Organic Chemistry

Section 9.3 Factors that Determine Product Distribution 341

the overall reaction is hydrogen atom abstraction, we conclude that it must be easier
to abstract a hydrogen atom from a secondary carbon than from a primary carbon.
Alkyl radicals have different stabilities (Section 4.10), and the more stable the
radical, the more easily it is formed because the stability of the radical is reflected in
the stability of the transition state leading to its formation. Consequently, it is easier
to remove a hydrogen atom from a secondary carbon to form a secondary radical than
it is to remove a hydrogen atom from a primary carbon to form a primary radical.

When a chlorine radical reacts with butane, it can abstract a hydrogen atom from an
internal carbon, thereby forming a secondary alkyl radical, or it can abstract a hydro-
gen atom from a terminal carbon, thereby forming a primary alkyl radical. Because it
is easier to form the more stable secondary alkyl radical, 2-chlorobutane is formed
faster than 1-chlorobutane.

After experimentally determining the amount of each chlorination product obtained
from various hydrocarbons, chemists were able to conclude that at room temperature
it is 5.0 times easier for a chlorine radical to abstract a hydrogen atom from a tertiary
carbon than from a primary carbon, and it is 3.8 times easier to abstract a hydrogen
atom from a secondary carbon than from a primary carbon. (See Problem 17.) The pre-
cise ratios differ at different temperatures.

To determine the relative amounts of products obtained from radical chlorination of
an alkane, both probability(the number of hydrogens that can be abstracted that will
lead to the formation of the particular product) and reactivity(the relative rate at which
a particular hydrogen is abstracted) must be taken into account. When both factors are
considered, the calculated amounts of 1-chlorobutane and 2-chlorobutane agree with
the amounts obtained experimentally.

number of hydrogens × reactivity

6 × 1.0 = 6.0

percent yield =

21

= 29%

relative amount of 1-chlorobutane relative amount of 2-chlorobutane

6.0

number of hydrogens × reactivity

4 × 3.8 = 15

percent yield =

21

(^15) = 71%
tertiary >
5.0
secondary >
3.8
primary
1.0
relative rates of alkyl radical formation by a chlorine radical at room temperature
increasing rate of formation
a secondary 2-chlorobutane
alkyl radical
a primary
alkyl radical
1-chlorobutane
Cl Cl 2
Cl
CH 3 CH 2 CH 2 CH 3
CH 3 CH 2 CHCH 3 CH 3 CH 2 CHCH 3
Cl

• HCl


• HCl
  CH 3 CH 2 CH 2 CH 2 CH 3 CH 2 CH 2 CH 2 Cl
  Cl (^2) + Cl


• Cl

  
  

  R
  RC
  R
  tertiary radical

  
  

  R
  RC
  H
  secondary radical

  
  

  H
  RC
  H
  primary radical
  H
  HC
  H
  methyl radical
  relative stabilities of alkyl radicals
  most
  stable
  least
  stable