4. Conformations

Conformations

Different spatial arrangements of atoms in a molecule that can be converted into one another by rotation around a single bond.

Alkanes

• Carbon-carbon sigma (σ) bonds.

Torsional strain

Repulsive interaction between adjacent bonds in a molecule due to rotation around a single bond.

• conformational isomers

• This rotation results into infinite number of spatial arrangements of hydrogen atoms attached to one carbon atom with respect to the hydrogen atoms attached to the other carbon atom.

eclipsed conformation

• In which hydrogen atoms attached to two carbons are as closed together as possible

Sawhorse projections

• A way to represent a molecule by viewing it along the molecular axis and projecting it on paper with the central bond as a longer line, showing the carbon atoms and hydrogen atoms attached to them.

• The lines are inclined at an angle of 120° to each other.

Newman projections

A way to represent a molecule by viewing it at the bond head on, with one carbon atom represented by a point and the other carbon atom represented by a circle, showing the hydrogen atoms attached to each carbon atom.

Relative stability of conformations

The stability of different conformations of a molecule based on the repulsive forces between electron clouds of carbon-hydrogen bonds.

Torsional strain

The repulsive interaction between electron clouds in a molecule that affects the stability of a conformation.

Dihedral angle

The angle of rotation about a carbon-carbon bond.

Staggered conformation

A conformation of a molecule where the electron clouds of carbon-hydrogen bonds are as far apart as possible, resulting in minimum repulsive forces and maximum stability.

Eclipsed conformation

A conformation of a molecule where the electron clouds of carbon-hydrogen bonds come closer to each other, resulting in an increase in electron cloud repulsions and decreased stability.

Preferred conformation

The most stable conformation of a molecule, in this case, the staggered conformation of ethane.

Energy barrier

The amount of energy required to overcome the repulsive forces between electron clouds and transition from one conformation to another.

Intermolecular collisions

Collisions between molecules that provide enough thermal or kinetic energy to overcome the energy barrier and allow rotation around a carbon-carbon bond in ethane.

Conformational isomers

Different spatial arrangements of atoms in a molecule due to rotation around a single bond, resulting in different conformations.