Question

How do you draw a newman projection of the most stable conformation of 2R,3S-dibromobutane sighting down the `C_2-C_3` bond? Is this optically active, racemic, dextrorotatory, or a meso compound?

Answer 1

Based on the name, I've drawn out the structure like this:

Rotating the `"C"_2-"C"_3` bond `120^@` counterclockwise with your right hand gives you a structure where you can see the vertical mirror plane bisecting the two stereocenters on this eclipsed conformation, which jointly indicate a meso compound.

Although both carbon 2 and carbon 3 are chiral carbons, their optical activities cancel out (they rotate plane-polarized light to the same extent in opposite directions), so there no net optical activity for this meso compound.

To make the Newman projection, it helps to progress to a Sawhorse projection, and that should help you see where you can go to condense the structure down into the Newman projection.

1) From the first, left structure, rotate the `"C"_2-"C"_3` bond `60^@` counterclockwise with your right hand to achieve the staggered conformation.

2) Then rotate the entire molecule along the axis (depicted in the image directly above) that is coplanar with the mirror plane approximately `90^@` counterclockwise to get the Sawhorse projection.

3) Then, it can be condensed into the Newman projection fairly easily if it's already oriented; try picturing yourself squishing the `"C"_2-"C"_3` bond into nothing, and replacing it with a circle. That circle indicates that you are looking down the `"C"_2-"C"_3` bond.

Now is this the most stable one? The most stable one would have the rear hydrogen in between the front methyl and bromine in a staggered conformation to minimize [lone-pair]-[bonding-electron] repulsions.

4) Rotate the rear groups on the `"C"_2-"C"_3` bond `120^@` counterclockwise from the Newman projection shown, and you'll have it. Each bromine, methyl, and hydrogen should be trans from the other bromine, methyl, and hydrogen, respectively.

CHALLENGE: Can you draw out the other four Newman projections? Three are eclipsed conformations, and the last one is the third staggered conformation that I didn't show.