Question

When drawing a Newman projection looking down C 1 & 2, does it matter which groups are placed where in the back? I.e. I have it labeled (going clockwise) as methyl, hydrogen, propane, but would it be incorrect to have methyl, propane, hydrogen? Thanks so much.

Answer 1

Yes it will be incorrect if you change the order. I will explain stepwise as to why that would be incorrect using a similar example to that of your molecule.

Lets take a look at this molecule and its Newman Projection.

Looking down from C1 to C2, we have methyl, hydrogen and propyl group in clockwise manner. Now, C2 is a chiral center as it has 4 different groups. If you know how to give R,S configuration to chiral center, you will notice that C2 is S.

Now, when we change the order of the groups in the back.

Here, the configuration of C2 is R.

So, what you are doing by reversing is essentially changing the configuration of the chiral center. So, the original molecule is not conserved but now is a configurational isomer of the former.

But since Newman projection keeps the configuration same and it only changes the conformation of the molecule, what you are doing is incorrect.

Hope that clears the doubt.