What do you mean by dihedral angle or torsional angle?

On another planet,the gauche and anti forms of $1, 2-$dichloroethane freeze and single bond rotation stops. Under these conditions,these two forms can be considered as:

Which of the following describes the best relationship between the methyl groups in the chair conformation of the substance shown below?

The most stable conformation of ethylene glycol is .....

The Newman projection formula of the most stable conformation of $3$-hydroxypropanal is gauche. It is stable due to

The structures shown below represent .......