2014 Midterm Q7 E & F

[Rhea Kundan_2G]

7E says: The molecule 3-chloro-4,4,4-trihydroxy-3-methylbutanenitrile was produced. Draw the lowest energy Newman diagram for this molecule looking down the C2-C3 bond axis.
and 7F is almost the same but asks about the second lowest energy conformation.

Can someone explain why the lowest energy diagram needs to have C(OH)3 gauche to both hydrogens for the lowest energy diagram? And why the C(OH)3 has to be gauche to CN and H for the second lowest energy diagram? Why do we look at C(OH)3 rather than CH3 or Cl? Thank you!!

[nickrodgers]

The lowest energy Newman diagram is when the largest groups are farthest apart. CN and C(OH)3 are the two largest groups (nitrogen atom is larger than 3 hydrogen atoms) so the lowest energy diagram will be when they are in anti-conformation.