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Primary and Secondary Steric Effects

Karuna Samuel Finch, Korry Barnes
  • Author
    Karuna Samuel Finch

    Karuna has taught Middle and High School level Chemistry and Biology for over 10 years. She have a Masters in Chemistry, a professional degree in Education and, is currently pursuing a PhD in Education.

  • Instructor
    Korry Barnes

    Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

Learn about steric effect (both primary and secondary) and their effects. See examples of each effect identified. Updated: 02/15/2022

Steric Effect

All matter has mass and takes up space. This is true for all bodies. So, despite the fact that electrons, atoms, and molecules appear to be too small to occupy space, they do. This behaviour becomes more relevant during chemical reactions and molecular configurations due to a phenomenon exhibited by them called the steric effect. The term steric derives from the Greek term stereos, which means ''solid.'' The meaning of steric in chemistry centers around the 3D or spatial arrangement of things. Steric effect may occur as a result of interactions between valence electrons of non-bonded atoms in relation to their 3D layout, or when atoms/molecules surrounding an active site unintentionally shield it due to the way they are situated around it. The former is known as the primary steric effect, whereas the latter is known as the secondary steric effect. Both of these are covered in depth in the upcoming sections. Below is a list of the effects of primary and secondary steric effects:

  1. Steric interactions
  2. Steric repulsions
  3. Steric hindrance
  4. Steric factor
  5. Steric crowding
  6. Steric strain (or steric tension)

Steric interactions, whether between electrons or atoms, always result in a rise in the energy of a system. Systems with higher energies are more unstable than systems with lower energies. Figure 1 depicts cyclohexane conformations as an example of this.


Figure 1. Chair and boat conformers of the molecule cyclohexane.

An image showing the arrangement of bonds in space for the chair and boat conformers of cyclohexane.


The chair and boat forms are the polar opposites of all the other conformers that the molecule can adopt. All of the hydrogen in the molecule can be classed as either axial (indicated in the figure by letter a) or equatorial (shown by letter e in the figure). The "up" and "down" bonds are referred to as axial, while the others are referred to as equatorial. According to studies, the chair conformation is a low-energy structure; whereas the boat form is a higher-energy form. This is due to the fact that the boat form is extremely packed, and all of the axial hydrogens ''eclipse'' one another. The eclipsing causes steric repulsions, making the boat-form relatively unstable.

Steric Factor

According to the collision theory, the pace of a chemical reaction is a direct indication of the effective collisions occurring in the reaction mixture. When the colliding molecules have suitable orientation and energy levels, the collision is said to be effective. They will subsequently be converted to products. If two NO2 molecules must react to generate N2O4 according to the equation, {eq}NO_2 + NO_2 \rightleftharpoons N_2O_4 {/eq}, then the collisions must cause both nitrogen atoms to come into contact. In other words, the orientation of the molecules must be appropriate. Only then will this collision be effective. Apart from the activation energy, this reaction is concerned with the spatial orientation of the NO2 molecules and therefore has a steric factor connected with it.


Figure 2. An effective collision between two NO2 molecules.

An image showing the orientation of NO2 molecules for an effective collision.


The steric factor in a reaction mixture is the ratio of total collisions to effective collisions. It is fundamental throughout chemical kinetics and is symbolized by the character {eq}\rho {/eq} (rho). Lower steric factor values are associated with sophisticated reactants exhibiting severe steric crowding. 2-Methylpropan-2-ol, a tertiary alcohol, is an example of a sterically crowded molecule. The core carbon in this molecule (shown with a yellow outline in figure 3) is surrounded by three bulky methyl groups and a hydroxyl group. It is safe to predict that any reaction that requires the core carbon to collide effectively with another reactant molecule will have to go through several collisions.


Figure 3. 2-Methylpropan-2-ol, a molecule with steric crowding.

A ball-and-stick model of  2-Methylpropan-2-ol, a sterically crowded molecule.


In other circumstances, if the point of interest, or the location where the collision must occur, is extremely well shielded, then the reaction does not occur at all.

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!

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  • 0:04 Crowding Effect
  • 0:51 Definition of Steric Effect
  • 1:36 Example: Grignard Reaction
  • 2:13 Example: Butane
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Examples of Steric Effect

As previously stated, primary and secondary steric factors govern molecular geometries and reaction rates. This section offers examples of each of these.

Steric Effect of Butane

Butane exhibits a wide range of conformations, all of which can be divided into two categories: eclipsed and staggered.


Figure 4. A comparison of the energy levels of the main conformers of butane

An energy level graph making a comparison of the energy levels of the main conformers of butane


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
null


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
null


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Video Transcript

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
null


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
null


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Frequently Asked Questions

What is steric effect in organic chemistry?

In organic chemistry, many reactions are either slowed down or do not happen at all due to bulky groups surrounding the atoms that must collide for the reaction to proceed. This is a form of a secondary steric effect and it controls the route in reactions mechanisms.

What causes steric effect?

The steric effect can be caused by interactions between the valence electrons of non-bonded atoms in relation to their 3D layout, or when atoms/molecules surrounding an active site accidentally shield it due to their placement. The first is referred to as the primary steric effect, while the second is referred to as the secondary steric effect.

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