generalchem_ls_24.pdf

8/10/2019 GeneralChem_LS_24.pdf

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BITSPilani, Pilani Campus

CHEM F111 General ChemistryLecture 24

BITSPilaniPilani Campus

Dr. Ajay K. Sah


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CONFORMATIONS



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Conformations

Two methyl groups are not fixed in asingle position.

They rotate about the sigma bond

connecting two carbon atoms, maintainingthe linear bonding overlap.

The different arrangements formed by

rotation about single bond are calledconfomations.



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Conformations



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Conformations

H

H

H

H

H H

Newman

projection sawhorse

model

Conformations are represented by

two kinds formulas

(1)Sawhorse formula

(2) Newman Projection formula



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Viewed from end

Newman Projection



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Dihedral angle ():Angle between the C-H

bond on the front carbon atom and C-H

bond on the back carbon atom in Newman

projection.

Eclipsedconformation: (= 00)

Staggered conformation: (= 600)

Skewconformation: (= anything else)

Conformations



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Eclipsed Staggered Skew

Newman Projections



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Sawhorse Structures

Eclipsed Staggered Skew



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Conformations

Conformational analysis: study ofenergetics of different conformations.

Potential Energy of the molecule:

Minimum for the staggered conformation

Increases with rotation

Maximum at the eclipsedconformation



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Conformational nalysis of ethane



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Torsional Strain:

It is the resistance to twisting (torsion)

as ethane rotates toward an eclipsed

conformation, the energy corresponding to

this is called torsional energy.

For ethane molecule torsional energy

is about 12.6 kJmol-1.

The lowest-energy conformation is

most prevalent.

Conformations



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As the Hsof ethane are replaced by

other atoms or group of atoms, otherfactors affecting the stability of

conformations appear: van der Waals

attraction or repulsion, dipole-dipoleinteraction, hydrogen bonding etc.

But tendency for the bond orbitals

on adjacent carbons to be staggeredremains, and any rotation away from the

staggered conformation is accompanied

by torsional strain.

Conformations



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Coformations of propane

Increase in torsional strain (13.8kJ/mol)

due to the more bulky methyl group.



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Conformational Analysis of propane



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n-Butane Conformation

Highest energy has methyl groups eclipsed.

Steric hindrance Dihedral angle = 0 degrees

Totally eclipsed

Conformations of n-Butane

Free rotation around C2-C3 bond



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Gauche, staggered conformation Methyls closer than in anti conformation

Dihedral angle = 60 degrees

gauche



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Methyl groups eclipsed with hydrogens Higher energy than staggered conformer


eclipsed



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Lowest energy has methyl groups anti.


anti



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Conformational analysis of n-butane



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Conformational analysis of n-butane

Totally eclipsed conformation is

about 6 kJ higher in energy than other

eclipsed confomations because of steric

strain or steric hindrance.



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Higher Alkanes

Anti conformation is lowest in energy.

Straight chain actually is zigzag.

CH3CH2CH2CH2CH3

C

H C

CC

C

H H H H

H H

HH

HH

H

Since conformations differ from each other

only in the way their atoms are oriented in

space, they too are stereoisomers.



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Conformations of Cycloalkanes

In cyclopropane or cyclobutane, one

pair of bonds to each carbon cannotassume the tetrahedral angle, but must be

compressed to 600or 900to fit the

geometry of the ring.

These deviations of bond angles

from the normal tetrahedral value cause

the molecule to be strained, and hence to

be unstable compared with molecules in

which the bond angles are tetrahedral.



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CyclopentaneCyclobutane

Conformations of Cycloalkanes

Cyclopropane



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Conformations Of Cycloalkanes

Angle strain



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Torsional strainbecause of eclipsed hydrogens




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Torsional strain partially relieved by ring-puckering


Ring Strain= Angle strain + Torsional strain



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Deviation of the bond angles

Cyclopropane (109.50 - 600= 49.50)Cyclobutane (109.50 - 900= 19.50)

Cyclopropane and cyclobutane undergo

ring opening reactions since these relieve

the strain and yield the more stable open

chaincompounds.

Cyclopropane is more highly strained,

more unstable, and more proneto undergo

ring opening reactions.BITSPilani, Pilani Campus


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Conformations Of Cyclohexane

The ring is not planarbut is puckered, so

as to relieve itself of the ring strain.



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The angles of regular pentagon(1080

)are very close to the tetrahedral angle

cyclopentane therefore, should be virtually

free of angle strain.The angles of a regular hexagon

(1200) considered to be planar, are

somewhat larger than the tetrahedralangle, as such, cyclohexane molecule

should have in it an angle strain of 10.50.



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The chair form is the most stableconformation of cyclohexane, and

indeed, of nearly every derivative of

cyclohexane.

The chair conformation has 109.5

bond angles and all hydrogens are

staggered.

No anglestrain and no torsionalstrain.

Chair Conformations of Cyclohexane

generalchem_ls_24.pdf

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