222 CHAPTER 5 Stereochemistry
Because the products of the preceding reaction are diastereomers, the transition
states that lead to them are also diastereomeric. The two transition states, therefore,
will not have the same stability, so different amounts of the two diastereomers will be
formed. The reaction is stereoselective—more of one stereoisomer is formed than of
the other.Addition Reactions that Form Products
with Two Asymmetric Carbons
When a reactant that does not have an asymmetric carbon undergoes a reaction that
forms a product with twoasymmetric carbons, the stereoisomers that are formed de-
pend on the mechanism of the reaction.- Addition Reactions that Form a Carbocation Intermediate
or a Radical Intermediate
If two asymmetric carbons are created as the result of an addition reaction that forms a
carbocation intermediate, four stereoisomers can be obtained as products.
In the first step of the reaction, the proton can approach the plane containing the
double-bonded carbons of the alkene from above or below to form the carbocation.
Once the carbocation is formed, the chloride ion can approach the positively charged
carbon from above or below. As a result, four stereoisomers are obtained as products:
The proton and the chloride ion can add from above-above, above-below, below-
above, or below-below. When the two substituents add to the same side of the double
bond, the addition is called syn addition. When the two substituents add to opposite
sides of the double bond, the addition is called anti addition. Both syn and anti addi-
tion occur in alkene addition reactions that take place by way of a carbocation inter-
mediate. Because the four stereoisomers formed by the cis alkene are identical to the
four stereoisomers formed by the trans alkene, the reaction is not stereospecific.
Similarly, if two asymmetric carbons are created as the result of an addition reac-
tion that forms a radical intermediate, four stereoisomers can be formed because
both syn and anti addition are possible. And because the stereoisomers formed by
the cis isomer are identical to those formed by the trans isomer, the reaction is not
stereospecific—in a stereospecific reaction, the cis and trans isomers form different
stereoisomers.Fischer projections of the stereoisomers of the productCH 2 CH 3Cl
CH 3 HCH 3CH 2 CH 3CH 2 CH 3CH 3
CH 3Cl
HHCH 2 CH 3CH 2 CH 3CH 3CH 3 ClCH 2 CH 3CH 2 CH 3CH 3
CH 3Cl
HCH 2 CH 3HCl CH 3 CH 2 CH CCH 2 CH 3CH 3 CH 3Cl+CH 3 CH 2 CH 2 CH 3H 3 C
cis-3,4-dimethyl-3-hexene 3-chloro-3,4-dimethylhexaneCH 3new asymmetric
carbonsCCperspective formulas of the stereoisomers of the productCl
CH 3CCCH 3 CH 2CH 3 CH 2 CH 3
H
CH 3
ClC CCH 2 CH 3CH 3 CH 2 CH 3
H CH
3
ClCCCH 3 CH 2CH 3 CH 2 CH 3
H
Cl
CH 3C CCH 2 CH 3CH 3 CH 2 CH 3
H