Conformational Isomers

In alkanes, the allocation of electrons in sigma molecular orbital is balanced around the internuclear axis of the C-C bond. Therefore, it allows them to freely rotate around the C-C single bond. Due to this rotation, different spatial arrangements of carbon atoms in space are seen which can transform into one another. Such spatial arrangement of carbon, hydrogen atoms which can be changed into one another by rotation around a C-C single bond is known as conformation. Alkanes can accordingly have an infinite number of conformations by rotation around C-C single bonds. Yet, this rotation is not completely free due to repulsive interactions between the electron clouds of C-H bonds. This repulsive interaction is called the torsion strain.

For any molecule, Diverse conformations can take place in which one covalent-bond connects two polyatomic groups, in each of which at least one atom does not lie along the axis of the single bond being referred to. Hydrogen peroxide is the simplest type of molecule, in which the 2 hydroxyl groups can rotate with respect to one another about the axis of the oxygen-oxygen bond. The occurrence of more than one such single bond in a molecule causes complications in the situations without changing its nature. One example of this type of molecule is propane (CH₃―CH₂―CH₃). 

Generally, each noticeable conformation of a molecule represents a position of different potential energy because of the process of attractive or repulsive forces that differ with the distances between different parts of the structure. If these forces were not present, all conformations would have the same energy, and rotation around the single bond would be completely free or unrestricted. If the forces are strong, different conformations vary greatly in energy or stability: the molecule will normally occupy a steady state (one of low energy) and experience a change to another steady state only upon absorbing enough energy to reach and pass through the uneven intervening conformation.

The intra-molecular forces in ethane, for instance, are so weak that their survival can be concluded only by subtle effects on thermodynamic properties like enthalpy and entropy. (The three most stable conformations in ethane are indistinguishable, even if internal rotation in ethane were severely restricted). The molecular integrity and structures of some of the extra complex compounds will impose such a strong and significant barrier to a rotation that these stereoisomers will form to be more than stable to be able to isolate them.

Conformational Isomers

Let's understand the fundamentals of conformation with the examples of ethane. If we monitor the ball and stick model of ethane and rotate one carbon atom keeping another carbon atom still around the C-C axis. We will see that the rotations will result in an infinite number of spatial arrangements of Hydrogen atoms connected to one carbon atom with respect to the hydrogen atoms attached to the other carbon atom. These different arrangements are commonly known as conformational isomers or conformers.

Mainly, these can be largely classified into 2 different cases:

1. Eclipse Conformation

Eclipse Conformation is a conformation in which the hydrogen atoms are connected to 2 carbon areas closest to each other possible.

In chemistry an eclipsed conformation is a conformation in which two substituent X and Y on neighboring atoms A, B are in the closest distance, showing that the torsion angle X–A–B–Y is 0°. Such a type of conformation exists in any open chain, where a single chemical bond is connecting two sp3-hybridised atoms, and it is usually the highest level of conformational energy. This maximum level is often explained by steric hindrance, but its origins now and then lie in hyper conjugation (as when the eclipsing interaction is of two hydrogen atoms).

In the case of ethane in Newman projection, it shows that the rotation around the carbon-carbon bond is not completely free but that an energy barrier exists. The ethane molecule in the eclipsed conformation is said to undergo the torsion strain and by a rotation around the carbon-carbon bond to the staggered conformation around 12.5 kJ/ mol of torsion, energy is released.

2. Staggered Conformation

Staggered Conformation is a kind of conformation in which the hydrogen atoms that are connected to 2 carbons are as far as possible to each other. The staggered conformation is therefore relatively more stable as compared to the eclipse conformation as there are minimum repulsive forces, minimum energy due to much division between the electron clouds of C-H bonds.

These conformations exist in any open chain single chemical bond connecting 2 sp3-hybridised atoms, and usually, have a very low level of conformational energy. For some molecules such as n-butane, there can be particular versions of staggered conformations called gauche and anti.

Representation of Eclipsed and Staggered Conformation:

Sawhorse Projections:

In this type of projection, the bond between carbon atoms is shown as a long straight line. The lower part of the line classifies the front carbon atom whereas the upper part classifies the rear carbon atom. Since each carbon atom in ethane is connected to three hydrogen atoms; each carbon atom has three lines connected designating C-H bonds elevated at an angle of 120° to each other.

Newman Projections:

In this projection, out of the 2 carbon atoms present in ethane, one which is closer is shown as a dot whereas the rear carbon atom is shown as a circle. The three hydrogen atoms connected to each carbon atom are represented with the help of three lines either bulging out of the circle or diverging of the dotted lines. These lines are elevated to each other at an angle of 120° to each other.

Main Difference – Staggered Vs Eclipsed Conformation

The term conformation describes diverse forms of projections of a molecule. In other words, it is the name given for various positions that a molecule can be bent into. The arrangement of atoms in a molecule has a vast effect on the strain of the molecule. The steadiness of a molecule is high if it has a low strain conformation. Newman projections, which are usually used in Alkane stereo-chemistry, demonstrate the conformation of the molecule when seen through the C-C bond in the front-back direction. Staggered conformation and eclipsed conformation are 2 types of Newman projections that illustrate the spatial arrangement of atoms. The main difference between eclipsed conformation and staggered conformation is that staggered conformation has lesser potential energy whereas eclipsed conformation has the highest potential energy.

