conformations - p.g. college · 2015. 6. 18. · if cyclobutane ring were planar, the angle strain...

1

CONFORMATIONS

Dr. Mishu Singh

Chemistry Department

Maharana Paratap Govt. P.G College

Hardoi.

Conformations

The infinite number of arrangements of the atoms or groups of a

molecule in three dimentional space which are interconvertible into

each other by rotation about single bond are called Conformations or

Rotational Isomers or simply Rotamers.

These conformers have different internal dimensions (atom-to-atom

distances, dihedral angles, dipole moment etc.)

.

The energy barrier for rotation of carbon-carbon single bonds

(conversion of different spatial arrangements) is normally small, < 0.6

kcal/mol and >16 kcal/mol.

2

Rotation about Carbon–Carbon Bonds

3

Newman & Sawhorse Projections

4

5

Staggered conformation:

A conformation about a carbon-carbon single

bond in which the atoms or groups on one

carbon are as far apart as possible from the

atoms or groups on an adjacent carbon

H

H H

H H

H

6

Eclipsed conformation:

A conformation about a carbon-carbon single bond

in which the atoms or groups of atoms on one

carbon are as close as possible to the atoms or

groups of atoms on an adjacent carbon

H

H H

H

HH

7

•

Eclipsed conformation Staggered conformation

• Each hydrogen on one

carbon as close as

possible to one

hydrogen on the other

carbon

• Hydrogen on one carbon

as far from the hydrogen

from other carbon

A Staggered conformation is more stable than an

eclipsed conformation

8

Types of Strain

Steric - Destabilization due to the repulsion between the electron clouds of atoms or groups. Groups try to occupy some common space.

Torsional - Destabilization due to the repulsion between pairs of bonds caused by the electrostatic repulsion of the electrons in the bonds. Groups are eclipsed.

Angle - Destabilisation due to distortion of a bond angle from it's optimum value caused by the electrostatic repulsion of the electrons in the bonds. e.g. cyclopropane

9

Torsional strain

Also called eclipsed interaction strain.

Strain that results from eclipsed bonds.

Strain that arises when non-bonded atoms/groups,

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

10

60o Rotation Causes Torsional or

Eclipsing Strain

11

Dihedral angle (Ɵ)

The angle created by two intersecting planes

12

Conformers of Alkanes

Structures resulting from the free rotation of a C-C

single bond

May differ in energy. The lowest-energy conformer

is most prevalent.

Molecules constantly rotate through all the possible

conformations.

13

Conformations of Ethane

• Staggered conformer has lowest energy.

• Dihedral angle = 600

H

H

H

H

H H

Newman projection Sawhorse Projection

14

Rotational Conformations of Ethane

15

16

Ethane as a function of dihedral angle

17

18

The origin of torsional strain in ethane: Originally thought to be caused by repulsion between eclipsed

hydrogen nuclei

Alternatively, caused by repulsion between electron clouds of eclipsed C-H bonds

Theoretical molecular orbital calculations suggest that the energy difference is not caused by destabilization of the eclipsed conformation but rather by stabilization of the staggered conformation

This stabilization arises from the small donor-acceptor interaction between a C-H bonding MO of one carbon and the C-H antibonding MO on an adjacent carbon; this stabilization is lost when a staggered conformation is converted to an eclipsed conformation

Anti - Description given to two substitutents attached to

adjacent atoms when their bonds are at 180o with respect to

each other.

Syn - Description given to two substitutents attached to

adjacent atoms when their bonds are at 0o with respect to each

other.

Gauche - Description given to two substitutents attached to

adjacent atoms when their bonds are at 60o with respect to

each other.

CH3

CH3

anti

CH3

CH3

gauche

CH3CH3

eclipsed

0o

180o

60o

20

Conformations of Propane

21

Conformations of Butane

25

2 Different Eclipsed Conformations

26

27

Butane has Steric and Torsional

strain when Eclipsed

The totally eclipsed conformation is higher in energy because it forces the two end methyl groups so close together that their electron clouds experience a strong repulsion.

2 | 28

Three valleys (staggered forms) 120 apart; Three hills (eclipsed) 120 apart.

Extra slide

30

Draw staggered and eclipsed conformers of

1-Chloropropane?

Draw the Rotational profile of 2-methylbutane about

C2-C3.

Eclipsed Structures:

Me

H

Me

H

Me

H

This was the

high energy

staggered

structure, 180 0

Me

H

Me

H

H

MeMe

H

Me

H

Me

H

1200 2400 1800

Me

H

Me

Me

H

HMe

H

Me

Me

H

H

00 3600

Now relative energies…..

