Conformations of Mono & Disubstituted Cyclohexanes | Organic Chemistry PDF Download

Monosubstituted cyclohexane:-

1. When methyl group is axial it is sufficient ly closer to the Syn axial Hydrogen’s to undergo 1,3-diaxial interactions and is repelled by them ( This is another example of trans-annular strain)

2. This 1, 3-diaxial interact ion is similar to that in gauche conformat ion o f butane. The axial methyl group in methyl cyclohexane is thus gauche to two ring carbons and when in equatorial positions it is anti to same nuclei.

3. When methyl group is equatorial, there are no 1, 3-diaxial interaction.

4. Equatorial methyl group don’t show any gauche butane like interactions.
Monosubstituted cyclohexane exists in two non-equivalent diastereomeric chair conformations.

I) is less stable than (ii) because of the presence of 1, 3-diaxial interactions.

Disubstituted cyclohexane

Stability:- A > C=D > B

Enantiomerism in 1, 2-dimethyl cyclohexane: - In 1, 2-dimethylcyclohexane two chiral centers are present and hence can have more than four stereoisomers but actually it has only three. The Cis-1, 2-dimethylcyclohexane molecule is not superimposable on its mirror image but the molecule and its mirror  image are readily interconvertible by flipping one chair conformation into another. Hence these are conformational enantiomers. There is a rapid Interconversion at room temperature.

                                      Axial-equatorial cis-1, 2-dimethylcyclohexane 

The trans-1, 2-dimethylcyclohexane molecule and its mirror image are not superimposable, hence constitute an enantiomeric pair. These are not interconvertible by flipping one chair form into another. On flipping the equatorial is converted into axial. The two isomers are known as configurational isomers. Each isomer is optically active and mixture is resolvable. 

                            Trans -1, 2-dimethylcyclohexane 

In summary 1, 2-dimethylcyclohexane exists as a pair of Diastereomers, the Cis and trans isomers. The Cis isomer exists as a pair of conformational enantiomer, whereas the trans isomer exists as a pair of configuration Diastereomers.

Chirality and optically activity 
All 1, 2-disubstitutedcyclohexanes are achiral due to presence of plane of symmetry and two fold rotational axis hence all are optically inactive.

1, 3-disubstitutedcyclohexane

Stability: - A>C=B>D

1, 3-dimethylcyclo hexane has two chiral centers, and can have  four stereoiso mers (2²=4). Actually there are only three, the cis-1, 3-dimethylcyclohexane has a plane of symmetry and is achiral. Trans isomer has a twofold rotational axis hence it is also achiral. If different substituents are present all will be chiral. 

1, 4-disubstututedcyclohexane

Stability: - A > C = D > B

In 1, 4-dimethylcyclohexane it does not have any chiral centre. It exists as Cis and Trans Diastereomers. Neither Cis nor Trans form is chiral because both have a plane of symmetry.
 

Chirality:- All are achiral due to the plane of symmetry If different substituent’s then all 1, 4-disubstituted will be chiral.

Factors affecting stability of cyclohexane derivatives

1. Steric strain α 1/ stability
2. Torisonal strain α 1/ stability

3. Dipole moment:- If µ > 0 less stable, If µ=0 more stable If electronegative group is present then 1,2-diaxial is more stable than 1,2-diequatorial due to dipoledipole repulsions.

4. Hydrogen bonding:- In cases where  hydrogen bonding is possible, gauche form is more stable than the staggered form as in case of ethylene glycol.

Stable conformations of cyclohexandiols 

A. 1,4-diol :-

B. 1,2-diols:-

C. 1,3-diols:-

Problem:- Draw the most stable chair conformation of following cyclohexanes

Decalins- bicyclo[4,4,0]decane

Trans- decalin:

Trans decalin is obtained by fusion between two equatorial bonds of cyclohexanes with 4 carbon system. In this two hydrogen atoms on the bridge head carbon are opposite to each other. Achiral, more stable due to diequatorial type structure.

Cis-decalin :

When one axial and one equatorial bond of cyclohexanes ring are used for fusing 4 carbon chain system a decalin molecule results which has two hydrogen on the same side of bridgehead carbon.

(i) Less stable due to non-bonding interactions
(ii) No plane of symmetry
(iii) Molecule is chiral

Cis- and trans- isomers are conformational isomers. Energy of trans decalin is 25 kj/mol lesser than cis decalins

Stability Trans decalins are generally more stable. If 1 position of decalin is substituted by Me- group then stabilit y is reversed