Conformation Al Analysis

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R. B. Sunoj

CH-103

Conformational Analysis



R. B. Sunoj

CH-103Internal Coordinates

C1 C2

H1

R 1

R 2

θ

H2

C1

C2

H2

=

H1

View

H1

C1

C2

H2

Bond length between 2 atoms

Bond angle between 3 atoms

Dihedral angle between 4 atoms

R B S j



R. B. Sunoj

CH-103Conformational Analysis

Conformational Analysis is the study of energetics of different

conformers.

Energy versus tortional angle (dihedral angle)

Conformers arise due to the possible free rotation around single

bonds

Refers to changes in “Shape”n-octane

R B S j



R. B. Sunoj

CH-103Conformational Analysis, Example-1: Ethane

Ethane has two major conformers – Staggered and Eclipsed

H

H H

H

HH

H

HH

Staggered Eclipsed

H

H

H

60 0Dihedral angle, θ

H

HH

H

H

H

2.4 A

H H

H H

H

H

2.2 A

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R. B. Sunoj

CH-103

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R. B. Sunoj


The Eclipsed conformer is 3.0 kcal/mol higher in energy than the

staggered conformer

Higher energy of the eclipsed conformer is due to

(1) Bond pair-bond pair repulsion between C-H bondsH

H H

H

H

H σC-H repulsion with σC-H

or filled-filled repulsive interaction

(2) Improved delocalization in the staggered conformer

σC-H interaction with σ∗C-H

or filled-unfilled attractive interaction

H

HH

H

H

H

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The Eclipsed conformer is 3.0 kcal/mol higher in energy than the

staggered conformer

Courtesy: http://www.cem.msu.edu/~reusch/

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R. B. Sunoj

CH-103Torsional Barrier for C-C bond rotation

Contribution to

Barrier height (kcal/mol)Eclipsing Group

0.9

1

1.32.9

3.4

3.9

4.7

(CH3) … (CH3) gauche

(C-H) … (C-H)

(C-H) … (CH3)(CH3) … (CH3)

(CH3

) … (CH2

CH3

)

(CH3) … (CH(CH3)2)

(CH3) … (C(CH3)3)

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R. B. Sunoj

CH-103Conformational Analysis, Example-2: n-Butane

H

H3C

H

CH3

HH

H H

HH

CH3

CH3

H

H CH3

HH

CH3

H

H

CH 3

HH

CH 3

D

H

H

HH

CH3

CH3

H

H3C H

HH

CH3

C

A

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R. B. SunojCH-103Conformational Analysis, Example-2: n-Butane

H

H3C

H

CH3

HH

DEclipsed Syn-periplanar, θ = 0°4.9

H

H CH3

HH

CH3

H

H

CH 3

HH

CH 3

B Anticlinal, θ = 120°Eclipsed3.6

H

H

HH

CH3

CH3

C

H H

HH

CH3

CH3

A

H

H3C H

HH

CH3

GaucheSynclinal, θ = 60°

0.9

Anti-periplanar, θ = 180°Staggered 0.0

Synclinal= slope towards syn

Barrier of 4.9 kcal/mol is equivalent to 2 x 109 rotations/sec (RT)!!

R. B. Sunoj



R. B. SunojCH-103Conformers and Conformations

H

H3C

H

CH3

HH

Conformations: Can take anyvalue of θ, includes maxima as

well.

Infinite number of conformations!Can not be isolated

D

4.9

H

H CH3

HH

CH3

H

H

CH 3

HH

CH 3

B3.6

H

H

HH

CH3

CH3

C

H H

HH

CH3

CH3

A

H

H3C H

HH

CH3

0.9Conformers: Minima on the

Potential Energy Curve

Could be isolated0.0

R. B. Sunoj



. . Su ojCH-103Important Kinds of Molecular STRAINS

Torsional Strain: Extra energy of eclipsed conformation arising

due to the repulsion between bonding electrons of one substituent

with that of the other as they pass close to each other

Steric Strain: Strain induced when two atoms or groups in a

molecule are too close to each other, arising due to repulsion

between electron clouds of interaction atoms/groups

Angle Strain: Strain induced in molecules when the bond angles

are different from the desired tetrahedral bond angle of 109.5°

R. B. SunojS ll Ri C d (1) l



jCH-103Small Ring Compounds – (1) cyclopropane

PropanePropane CyclopropaneCyclopropane

H

H

Cyclopropane

H

H

H

H

Deviation of 49.5° per carbon

Torsional strain: (C-H) … (C-H)

eclipsing interactions

Angle strain : None

(all angles are nearly tetrahedral)

Steric strain: None

(Fully staggered conformer)

Cyclopropane suffers from angle strain and torsional strain

Renders higher reactivity, Weaker C-C and C-H bonds

R. B. SunojS ll Ri C d (2) l b t



jCH-103Small Ring Compounds – (2) cyclobutane

Planar? Problems are: Angle strain plus severe

torsional strain90

Compromise is

Ring puckering

Relieves eclipsing interactions with only a slight

increase in angle strain (90 to 88°)

HH

H

H

H

H

H H

HH

H H

H

H

H

H

Flipping

Cyclobutane has a ‘wing shaped’ or ‘puckered’ conformer

R. B. SunojSmall Ring Compounds (3) Cyclopentane



jCH-103Small Ring Compounds – (3) Cyclopentane

Planar? Though the angle strain is very little,there will be 10 eclipsing interactions!

