Conformations of Cyclohexane handout PDF

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Summary

shows you how to draw out the conformations of cyclohexane...

Description

Cycloalkanes Initially, in 1885, it was believed that cycloalkanes are planar and they represented these saturated hydrocarbons with polygons (triangle for cyclopropane, square for cyclobutane, pentagon for cyclopentane, hexagon for cyclohexane, etc). Due to the distortion of the sp3 hybridized carbon angles from 109.5°, there was considerable ANGLE STRAIN in these cyclic molecules. It was assumed that cyclopentane (bond angle 108°) was most stable (closest angle to 109.5); however these molecules are NOT planar but rather adopt bent geometric shapes to overcome two other types of strain: TORSIONAL and VAN DER WAAL’S. Through heat of combustion studies it was determined that cyclohexane is the most stable ring (653 kJ/mol per CH2 in ring), so the greatest focus is on the conformations of cyclohexane, which is prevalent in nature and many organic molecules.

Boat and Chair Conformations of Cyclohexane 1. To be free of angle strain, tetrahedral bond angles are necessary. So nonplanar geometry is required-cycloalkanes ARE NOT FLAT!!! Two conformations meet this requirement:

2. To be free from torsional strain, the bonds in cyclohexane should be staggered (like the chair conformer), not eclipsed (like they are in the boat form).

In the chair, all bonds are staggered so no torsional strain; in the boat, the hydrogen atoms are all eclipsed, so high torsional strain. You can twist the carbon backbone in the “boat” conformation to release some of the torsional strain that results from the eclipsing of the hydrogen atoms. This forms the “twist-boat” conformation that is still much less stable than the chair conformation where all the hydrogen atoms are staggered.

See full energy diagram on last page of lecture notes.

Axial and Equatorial bonds in Chair Conformer:

Conformational Inversion: Ring flipping (chair to chair interconversion) is a rapid process (needs about 11 kcal/mol of energy). In ring flipping, equatorial and axial bonds switch positions, so: ALL AXIAL BONDS Æ EQUATORIAL; ALL EQUATORIAL BONDS Æ AXIAL

A

E C B

F

RING FLIP A

F

E CD B

D

Equatorial Bonds

Axial Bonds

Most stable position for substituents on a chair: Equatorial position

Conformations of Disubstituted Cyclohexanes: Recall “geometric” or “cis-trans” isomers. In rigid rings, if both substituents are on the SAME SIDE (FACE) of the ring, this is a “cis” geometric isomer; if two substituents are on OPPOSITE SIDE (FACE) of the ring, this is a “trans” geometric isomer. Disubstituted cyclohexanes can be referred to as “cis” or “trans” since they can have the two groups on the same side of the ring (cis) or on opposite sides (trans). To determine stability, 1,3-diaxial interactions must be considered.

1,4-cis:

1,4-trans: H

H H 4

1

ring flip CH 3

CH 3

H3 C

1 H

H3C 4

CH3

H

4

equal stability 1,4: a,e

1,4: e,a

1

ring flip CH3

H

4

1 H CH3

1,4: a,a

1,4: e,e MORE STABLE

1,3-cis: H

CH3

H

1,3-trans: H

1 CH3 3 CH3 1,3: e,e MORE STABLE

ring flip

1 H

H

3 CH3 CH3 1,3: a,a

CH3 3 H

H

ring flip

1 CH3

1,3: e,a

H3C

1 H 3 H

equal stability 1,3: a,e

CH3

1,2-cis:

1,2-trans:

H 1 HCH3 2 CH3

ring flip

H3C 2 equal stability

1,2: e,a

H 1 CH3 CH3 2 H

H 1 H CH3

1,2: a,e

ring flip

1,2: e,e MORE STABLE

Practice Exercises: 1. myo-Inositol acts as a growth factor in both animals and microorganisms. Draw the most stable chair conformation of myo-inositol.

OH OH OH OH OH

OH

myo-inositol

2. Draw the two chair conformations of trans-1-chloro-2-methylcyclohexane. Which is more stable?

3. Draw the two chair conformations of 1,1,3-trimethylcyclohexane and estimate the amount of strain energy in each (use the tables given in lecture notes). Which conformation is favored?

CH3 1 H H 2 CH3 1,2: a,a...