Me

H

Me

H

Me

H

Me

H

Me

Me

H

H

Me

H

Me

H

H

Me

1200 600 3000

Staggered Structures:

33

Conformations

in

Cycloalkane

Stability of Cycloalkanes: Ring Strain

Rings larger than 3 atoms are not flat

Cyclic molecules can assume nonplanar conformations to minimize angle strain and torsional strain by ring-puckering

Larger rings have many more possible conformations than smaller rings and are more difficult to analyze

The Baeyer Strain Theory

Baeyer (1885): since carbon

prefers to have bond angles

of approximately 109°, ring

sizes other than five and six

may be too strained to exist

Rings from 3 to 30 C’s do

exist but are strained due to

bond bending distortions and

steric interactions

36

Summary: Types of Strain

Angle strain - expansion or compression of bond angles away

from most stable

Torsional strain - eclipsing of bonds on neighboring

atoms/gps

Steric strain - repulsive interactions between nonbonded

atoms in close proximity

Cyclopentane

38

angle strain: the C-C-C bond angles are

compressed from 109.5° to 60°

torsional strain: there are 6 sets of eclipsed

hydrogen interactions

strain energy is about 116 kJ (27.7 kcal)/mol

39

Cyclobutane

The ring strain of a planar cyclobutane results from two factors:

1.angle strain from the compressing of the bond angles to 90° rather than the tetrahedral angle of 109.5°

2. torsional strain from eclipsing of the bonds.

Internal bond angle ~88o (~21o deviated from the normal

109.5o tetrahedral angle)

Cyclobutane ring is not planar but is slightly folded. It is

slightly bent out of plane - one C atom is about 25°

above.

If cyclobutane ring were planar, the angle strain would be

somewhat less (the internal angles would be 90o instead

of 88o), but torsional strain would be considerably larger

because all eight C–H bonds would be eclipsed

puckering from planar cyclobutane reduces torsional

strain but increases angle strain

the conformation of minimum energy is a puckered

“butterfly” conformation

strain energy is about 110 kJ (26.3 kcal)/mol

42

Cyclopentane

Planar cyclopentane would have no angle strain but very high torsional strain

Actual conformations of cyclopentane are nonplanar, reducing torsional strain. Puckering from planar cyclopentane reduces torsional strain, but increases angle stain

Four carbon atoms are in a plane

The fifth carbon atom is above or below the plane – looks like an envelope

the conformation of minimum energy is a

puckered “envelope” conformation

strain energy is about 42 kJ (6.5 kcal)/mol

Measuring Strain in Cycloalkanes

Heats of combustion can be used to compare stabilities of

alkanes & cycloalkanes.

Heats of combustion increase as the number

of carbon atoms increase.

Therefore, divide heat of combustion by number

of C’s and compare heats of combustion

on a "per CH2 group" basis.

45

CnH2n + O2 n CO2 + (n+1) H2O + heat cycloalkane (can be measured)

Total Strain

Energy =

Sample

ΔHcomb per -CH2- _

Reference

ΔHcomb per -CH2- • n

Heats of Combustion of Cycloalkane: the more strained a compound is, the more is the heat released upon combustion

Cycloalkane

Cyclopropane

Cyclobutane

Cyclopentane

Cyclohexane

Cycloheptane

Cyclooctane

Cyclononane

Cyclodecane

Cyclohexadecane

Alkane reference

Ring Size (n)

3

4

5

6

7

8

9

10

16

Hcomb KJ/mol

2091

2721

3291

3920

4599

5267

5933

6587

10466

Hcomb per CH2- KJ/mol

697

681

658

654

657

658

659

659

654

654

Total Strain Energy

129

108

20

0

21

32

45

45

0

0

strained rings

commonrings

mediumrings

large rings (> 12)

(43)

(27)

(4)

(0)

(3)

(4)

(5)

(5)

(0)

According to Baeyer, cyclopentane should have less

angle strain than cyclohexane.

Cyclopentane 3,291 658

Cyclohexane 3,920 653

The heat of combustion per CH2 group is less for

cyclohexane than for cyclopentane. Therefore,

cyclohexane has less strain and more stable than

cyclopentane.

Heat of combustion suggests that angle strain

is unimportant in cyclohexane.

Tetrahedral bond angles require nonplanar

geometries.

Conformations of Cyclohexane

Cyclohexane is by far the most common

cycloalkane in nature and also in organic

chemistry.

The cyclohexane ring is free of angle strain and

torsional strain. Zero ring strain implies the bond

angles must be 109.5°. (no angle strain) and also

no eclipsing interactions between the C-H bonds

(no torsional strain).

