Allylic Strain Energy in Organic Chemistry

Instructor: Korry Barnes

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

Can molecules feel strain? In this lesson we'll learn about a specific type of molecular strain called allylic strain and how it affects the energy and shape of a molecule.

Strain Happens to Everyone

Have you ever felt overwhelmed the night before a big exam or an important job interview? Those types of things can be a big source of stress and can cause strain until they are behind us or resolved.

Did you know that sometimes organic molecules can actually feel strain too? Because of the certain conformations (shapes) a molecule can adopt and the different groups of atoms it can contain, it can feel a type of strain that results in the molecule being at a high energy state.

The specific type of strain we are going to be discussing today is called allylic strain and involves compounds that contain carbon-carbon double bonds. Let's get started!

Definition of Allylic Strain

Allylic strain is a type of molecular strain energy associated with a compound that results from the interaction between the substituents (atom or group of atoms) on the ends of an alkene. An alkene is a type of organic compound that contains a carbon-carbon double bond.

The term 'allylic' here refers to a very specific carbon atom, namely the carbon atom that is directly adjacent to the carbon-carbon double bond. Take for example the compound propene. The methyl group is directly adjacent to the carbon-carbon double bond, making it an allylic carbon.

The allylic carbon atom is defined as the carbon directly adjacent to a carbon-carbon double bond

A compound experiences allylic strain when the groups on both ends of the double bond interact with one another so that a higher energy state of the molecule is achieved (a bad thing).

What Causes the Strain?

What exactly causes allylic strain and what type of strain is it? It's actually a type of steric strain, which is the strain a molecule experiences when two or more of the atoms are forced to occupy the same three-dimensional space.

Since atoms have mass and occupy physical space, a repulsive force results when they are brought into close proximity of one another.

This induced strain always results in a high-energy state of the molecule. This energy 'cost' of the molecule isn't favorable and that's why compounds that experience the strain tend to be unstable and even particularly reactive in some cases in order to alleviate the strain.

An Example of Allylic Strain

Let's consider an alkene that contains two general 'R' groups (any carbon-based group) on each end of the molecule, one directly attached to the carbon-carbon double bond and one at the allylic position.

Because of the free bond rotation at the allylic position, one of the R groups can rotate freely. This rotation results in two different conformations (shapes) of the molecule. Notice that in one of the conformations the R groups are almost 180 degrees apart from one another, but in the other conformation, they are right next to each other.

An example of allylic strain. Left conformation show R groups at 180 degrees apart; right conformation shows R groups next to each other.

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