Page 400 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

O OH 381
d. k 1
PhCCH(CH ) PhC C(CH ) 2 PROBLEMS
3 2
k –1 3
OH
k
+ Cr(VI) 2 products
)
PhC C(CH 3 2
–1
2
making no assumptions about the relative magnitude of k 1 , k , or k .
3.9. One method of estimating “aromatic stabilization” is to compare the heats of
hydrogenation of cyclic conjugated systems with an acyclic molecule having
the same number of conjugated bonds and electrons. The table below gives
the AM1 calculated heat of hydrogenation for various cyclic conjugated systems
and for the corresponding polyene [ H H2 (ref.)] For example, for benzene the
comparison would be with 1,3,5-hexatriene. Calculate the aromatic stabilization
or antiaromatic destabilization for each system. What conclusions do you draw?
Relate your conclusions to HMO theory in Chapter 1. How does this computation
deal with “strain?”

System H H2 (ref.) H H2 (cyclic system)
Cyclopropenyl cation −13 3 10 6
Cyclopropenyl anion 4 7 −59 6
Cyclobutadiene −50 7 −101 9
Cyclopentadienyl cation −19 4 −72 9
Cyclopentadienyl anion −0 3 −2 9
Benzene −72 2 −45 0
Cycloheptatrienyl cation −31 4 −23 9
Cycloheptatrienyl anion −11 4 −26 4

3.10. A number of experimentally inaccessible H values have been computed at
the G2(MP2) level and are given below in kcal/mol. Taking for comparison
H H2 of ethene as −32 4kcal/mol, of 1-butene as −30 3kcal/mol, and of
cyclopropane to propane as −38 6kcal/mol, indicate what factors lead to the
observed differences in each step of the sequences shown.
a. CH 3 H 2 CH 3 H 2 CH 3

– 62.15 –29.9
CH
CH 3
b. H 2 3 H 2 CH 3
–46.2 CH 3 –37.6 CH 3 CH 3

c. H 2
–3.0 –13.3 –7.7 –28.3
3.11. The Cannizzaro reaction is a disproportionation that takes place in strongly basic
solution and converts aromatic aldehydes to the corresponding benzyl alcohol
and sodium benzoate.
2ArCH=O+NaOH −→ ArCH OH+ArCO Na
2
2