48

Cyclohexane adopts a puckered structure. A planar arrangement of the six methylene groups in cyclohexane

does not give a tetrahedral shape for every carbon atom - this is

achieved by puckering the ring. Cyclohexane does this by

adopting mainly two conformations the CHAIR and the BOAT.

49

Chair Conformation

Most stable conformation. Each carbon is in the

staggered conformation

All the bond angles are 109.5° and all the C-H bonds

are staggered. (Zero ring strain) .

More stable than a boat conformation by 27 kJ (6.5

kcal)/mol.

50

Boat Conformation

51

52

The boat is just a chair with the footrest flipped up.

C-1, C-4 are bent toward each other.

Four sets of eclipsed C-H interactions & one

flagpole interaction

This also has bond angles of 109.5° and thus avoids

any angle strain, but there is torsional strain.

The two hydrogens at the ends of the boat are in

close contact, causing torsional strain. These flagpole

hydrogens are eclipsed.

53

Twist-boat conformation To avoid these unfavorable interactions, the boat

conformation skews slightly, giving a twist boat

conformation. The twist boat conformation has a

lower energy than the pure boat conformation, but is

not as stable as the chair conformations

approximately 41.8 kJ (5.5 kcal)/mol less stable

than a chair conformation

approximately 6.3 kJ (1.5 kcal)/mol more stable

than a boat conformation

Half-chair

Half-chair

Skew boat

Half-chair

Skew-boat

45

kJ/mol

45

kJ/mol

23

kJ/mol

59

The chair is the lowest energy conformation, although since

the energy barrier to ring flip is fairly small, there will always

be some other conformations present.

The half chair is the point of highest energy, and is not a

stable conformation.

Axial and Equatorial Bonds in

Cyclohexane

The chair conformation has two kinds of positions for substituents on the ring: axial positions and equatorial positions

Chair cyclohexane has six axial hydrogens perpendicular to the ring (parallel to the ring axis) and six equatorial hydrogens near the plane of the ring

61

• Each carbon atom in cyclohexane has one axial and one equatorial hydrogen

• Each face of the ring has three axial and three equatorial hydrogens in an alternating arrangement

How to Draw Cyclohexane

Step 1: Draw two parallel lines slanted

downward

Step 2: Draw two lines starting from the

parallel lines slanting upward

and intersecting at a point.

Step 3: Draw two lines downward

starting from the other end of

the parallel lines and intersecting

at another point.

63

How to make Axial bonds and

Equatorial bonds

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Chair–Chair Interconversion/

Ring Flip

An most important phenomenon in chair

conversion is that any substituent that is axial in

the original conformation becomes equatorial in

the new conformation (exchange of axial and

equatorial positions by a ring-flip )

65

All axial bonds become equatorial

All equatorial bonds become axial

All “up” bonds stay up

All “down” bonds stay down

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Example:

Axial-up becomes Equatorial-up

67

Equatorial Conformation is Preferred……WHY????

A Conformational Analysis of Methyl cyclohexane

Substituted cyclohexane

• Exists in two different chair forms

H

G

HG

69

Axial Methyl in Methylcyclohexane

70

Equatorial Methyl Group

71

Cyclohexane ring rapidly flips between chair conformations at room temp.

Two conformations of monosubstituted cyclohexane aren’t equally stable.

The equatorial conformer of methyl cyclohexane is more stable than the axial by 7.6 kJ/mol

72

1,3-Diaxial Interaction

5% 95%

Van der Waals/ steric repulsions between axial

substituents on a cycloalkane ring

73

The axial substituent interferes with the axial

hydrogens on C1 and C3. This interference is called

a 1,3-diaxial interaction.

Hydrogen atoms of the axial methyl group on C1 are

too close to the axial hydrogens, three carbons away

on C3 and C5, resulting in 7.6 kJ/mol of steric strain

. Difference between axial and equatorial conformers

is due to steric strain caused by 1,3-diaxial

interactions

74

Tert-butylcyclohexane

Substituents are less crowded in the equatorial

positions.

Mono substituted Cyclohexane

Less than 0.01% Greater than 99.99%

40% 60%

Crowding is less pronounced with a "small"

substituent such as fluorine.

Size of substituent is related to its branching.

Fluorocyclohexane

F

F

Keq = [equatorial conformer]/[axial conformer]

• The larger the substituent on a cyclohexane ring, the

more the equatorial substituted conformer will be

favored

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Substituent Axial – equatorial energy

difference kJ mol-1

% equatorial

H 0 50

OMe 2.5 73

Me 7.3 95

Et 7.5 95

iPr 9.3 98

tBu >20 >99.9

110 11.7 99

Substituted cyclohexanes:energy difference

78

Chapter 4

Disubstitued Cycloalkanes Can exist as pairs of cis-trans stereoisomers

– Cis: groups on same side of ring

– Trans: groups on opposite side of ring

80

Cis-1,3-dimethylcyclohexane

Cis-1,3-dimethylcyclohexane can have both methyl

groups in axial positions or both in equatorial positions.

