Download - stereo new 2
![Page 1: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/1.jpg)
Lecture 3 - Recap
Different types of isomersStereochemistrySymmetry and asymmetryChiralityAsymmetric centre/ Chiral CarbonStereogenic CentreEnantiomersPlane of symmetry & center of symmetry
![Page 2: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/2.jpg)
Stereochemistry-II
![Page 3: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/3.jpg)
ConfigurationConfiguration of a molecule is the detailed
stereochemical picture including how the atoms are arranged in space.
It is represented (specified) by letter ‘R’ or ‘S’.For doing this, we assign priority numbers
according to Cahn-Ingold-Prelog system and see which way the priority numbers are arranged.
If priority numbers are arranged clockwise, then specification is ‘R’.
If priority numbers are arranged anticlockwise, then specification is ‘S’.
![Page 4: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/4.jpg)
Wedge – Dash & Fischer Projections
How do we represent a three dimensional molecule on to a two dimensional plane i.e. blackboard or paper?
By, Wedge-and-dash drawing (perspective diagram) or
By Fischer’s projection formula (also called cross formula).
We will concentrate in our discussions on molecular representation by Fischer’s formula mainly as it is difficult to represent molecules with many chiral carbon atoms by Wedge and dash form.
![Page 5: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/5.jpg)
Wedge – Dash RepresentationUsually drawn with two bonds in the plane of the page, one in front, and one behind to give the molecule perspective. When drawing wedge-dash it is a good practice to visualize the tetrahedral arrangement of the groups and try to make the diagram "fit“ this.
![Page 6: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/6.jpg)
Fischer ProjectionsHow is Fischer’s formula drawn?
We draw two lines (one vertical and one/( two or more) horizontal line(s)) depending upon the number of stereocenter/(s) in the molecule.
The point of intersection of these lines is considered as representing the stereocenter.
The formula can be drawn only for molecules with stereocenter.
Fischer’s formula for Lactic acid is
![Page 7: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/7.jpg)
Fischer ProjectionsThe horizontal lines represent wedges (bonds) coming out of the plane of paper.
The vertical lines represent dashed lines (bonds) going into the plane of the paper.
When drawing Fischer Projection the carbon chain is drawn along the vertical line of the projection, with the most highly oxidized carbon substituent at the top.
![Page 8: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/8.jpg)
Conversion of Wedge – Dash to Fischer Projections
![Page 9: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/9.jpg)
ConfigurationWhile assigning configurations using fisher Projections:
If the lowest priority number is along vertical line (may be above or below) then assigned specification remains same.
If the lowest priority number is along horizontal line (on the left or right of the stereocenter) then assigned specification will be opposite to the specification we got.
![Page 10: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/10.jpg)
Cahn-Ingold-Prelog RulesAssign a priority number to each group
attached to the chiral carbon.Atom with highest atomic number -
assigned the highest priority #1.In case of ties, look at the next atoms along
the chain.Double and triple bonds are treated like
bonds to duplicate atoms.
![Page 11: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/11.jpg)
Cahn-Ingold-Prelog Rules
Rule 1: Priority number isassigned on the basis ofatomic number of atombonded to stereocenter. I > S > O > N > 13C > 12C > Li > 3H (T) > 2H (D) > H
Example 1 In this example the priority numbers are arranged anticlockwise and the lowest priority number is along vertical.
Therefore specification is ‘S’ (Latin, sinister = left).
![Page 12: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/12.jpg)
Cahn-Ingold-Prelog Rules
Rule 2: Priority number isassigned on the basis of mass number if isotopes are bonded to stereocenter.
Example 2 In this example the priority numbers are arranged clockwise and the lowest priority number is along vertical.
Therefore specification is ‘R’ (Latin, rectus = upright).
2CH3
H
1Cl
3D
![Page 13: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/13.jpg)
(R) and (S) Assignments
(a)The molecule is drawn is 3 dimensions, and arranged in such a way that the bond between the chiral carbon and the lowest priority groupheads back into the paper.
(b) Draw an arrow from the group of highest priority, to the second, to the third priority group.
NH2
CH
CO2H
C*
NH2
CO2HH3C
*
NH2
CH
HO2C
C*
NH2
CH3HO2C
*
(R) enantiomer (S) enantiomer
H3CCH3
(1)
(2)
(3) (4)
(1)
(4)
(2)
(3)
(1) (1)
(2)(2) (3)(3)
unnatural alanine
naturalalanine
![Page 14: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/14.jpg)
Assign (R) or (S)Working in 3D, rotate molecule so that lowest
priority group is in back.Draw an arrow from highest to lowest
priority group.Clockwise = (R), Anticlockwise = (S)
![Page 15: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/15.jpg)
Assign (R) or (S)
![Page 16: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/16.jpg)
CH3
C3H7C2H5
12
3
R
2
![Page 17: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/17.jpg)
Cahn-Ingold-Prelog Rules
Rule 3: In the case of the same atoms being bound directly to the chiral carbon, we go to the next atoms along the chain till Priority number is determined.
Example 3 In this example the specification is ‘S’.
2C
1Cl
H
3CHCH3
CH3
CH3
CH3CH3
-CH2Br > -CHCl2 > -C(CH3)3 > -CH(CH3)CH2F > -CH(CH3)2 > -CH2CH3
![Page 18: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/18.jpg)
C
Cl
H
CCCH
C
HC
C
ClH
H
H H
CH3OH
OH
H H
HH
1
4
??
C(C,C,H) and C(C,C,H) do not show any difference
Example
![Page 19: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/19.jpg)
C
Cl
H
CCCH
C
HC
C
ClH
H
H H
CH3OH
OH
H H
HH
C(C,C,H) and C(C,C,H) do not show any difference
Two sub-branches on right are arranged:
C(O,C,H) >
C (O,H,H)
Two sub-branches
on left are
arranged: C(Cl,H,H)
> C(H,H,H)
Considering senior sub-branches it yields C(Cl,H,H) > C(O,C,H). (No need to consider junior sub-branches).
Example contd..
![Page 20: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/20.jpg)
C
Cl
H
CCCH
C
HC
C
ClH
H
H H
CH3OH
OH
H H
HH
1
4
32
Priority of left hand side group is higher than right hand side
1
2 3
S
Example contd..
![Page 21: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/21.jpg)
Cahn-Ingold-Prelog RulesRule 4: Double and triple bonds are treated as if each bond were to a separate atom. (Imagine that each bond is broken and that the atoms at both ends were duplicated. When a bond is broken only two imaginary atoms are added).
![Page 22: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/22.jpg)
Cl
CH
CH2CH3
H
H2COH
H
O OH
O
HO
S
SH3CH2C
CH2CH3H3C
H
HCH3
3S and 4S
![Page 23: stereo new 2](https://reader034.vdocuments.net/reader034/viewer/2022050815/5471d69eb4af9f43438b45c1/html5/thumbnails/23.jpg)
NH
CH3
Cl
O
HO
HO
CH2NHCH3
H
OH
S R