CHAPTER 1: Question 6
3D Explanations
a) First rotate the structure so that the C-CH3 and C-CO2H
bonds are vertical and pointing away from you with the acid group at the
top, whilst the C-H and C-NH2 bonds are horizontal and pointing
towards you. This can be achieved by rotating the 3D structure shown below.
It is important that the acid group is at the top of the structure, as
the carbonyl carbon would be numbered 1 in organic nomenclature. This is
the difficult part of the process, and a molecular model may be very useful.
Next redraw the wedges and hashes as normal bonds, then delete bonds to
hydrogen atoms to give the Fischer projection. The process is shown below.
b) First rotate the structure so that the C-CH2OH and C-CO2H
bonds are vertical and pointing away from you with the acid group at the
top, whilst the C-H and C-NH2 bonds are horizontal and pointing
towards you. This can be achieved by rotating the 3D structure shown below.
It is important that the acid group is at the top of the structure, as
the carbonyl carbon would be numbered 1 in organic nomenclature. This is
the difficult part of the process, and a molecular model may be very useful.
Next redraw the wedges and hashes as normal bonds, then delete bonds to
hydrogen atoms to give the Fischer projection. The process is shown below,
though note that the orientation of the OH group attached C3 is not important
as C3 is not a stereocentre.
c) First rotate the structure by 180o about the midpoint
of the C2-C3 bond so that the acid group is at the top of the diagram.
This can be achieved by rotating the 3D structure shown below. Next, rotate
C3 about the C2-C3 bond so that it is pointing in the same direction as
the acid group. View the molecule from the side opposite to the acid and
methyl groups which will give a projection in which the C1-C4 bonds are
vertical and directed away from you, whilst the other bonds are horizontal
and directed towards you. Next redraw the wedges and hashes as normal bonds,
then delete bonds to hydrogen atoms to give the Fischer projection. The
process is shown below.
d) First rotate the structure anti-clockwise by 90o, so
that the aldehyde is at the top. If the resulting structure is viewed from
the left side, then the bonds around C2 and C4 are correctly oriented to
allow a Fischer projection to be drawn, whilst if the molecule is viewed
from the right side then the bonds around C3 are correctly located to allow
a Fischer projection to be drawn. This can be visualized by rotating the
3D structure shown below. Finally, redraw the wedges and hashes as normal
bonds, then delete bonds to hydrogen atoms to give the Fischer projection.
The process is shown below.
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