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850 Chapter 28
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c.
Under photochemical conditions, a compound with four conjugated
p
bonds undergoes disrotatory
ring closure.
d.
Because ring closure is disrotatory and the substituents point in opposite directions, the product will
have the cis configuration.
7.
a.
correct
b.
correct
c.
correct
8.
1. a.
conrotatory
b.
trans
2. a.
disrotatory
b.
cis
9.
Solved in the text.
10.
The reaction of maleic anhydride with 1,3-butadiene involves three
p
bonds in the reacting system.
Such a reaction under thermal conditions involves suprafacial ring closure.
O
O
O
1,3-butadiene
The reaction of maleic anhydride with ethylene involves two
p
bonds in the reacting system, and such
a reaction under thermal conditions involves antarafacial ring closure, which cannot occur with a four-membered ring.
O
O
O
ethene
11.
Because we are dealing with formation of a small ring, ring closure must be suprafacial. Under
photochemical conditions, suprafacial ring closure requires an even number of
p
bonds in the reacting
system. Therefore, a concerted reaction will occur but it will use only one of the
p
bonds of 1,3-butadiene.
CH
2
CH CH
+
CH
2
O
CH CH
2
hv
O
12.
a. 1.
[1,7] sigmatropic rearrangement
2.
[1,5] sigmatropic rearrangement
3.
[5,5] sigmatropic rearrangement
4.
[3,3] sigmatropic rearrangement