120 CHAPTER 3 Alkenes• Thermodynamics and KineticsThe greater the atomic number of the
atom bonded to an carbon, the high-
er is the priority of the substituent.sp^2Br has a greater atomic number than H, so Brhas a higher priority than H. The
other carbon is bonded to a Cl and to a C. Cl has the greater atomic number,
so Clhas a higher priority than C. (Notice that you use the atomic number of C,
not the mass of the group, because the priorities are based on the atomic
numbers of atoms,noton the masses of groups.) The isomer on the left has the
high-priority groups (Br and Cl) on the same side of the double bond, so it is the
Zisomer. (Zee groups are on Zee Zame Zide.) The isomer on the right has the
high-priority groups on opposite sides of the double bond, so it is the Eisomer.- Rule 2.If the two substituents bonded to an carbon start with the same atom
(there is a tie), you must move outward from the point of attachment and consid-
er the atomic numbers of the atoms that are attached to the “tied”atoms.
In the following compounds, one of the carbons is bonded both to a Cl
and to the C of a group:
Cl has a greater atomic number than C, so the Cl group has the higher priority.
Both atoms bonded to the other carbon are C’s (from a group and an
group), so there is a tie at this point. The C of the group is
bonded to O,H, and H, and the C of the group is bonded to C,C, and
H. Of these six atoms, O has the greatest atomic number, so has a high-
er priority than (Note that you do not add the atomic numbers; you
take the single atom with the greatest atomic number.) The Eand Zisomers are
as shown.- Rule 3.If an atom is doubly bonded to another atom, the priority system treats
it as if it were singly bonded to two of those atoms. If an atom is triply bonded to
another atom, the priority system treats it as if it were singly bonded to three of
those atoms.
For example, one of the carbons in the following pair of isomers is bond-
ed to a group and to a group:
Because the atoms immediately bonded to the carbon are both C’s, there is a
tie. The first carbon of the group is bonded to C,H, and H. The first
carbon of the group is bonded to an H and doubly bonded to a C.
Therefore, it is considered to be bonded to C,C, and H. One C cancels in each
of the two groups, leaving H and H in the group and C and H in the
group. C has a greater atomic number than H, so has a
higher priority than Both atoms that are bonded to the other car-
bon are C’s, so there is a tie there as well. The triple-bonded C is considered to
be bonded to C,C, and C; the other C is bonded to O,H, and H. Of the six
atoms, O has the greatest atomic number, so has a higher priority than- Rule 4.In the case of isotopes (atoms with the same atomic number, but differ-
ent mass numbers), the mass number is used to determine the relative priorities.
C‚CH.CH 2 OHCH 2 CH 3. sp^2CH“CH 2 CH“CH 2CH 2 CH 3CH“CH 2CH 2 CH 3sp^2CH CH 2CH 2 CH 3C
HC CC
HC CCHOCH 2 CH 2 CH 3
C
CH CH 2HOCH 2the Z isomer the E isomerCH 2 CH 3 CH“CH 2sp^2CH(CH 3 ) 2.CH 2 OHCH(CH 3 ) 2CH(CH 3 ) 2 CH 2 OHsp^2 CH 2 OHClCH 3CHCH 3CH 2 OHthe Z isomer the E isomerC CClCH 2 ClCH 3CHCH 3CH 2 OHC C
ClCH 2CH 2 Clsp^2sp^2CH 3sp^2If the atoms attached to carbons are
the same, the atoms attached to the
“tied” atoms are compared; the one
with the greatest atomic number
belongs to the group with the higher
priority.sp^2If an atom is doubly bonded to another
atom, treat it as if it were singly bonded
to two of those atoms.
If an atom is triply bonded to another
atom, treat it as if it were singly bonded
to three of those atoms.
Cancel atoms that are identical in the
two groups; use the remaining atoms to
determine the group with the higher
priority.BRUI03-109_140r4 24-03-2003 11:53 AM Page 120