Transcript Conversion of Fischer projection to RS configuration

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Conversion of Fischer projection to R-S
configuration
This Learning object shows you how to convert the Fischer
projection of a given molecule to its R-S configuration.
Author: Rachit Agarwal
Department of chemistry, IIT Bombay
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Learning objectives:
After interacting with this Learning Object, the learner will be able to:
 convert any molecule given in Fisher projection to R-S
configuration
Definitions of the components:
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Stereocenter or stereogenic center:
It is any point in a molecule, though not necessarily an atom, bearing
groups, such that an interchanging of any two groups leads to
a stereoisomer (the atoms making up the isomers are joined up in the
same order but have a different spatial arrangement.)
Chirality center:
It is a stereocenter consisting of an atom holding a set of atoms or
groups of atoms in a spatial arrangement which is not superimposable on
its mirror image.
A chiral center is a generalized extension of an asymmetric carbon atom,
which is a carbon atom bonded to four different entities, such that an
interchanging of any two groups gives rise to an enantiomer.
Fischer projection:
Fischer Projections are abbreviated structural forms that allow one to
convey valuable stereochemical information to a chemist without them
having to draw a 3D structural representation of a molecule. These
representations are only used for molecules that contain stereogenic
centers, which are then represented as simple crosses.
They can be derived by considering the more accurate 3D representation
using wedges and assuming the convention that horizontal lines
represent bonds coming out of the plane of the paper and vertical lines
represent bonds going behind the plane of the paper.
Sequence rules:
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There are certain rules that are to be followed while assignment of the
R/S to Fischer projection
1) To assign the priority of the atoms around the chiral center
a) Look at the atoms directly attached to the chiral atom, the
higher atomic no gets higher priority. In case of isotopes,
higher atomic number gets higher priority.
b) If directly attached atoms are same then go for the next atom
attached to this atom , and higher atomic no gets higher
priority
2) In Fischer projection the atoms/bonds which are in the horizontal
line should come out of the plane while those are in vertical line
should be going inside the plane
3) If the lowest priority group is in the vertical plane then R remains R
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S remains S and If the lowest priority group in the horizontal plane
then S becomes R , R becomes S.
4) While checking the direction of the curl around the atom, the lowest
priority is not considered.
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Notations:
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 S stands for sinister which means the anti clock wise rotation of
the curl around the chiral center if the lowest priority group is
away from viewer
 R stands for rectus which means the clock wise rotation of the
curl around the chiral center if the lowest priority group is away
from viewer
Arrow showing the sense of the rotation of the curl (anti clock wise)
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Arrow showing the sense of the rotation of the curl (clock wise)
R / S are simple notation and are used at each cross section.
(carbon atom)
Numerical 1,2,3,4 are used for the notation of each
atom around a particular cross section (carbon atom)
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Step 1: Molecule in a Fisher projection
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CH3
F
2
Cl
3
4
5
OH
H
CH3
Description
Audio Narration
Show the image as
above.
This is a molecule in its Fisher projection.
The horizontal lines represent bonds coming
out of the plane of the paper and vertical
lines represent bonds going behind the
plane of the paper.
Step 2: Naming the chiral centers
1
CH3
F
2
OH
C1
Cl
3
4
5
H
C2
CH3
Description
Audio Narration
1) Show the plum colour arrow, then the
text appears.
Each chiral center is
named separately as C1
and C2
2) Show the blue colour arrow and then
the text.
Step 3: Assignment of C1
1
CH3
2
F
3
OH
C1
Cl
H
C2
CH3
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Description
1) Make the figure in Step 2 smaller and
bring it to the lowermost left corner.
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2) Make the circle appear.
3) Then show the magnified view
Magnified view
Audio Narration
Step 4: Assignment of C1
1
C,HHH
2
3
F
OH
CH3
F
OH
C,ClCH
C1
Cl
H
C2
CH3
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5
Description
Audio Narration
1) Make the magnified figure in Step 3
smaller, and bring it to the uppermost left
corner.
