Strain in Organic Chemistry

Core Concepts

Strain is an important concept in organic chemistry, often the most important factor in determining molecular structure. In this article, we explore the different types of strain and how each affects structure and molecular energy.

Topics Covered on Other Articles

What is Strain in Organic Chemistry

Chemists use the term “steric strain” or just “strain” to describe forces of repulsion between parts of a molecule, resulting in a particular arrangement which limits this repulsion. Within molecules, every atom has a negatively charged electron cloud, composed of electron lone pairs and bonds between atoms. These lone pairs and bonds, collectively called “electron terminals”, arrange themselves spatially on an atom in a way that maximizes distance, and thus minimizes repulsion, between electrons. This spatial arrangement, or molecular geometry, comes directly from an atom’s hybridization.

At four electron terminals, sp3 hybridized atoms adopt a maximum angle of 109.5 degrees away from one another, forming a tetrahedral geometry. At three electron terminals, sp2 hybridized atoms adopt a 120 degree angle between terminals, resulting in a trigonal planar geometry, while sp hybridized atoms have a 180 degree angle between their two electron terminals.

sp3 hybridization
tetrahedral geometry (sp3 hybridization)
sp2 hybridization
trigonal planar geometry (sp2 hybridization)
sp hybridization
linear geometry (sp hybridization)

When incorporated into large molecules, these molecular geometries become strained, leading to instability in the molecule. With this in mind, we can define strain in organic chemistry as the increase in molecular potential energy caused by structural deviations from ideal geometries or by repulsive forces between atoms, and more specifically, electron clouds. This strain can fall into 4 different, but overlapping categories: ring, angle, torsional, and steric.

Torsional Strain

Torsional strain is the result of repulsive dispersion forces between atoms, hindered by limited conformational mobility within a molecule. An example can be visible in the Ethane conformations. Ethane is composed of two carbon atoms bonded by a single bond. When ethane adopts different conformations, such as the staggered and eclipsed conformations, torsional strain arises from the interaction of the electron clouds of the atoms or groups involved.

ethane structure
Ethane Molecule

In the staggered conformation of ethane, the two methyl groups position themselves as far apart as possible, minimizing steric hindrance and reducing repulsive interactions between the electron clouds. This results in lower torsional strain and a more stable conformation.

staggered confirmation, which limits strain
Staggered Conformation

In the eclipsed conformation of ethane, aligned methyl groups cause maximum repulsion between their electron clouds. This leads to increased torsional strain and a higher energy state.

eclipsed confirmation, which comes with strain
Eclipsed Conformation

Torsional strain restricts the mobility of ethane as it creates an energy barrier that molecules must overcome to transition between conformations. Specifically, transitioning from the lower energy staggered conformation to the higher energy eclipsed conformation requires additional energy to surpass the torsional strain energy barrier. Consequently, ethane prefers to adopt the staggered conformation at room temperature due to its lower torsional strain and greater stability.

Angle Strain

Angle strain occurs when there is a deviation from ideal bond angles. However, due to the constraints imposed by molecular structure, especially in ring structures, many molecules experience distortion in bond angles, leading to a deviation from the ideal value. This deviation creates increased potential energy within the molecule, contributing to the overall instability.

In cycloalkanes, it originates from the fact that carbon atoms in a ring structure are not able to adopt their ideal tetrahedral geometry due to the constraints of the cyclic arrangement. One example can be observed in cyclobutane, a cycloalkane with four carbon atoms forming a ring.

cyclobutane structure, which involves strain
Cyclobutane’s square shape

The close proximity of the carbon atoms in the ring attributes the angle strain in cyclobutane. The compressed bond angles generate increased electron-electron repulsions between the adjacent carbon atoms, leading to higher energy levels compared to a hypothetical acyclic compound with the same carbon skeleton.

Non-cyclic structures, like cis-alkenes, can also experience angle strain as a result of rigid bonds that force repulsive groups close to each other.

Ring Strain

Ring strain encompasses both angle and torsional strain. It refers to the increase in potential energy in a cyclic molecule that deviates from the ideal geometry experiencing repulsive forces. In cycloalkanes, the ring strain is primarily due to the constraints imposed by the ring structure. As mentioned earlier, cyclohexane, with its six-membered ring, has the least ring strain among cycloalkanes and is the most stable. Cyclopropane and cyclobutane, due to their small ring sizes, exhibit higher ring strain, making them less stable.

The bond angles in cyclobutane are significantly compressed, measuring approximately 88 degrees, contrasting the ideal bond angle in a tetrahedral geometry of 109.5 degrees. This reduction in bond angle causes considerable strain within the molecule, creating a ring conformation for cyclobutane named “Butterfly Conformation”. Further the hydrogens on each carbon are held in rigid eclipsed conformations with one another, resulting in torsional strain as well.

cyclobutane confirmation that reduces strain
Butterfly Ring Conformation

Another example of the presence of this concept is the Conformations of Cyclohexane, where we have different ways this molecule can fold, and depending on them we get a higher or lower ring strain.

Ring Conformations

Cyclic compounds, with their closed-ring structures, often exhibit intriguing properties and behaviors due to the steric strain resulting from the close proximity of atoms within the ring. Steric hindrance arises when bulky substituents or atoms within a cyclic molecule experience repulsive interactions that interfere with the optimal spatial arrangement of the molecule. As a result, rings like cyclohexane may adopt a variety of arrangements, or conformations, to accommodate the repulsion imposed by their substituents.

In cycloalkanes, it becomes more pronounced when these substituents occupy neighboring positions in the ring. This proximity leads to repulsive interactions between the electron clouds, resulting in destabilizing forces. One of the classic examples in cyclic compounds is observed in the chair conformation of cyclohexane.

chair conformational isomer, which limits steric strain
Chair Conformation

In the chair conformation, each carbon atom forms bonds with two other carbon atoms and two hydrogen atoms. However, the spatial arrangement of these substituents can lead to steric interactions and hindrance. Specifically, it occurs when bulky substituents occupy axial positions, leading to repulsive interactions with neighboring atoms.

tertbutyl group on cyclohexane producing strain
Tert-butyl occupies the axial position (red) and equatorial position (blue)

This strain arises due to the close proximity of the bulky groups, increasing its energy. If they are occupying the equatorial positions steric it minimizes, resulting in a more stable conformation.