Difference Between Staggered and Eclipsed Conformation

Definition

Staggered Conformation: Staggered conformation is the collection of atoms or groups of atoms in a molecule resulting in a 60⁰ dihedral angle.

Eclipsed Conformation: Eclipsed conformation is the collection of atoms or groups of atoms in a molecule resulting in a 0⁰ dihedral angle.

Potential Energy

Staggered Conformation: Staggered conformation has lesser potential energy than eclipse conformation.

Eclipsed Conformation:  Eclipsed conformation has higher potential energy. 

Strain

Staggered Conformation: Staggered conformation known as a low strain structure.

Eclipsed Conformation: Eclipsed conformation known as a high strain structure.

Stability

Staggered Conformation: The steadiness of staggered conformation is high.

Eclipsed Conformation: The steadiness of eclipsed conformation is low.

Conformations of Ethane

A form of stereoisomerism where interconversions of isomers are possible by rotations referring to single bonds is called as Conformational Isomerism. These isomers are termed as Conformational isomers. In case of single bond rotation, Rotational Energy acts as a barrier Conformational Isomerism to occur, the energy barrier must be a small one. There are numerous types of conformational isomers, such as Ethane and Butane.

Conformations of Ethane

As we have seen before there can actually be an infinite number of these conformations when looking at ethane due to the C-C single bond. But now we will focus on the two important conformers of the gas. These two conformations are known to be the most opposite the most extreme conformations of ethane.

When we notice a structure of ethane we will notice a single C-C bond and every atom tends to be bonded to three atoms of hydrogen per bond. In the first conformation, hydrogen is bonded to the first carbon atom and they will line up with the hydrogen atom of the second carbon. The dihedral angle which is the angle between two planes is 0°. If we notice the first hydrogen atom, it can be said to be circling the second one and is thus known as an Eclipsed Conformation. There is also another opposite conformation. In this conformation, the hydrogen atoms bond to the carbon atoms and are as far away as they can be. Here the dihedral angle between the two. This conformation is known as ‘staggered conformation'.

In an ethane conformation, the eclipsed conformation is known to be very unstable. There is an unfavorable interaction between the two hydrogen atoms they are lined up together and they repel one another. Less energy is required in staggered conformation to maintain it and is also more common. 

Conformation of Alkanes

As we have come to know, alkanes have a very simple C-C single bond in the higher alkanes. A bond does like this does not exist in methane, but onwards to ethane, all alkanes have a single bond of C-C. This will allow the rotation of these single bonds to form different spatial arrangements of the carbon atoms, which in turn form different conformations of these alkanes.

Well, ideally this rotation should be limitless and an infinite number of conformations should form from this arrangement but there is a strain called the torsional strain which we need to consider. This acts as a resistance to the bond rotation. This strain tends to occur due to a repulsive interaction of other bonds in alkanes; a C-H bond. In the end, this will limit the conformation number which is able to form.

Conformation of Butane

• Now let us consider butane, a little bigger molecule. There are now 3 rotating carbon-carbon bonds to consider, but we will focus on the center bond between C₂ and C₃. Below are two examples of butane in a conformation which puts the 2 CH₃ groups (C₁  and C₄) in the eclipsed position. 

• This is the maximum energy conformation for butane, due to what it is called as ‘van der Waals repulsion’, or ‘steric repulsion’, between the 2 rather bulky methyl groups. 

• For instance: you probably like to be near your friends, but no matter how close you are you probably don’t want to split a one-room apartment with 5 of them. When the 2 methyl groups are brought too close together, the overall resulting non-covalent interaction is repulsive rather than attractive. The result is that their own electron densities repel one another.

• If we turn the front, (blue) carbon by 60°clockwise, the butane molecule is now in a staggered conformation.

• This is more exclusively referred to as the ‘gauche’ conformation of butane.  Notice that even though they are staggered, the 2 methyl groups are not as far apart as they could possibly be.  There is still major steric repulsion between the 2 bulky groups.

• Further rotation of 60°results into a second eclipsed conformation (B) in which both methyl groups are lined up with hydrogen atoms.

• Because of the steric repulsion between methyl and hydrogen substituent, this eclipsed conformation B is superior in energy than the gauche conformation.

• However, because there is no methyl-to-methyl eclipsing, it is inferior in energy than eclipsed conformation A. 

• One more 60 rotation gives us the ‘anti’ conformation, where the 2 methyl groups are positioned opposite each other and steric repulsion is minimized. 

• This is the lowest energy conformation for butane. 

• The diagram shown below summarizes the relative energies for the different eclipsed, staggered, and gauche conformations.

• At room temperature, butane is probably to be in the lowest-energy anti conformation at any given moment in time, however, the energy barrier between the anti and eclipsed conformations is not high enough to stop constant rotation except at very low temperatures. For this reason, it is conventional to draw straight-chain alkanes in a zigzag form, which implies anti conformation at all carbon-carbon bonds.