What is the Steric Effect in Organic Chemistry? - Definition & Examples

Instructor: Joelle Mumley
The goal of this lesson will be to gain an understanding of what's called the steric effect in organic chemistry by studying its definition and then looking at specific molecular examples in which the effect is observed.

Crowding Effect

Have you ever been in a crowded room or area before where it was shoulder-to-shoulder people? Maybe it was one of those rock-and-roll concerts where people jam-pack together on the floor and try to get as close to the stage as possible. Or maybe you've had to endure an airplane ride where the plane seemed so small everyone was sitting on top of one another with barely any arm or leg room. Regardless of the case, everyone's general response, when placed in physically crowded situations, is to try and either get out or make some space somehow.

Did you know that organic compounds need space too? It turns out they don't like to be crowded either and the atoms within a molecule tend to want to maximize the space between them. This particular concept is called the steric effect. Let's get more familiar with this concept and look at some specific example structures that have the effect at play. Let's make some room!

Definition of Steric Effect

In general, the steric effect refers from the fact that the atoms composing molecules occupy some degree of space, and when atoms come too close together there's a rise in the energy of the molecule due to the atoms being forced to occupy the same physical space.

It turns out that steric effects can have a dramatic effect on the observed or preferred shape of a molecule and in some cases even its chemical reactivity. In its very simplest form, steric effects can be thought of as a 'crowding' effect kind of like that crowded room of people we mentioned earlier.

Consider as an example the molecule known as tert-butanol. The three bulky methyl groups labeled in blue are what makes this molecule so sterically crowded. Both the central carbon atom and the alcohol group are what we would call 'sterically shielded' due to the presence of the large methyl groups.

Tert-butanol is an example of a very sterically crowded molecule

Example: Grignard Reaction

Now that we're a bit more familiar with the steric effect, let's take a look at some example compounds to get a better feel for it.

Sometimes if two molecules are very sterically demanding (take up lots of space) a chemical reaction that you think would be trivial actually won't occur at all. An example would be a reaction called a Grignard addition, which occurs between a ketone and a magnesium-bromide reagent, called the Grignard reagent.

In this specific Grignard reaction, no chemical reaction occurs at all because of steric effects. Both the ketone and the Grignard reagent are so crowded due to the bulky methyl groups that no product is observed when the two reactions are mixed.

This Grignard reaction does not work due to steric effects

Example: Butane

Sometimes steric effects can cause a certain conformation (shape) of a molecule to predominate over another conformation. For example, the molecule butane (used as lighter fluid) can adopt one of two conformations due to bond rotation about the carbo-2-carbon-3 bond. When the molecule is in what's called an eclipsed conformation, the two methyl groups are close enough in space to induce steric strain on the molecule.

Have you ever seen a solar eclipse? That's when the moon moves in front of the sun and blocks the sunlight. The same type of phenomenon is happening here with the methyl groups. The two groups are directly in line with one another, which is why we say they are eclipsed.

The eclipsed and anti conformations of butane

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