What are steric effects? What’s an example?

What are steric effects? What’s an example?

Steric effects describe any effect that is due to the sterics of a molecule.

For an answer that isn't so tautological, we need to know what sterics are.

Sterics are simply the way a molecule is shaped. It's very similar to molecular geometry. Sterics also concerns itself also with the size of side chains and functional groups.

So, here are a few examples of steric effects:

1. TEMPO is a stable radical species, partly due to the steric effect of the 4 methyl groups. It makes it hard for large substituents to attack the oxygen radical as they have to push the methyl groups out of the way!

2. Biaxial substituents on cyclohexane rings (in the 1 and 3 positions) are less preferred to Biequatorial ones. This is because the two substituents push each other apart, so the equatorial form is slightly more stable. Note that if these substituents are capable of hydrogen bonding, the reverse is often observed- Axial hydrogen bonding can overcome steric repulsion.

3. Several reaction mechanisms contain a steric component. For example, SN1 reactions occur on triply-substituted molecules, due to the sterics of the nucleophile attacking such a bulky site as in an SN2 reaction. Instead, the leaving group falls off, then the nucleophile attacks the trigonal planar carbon. As a second example, the selectivity of Witting reactions with stabilised and unstabilised Witting reagents is due to sterics.

Hopefully that makes things clear!