1- Stereo Chemistry-Notes (1)
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Transcript of 1- Stereo Chemistry-Notes (1)
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Stereo Chemistry
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Fisher configuration is the 2D representation of a 3D molecule
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http://www.youtube.com/watch?v=tTc19BDXVG8
http://www.youtube.com/watch?v=tTc19BDXVG8
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In a Fisher Configuration,
Horizontal lines are towards the observer
Vertical lines are away from the observer
A fisher configuration should not be taken out of the plane of paper while making some changes to it
Longest C chain or parent chain has to be vertical
C with highest oxidation number (most oxidized C) should be at the top of the vertical line
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H
Cl
Br I
Cl
H
I Br
1800
A
B
A
HCl
Br
I
900
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AA
SUPERIMPOSABLE
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BA
BB
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BA
NON-SUPERIMPOSABLE
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BA
BA
mirror
Enantiomers: Non superimposable mirror images
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Rules in handling Fisher Configuration,
1800 rotation to the whole structure or on any C center will yield the same Fisher configuration
A 900 rotation will yield its stereo-isomer (enantiomer)
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NEWMAN projection
http://www.youtube.com/watch?v=tEXtJLTmdDI
http://www.youtube.com/watch?v=tEXtJLTmdDI
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NEWMAN projection
Staggered representation: dihedral angle is 60
Eclipsed representation: dihedral angle is 0
Skew representation: any conformation between staggered & eclipsed form
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SAWHORSE projection
Views a C-C bond from an oblique angle
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Imagine the black dot to be CH3 &
white dot to be H
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Few Terms
Staggered representation: dihedral angle is 60
Eclipsed representation: dihedral angle is 0
Skew representation: any conformation between staggered & eclipsed form
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Anti form (Just like trans form) : is a staggered representation where the similar gps on adjacent C atoms are oriented with a dihedral angle of 1800
Syn form ( Just like cis form) : is an eclipsed representation where the similar gps on adjacent C atoms are parallel to each other
Gauche form : is a staggered representation where the similar gps on adjacent C atoms are oriented with a dihedral angle of 600
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CH3
H H
CH3
H H
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Interconversions
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Use of projections to study the stereochemistry of molecules
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Classification of Stereoisomers:
Conformational isomers (Rotational isomers/Rotamers): Stereoisomers that can be interconverted by simple C-C sigma bond rotation
Configurational isomers: Stereoisomers that CAN NOT be interconverted by simple C-C sigma bond rotation
Enantiomers: Are stereoisomers which are non-superimposable mirror images
Diastereomers: Are stereoisomers which are not mirror images of each other.
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Conformational Analysis
Definition: The study of the energetics between different conformational isomers (rotamers)
Insight: This analyses gives us the idea about the least energy (most stable) rotamer of all, with which one can explain why few conformations are possible and why few not.
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Revisiting few definitions
Staggered representation: dihedral angle is 60 (substituents are far away from each other, so repulsion is minimum low energy state)
Eclipsed representation: dihedral angle is 0 ( substituents are close to each other, repulsion is max high energy state)
Skew representation: any conformation between staggered & eclipsed form
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Anti form (Just like trans form) : is a staggered representation where the similar gps on adjacent C atoms are oriented with a dihedral angle of 1800 (00)
Syn form ( Just like cis form) : is an eclipsed representation where the similar gps on adjacent C atoms are parallel to each other
Gauche form : is a staggered representation where the similar gps on adjacent C atoms are oriented with a dihedral angle of 600
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Few terms
# Dihedral angle (Torsional angle):- Angle between substituents on adjacent C atoms {Angle between two imaginary planes in the molecule}
# Torsional strain:- Strain created due to the repulsion between electron clouds of groups on adjacent C atoms. Torsional strain creates barrier for C-C bond rotation
{Strain created due to the energy difference between staggered and eclipsed form which impart rotational barrier across C-C bond}
# Steric strain or vander Waals strain:- Strain due to the proximity of groups in space
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Conformations in ethane
http://www.youtube.com/watch?v=oG1aCQvkkD0
D:/BASE/PPT/Stereochemistr/trim/Conformational Analysis of Ethane Using Newman Projections.wmvhttp://www.youtube.com/watch?v=oG1aCQvkkD0
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Conformations in propane
CH3
H H
H
HH
H
H
CH3
H
HHH3C
H
H
H
HH
00 600 1200
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H
H3C
H
CH3
HH
H
H3C H
CH3
HH
H
H CH3
CH3
HH
CH3
H H
CH3
HH
CH3
H H
CH3
HH
H
H
CH3
CH3
HHH3C
H
H
CH3
HH
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Conformations in butane
http://www.youtube.com/watch?v=xXci5VGousQ
D:/BASE/PPT/Stereochemistr/trim/Conformational Analysis of Butane Using Newman Projections.wmvhttp://www.youtube.com/watch?v=xXci5VGousQ
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Non-bonded interactions
Anti: Preferred due to min dipoledipole repulsion
Staggered -Gauche: Preferred due toH-bonding interactions in Aprotic solvent
Then, what is the preferred orientation in protic solvent?Ans: Staggered-Anti
OH
H H
OH
HH