Ring Strain in Cycloalkanes

The normal bond angle for a tetrahedral is 109.5˚. When we deviate from this angle of 109.5˚, the molecule experiences tension, which we call ring strain or angle strain. Ring strain in cycloalkanes is an important foundational concept to understand.

If we look at a cyclopropane molecule, it is clear that the bond angles are ~60˚.

 

The ring strain in cycloalkanes can be large as illustrated by this cyclopropane molecule.

 

Because this is so far from the normal 109.5˚, the molecule feels a large amount of strain. Ring strain in cycloalkanes, also known as angle strain, is essentially pent-up energy in the molecule that can be immediately released if the ring is broken.

If we now look at cyclobutane, we see the bond angles are ~90˚. They aren’t exactly 90˚ because the molecule prefers to take a puckered comformation; however, it is clear that the bond angles are closer to 109.5˚ than cyclopropane. They being said, they still are fairly far away from the ideal tetrahedral number.

 

The ring strain in cycloalkanes can be large as illustrated by this cyclobutane molecule.

 

Cyclopentane is much closer to 109.5˚ than cyclopropane, but it still has some deviation from the perfect value of 109.5˚ as well. Note the bond angle is ~108° as the molecule changes configuration from the pentagon shape shown on the 2-D structure below.

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Keep it Simple

The best way to really understand the concept of ring strain in cycloalkanes is by using a molecular model kit. Try to make a cyclopropane molecule. It should be very difficult to make. In fact, you might not even be able to do it (use caution to not break your model set). If you can create the cyclopropane, undo one of the bonds, getting rid of the cyclic structure. You should feel a strong “pop,” as all of the ring strain energy is released. If you then create a cyclobutane, it should be easier to create, and there should also be a smaller “pop,” as less energy is released when the structure is broken. Try this with cyclopentane and cyclohexane as well.

The reason this is a great demonstration is because the model kit bonds are locked at 109.5˚. Any deviation from this number will result in energy being stored in the bonds, which can be felt by creating the bonds or by breaking the bonds suddenly.

 

If we look at a six-membered ring, it actually conforms in a way that is strain-free (see next section). This means that its bond angles are indeed the ideal bond angle of 109.5˚.

Just as we had ring strain for angles less that 109.5˚, we can have strain for larger molecules as well. Cycloheptane and cyclooctane are subject to this strain, for example.

 

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Fun Fact

Octanitrocubane (see structure below) is an extremely strong and effective explosive. Explosives work by having extremely high energy reactants, which can quickly convert to very low energy products. A large amount of energy is lost in a very short amount of time, which causes the explosion.

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By analyzing the structure, we can see why octanitrocubane is such a high energy molecule. The cubane structure contains 6 different cyclobutane groups, which give the overall molecule a large amount of ring strain. All of this ring strain creates a high energy reactant, which can quickly release its energy to create the explosion.