Construct a qualitative potential-energy diagram for rotation about the C—C bond of 1,...

Question

Construct a qualitative potential-energy diagram for rotation about the C—C bond of 1, 2-dibromoethane. Which conformation would you expect to be more stable? Label the anti and gauche conformations of 1, 2-dibromoethanc.

Robert · Accepted Answer

Lesson13.ppt
1,2-Dibromoethane 
Conformations 
For ethylbromide, each conformational 
minimum had an identical structure.  That is, 
the structure at the 60˚ minimum cannot be 
distinguished from that at 180˚.  Because 
these conformational isomers have 
indistinguishable structures, they also 
possess identical energies (i.e., each one of 
the conformational states are equally 
populated).   
The situation is different with 1,2-dibromoethane. 
In MarvinSketch (http://butane.chem.uiuc.edu/jsmoore/
marvin_scripts/MarvinSketch_Practice.html) draw 1,2-
dibromoethane.  Use the conformation tool as 
shown by these screenshots to find the 
conformational states of this molecule (the default 
settings may differ slightly for PC vs. Mac 
platforms). 
Minimum Energy Conformations of 1,2-Dibromoethane 
For 1,2-dibromoethane, MarvinSketch finds three 
staggered conformations, as shown. Each structure has 
been oriented in order to present a view comparable to a 
Newman projection.  The energy is listed above each 
conformation.  The lowest energy conformation is 
assigned a value of 0.75 kcal/mole while the other two 
conformations have values of 0.95 kcal/mol each.  
Conformations labeled 2 and 3 look similar, but in fact 
they cannot be superimposed one on the other;

Construct a qualitative potential-energy diagram for rotation about the C—C bond of 1, 2-dibromoethane. Which conformation would you expect to be more stable? Label the anti and gauche conformations...

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