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

Homolytic bond dissociation energies: C6H5CH2—H 85 Kcal/mole HO—Cl 60 Kcal/mole C6H5CH2—Cl 68 Kcal/mole HO—H 119 Kcal/mole

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 ΔH = + 145 – 187 = -42 Kcal/mole

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

Initiating step: 1) HO—Cl  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

2) C6H5CH3 + •OH  C6H5CH2• + HOH then: C6H5CH2• + ClOH  C6H5CH2Cl + •OH then 2), 3), 2), 3)… Propagating steps.

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

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

3. Calculate the enthalpy changes for each step in the mechanism. HO—Cl  HO• + •Cl +60 ΔH = +60 C6H5CH3 + •OH  C6H5CH2• + HOH +85 -119 ΔH = -34 C6H5CH2• + ClOH  C6H5CH2Cl + •OH +60 -68 ΔH = -8 4) HO• + •Cl  HOCl -60 ΔH = -60

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 

5. Estimate the Energies of Activation (Eact) for each of the propagating steps in the mechanism. C6H5CH3 + •OH  C6H5CH2• + HOH +85 -119 ΔH = -34 C6H5CH2• + ClOH  C6H5CH2Cl + •OH +60 -68 ΔH = -8 step 2) ΔH = -34 Kcal/mole; estimated Eact > 0 step 3) ΔH = -8 Kcal/mole; estimated Eact > 0

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!

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