• The different arrangements of the atoms in space that result from rotations of groups about single bonds are called conformations of the molecule.• An analysis of the energy changes that a molecule undergoes as groups rotate about single bonds is called conformational analysis.

CO2HMe

H

CO2HMe

H

CO2HMe

H

MeH

HO2C

HO

Et

Me

OH

Me

Et

Conformational analysis

CO2HMe

H

CO2HMe

H

CO2HMe

H

MeCO2H

H

HO

Et

Me

OH Et

Me

Different conformations

Different configurations

Conformation of ethane

C C

H

HH

H

H

600

C C

H

HH

H

H

H

600

600

Staggered conformation Eclipsed conformation

Wedge-and-dash structures

Sawhorseprojections

Newmanprojections H

H

H

H

HH

H

H H

H

HH

H

H H

H

H HH

H H

H

HH

H

• The energy level diagram shows that staggered conformation as a potential energy minimum whilst the eclipsed conformation represents an energy maximum (staggered conformation is lower in energy than the eclipsed by 2.9 kcal/mole (12 kJ/mole))

Torsion angles in degrees

Potential energy of ethane as function of torsion angles

torsion angles also called dihedral angles

•The H-atoms are too small to get in each other’s way-steric factors make up < 10% of the rotational barrier in ethane

Why is the eclipsed conformation higher in energy than the staggered conformation?

• Torsional strain

Caused by repulsion of the bonding electrons of one substituent with the bondingelectrons of a nearby substituent

filled orbitals repel

• Stabilizing interaction between filled C-H bond

The real picture is probably a mixture of all 3 effects

• The rotational barrier is (12 kJ/mol) small enough to allow the conformational isomers to interconvert million of times per second

Conformation of butane

Torsion angles in degrees

Potential energy of butane as a function of torsion angle

A "synclinal" or "gauche"B "anticlinal"

C "anti-periplanar" or "anti"

D "syn-periplanar" or "fully eclipsed"

• Calculations reveal that at room temperature ~72% of the molecules of butane are in the “anti” conformation, 28% are in “gauche” conformation

"anti" conformation no torsional strain as the groups are staggered and CH3 groups far apart

''gauche'' conformation van der Waals forces between two CH3 groups are repulsive: the electron clouds repel each other, accounts for 0.9 Kcal/mole more energy compared to "anti" conformer

"eclipsed" or "syn-periplanar" conformation

greatest energy due to torsional strain and large van der waals repulsive force between tne CH3 groups

Conformations and Conformers

Butane can exit in an infinite number of conformations (6 most important have been considered), but has only 3 conformers (potential energy minima)-the two “gauche”conformations and the “anti” conformations

• The preference for a staggered conformation causes carbon chains to orient themselves in a zig zag fashion, see structure of decane

Cycloalkanes:

Ring strainNo of atoms in

ringInternal angle in planar ring

109.50-internal angle*

345678

600

900

1080

1200

128.50

1350

49.50

19.50

1.50

-10.50

-190

-25.50

All internal angles 109.5 0C

* a measure of strain per C-atom

•Ring strain largest for 3-membered rings, then decreases through a 4-membered ring and reaches a minimum for 5-memberd ring.

•Prediction: planar 5-membered ring should have the minimum level of ring strain.

•The ring strain keeps on increasing as the rings get larger after the minimum at 5

109.50-internal angle49.50

19.50

1.50

-10.50

-190

-25.50

H

• the difference between any two in series is very nearly constant at around –660 kJ/mole

• assuming there is no strain in the straight-chain alkanes, each extra –CH2 group contributes an extra 658.7 kJ/mole to the heat of combution

• if cycloalkane is strain free, its heat of combustion should be n X 658.7 kJ/mole. If there is some strain,more energy is given out on combustion

Look at the graph which shows, for each ring size: (a) angle strain per CH2 group(b) heat of combustion per CH2 group

• The greatest strain is in the 3-membered ring.• The strain decreases with ring size and reaches a minimum for 6-membered ring. • The strain then increases, but not as quickly as the angle calculation suggested: it reaches a maximum at 9 and then decreases.• The strain remains constant at ~14, not increases steadily as the angle-strain suggested.

Q • Why 6-membered ring is more strain free?• Why there is still some strain in 5-membered ring?

