Winter 2014 final question 7 part F newman projections

[Franca Park 3J]

For the molecule 3-chloro-4,4,4,-trihydroxy-3-methylbutanenitrile draw the second lowest energy conformation newman diagram, looking down the C2-C3 bond axis.

the answer drawn shows the newman projection with C(OH)3 gauche to CN and H, Cl gauche to H and H, and CH3 gauche to CN and H.
why can't the answer be C(OH)3 gauche to CN and H, CH3 gauche to H and H, and Cl gauche to CN and H?

[Eduardogonzalez1G]

That wouldnt be the answer simply because that wouldnt the second lowest energy conformation. The positioning of the CN is crucial, as CH3 is gauche to CN and H, where this would be altered to CH3 gauche with H and H. For low energy conformation, you want the biggest groups to have some space to hold true that this gauche model does indeed have relatively low energy compared to when its eclipsed but higher energy than when its anti

[Rachel Colbath 2G]

How would we know that the chlorine is bigger than the methyl group when arranging the second most stable conformation?

[Michelle Yu 1L]

How would we know that the chlorine is bigger than the methyl group when arranging the second most stable conformation?

Chlorine has an atomic mass of 35.45 while methyl (CH3) is about 15 (add up the all the atomic mass. C=12 H=1). Therefore Chlorine is bigger.