Let us check all the four
atoms attached to chiral
atom for the priority order.
2) show the yellow arrow
3) Then show the figure on the right side
Step 5: Assigning priority numbers
1
C,HHH
2
3
OH
F
CH3
F
OH
C,ClCH
C1
Cl
H
C2
CH3
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5
Interactivity type
Instructions to
the user
Drag and
drop
(multiple
attempts)
Drag and drop
the appropriate
number in the
blue boxes
according to the
priority
Instructions to the animator
1) Figure from step 4
continues on screen
2) Show the numbers
1,2,3,4 and then blue
boxes
3) Display the instructions
to the user.
4) Once all the numbers
are correct display the
feedback as given on
next slide.
Step 5: Answer and feedback
1
C,HHH
2
3
F
OH
CH3
F
OH
C,ClCH
C1
Cl
H
C2
CH3
4
5
Feedback /Output
Atomic number of fluorine is 9, the combined atomic number of OH is 9 but
primary atoms get highest priority so fluorine gets 1st priority whereas OH gets
2nd priority. That leaves carbon at both ends so we check for the next attached
atom to them. Chlorine has higher atomic number than hydrogen so C,Cl,C,H
gets 3rd priority.
Step 6: Removing the lowest priority group
1
2
3
CH3
F
OH
C1
Cl
H
C2
CH3
4
5
Description
Audio Narration
1) Keep the image from Step 5
(Answer and feedback) for some
time.
While checking the direction of the
curl around the atom, the lowest
priority is not considered.
2) Display the image as shown
(right side) to show that 4th part
disappears.
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Step 7: Giving the direction of rotation and
assignment of R/S
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3
CH3
F
OH
C1
Cl
H
C2
CH3
4
Description
Audio Narration
1) Keep the image from Step 6 for some
time.
The lowest priority group (# 4) is on a
vertical bond; the circle connecting the
groups with priorities 1 to 2 to 3 goes
clockwise, so the stereocenter in this
Fischer projection has an Rconfiguration.
2) Display the arrow from 1 – 2 – 3
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3) Display text “R”
Step 8: Assignment of C2
1
CH3
2
F
3
OH
C1
Cl
H
C2
CH3
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5
Magnified view
Description
Audio Narration
1) From the previous screen(slide) keep
only the lowermost left corner figure with
the text “R” as shown above
Now let us apply the same
steps to C2.
2) Make the circle appear.
3) Then show the magnified view
Step 9: Assignment of C2
1
C,FOC
2
H
Cl
3
CH3
F
OH
C1
Cl
H
C2
CH3
4
5
C,HHH
Description
Audio Narration
1) Make the magnified figure in Step 8
smaller, and bring it to the uppermost left
corner.
Let us check all the four
atoms attached to chiral
atom for the priority order.
2) show the yellow arrow
3) Then show the figure on the right side
Step 10: Assigning priority numbers
1
C,FOC
2
3
Cl
H
CH3
F
OH
C1
Cl
H
C2
C,HHH
CH3
4
5
Interactivity type
Instructions to
the user
Drag and
drop
(multiple
attempts)
Drag and drop
the appropriate
number in the
blue boxes
according to the
priority
Instructions to the animator
1) Figure from step 9
continues on screen
2) Show the numbers
1,2,3,4 and then blue
boxes
3) Display the instructions
to the user.
4) Once all the numbers
are correct display the
feedback as given on
next slide.
Step 11: Answer and feedback
1
C,FOC
2
3
Cl
H
CH3
F
OH
C1
Cl
H
C2
C,HHH
CH3
4
5
Feedback /Output
Atomic number of chlorine is 17 and that of carbon is 6. So, Cl gets first priority.
At two ends there are Carbon atoms so we check for the secondary atoms.
Fluorine has higher atomic number than hydrogen. So C, F, O, C gets second
priority and the other carbon atom gets third priority and hydrogen gets 4th
priority.