Cyclic compounds twist and bend to minimize the 3 different kinds of strain1. Angle strain 2. Torsional strain 3. Steric strain

• banana bonds poor orbital overlap

•Torsional strain

Good overlapStrong bond

Poor overlapWeak bond

Cyclopropane

The “bent” or “envelope” conformation of cyclopentane. In this structure the front carbon atom is bent upwards. The additional bond angle strain in this structure is more than compensated by the reduction in eclipsed hydrogens. With little torsional strain and angle strain, cyclopentane is as stable as cyclohexane. The molecule is flexible and shifts conformation constantly.

Cyclobutane

Cyclopentane

to reduce torsional angle

Chair conformation

Sum of the van der

Waals radii = 2.4 A0

Boat conformation

H

HH

H

H

H

H

H

Newman projection of the

boat conformation

HA

HB

HB

HAring-flip

1.8 A0

H H

flagpole hydrogens

The easiest way to draw a twist-boat conformation

• The energy difference between the chair, boat, and twist conformation of cyclohexane are low enough to make their separation impossible at r.t. At room temperature approx. 1 million introversions occur each other second.• More than 99% of the molecules are estimated to be in chair conformation at any given time

X

X

Monosubstituted cyclohexane

This conformation is lower in energyWhy?

X

H

H For methylcyclohexane (X=CH3), the conformer with the methyl group axial is 7.3 kJ/mol higher in energythan the conformer with the methyl group equatorial.Result: 20:1 ratio of equatorial:axial conformer at 200 C

1,3-diaxial interaction

X

H

HH

H

XH

H

H

X

HH

H

HH

H

HX

The black bonds are anti-periplanar (only one pair shown)

The black bonds are synclinal(gauche) (only one pair shown)

X

X

K=Conc. of equatorial conformer

Conc. of axial conformer

Equilibrium

constant

Energy diff. between axial and equatorial conformers

kJ/mol

% with substitutent equatorial

H

Me

Et

i-Pr

t-Bu

OMe

Ph

1

19

20

42

>3000

2.7

110

0

7.3

7.5

9.3

>20

2.5

11.7

50

95

95

98

>99.3

73

99

X

OH

Preferred conformation

OH

OH

Preferred conformation

OH

OH H H

disfavouredtwist-boat conformation with botht-Bu groups pseudoequatorial

t-butyl group- a locking group

Me

i-Pr

OH

Q. Write the prefered conformation for

OH

Me

Me

OH

favoured

•It should form a ()- pair

It exists as a dl-pair, but since barrier to rotation is low to allow separation.Therefore the ()- pair is inseparable and hence the compound is optically inactive.

CH3

CH3

CH3CH3

CH3

CH31 gauche-butane interaction0.9 kcal/mol

4 gauche-butane interaction4 x 0.9 kcal/mol = 3.6 kcal/mol

Difference in stability between the conformational isomers

3.6 - 0.9 = 2.7 kcal/mol

CH3

CH3

CH3

CH3

CH3

H3C

CH3

CH3

This has 3 gauche-butaneinteractions

The two conformational isomers are mirror images

CH3

Very bad steric situation ~ 5.5 kcal/mol (4 x 0.9 = 3.6 kcal/mol + Methyl-Methyl interaction)

CH3

CH3 CH3H3C

CH3

CH3

CH3

CH3

CH3H3C

CH3

It is a resolavable molecule2-gauche-butane interaction = 1.8 kcal/mol

CH3

CH3

plane of symmetry : not resolavable

CH3

CH3H3C

CH3

2-gauche-butane interaction, 2 x 0.9 = 1.8 kcal/mol

CH3 plane of symmetry

CH3

CH3

CH3

H3CCH3

Why is the eclipsed conformation higher in energy than the staggered conformation?

As two nonpolar groups approach each other,the van der Waals attractive force increases toa maximum,then decreases and becomes repulsive

The van der Waals radius one-half thedistance between two equivalent atoms at the point of the energy minimum

The H-atoms are too small to get in each other’s way-steric factors make up < 10% of the rotational barrier in ethane

van der Waals radii, A0

H N O F

1.2 1.5 1.4 1.35

CH2 P S Cl

2.0 1.9 1.85 1.8

CH3Br

2.0 1.95

I2.15