Conformation Of N-Butane-1 - Stereochemistry | Organic Chemistry PDF Download

Conformation of n-butane:-

Butane may be treated as a derivative of ethane where one hydrogen on each carbon is replaced by a methyl group. The conformat ion of butane will be symmetrical only if the rotation will be about C₂-C₃ bond.

Butane has three conformations which are staggered (B, D, F). (D) in which the two methyl groups are as far apart as possible is more stable than the other two staggered conformations (B) and (D). The most stable of the staggered is called anti conformation and other two conformations are called gauche conformation.
In the anti conformation the larger substituents are opposite to each other, in the gauche conformer, they are adjacent. The two gauche conformers have the same energy barrier but each is 0.9 kcal/mo le less stable than the anti conformation.
Anti and gauche conformers do not have the same energy because of steric strain. Steric strain is the strain put on a molecule when its atoms or groups are large in size and due to this they are too close to each other, which cause repulsion between the atoms or groups.
All other eclipsed conformation has torsional and steric strain. The (A) conformation is much unstable because the two methyl groups eclipsed each other and cause much steric strain.

Anti(D) > gauche (B, F) > eclipsed (C, E) > fully eclipsed (A).

Cyclopropanes 

1. The molecule of Cyclopropane is planar
2. Its bond angle is 60⁰(C-C-C).
3. Bent bond between C-C atoms.
4. It has been suggested that in cyclopropane , carbon uses Sp² orbital’s for (C-H) ( which are short and stronger) but it uses orbital’s having greater p-character for the format ion of C-C bonds  (Sp⁴ to Sp⁵)
5. The greater p-character (Sp⁴ to Sp⁵) makes these orbital’s larger in size and consequent ly the C-C bond in cyclopropane is longer than the C-H bond.
6. The orbital’s (used for C-C bonds) are capable of bending and their overlap gives bent bonds (also called banana bond) the bending of orbital’s permits their weak overlap only thereby making the ring unstable.
7. Because of bending of orbital’s a considerable part of the overlapping orbitals is protruded outside the ring and hence becomes susceptible to the attack of electrophile reagents.

Cyclobutane 

1. Bond angle 90⁰ (less angular strain as co mpare to cyclopropane).
2. Four strained bonds are present rather than three.
3. Eight pairs of eclipsed hydrogen’s are rather than six in cyclopropane. So, total ring strain in the two compounds is almost the same.

Folding of a methylene group in Cyclobutane.

Cyclopentane 

1. Planar Cyclopentane ring, the interior C-C-C bond angle of 108⁰ approaches the normal tetrahedral angle of 109⁰28⁰. So, expected free from angle strain.
2. All hydrogen’s are eclipsed. So, there is a torsional strain.

Actually, Cyclopentane exists in “Envelope” shape. By doing this the ring is relieved of considerable torsional strain at the expense of a little angle strain.

Cyclohexane and its derivative

A.Chair conformation of cyclohexane

This conformation of cyclohexane resembles a chair and hence is called chair form.

1. This conformation is free from angle strain.
2. The close resemblance of the chair conformation of cyclohexane with the staggered conformation of ethane can be well appreciated from the following diagrams..

Resemblance between chair conformation of cyclohexane and ethane in staggered form.
1. The chair form of cyclohexane represents the staggered conformation and hence it is free from torsional strain.
2. The chair form, therefore lies at the energy minimum and is the most stable conformation of cyclohexane and of almost all its derivatives.

B. Boat conformation of cyclohexane

If we flip up the lower end of the model of the chair form we will get the boat conformation

1. It is obvious that these results fro m rotation about single bonds, hence we are indeed dealing with the conformation of cyclohexane.
2. If we sight the model of the boat conformation from the front side, we find the hydrogen’s on two sets of C-C bonds in perfectly eclipsed conformation 

Boat conformation of cyclohexane 

1. The boat conformat ion of cyclo hexane is expected to have considerable torsional strain ~ approximately equivalent to the two eclipsed ethane molecules (2*3=6 kcal/mole). The result however indicates the boat conformation to be less stable than the chair form by about 7.1 kcal/mole. It leads to the conclusion that beside torsional strain, some other factor is also contributing to the boat conformation.
2. If we look closely at the model o f the boat conformat ion fro m the front side, it is found that the t wo hydrogen’s are very close to each other. These hydrogen’s are called flagpole hydrogen’s. It has been found that the flagpo le hydrogen’s lie only 1.83 A⁰ apart, considerably closer than the sum o f their Vander Waals radii. This leads to the development of Vander Waals repulsion between two flagpole hydrogen’s. The combined effects of torsional strain and Vander walls repulsion between flagpole hydrogen’s make the boat conformation considerably less stable then the chair conformation.

C. Twist boat conformation of cyclohexane 

1. If we twist the two flagpo le hydrogen’s in the mo del o f the boat conformat ion of cyclo hexane in the opposite direction a new conformation results, which is known as twist-boat conformation.

2. The hydrogen’s on two sets of C-C bonds are not in perfect ly eclipsed posit io ns, which relieves the torsional-strain considerably. The distance between flagpo le hydrogen’s also increases, as a result of which the Vander Waals repulsion between them decreases. The twist boat conformation is therefore more stable than the boat conformation.

D. Half-chair conformation of cyclohexane

1. If we flip the lower end of chair form then new conformation results known as half chair. In it H atoms of adjacent carbon atoms are fully eclipsed.

2. In half chair there is angular strain. So , half chair conformation is less stable.
 

Stability order:-Chair > Twist boat > Boat > Half-Chair

Conformational free energy

The two diastereomeric chair forms are of unequal energy and so are differently populated, the equilibrium constant K being given by the equation:-

∆G⁰=-RT lnK

∆G⁰ is the difference of free energy between the Equatorial and axial conformers and -∆G⁰ is known as conformational free energy of the substituent. (Sometimes known as A value).
It determines the equatorial preference of the substituent in the subst ituted cyclohexane. The conformational free energies (-∆G⁰ values) of a number of common subst ituent are given below

*a=aprotic so lvent; b= protic so lvent.