actually, both atoms of
A
A
C9O
a multiple bond are duplicated, or triplicated, so that C" O is treated as , that
O C
C9O O9C
C
N
is and , and C# N is treated as . A phenyl carbon
(O) (C)
(N) (N) (C) (C)
CH
becomes 9C C . Only the double-bonded atoms themselves are duplicated, not the atoms or
CH
groups attached to them. The duplicated atoms (or phantom atoms) may be considered as carrying
atomic number zero. For example, among the groups OH, CHO, CH 2 OH, and H, the OH group has
the highest priority, and the C(O, O, H) of CHO takes priority over the C(O, H, H) of CH 2 OH.
1.1.4.4 Chirality and Optical Activity. A compound is chiral (the term dissymmetric was formerly
used) if it is not superimposable on its mirror image. A chiral compound does not have a plane of
symmetry. Each chiral compound possesses one (or more) of three types of chiral element, namely,
a chiral center, a chiral axis, or a chiral plane.