(a) Draw sawhorse projection formulae for the two eclipsed conformations of butane.
(b) What is the dihedral angle between adjaccent methyl groups? (c) Which conformer has greater torsional strain?

Explanation

(a)
(b) Dihedral angle `0^(@)` Dihedral angle `120^(@)`
(c) Greater torsional strain because two Me's are exzctly eclipsing each other. Lesser trorsional strain because two Me's are fathest apart.
Dihedral angle is the angle between from the H on one C and H of other C as seen along the (C --- C) bond. The dihederal angle in eclipes and staggered forms is `0^(@)` and `60^(@)`, respectively.