Page 1161 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

1146 (d) CH
(a) H 3
CHAPTER 12
Photochemistry CH 3
H H O
CH 3
O
(b) CH 2 CH HC CH 2
(e)

(c)
CO CH 3 (f)
2

12.3. Suggest reasonable mechanisms for the following observations:

a. The optically active allene, 2,3-pentadiene, is racemized under toluene-
sensitized photolysis.
b. Direct photolysis of diene 3-A at 254 nm produces a photostationary state
containing about 30% 3-A and 70% 3-B. When photolysis is carried out
at 300 nm, photocyclization to 3-C occurs and little 3-B is present in the
mixture.
H H
H
hv hv
300 nm 254 nm
H 3-A H 3-B
3-C H

c. Photocyclization of the acetal 3-D to 3-E is catalyzed by benzoic acid.
O O
H
hv, benzene
H O PhCO H O
2
H
3-D O O 3-E O O
Ph Ph
12.4. Provide a mechanistic rationalization for each of the following reactions:

a.
O
1) hv
CH 3
pentane CH + (CH ) C CHCH CHCH O
3
CH 3 2) CH OH CH 3 CH CO CH 3 3 2
2
2
3
b.
(CH ) CH CHCH 3
2 3
hv
H
CH
H 3