Torsional Strain [ENDORSED]

[Dianna Grigorian 1C]

Does torsional strain only occur when the conformation is eclipsed?

[Jocelyn_Limas_2G]

I believe so, the Organic Chemistry book says "Torsional strain results from bonds on adjacent atoms being eclipsed".

[ChiragThadaniDis2M]

Can someone explain the difference between torsional strain and steric strain?

[AnkitaNair1E]

Hi! So torsional strain happens when a molecule in a the stable staggered form becomes eclipsed because of collisions. When a molecule becomes eclipsed, the hydrogens end up becoming closer to each other. (see the two diagrams of ethane. Notice how the two hydrogens are closer together on the eclipsed Newman projection?)

2UumA.png (4.06 KiB) Viewed 816 times

As a result, because the hydrogens are closer together, there is an electrostatic repulsion between the electrons which eventually will cause the eclipsed molecule to go back to the more stable staggered form. This repulsion is called torsional strain and explains why when molecules collide, briefly can stay in eclipsed form, but usually go back to being staggered.

Steric Strain on the other hand happens when you have two atoms that are so close that they are trying to occupy the same space. This usually happens with larger molecules like butane for example.

butane_0.png (3.85 KiB) Viewed 816 times

Notice how in this Newman projection, the the two CH3's are directly on top of each other. Well as a result, if you were to draw out butane you would realize that two hydrogens were incredibly close to each other, so close they'd practically be occupying the same place. When this happens, there is steric strain because the hydrogens are practically bumping in to each other. As a result, steric strain leads the eclipsed confirmation to have the highest energy and be less stable.

So to recap, torsional strain has more to do with the electrostatic forces (the repulsions of electrons) and explains why molecules in collision often so not stay in eclipsed form very long. steric strain deals with the physical closeness of atoms and explains why the eclipsed confirmation of a molecule is more unstable/has higher energy than the gauche or anti confirmation molecule. Hope this helps!

[Chem_Mod]

Excellent answer.

[Yamilex Velgara 2I]

In lecture, Professor Lavelle mentioned that the most stable conformations have the least energy, and the least stable conformations have the most energy. Can somebody explain why there is this inverse relationship between energy and stability in conformations?