[maccha]

I'm just starting organic chemistry and I have a bit of a confusion. I understand that there is an energy barrier for a molecule to go from the staggered conformation to the eclipsed conformation. I don't understand why a molecule would go to the eclipsed conformation, then, if staggered is so much more stable? And where does it get the energy to do so?

[Arctic-Nation]

The energy available is thermal energy. For conformational changes (rotation around a single bond), the energy available at room temperature is much higher than the energy needed to overcome the repulsive forces when moving through an eclipsed conformation (the molecule will not stay eclipsed for any appreciable amount of time). The exact amount of energy needed depends strongly on the size and interaction of substituents on either side of the bond in question, and for multi-substituted compounds certain staggered conformations usually are more stable than others.

Of course, all this means that if you lower the temperature and thus decrease the amount of energy available for conformational changes, the molecule will get 'locked' into the different staggered conformations. Using NMR, you might even be able to tell them apart.