Conformational Analysis and Newman Projections

Flashcards covering concepts of conformational isomers, Newman projections, torsional angles, energy barriers, and strain in ethane, propane, and butane.

Newman projection

A visual representation of a carbon-carbon bond where a circle represents the front carbon, and lines ending at the center of the circle represent bonds to that front carbon.

Torsional angle

The angle of rotation around a central carbon-carbon bond in a molecule, used to track changes between different conformational isomers.

Eclipsed conformational isomer

A conformer where the bonds on adjacent carbons directly align or overlap from front to back, maximizing electron repulsion and placing the molecule at an energy maximum.

Staggered conformational isomer

A conformer where the bonds on the front carbon sit directly in between the bonds on the back carbon, keeping bonding electrons as far apart as possible and placing the molecule at an energy minimum.

Steric strain

Repulsive strain that occurs when non-bonded atoms or groups get too close to each other in three-dimensional space.

Anti conformational isomer

A staggered conformer in which the two largest alkyl groups (such as methyl groups) are positioned on opposite sides ($180^\bullet$ apart), resulting in the lowest energy and highest stability.

Gauche conformational isomer

A staggered conformer where two bulky groups (such as methyl groups) are adjacent to each other ($60^\bullet$ apart), introducing steric strain while maintaining staggered bond geometry.

Ethane rotational energy barrier

The $12\,\text{kJ/mol}$ energy difference between the staggered and eclipsed conformers of ethane, where each eclipsed hydrogen-hydrogen interaction contributes $4\,\text{kJ/mol}$ of strain.

Propane rotational energy barrier

The $14\,\text{kJ/mol}$ energy difference between the staggered and eclipsed conformers of propane, where the single carbon-hydrogen to carbon-methyl eclipsed interaction contributes $6\,\text{kJ/mol}$ of strain.

Butane C-H/C-C eclipsed isomer energy

An eclipsed conformer of butane featuring one C-H to C-H interaction ($4\,\text{kJ/mol}$) and two C-H to C-methyl interactions ($6\,\text{kJ/mol}$ each), totaling $16\,\text{kJ/mol}$ of strain energy above the anti conformer.