stereochemistry of butane

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Topic:- conformational analysis of n-

BUTANE Made by:- Sophia Mubashir

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Conformational Analalysis of n-BUTANE

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Isomerism‣Isomers are non-identical compounds with the same molecular formula.

Types‣The two main classes of isomers are called structural isomers andstereoisomers.‣Structural (constitutional) isomers‣Stereoisomers

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ISOMERS

Compounds with thesame molecular formula

ConformationalIsomers

rotation aboutsingle bonds

with chiral centers

Stereoisomers

MesoCompounds

Enantiomers

ConstitutionalIsomers

Cis,Trans (E,Z) Isomers(can be calleddiastereomers)

Conformations

rotation restricted

differentconnectivity

Diastereomers

stereocentersbut no chiral centers

Enantiomers

one chiral centerm ore than

one chiral center

chiralachiral

not mirror images

mirror images

Atropisomers

sameconnectivity

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Conformations or Conformers or Rotamers:-

‣Different spatial arrangements of a molecule that are generated by rotation about single bonds.

Conformational analysis :-

‣Conformational analysis is the study of the effect of rotation on the properties of a molecule.

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Andiron or SawhorseSawhorse Projections are very similar to Newman Projections, but are used more often because the carbon-carbon bond that is compressed in a Newman Projection is fully drawn out in a Sawhorse Projection.

Newman ProjectionThe molecule is viewed from front to back in the direction of bond linking the two carbon. The bond joining the two carbons is hidden. ). The front carbon atom is called proximal, while the back atom is called distal

Representation:-

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Sawhorse Projection

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Newman Projection

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Conformations Torsional strain

◦ also called eclipsed interaction strain◦ strain that arises when nonbonded atoms separated

by three bonds are forced from a staggered conformation to an eclipsed conformation

◦ the torsional strain between eclipsed and staggered ethane is approximately 12.6 kJ (3.0 kcal)/mol

+12.6 kJ/mol

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Conformations Steric strain (nonbonded interaction strain):

◦ the strain that arises when atoms separated by four or more bonds are forced closer to each other than their atomic (contact) radii will allow

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Angle strain:◦ strain that arises when a bond angle is either

compressed or expanded compared to its optimal value

The total of all types of strain can be calculated by molecular mechanics programs◦ such calculations can determine the lowest energy

arrangement of atoms in a given conformation, a process called energy minimization

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Conformations Dihedral angle ( ):Q the angle created by two

intersecting planes

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Anti Butane Energy-minimized anti conformation

◦ the C-C-C bond angle is 111.9° and all H-C-H bond angles are between 107.4 and 107.9°

◦ the calculated strain is 9.2 kJ (2.2 kcal)/mol

CH3

H H

H H

CH3

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Eclipsed Butane◦ calculated energy difference between (a) the non-

energy-minimized and (b) the energy-minimized eclipsed conformations is 5.6 kJ (0.86 kcal)/mol

H

H H

H

HH

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C C

CH3

H3C

H

H

H

HHH

CH3

H H

H3C

Staggered: anti

C C

CH3

H

H3C

H

H

HCH3H

H

H H

H3C

Staggered: gauche3 KJ/mol

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Butane Conformers C2-C3

Highest energy has methyl groups eclipsed. Steric hindrance Dihedral angle = 0 degrees

totally eclipsed

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Butane Conformers (2)

Lowest energy has methyl groups anti. Dihedral angle = 180 degrees

anti

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Butane Conformers (3) Methyl groups eclipsed with hydrogen Higher energy than staggered conformer Dihedral angle = 120 degrees

eclipsed

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Butane Conformers (4)

Gauche, staggered conformer Methyl closer than in anti conformer Dihedral angle = 60 degrees

gauche

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Conformational Analysis

=>

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E2 is an anti-elimination. They are stereo specific. The hydrogen and the halogen must be on opposite sides of the molecule before the E2 elimination can take place. This makes sense as both the base and the leaving group are negatively charged. Therefore they would try to be as far apart as possible. In addition, the leaving group is large and there is more room for the removal of the adjacent proton if it is on the opposite side from the leaving group.

Reactions

If the anti-arrangement is not possible, syn-arrangement may take place.

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Mechanism = elimination, bimolecular E2

100% anti-elimination!

base:

C

X

C

H

C C + H:base + :XRDS