CHAPTER 3: Question 5
2D explanations
These compounds all contain a stereogenic axis which is indicated by
the red line in the diagrams shown below. It is this stereogenic axis which
causes the molecule to be chiral.
a) To assign the appropriate stereochemical prefix (which will be R
or S), the molecule must first be viewed along the stereogenic axis
as shown below. It does not matter which end of the axis is to the front,
but this answer will assume that the nitrogen as at the front. The four
substituents attached to the stereogenic axis are
then compared. In this case, three of the substituents are chemical
groups which start with nitrogen, carbon and hydrogen atoms respectively
(shown in red below), and the fourth substituent is a lone pair of electrons.
CIP rule 0 says that front substituents have priority over rear substituents,
so the nitrogen atom and lone pair of electrons will be ranked 1 and 2
(though not necessarily in that order) and the carbon and hydrogen atoms
will be ranked 3 and 4 (though not necessarily in that order). To decide
which of the front substituents should be ranked 1, CIP rule 4 which says
that a lone pair of electrons always has the lower priority is used, allowing
the nitrogen atom to be ranked 1 (and hence the lone pair 2). To rank the
two rear substituents, CIP rule 1 is used and since carbon has a higher
atomic number than hydrogen, the carbon substituent is ranked 3 and the
hydrogen 4. Since the three groups of highest rank decrease in order of
priority in an anti-clockwise direction, the correct stereochemical prefix
is S.
b) To assign the appropriate stereochemical prefix, the molecule must
first be viewed along the stereogenic axis as shown below. It does not
matter which end of the axis is to the front. The four substituents attached
to the stereogenic axis are then compared. CIP rule 0 says that front substituents
have priority over rear substituents, so the two oxygen atoms of the front
ring will be ranked 1 and 2. To determine which of these two substituients
should be ranked 1, CIP rules 1-3 are used. The first atoms (oxygen shown
in red) are identical, so rule 2 is used to compare the next atoms along
each chain, both of which are carbon atoms shown in blue. Finally, CIP
rule 3 is used to convert the C=O and C=C into multiple single bonds, and
the third atoms in each chain can then be compared. These are shown in
magenta and are O,O,C for the top of the ring and C,C,C for the bottom
of the ring. Compring these starting with the atoms of highest atomic number
allows the top of the ring to be ranked 1 as shown below. Exactly the same
procedure is used to rank the two substituents attached to the rear of
the stereogenic axis since they are the same as the substituents attached
to the front. Since the three groups of highest rank decrease in order
of priority in a clockwise direction, this is the R-isomer of the
compound.
c) To assign the appropriate stereochemical prefix, the molecule must
first be viewed along the stereogenic axis as shown below. It does not
matter which end of the axis is to the front. The four substituents attached
to the stereogenic axis are then compared. CIP rule 0 says that front substituents
have priority over rear substituents, so the nitrogen and carbon atoms
attached to the front of the stereogenic axis will be ranked 1 and 2 respectively
(they are ranked by rule 1). Similarly, the nitrogen and carbon atoms attached
to the rear of the stereogenic axis will be ranked 3 and 4 respectively.
Since the three groups of highest rank decrease in order of priority in
a clockwise direction, this is the R-isomer of the compound.
d) To assign the appropriate stereochemical prefix, the molecule must
first be viewed along the stereogenic axis as shown below. It does not
matter which end of the axis is to the front. The four substituents attached
to the stereogenic axis are then compared. CIP rule 0 says that front substituents
have priority over rear substituents, so the oxygen and carbon atoms attached
to the front of the stereogenic axis will be ranked 1 and 2 respectively
(they are ranked by rule 1). Similarly, the oxygen and carbon atoms attached
to the rear of the stereogenic axis will be ranked 3 and 4 respectively.
Since the three groups of highest rank decrease in order of priority in
a clockwise direction, this is the R-isomer of the compound.
e) To assign the appropriate stereochemical prefix, the molecule must
first be viewed along the stereogenic axis as shown below. It does not
matter which end of the axis is to the front. The four substituents attached
to the stereogenic axis are then compared. CIP rule 0 says that front substituents
have priority over rear substituents, so the carbon atom and lone pair
attached to the nitrogen at the front of the stereogenic axis will be ranked
1 and 2 respectively (they are ranked by rule 4). Similarly, the carbon
atom and lone pair attached to the nitrogen atom at the rear of the stereogenic
axis will be ranked 3 and 4 respectively. Since the three groups of highest
rank decrease in order of priority in a clockwise direction, this is the
R-isomer of the compound.
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