1. 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:
C6H5CH HOCl, hv  C6H5CH2Cl HOH

2. Homolytic bond dissociation energies:
C6H5CH2—H Kcal/mole
HO—Cl 60 Kcal/mole
C6H5CH2—Cl 68 Kcal/mole
HO—H 119 Kcal/mole

3. Calculate the enthalpy change for the reaction of HOCl with toluene.
C6H5CH2--H HO--Cl, hv  C6H5CH2--Cl HO--H
ΔH = – 187 = -42 Kcal/mole

4. 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?
C6H5CH2—H  C6H5CH2• •H 85 Kcal/mole
HO—Cl  HO• •Cl 60 Kcal/mole

5. Initiating step:
1) HO—Cl  HO• •Cl 60 Kcal/mole
? C6H5CH •Cl  C6H5CH2• HCl
HCl is not a product of this reaction! Therefore this step does not occur.
2) C6H5CH •OH  C6H5CH2• HOH

6. 2) C6H5CH •OH  C6H5CH2• HOH
then:
C6H5CH2• ClOH  C6H5CH2Cl •OH
then 2), 3), 2), 3)… Propagating steps.

7. Terminating steps:
4) HO• •Cl  HOCl
C6H5CH2• •Cl  C6H5CH2Cl
etc.

8. Mechanism:
initiating step:
HO—Cl  HO• •Cl
propagating steps:
2) C6H5CH •OH  C6H5CH2• HOH
C6H5CH2• ClOH  C6H5CH2Cl •OH
terminating steps:
HO• •Cl  HOCl
C6H5CH2• •Cl  C6H5CH2Cl
etc.

9. 3. Calculate the enthalpy changes for each step in the mechanism.
HO—Cl  HO• •Cl
ΔH = +60
C6H5CH •OH  C6H5CH2• HOH
ΔH = -34
C6H5CH2• ClOH  C6H5CH2Cl •OH
ΔH = -8
4) HO• •Cl  HOCl
ΔH = -60

10. progress of the reaction 
4. Draw a graph of the potential energy changes for the third step in the mechanism. Label all parts.
potential energy 
Eact ΔH
C6H5CH2• + ClOH  C6H5CH2Cl + •OH
progress of the reaction 

11. 5. Estimate the Energies of Activation (Eact) for each of the propagating steps in the mechanism.
C6H5CH •OH  C6H5CH2• HOH
ΔH = -34
C6H5CH2• ClOH  C6H5CH2Cl •OH
ΔH = -8
step 2) ΔH = -34 Kcal/mole; estimated Eact > 0
step 3) ΔH = -8 Kcal/mole; estimated Eact > 0

12. 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) ΔH = -34 Kcal/mole; estimated Eact > 0
step 3) ΔH = -8 Kcal/mole; estimated Eact > 0
Cannot tell from the available information!

13. 7) Draw a hypothetical structure for the transition state for step 2) of the mechanism.
C6H5CH2--H •OH  C6H5CH2• H--OH ‡
C6H5CH H OH
δ• δ •