In the example of staggered ethane in Newman projection, a hydrogen atom on one carbon atom has a 60?"'torsional angle "'or "'torsion angle "'with respect to the nearest hydrogen atom on the other carbon so that steric hindrance is minimised.
12.
The preference for the formation of the threo isomer can be explained by the rule stated above by having the active nucleophile in this reaction attacking the carbonyl group from the least hindered side ( see Newman projection "'A "') when the carbonyl is positioned in a gauche orientation and phenyl as the most bulky group in the anti conformation.
