Chemical conformation
From Freepedia
In chemistry, a chemical conformation is the spatial arrangement of atoms in a molecule. Molecules in which atoms are linked together in the same way, but in which their spatial arrangement are different, are called conformational isomers or conformers (contrast to structural isomers and Stereoisomerism). Different conformers can interconvert by rotation around single bonds, without breaking chemical bonds. Since the rotations have different energy barriers, some conformations are more stable than others. An example of where this can easily be see is protein folding, where some shapes are stable and functonal, but others are not. Butane is another classic example. It has a total of six possible conformations, of which only three are stable.
Chemical conformations can be drawn in several ways. Sawhorse representation views a C-C bond from an oblique angle. Newman projections, developed by Melvin S. Newman of The Ohio State University, view a C-C bond from front to back, front carbon as a dot and back carbon as a circle.
| Molecule of butane in syn-clinal (-sc) conformation | |
| Image:Sawhorse projection butane -sc.png | Image:Newman projection butane -sc.png |
| sawhorse projection | Newman projection |
Chemical conformations exist in many classes of molecules:
- Alkanes can be seen to have two possible conformations between each of their C-C bonds. Staggered is a conformation where hydrogens of on carbon is rotated 60 degrees from the hydrogens on the other carbon. Eclipsed is a conformation where the hydrogens on the different carbons are superposed. Staggered versus ecplised follows a Boltzmann distribution from pure staggered to pure eclipsed.
- <math> \frac{N_i}{N_j} = \frac{g_i}{g_j} \exp \left (\frac{-(E_i-E_j)}{RT} \right) </math>
The subscript i and j represent the highest and lowest energy, ecplised and staggered. g is the number of conformations found at that particular energy, the degeneracy. N is the population of molecules in a particular conformation.
- Butane can adopt a two additional staggered conformations, the anti conformation and gauche conformation
- in cycloalkanes puckered conformations exist
- in cyclohexanes, a substituent forms a equatorial bond or a axial bond