Ring puckering

Relieves eclipsing interactions with only a slight

increase in angle strain

HH

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

Four coplanar

carbon atoms

Cyclopentane has an ‘envelope’ conformer

R. B. SunojSmall Ring Compounds (4) Cyclohexane



CH-103Small Ring Compounds – (4) Cyclohexane

One of the most abundant and important structural units in organic

chemistry (and nature!)

Both angle and torsional (eclipsing interactions) strains can beavoided by ring puckering.

Ring puckering leads to “CHAIR ” and “BOAT” conformers

H

H

H

H

H

H

H

H

H

H

H

H

HH

H

H

H

H

H

H

H

H

H

H

R. B. SunojDrawing Cyclohexanes



CH-103Drawing Cyclohexanes

1

22 1

Axial Bonds

Equatorial Bonds

Look for “W” and “M”

Look for ‘parallel’ bondsTips:

Courtesy L. G. Wade, Organic Chemistry, 4e, Pearson Eduction

R. B. SunojCH 103Examples of chair conformers (D) Glucose



CH-103Examples of chair conformers – (D)-Glucose

β-D-(+)-glucose

H

O

H

OH

HO

HO

OH

OH

H

H

H

Equatorial Region

Polar face

Axial Region

Apolar face

R. B. SunojCH 103Ring Inversion or Ring Flipping



CH-103Ring Inversion or Ring Flipping

In monosubstituted (small groups) cyclohexanes the ring flipping is of the order of 104 to 105 inversions/second

R. B. SunojCH 103A grossly approximate comparison



CH-103A grossly approximate comparison

Molecules ‘move around’ on potential energy surfaces

Joyrides on PES

Picture courtesy: http://www.joyrides.com

R. B. SunojCH 103Ring Inversion or Ring Flipping



CH-103Ring Inversion or Ring Flipping

0.0

10.8 5.5

tb b

1 kcal/mol

Low-activation energy flexing

motion: pseudorotation

The net result of

cyclohexane ring inversionappears as if the boat form

is rotating by 60º at a time!

kcal/mol

R. B. SunojCH 103Boat conformer of cyclohexane



CH-103Boat conformer of cyclohexane

Flag pole

Bowsprit

Summary

Maxima (1) Half-chair (2) Boat

Minima (1) Chair (2) Twist-boat

Courtes icture : htt ://www.all-model.comR. B. SunojCH-103Points to remember while ring-inversion is done



CH 103Points to remember while ring inversion is done

(i) Groups which are above the avg. plane would continue to be above

the avg. plane even after ring flipping

CH3

CH3

(ii) A cis-isomer would remain as cis-isomer, so is trans

CH3

CH3

CH3

CH3

R. B. SunojCH-1031,3-diaxial interactions in substituted cyclohexanes



CH 103,3 d a a te act o s subst tuted cyc o e a es

Courtesy L. G. Wade, Organic Chemistry, 4e, Pearson Eduction

R. B. SunojCH-1031 3-diaxial interactions



CH 1031,3 diaxial interactions

H

CH3

H H

H

H

van der Waals repulsion between the axial methylgroup and the axial hydrogens at C3 and C5 is

known as 1,3-diaxial interactions

CH3 is gauche to two C-C bonds

‘gauche-butane’ type interaction in

axial positionH

CH3

H H

H

H

H

H

ViewH2C

CH3

H

H

H

H2C

R. B. SunojCH-103A-value for mono substituted cyclohexanes



A value for mono substituted cyclohexanes

X group A value (kcal/mol)

Free energy difference between axial and equatorial conformers of

monosubstituted cyclohexanes are known as A-values

∆G = Gax - Geq

X

X

K % eq

H

CH3

CH(CH3)2

C(CH3)3

0

1.7

2.15

5

1

19

42

3000

50

95

98

99.9

R. B. SunojCH-103t-Butyl cyclohexanes



t Butyl cyclohexanes

The t-butyl group occupies the equatorial position in cyclohexane

Conformation of t-butylcyclohexane is said to be BIASED

but

NOT LOCKED

R. B. SunojCH-103Disubstituted cyclohexanes



y

1,2-disubstituted

cis trans

H

CH3

H

CH3

H

CH3

H

CH3

CH3

CH3

H

H

CH3

CH3

H

H

cis trans

H3C

CH3

H

H

trans

R. B. SunojCH-103Disubstituted cyclohexanes



1,3-disubstituted

CH3

H

H

H3C

H3C

H

H

H3

C

cis trans

H3C

H

H

CH3

cis

1,4-disubstituted

CH3

H

H

CH3

CH3

H

H

H3C

H3C

HH

CH3

cis trans

R. B. SunojCH-103Polycyclic compounds



Spiro cyclic compounds: Compounds that share one carbon atom

between two rings

Fused ring compounds: Compounds that share two adjacent carbon

atoms

Bicyclic ring compounds: Compounds that share two non-adjacent

carbon atoms

R. B. SunojCH-103



To be continued….

Time is too slow for those who wait Too swift for those who fear

Too long for those who grieve

Too short for those who rejoice

----Henry van Dyke

Conformation Al Analysis

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