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Section III
REASONING IN BIOLOGICAL AND PHYSICAL SCIENCES
UNIT 2
Questions 6 -
12
Some of the most important reactions seen in organic chemistry
involve the rearrangement of molecules via processes involving cyclic
intermediates. These so called pericyclic rearrangements can be split into a
number of different classes. In this section of questions we examine one of the
most common types of such reactions, sigmatropic rearrangements.
The
original sigmatropic rearrangement was seen when an aryl allyl ether was simply
heated. An example of this type of reaction is given below.
Looking at what happens here, the double
bond from the benzene ring forms a new bond with carbon 10. At the same time
the bond between oxygen 7 and carbon 8 breaks. The net result is that the
carbon chain is moved to be directly bonded on the benzene ring.
Question 6
By analogy the following molecule will give
which product on undergoing such a rearrangement.
Citral is an important compound in the
synthesis of vitamin A. The company BASF produce citral using a process which
involves two successive sigmatropic rearrangements. The second of these is
illustrated below.
Question 7
Which of the
following therefore represents the correct structure of citral.
Another compound which undergoes such a
sigmatropic arrangement is the pheromone used by brown algae to attract male
partners. The structure of this pheromone is outlined below.
Question 8
In this
reaction the double bond at position 11 will form a new bond with that at
position 5. This rearrangement produces a new carbon ring system which will
consist of how many carbon atoms.
A 6
B 8
C 10
D 12
Question 9
And how many
double bonds will the new compound contain.
A 1
B 2
C 3
D 4
The sigamatropic reactions we have seen
above have all involved carbon and oxygen. It is possible however for this type
of reaction to occur with other elements being present in the molecules. Such
an example is the so called Fischer indole synthesis. This involves the
rearrangement of the following intermediate.
Question 10
A similar
reaction occurs here so that carbon 4 becomes bonded to carbon 14. This gives
which product.
A final example of this type of
reaction is the pinacol rearrangement. This occurs when a 1,2 -diol is treated
with acid. Such an example is given below.
Question 11
Again by
analogy, which molecule could react in this way to form the compound shown
below.
Question 12
And between
which two carbon atoms has a new bond been formed.
A 4 and 5
B 2 and 3
C 2 and 7
D 8 and 9
SECTION
ANSWERS
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