Lecture Lecture 33

Symmetry elements :

One example when Cn is present but the molecule is chiral. .

COOH

COOH

H

OHH

HOC2

COOH

COOH

H

OHH

HO

180'2

1

1

2

The molecule is chiral

COOH

COOH

OH

OHH

H

C

C

OHH

OHH

COOH

COOH

C

C

OHH

OHH

COOH

COOH

S1

Thus

Relation between S, and i 1

2

3

4

4

3

2

1CH3

HH

CH3

H

CH3

CH3

H

CH3

HH

CH3

S2

180'

Thus i

i

1

2 3 4 1

234

Designation of R/S :

One simple way :

1) Covert the 3D structure into the Fischer projection2) Assign priority sequence3) By exchange in a group three, bring the group of lowest priority

to one of the vertical position4) Now check the R / S

HC

I

Cl

BrC

1 2

341

1

2

2

3

3

4

4

R Configuration

Groups occupying the vertical positions are away from the observer.

chiral

Y

X

CH3

B

A

X

CH3 CH3 CH3 CH3

X

X

Y

CH3

B

A

CH3CH3NO2 NO2

X

A

Y

B

A

B

X

Y

Atropisomerism : Enantiomerism arrising out of restricted rotation

planar and inactive(achiral)

provided :

Qn.

chiral or achiral ?

CX

Y

A

BC

X

Y

A

B

ALLENES

Plane containing X and Y is orthogonal to that containing A and B

The molecule will be chiral if A is not identical with B

And X , Y are not identical

CMe

H

Me

HC C

Me

H

Me

H

Are the following molecules Chiral?

Cycloalkanes

Heat of combustion per CH2 Cyclopropane 697 KJ/ mole

Cyclobutane 686 KJ/ mole

Cyclopentane 664 KJ/ mole

Cyclohexane 658.7 KJ/ mole

Cycloheptane 662.4 KJ / mole

Cyclooctane similar 3

4

5

6

7 8 9

Stability

Ring size

(CH2)n + 3/2 n O2 n CO2 + n H2O

Origin of strain in cyclopropane / cyclobutane :

Cyclopropane :600

C

C C

H

H H

H

H H

Bayer’s strain :

Angle strain : = normal angle – actual angle = (109.50 - 600)/ 2 = 24.750

+

torsional strain : adjacent H’s eclipsed.

MO theory : Angle strain exists because the sp3 orbitals of the carbon atoms can not overlap effectively. the bonds are called “bent” or “banana” bonds.

C

C

C

H

HH

H

HH

C

C

C

H

HH

H

HH

conformations of cyclohexane :

chair form is the most stable form.

analysis of chair form

Axial bonds : perpendicular to the average plane.Equatorial bond : lies around the perimeter of the ring.

C-1, C-3, C-5 in one planeC-2, C-4, C-6 in another plane.

chair half chair twist boat boat

back

front

1

234

5 6

a

e ae

axialequatorial

1

23

4

5 6

At each carbon in cyclohexane,One axial bond, one equatorial bondOne - bond, one – bond.

Flipping

in flipping axial becomes equatorial

equatorial becomes axialBut remains and remains .

The two chairs look like mirror images.

X

Y

B

A

1

234

5 6

Y

X

A

123

45

6a

a

a

ae e e

eB

Some Important Rules:

1. bulky substituents tend to occupy equatorial positions. Putting them in axial positions incorporates what is called “ 1,3 – Diaxial Interactions”

2. i) if two substituents on different atoms are both or both , the compound will be a cis compound. ii) if the susbstituents are and or and it will be called a trans compound.

H

X

1

23

“ 1,3 – Diaxial Interactions”

X

X

X

X

X

X

cis

trans

?