Stereo Chemistry

Fisher configuration is the 2D representation of a 3D molecule

http://www.youtube.com/watch?v=tTc19BDXVG8

http://www.youtube.com/watch?v=tTc19BDXVG8

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

H

Cl

Br I

Cl

H

I Br

1800

A

B

A

HCl

Br

I

900

AA

SUPERIMPOSABLE

BA

BB

BA

NON-SUPERIMPOSABLE

BA

BA

mirror

Enantiomers: Non superimposable mirror images

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)

NEWMAN projection

http://www.youtube.com/watch?v=tEXtJLTmdDI

http://www.youtube.com/watch?v=tEXtJLTmdDI

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

Imagine the black dot to be CH3 &

white dot to be H

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

CH3

H H

CH3

H H

Interconversions

Use of projections to study the stereochemistry of molecules

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.

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.

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

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

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

Conformations in propane

CH3

H H

H

HH

H

H

CH3

H

HHH3C

H

H

H

HH

00 600 1200

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

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

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