Chemistry Conformations, Relative stability of conformations

Topics to be covered

=> Conformations
=> Relative stability of conformations

Conformations of alkanes

Free rotation about color{red}(C–C) single bond is possible because of symmetric electron distribution of the sigma molecular orbital around the internuclear axis of the color{red}(C–C) bond which is not disturbed due to rotation about its axis. This rotation results into different spatial arrangements of atoms in space which can change into one another.

color{green}("𝐒𝐮𝐜𝐡 𝐬𝐩𝐚𝐭𝐢𝐚𝐥 𝐚𝐫𝐫𝐚𝐧𝐠𝐞𝐦𝐞𝐧𝐭𝐬 𝐨𝐟 𝐚𝐭𝐨𝐦𝐬 𝐰𝐡𝐢𝐜𝐡 𝐜𝐚𝐧 𝐛𝐞 𝐜𝐨𝐧𝐯𝐞𝐫𝐭𝐞𝐝 𝐢𝐧𝐭𝐨") color{green}("𝐨𝐧𝐞 𝐚𝐧𝐨𝐭𝐡𝐞𝐫 𝐛𝐲 𝐫𝐨𝐭𝐚𝐭𝐢𝐨𝐧 𝐚𝐫𝐨𝐮𝐧𝐝 𝐚 ") color{green}("𝐂-𝐂 𝐬𝐢𝐧𝐠𝐥𝐞 𝐛𝐨𝐧𝐝 𝐚𝐫𝐞 𝐜𝐚𝐥𝐥𝐞𝐝 𝐜𝐨𝐧𝐟𝐨𝐫𝐦𝐚𝐭𝐢𝐨𝐧𝐬 𝐨𝐫 𝐜𝐨𝐧𝐟𝐨𝐫𝐦𝐞𝐫𝐬 𝐨𝐫 𝐫𝐨𝐭𝐚𝐦𝐞𝐫𝐬")

Alkanes can thus have infinite number of conformations by rotation around color{red}(C-C) single bonds. However, rotation around a color{red}(C-C) single bond is not completely free but it is hindered by a small energy barrier of color{red}(1- 20 kJ mol^(–1)) due to weak repulsive interaction between the adjacent bonds. Such a type of repulsive interaction is called torsional strain.

color{green}("𝐂𝐨𝐧𝐟𝐨𝐫𝐦𝐚𝐭𝐢𝐨𝐧𝐬 𝐨𝐟 𝐞𝐭𝐡𝐚𝐧𝐞 :") Ethane molecule color{red}((C_2H_6)) contains a carbon – carbon single bond with each carbon atom attached to three hydrogen atoms. Considering the ball and stick model of ethane, keep one carbon atom stationary and rotate the other carbon atom around the color{red}(C-C) axis. This rotation results into infinite number of spatial arrangements of hydrogen atoms attached to one carbon atom with respect to the hydrogenatoms attached to the other carbon atom. These are called conformational isomers (conformers). Thus there are infinite number of conformations of ethane. However, there are two extreme cases:

• One such conformation in which hydrogen atoms attached to two carbons are as closed together as possible is called eclipsed conformation

•The other in which hydrogens are as far apart as possible is known as the staggered conformation.

• Any other intermediate conformation is called a skew conformation.

In all the conformations, the bond angles and the bond lengths remain the same. Eclipsed and the
staggered conformations can be represented by Sawhorse and Newman projections.

color{green}("𝟏. 𝐒𝐚𝐰𝐡𝐨𝐫𝐬𝐞 𝐩𝐫𝐨𝐣𝐞𝐜𝐭𝐢𝐨𝐧𝐬")

In this projection, the molecule is viewed along the molecular axis. It is then projected on paper by drawing the central C–C bond as a somewhat longer straight line. Upper end of the line is slightly tilted towards right or left hand side. The front carbon is shown at the lower end of the line, whereas the rear carbon is shown at the upper end. Each carbon has three lines attached to it corresponding to three hydrogen atoms. The lines are inclined at an angle of color{red}("120°") to each other. Sawhorse projections of eclipsed and staggered conformations of ethane are depicted in Fig. 13.2.

color{green}("𝟐. 𝐍𝐞𝐰𝐦𝐚𝐧 𝐩𝐫𝐨𝐣𝐞𝐜𝐭𝐢𝐨𝐧𝐬")

In this projection, the molecule is viewed at the color{red}(C–C) bond head on. The carbon atom nearer to the eye is represented by a point. Three hydrogen atoms attached to the front carbon atom are shown by three lines drawn at an angle of color{red}("120°") to each other. The rear carbon atom (the carbon atom away from the eye) is represented by a circle and the three hydrogen atoms are shown attached to it by the shorter lines drawn at an angle of color{red}("120°") to each other. The Newman’s projections are depicted in Fig. 13.3.

Relative stability of conformations:

• In staggered form of ethane, the electron clouds of carbon-hydrogen bonds are as far apart as possible. Thus, there are minimum repulsive forces, minimum energy and maximum stability of the molecule.

• But,when the staggered form changes into the eclipsed form, the electron clouds of the carbon – hydrogen bonds come closer to each other resulting in increase in electron cloud repulsions

• Tthe repulsive interaction between the electron clouds, which affects stability of a conformation, is called color{green}("𝐭𝐨𝐫𝐬𝐢𝐨𝐧𝐚𝐥 𝐬𝐭𝐫𝐚𝐢𝐧"). Magnitude of torsional strain depends upon the angle of rotation about color{red}(C–C) bond. This angle is also called color{green}("𝐝𝐢𝐡𝐞𝐝𝐫𝐚𝐥 𝐚𝐧𝐠𝐥𝐞 𝐨𝐫 𝐭𝐨𝐫𝐬𝐢𝐨𝐧𝐚𝐥 𝐚𝐧𝐠𝐥𝐞.")

• Of all the conformations of ethane, the staggered form has the least torsional strain and the eclipsed form, the maximum torsional strain.

• Thus it may be inferred that rotation around color{red}(C–C) bond in ethane is not completely free. The energy difference between the two extreme forms is of the order of color{red}(12.5 kJ mol^(–1)), which is very small. Even at ordinary temperatures, the ethane molecule gains thermal or kinetic energy sufficient enough to overcome this energy barrier of color{red}(12.5 kJ mol^(–1)) through intermolecular collisions.
Thus, it can be said that rotation about carbon-carbon single bond in ethane is almost free for all practical purposes. It has not been possible to separate and isolate different conformational isomers of ethane.