What is Conformation?
Conformation can be defined as the shape adopted by a molecule caused by the rotation around one or more single bonds. For instance, in the case of alkanes, there is a distribution of electrons around the internuclear axis of the C-C bond. This further permits free rotation around C-C single bonding which leads to different spatial arrangements of the carbon atoms. The process explained above is called conformation. Moreover, alkanes thereby also have an infinite number of conformations by rotation around single C-C bonds. The Conformational Isomers are also ranked differently. The ranking of conformational isomers is contingent upon the energy levels.
What are Conformational Isomers?
Conformational isomers are commonly referred to as stereoisomers as they can be converted into one another by rotation around the sigma single bond. For example, in the case of ethane, the different spatial arrangements of hydrogen atoms attached to one carbon atom concerning the hydrogen atoms attached to the other carbon atom are observed, and these are called conformational isomers or conformers.
As mentioned above, the ranking of conformational isomers depends on the energy levels. Considering the energy levels to be lowest to highest, the ranking of conformational isomers are: anti, gauche, eclipsed, and fully eclipsed.
Types of Conformational Isomers
Even though there are infinite spatial arrangements created by conformational isomers, there are two broad categories classified into two different cases as Eclipse conformation and Staggered Conformation.
Eclipsed Conformation
In the case of Eclipse conformation, hydrogen atoms are attached to two carbon areas that are as near to each other as possible. A torsion angle is created between two substituents such as X and Y with adjacent atoms such as A and B. This torsion angle between X-A-B-Y is of 0o. This is how conformed are formed by eclipsed conformation.
Staggered Conformation
On the other hand, in the case of Staggered conformation, hydrogen atoms are attached to two carbons that are as far as possible from each other. Moreover, in terms of stability, staggered conformation is relatively more stable and the repulsive forces are minimum, energy is minimum due to large separations between the electron clouds of C-H bonds.
Representation of Eclipsed and Staggered Conformation
There are two popular ways of representation of eclipsed and staggered conformation. These representations are known as Sawhorse projections and Newman Projections. Let's define sawhorse projections first.
Sawhorse Projections
In Sawhorse projections, carbon atoms are bonded as if they are in a long straight line. In this straight line, the lower end of the line designates the front carbon atom whereas the rear carbon atom is designated by the upper end. Another key characteristic of the Sawhorse projection is that the C-H bonds are inclined at an angle of 120° to each other.
Newman Projections
The second way of representation of eclipsed and staggered conformation is known as Newman's projections. A new man projection is also helpful in the stereochemistry of alkanes. For example, in the case of ethane, out of the two carbon atoms present, the closer one is marked as a dot while the rear carbon atom is represented as a circle. The front carbon is called proximal and the back carbon is called the distal.
A specific dihedral angle is illustrated in this type of representation between the proximal and distal atoms. The lines are again inclined at an angle of 120° to each other in such a way that the three lines, from three hydrogen atoms attaching to one carbon atom, seem to either bulge out of the circle or diverging from the dotted lines.
FAQs on Conformation
1. What is meant by conformation in chemistry, as per the CBSE Class 11 syllabus?
In chemistry, a conformation refers to any of the infinite number of possible spatial arrangements of atoms in a molecule that can be interconverted by rotation around single bonds. These different arrangements are also known as conformers or rotamers. For example, in an ethane molecule, the two methyl groups can rotate freely around the central carbon-carbon (C-C) single bond, leading to various three-dimensional shapes without breaking any bonds.
2. What are the two primary methods for representing the conformations of molecules?
The two most common methods used to represent the three-dimensional structures of conformers on a two-dimensional surface are:
- Sawhorse Projections: This method depicts the molecule from an oblique or side-on angle. The carbon-carbon bond is drawn diagonally, and all C-H bonds are shown. The front carbon is typically shown at the lower left, and the rear carbon at the upper right.
- Newman Projections: This method views the molecule directly down the axis of a specific carbon-carbon bond. The front carbon is represented by a dot, and the rear carbon is represented by a circle behind it. The bonds attached to each carbon are drawn as lines originating from the dot or the circumference of the circle.
3. What is the main difference between the staggered and eclipsed conformations of ethane?
The main difference lies in the arrangement of hydrogen atoms and the resulting stability:
- In the staggered conformation, the hydrogen atoms on the rear carbon are positioned exactly in between the hydrogen atoms on the front carbon. The dihedral angle is 60°, maximising the distance between them.
- In the eclipsed conformation, the hydrogen atoms on the rear carbon are directly behind the hydrogen atoms on the front carbon. The dihedral angle is 0°, minimising the distance between them.
4. Which conformation of ethane is more stable, and why?
The staggered conformation of ethane is more stable than the eclipsed conformation. This is because the staggered arrangement minimises the repulsive forces between the electron clouds of the C-H bonds on adjacent carbon atoms. This repulsion, known as torsional strain, is at its maximum in the eclipsed conformation where the bonds are closest together, making it less stable and higher in energy.
5. Why can't different conformers of a molecule like ethane be isolated as separate compounds at room temperature?
Different conformers of ethane cannot be isolated because the energy barrier required to rotate around the C-C single bond is very low (approximately 12.5 kJ/mol). The thermal energy available at room temperature is sufficient to overcome this barrier easily. As a result, the staggered and eclipsed conformations interconvert millions of times per second, making it impossible to separate them as distinct, stable compounds.
6. What is the difference between 'conformation' and 'configuration' in stereochemistry?
This is a crucial distinction in stereochemistry. Conformations are different spatial arrangements of the same molecule that can be interconverted simply by rotation around single bonds. No bonds are broken in this process. In contrast, configurations (such as E/Z isomers or R/S enantiomers) are different spatial arrangements that can only be interconverted by breaking and re-forming chemical bonds. This requires a significant amount of energy, and configurational isomers are distinct, isolable compounds.
7. How does the conformational analysis of butane differ from that of ethane?
The conformational analysis of butane is more complex than ethane because it involves rotation around the central C2-C3 bond, where each carbon is bonded to methyl (CH₃) groups, not just hydrogen. This introduces a new type of strain called steric strain (repulsion between bulky groups). Butane has unique conformers not seen in ethane, such as:
- Anti-conformation: The most stable form, where the two methyl groups are 180° apart, minimising both torsional and steric strain.
- Gauche conformation: A type of staggered form where the methyl groups are 60° apart. It is less stable than the anti-form due to steric hindrance between the methyl groups.
8. What is the primary purpose of performing a conformational analysis on a molecule?
The main purpose of conformational analysis is to study how the potential energy of a molecule changes with rotation around its single bonds. By doing this, we can identify the most stable conformation (the one with the lowest energy) and the energy barriers between different conformations. This understanding is critical for predicting a molecule's physical properties, shape, and chemical reactivity, as molecules tend to exist predominantly in their lowest-energy state.
