[rogerwilco]

If you have a compound with chiral carbon(s), such as 2,3-dibromopentane, and are drawing a Newman Projection for C2 and C3, does it matter in which order around the carbon you place the substituents? (in other words, does it matter for each carbon whether it's S or R configuration since they're both chiral?)  Further, when drawing an energy profile for the rotation about this bond, is a hydrogen and methyl group overlapping at the same energy level as a hydrogen and an ethyl group overlapping (and assuming that the other two substituents overlapping are the same in both cases)?

Thanks to anyone here who can *delete me*

[Dan]

does it matter in which order around the carbon you place the substituents?

Yes it does. RR is not the same as RS is not the same as SS

when drawing an energy profile for the rotation about this bond, is a hydrogen and methyl group overlapping at the same energy level as a hydrogen and an ethyl group overlapping (and assuming that the other two substituents overlapping are the same in both cases)?

Et incurrs more strain than Me.

[rogerwilco]

Thank you for the reply, Dan.

Actually, after looking at it again, the issue I'm working on (2,3-dibromopentane) does not line up the way I said.  I'm doing an energy profile for the six different newman projections of this compound using C2 and C3 for the projection.  This is the first time I've done one with two chiral carbons, so I'm not sure exactly how to compare the energy levels for some of them.  I was wrong, when the hydrogen and Me group line up, and when the hydrogen and Et group line up, the other substituents are not the same in both cases I don't believe.  With two achiral carbons, an energy profile is pretty simple.  Starting with an anti staggered formation and working your way around, there is a symmetrical graph created; however, with 2,3-dibromopentane, I'm having trouble comparing the energy levels of the eclipsed formations.  I know the one where the bromine, ethyl, and hydrogen eclipse the bromine, methyl, and hydrogen respectively, that's the highest energy, but the other two I'm not sure how to compare. 

Thanks again to anyone who can help.  I'm kind of new to organic chem, but it seems pretty interesting so far. 

[movies]

The energy profile would not depend on which enantiomer you selected, but would depend on which diastereomer you selected.  So, if you are drawing a Newman projection to look at rotamer energies, then it doesn't matter which absolute configuration you pick, just that the relative configuration between the two stereocenters is consistent.

If you are drawing a Newman projection to predict the stereochemistry produced by a particular reaction (e.g., Grignard addition at a proximal carbonyl) it's always best to be consistent with your stereochemistry.  You should be very careful to be consistent about stereochemistry at all times, really, because it is easy to get confused when you start swapping between enantiomers at random!

[rogerwilco]

Hmmm.....does that mean it would be necessary to actually make two different (and different looking) energy profiles for 2,3-dibromopentane?  It seems to me one would follow the usual

             -                         
    -                -   
        -       -                         
-                        -

pattern, while the other would be something like this?


    -       -       -                                   
-       -       -       -



Am I way off base here?  Sorry for the newbie-ish questions, I'm still learning....                                               


   

[movies]

Yes, the two would be a little bit different, although I don't know for sure how different they would be.  One difference would be that in one diastereomer you could have a rotamer where the two alkyl groups and the two Br atoms are both eclipsing, while in the other diastereomer you would have a rotamer where each alkyl group eclipses a Br at the same time.  Those should be the highest energy conformers, by the way.