Homework

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Homework 6 The compound HOCl (hypochlorous
acid) reacts with the compound C6H5CH3 (toluene)
under the influence of ultraviolet light as shown
in the following equation C6H5CH3 HOCl,
hv ? C6H5CH2Cl HOH
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Homolytic bond dissociation energies C6H5CH2H
85 Kcal/mole HOCl 60 Kcal/mole C6H5CH2C
l 68 Kcal/mole HOH 119 Kcal/mole
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• Calculate the enthalpy change for the reaction of
  HOCl with toluene.
• C6H5CH2--H HO--Cl, hv ? C6H5CH2--Cl
  HO--H

85 60
-68 -119
145
-187 ?H 145
187 -42 Kcal/mole
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2. Write a reasonable mechanism for the reaction
of HOCl with toluene and untraviolet light.
step 1) When the reaction starts, only toluene
and HOCl are in the reaction container. Which
bond is easier to break with ultraviolet
light? C6H5CH2H ? C6H5CH2 H 85
Kcal/mole HOCl ? HO Cl 60
Kcal/mole
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Initiating step 1) HOCl ? HO
Cl 60 Kcal/mole
? C6H5CH3 Cl ? C6H5CH2
HCl HCl is not a product of this reaction!
Therefore this step does not occur.
2) C6H5CH3 OH ? C6H5CH2 HOH
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• 2) C6H5CH3 OH ? C6H5CH2 HOH
• then
• C6H5CH2 ClOH ? C6H5CH2Cl
  OH
• then 2), 3), 2), 3) Propagating steps.

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• Terminating steps
• 4) HO Cl ? HOCl
• C6H5CH2 Cl ? C6H5CH2Cl
• etc.

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• Mechanism
• initiating step
• HOCl ? HO Cl
• propagating steps
• 2) C6H5CH3 OH ? C6H5CH2 HOH
• C6H5CH2 ClOH ? C6H5CH2Cl
  OH
• terminating steps
• HO Cl ? HOCl
• C6H5CH2 Cl ? C6H5CH2Cl
• etc.

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3. Calculate the enthalpy changes for each step
in the mechanism.

• HOCl ? HO Cl
• 60 ?H 60
• C6H5CH3 OH ? C6H5CH2 HOH
• 85
  -119 ?H -34
• C6H5CH2 ClOH ? C6H5CH2Cl
  OH
• 60
  -68 ?H -8
• 4) HO Cl ? HOCl
• -60
  ?H -60

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4. Draw a graph of the potential energy changes
for the third step in the mechanism. Label all
parts.

potential energy ?
Eact
?H
C6H5CH2 ClOH ? C6H5CH2Cl OH
progress of the reaction ?
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5. Estimate the Energies of Activation (Eact)
for each of the propagating steps in the
mechanism.

• C6H5CH3 OH ? C6H5CH2 HOH
• 85
  -119 ?H -34
• C6H5CH2 ClOH ? C6H5CH2Cl
  OH
• 60
  -68 ?H -8

step 2) ?H -34 Kcal/mole estimated Eact gt
0 step 3) ?H -8 Kcal/mole estimated Eact gt 0
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6. Which step in the mechanism is the rate
determining step?
In a chain mechanism, the rate determining step
is the slowest propagating step in the mechanism.
Which of the propagating steps is slower?
step 2) ?H -34 Kcal/mole estimated Eact gt
0 step 3) ?H -8 Kcal/mole estimated Eact gt 0
Cannot tell from the available information!
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7) Draw a hypothetical structure for the
transition state for step 2) of the mechanism.

• C6H5CH2--H OH ? C6H5CH2 H--OH

C6H5CH2-------H-------OH d
d