Step 12: Removing the lowest priority group
1
C,FOC
2
3
Cl
CH3
F
OH
C1
Cl
H
C2
C,HHH
CH3
4
5
Description
Audio Narration
1) Keep the image from Step 11
(Answer and feedback) for some
time.
While checking the direction of the
curl around the atom, the lowest
priority is not considered.
2) Display the image as shown
(right side) to show that 4th part
disappears.
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Step 13: Giving the direction of rotation and
assignment of R/S
C,FOC
2
3
Cl
CH3
F
OH
C,HHH
C1
Cl
H
C2
CH3
4
Description
Audio Narration
1) Keep the image from
Step 12 for some time.
The lowest priority group (# 4) is on a horizontal bond,
then the structure as drawn has the lowest priority
group oriented toward you. Therefore, your perspective
on the structure is the opposite of what you need to
assign the configuration directly. Reverse the
configuration that you get from directly reading the
Fischer projection. so the stereocenter in this Fischer
projection has a S- configuration.
2) Display the arrow
from 1 – 2 – 3
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3) Display text “S”
Step 14: Combining both the chiral centres
1
CH3
CH3
2
OH
F
H
Cl
3
4
CH3
H
Cl
CH3
Description
Audio Narration
1) Display the left side image first
Thus we have assigned the R-S
configuration to a molecule’s Fischer
projection.
2) Display the second image then the text
“R” and “S”
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OH
F
Questionnaire:
Assign R and S to the following molecules by typing “R” or “S” appropriately
in the boxes given on the fischer projection of the molecule.
a)
Cl
Cl
H3C
OH
H
Check
H3C
(R)
H
OH
Questionnaire:
Assign R and S to the following molecules by typing “R” or “S” appropriately
in the boxes given on the fischer projection of the molecule.
CH3
CH3
b)
H
F
H
OH
I
D
H
C2H5
Check
I
H
(S)
(S)
(R)
C2H5
F
OH
D
Questionnaire:
Assign R and S to the following molecules by typing “R” or “S” appropriately
in the boxes given on the fischer projection of the molecule.
CH 3
CH 3
c)
H
H
I
H
Cl
HO
H
HO
H
CH 3
H
(S)
(R)
I
Cl
Check
HO
HO
(S)
(S)
CH 3
H
H
Summary
1) In Fischer A Fischer projection is a two-dimensional representation of
the sp3-hybridized carbon(s) in a molecule that allows the
configuration (i.e., the stereochemistry) of a tetrahedral carbon to be
shown without the use of wedges and dashes.
2) In every Fischer projection the two horizontal bonds are understood
to be projecting out of the plane toward you and the two vertical
bonds are projecting into the plane away from you.
3) Assigning R- and S-Configurations to Stereocenters in Fischer
Projections:
a) Start out by determining the relative priorities of the four
substituents attached to the stereocenter by using atomic number
priorities as usual.
b) Remember that to correctly assign a configuration at a stereocenter
you need to orient the structure so that the lowest priority group is
pointing away from you.
c) The lowest priority group (# 4) is on a vertical bond; the circle
connecting the groups with priorities 1 to 2 to 3 goes clockwise, so
the stereocenter in this Fischer projection has an R- configuration.
d) If the lowest priority group (# 4) is on a horizontal bond, then the
structure as drawn has the lowest priority group oriented toward you.
Therefore, your perspective on the structure is the opposite of what you
need to assign the configuration directly. In a case like this, draw a circle
to connect the groups with priorities 1 to 2 to 3 as before but now reverse
the configuration that you get from directly reading the Fischer projection.
Links for further reading
Books:
1) Ernest L. Eliel and Samuel H. Wilen, Stereochemistry of organic
compounds, John Wiley & Sons (2003)
2) Jonathan Clayden, Nick Greeves, Stuart Warren, and Peter Wothers Organic Chemistry, Oxford University Press (2001)
2) Solomons, Fryhle: Organic Chemistry, 8th Edition, Wiley-India (2009)
Weblinks:
http://cobalt.rocky.edu/~barbaroj/fischer-projections.pdf