The conformation with both methyl groups being

equatorial is more stable. However, both conformations

are equal in energy.

81

82

Trans-1,3-dimethylcyclohexane

Both conformations have one axial and one equatorial

methyl group so they have the same energy.

Methyl groups are on opposite faces of the ring Steric strain of 4 3.8 kJ/mol = 15.2 kJ/mol makes the diaxial conformation 11.4 kJ/mol less favorable than the diequatorial conformation trans-1,2-dimethylcyclohexane will exist almost exclusively (>99%) in the diequatorial conformation

both methyl groups equatorial

•no 1,3-diaxial interactions

•both methyl groups axial

• four 1,3-diaxial interactions

CH3

ring

flipH3C

CH3H3C

H3C

H3C CH3

(more stablebecause largegroup isequatorial)

(less stablebecause largegroup isaxial)

CH3

Trans-1-tert-Butyl-3-methylcyclohexane

2 | 84

Cis-1,3-Disubstituted Cyclohexanes

ring

flip

(more stable)

CH3

H

CH3

H

CH3CH3

H H

(less stable)

2 | 85

Trans-1,2-Disubstituted Cyclohexanes

ring

flip

trans-1,2-Dimethylcyclohexane

CH3

CH3(eq)

(ax)

(ax)

(eq)

CH3

CH3

diequatorial(much more stable)

diaxial(much less stable)

Cis-1,4-Disubstituted Cyclohexanes

H

HH

H3C

CH3 CH3

HCH3

ring

flip

Equatorial-axial Axial-equatorial

chair-chair

CH3

CH3

ring

flipH3C

CH3H3C

H3C

H3C CH3

(more stablebecause largegroup isequatorial)

(less stablebecause largegroup isaxial)

Cis-1-tert-Butyl-4-methylcyclohexane

89

Cis-1,4-ditertbutylcyclohexane

The most stable conformation of cis-1,4-di-

tertbutylcyclohexane is the twist boat. Both chair

conformations require one of the bulky t-butyl groups

to occupy an axial position.

90

cis 1,3-Dimethylcyclohexane

91

trans 1,3-Dimethylcyclohexane

92

cis 1-Chloro-4-t-butylcyclohexane

CH3

ring

flipCH3

CH3CH3

cis-1,2-Dimethylcyclohexane(equal energy and equallypopulated conformations)

(equatorial-axial) (axial-equatorial)

(eq)

(ax)

(eq)

(ax)

Cis-1,2-Disubstituted Cyclohexane

94

Cyclohexane Stereochemistry

Cis -Trans Isomers

Position cis trans

1,2 e,a or a,e e,e or a,a

1,3

1,4

a = axial; e = equatorial

e,a or a,e e,e or a,a

e,e or a,a a,e or e,a

Conformations of Polycyclic Molecules

Decalin consists of two cyclohexane rings joined to share two carbon atoms (the bridgehead carbons, C1 and C6) and a common bond

Two isomeric forms of decalin: trans fused or cis fused

In cis-decalin hydrogen atoms at the bridgehead carbons are on the same face of the rings

In trans-decalin, the bridgehead hydrogens are on opposite faces

Both compounds can be represented using chair cyclohexane conformations

Flips and rotations do not interconvert cis and trans

97

Trans-fused cyclohexane ring is more stable than cis-

fused cyclohexane ring

99

Problems

• Is this the most stable conformer?

101

Problem- 1

A. Draw both chair conformations of cis-1,2- dimethylcyclohexane, and determine which conformer is more stable?

B. Repeat for the trans isomer.

C. Predict which isomer (cis or trans) is more stable.

102

A. There are two possible chair conformations for the

cis isomer, and these two conformations interconvert

at room temperature. Each of these conformations

places one methyl group axial and one equatorial,

giving them the same energy.

103

B. There are two chair conformations of the trans isomer

that interconvert at room temperature. Both methyl

groups are axial in one, and both are equatorial in the

other. The diequatorial conformation is more stable

because neither methyl group occupies the more

hindered axial position.

104

C. The trans isomer is more stable. The most stable

conformation of the trans isomer is diequatorial and

therefore about 7.6 kJ/mol (1.8 kcal/mol) lower in

energy than either conformation of the cis isomer,

each having one methyl axial and one equatorial.

Remember that cis and trans are distinct isomers and

cannot interconvert.