Introduction
In 1885, Adolf Baeyar theorized on how to create stability of the first few cycloalkanes, which was derived from the idea that in tetrahedral geometry, there's a normal angle between a pair of carbon atom bonds in 109.28' metal molecules. In the subject of Tetrahedral Geometry, this concept became very vital and helped us find out that the bond angle for carbon atoms is 109.28' (or 109.50) methane molecules. Baeyar also found out that these cycloalkanes have distinct bond angles and also different properties and stability at the same time. This is when, based on this, he first thought of proposing Strain Theory.
Overview of Strain theory
This theory, when published, described the cycloalkane reactivity and its stability in great depths. It also told us that the optimum overlap of atomic orbitals is achieved for a bond angle of 109.50. So in a gist, from this theory, we can conclude that this is the best bond angle for alkanes.
This abundantly efficient and preferable overlap of atomic orbitals gives results where we achieve the highest bond strength and the vastly stable molecule.
The rings in this experiment cause distress on the bond angles as they deviate from the ideal. This also helps us observe that the higher the pressure, the more unstable the system is. Such a higher strain concludes in an increase in reactivity and heat combustion. As Baeyer stated, if we deviate from the bond angle from the perfect bond angle value, which is 109.50, it will create a strain in the molecule. This will result in a lower variance and a much less unstable solution.
Assumptions of strain theory
This theory is founded on the following assumptions:
Planar Rings are utilized in all of the ring structures. Unstable Cycloalkanes originate due to divergences from the general tetrahedral angles.
Large Ring Structures contain negative strains, but these do not exist.
Since these cycloalkanes have carbon rings with a puckered texture instead of a planar(flat) structure, their bond angles are around 109.50 or less—for example, Cycloheptane, Cyclooctane, and Cyclopentolate.
These assumptions form the ground basis for comprehending the instability in the cycloalkane ring system.
Baeyer's Strain Theory in Cycloalkanes
When carbon gets bound to two other carbon atoms in propane, which is an open-chain compound, it is s sp3 hybridized; these hybrid orbitals are usually utilized to form strong sigma bonds.
Since these carbon atoms are present in the cyclopropane, they do not use these hybrid orbitals to form any bonds; their bent-bond is weaker than a general carbon to carbon bond. This strain is known as the angle strain.
This ring produces strains with bond angles that deviate from the ideal. We can observe that higher strains result in increased volatility, combustion of heat and reactivity. In simple words, the deviation is directly related to the instability.
By assuming this, Baeyer discovered that a number of cycloalkanes have different types of bond angles and different properties and stabilities.
He proposed that the angle in the strain theory is based on this and this theory describes the stability and reactivity.
The cyclopropane ring is in the shape of a triangle and has a standard tetrahedral structure where the angle is between the two bonds that are compressed to 60oand each of these bonds involved is pulled in by 24.75⁰.
What happens is that all three angles become 60o instead of109.5o, which is the normal bond angle for a carbon atom.
The deviation or Angle strain of each and every bond is defined by the value of 24.75⁰.
In the same way, a cyclobutane is a square with the bond angles of 90⁰ and not 109.5⁰ to make the ring system square with an angle strain of 9.75⁰.
When we talk about the cyclopropane and cyclobutane ring systems, a ring pressure is caused by a usual tetrahedral angle.
In contrast to this Baeyer believed that cyclopropanes are highly stressed and unstable compounds.
As a result, the triangle ring can be opened up even with a slight provocation, with a release of tension with them. This is true as the cyclopropane undergoes Br₂ ring-opening reactions.
Cyclopentane, on the other hand, is known to be the least stressed and the most stable. This is why it also has no ring-opening reactions.
In Cyclohexane, the strain angle is bigger than it is in cyclopentane. This states that if the number of the ring increases, the strain increases with it.
So, in theory, cyclohexane and higher cycloalkanes become more reactive and unstable as time passes.
But in contrast to this prediction, cyclohexane and the members of this group turned out to be highly stable, which meant that they go through substitution instead of additional reactions.
As a result, this hypothesis only accounts for the first three adequately. Hence, Cyclopentane > Cyclobutane > Cyclopropane
Limitations
The Baeyar was not able to describe the impact of an angle pressure in the larger structures.
According to him, cyclohexane is less stable than cyclopentane, but the reality is the opposite of this.
He stated that due to negative pressure, larger ring structures are not possible, but they do exist and are highly stable.
For the removal of angle pressure, larger ring structures are wrinkled (puckered) instead of being planar (flat).
Did You Know?
Simple and larger cycloalkanes are very stable, similar to alkanes, and their reactions, such as radical chain reactions, are similar to alkane reactions. Due to Baeyer strain and ring strain, small cycloalkanes, especially cyclopropane, have lower stability. They react in the same way as alkenes, but instead of electrophilic addition, they react in nucleophilic aliphatic substitution. Ring-opening or ring-cleavage reactions of alkyl cycloalkanes are these reactions.
A Diels–Alder reaction followed by catalytic hydrogenation may produce cycloalkanes. Medium rings have higher rates in nucleophilic substitution reactions but lower rates in ketone reduction. This is due to the conversion of sp₃ to sp₂ states, or vice versa, and the preference for the sp₂ state in medium rings, which relieves some of the unfavorable torsional strain in saturated rings. Many redox or substitution reactions have linear associations with strain energy differences SI between an sp₂ and sp₃ state measured using molecular mechanics.
FAQs on Strain Theory
1. What is Baeyer's Strain Theory in chemistry?
Proposed by Adolf von Baeyer in 1885, the Strain Theory explains the relative stability of cycloalkanes. It states that the stability of a cycloalkane is related to the deviation of its internal bond angles from the ideal tetrahedral angle of 109.5°. According to this theory, any deviation creates an internal strain, known as angle strain, which makes the molecule less stable and more reactive.
2. What is meant by angle strain in cycloalkanes?
Angle strain is the increase in a molecule's potential energy that occurs when its bond angles are forced to deviate from their ideal values. For sp³ hybridised carbon atoms in cycloalkanes, the ideal angle is 109.5°. For example, in cyclopropane, the C-C-C bond angles are compressed to 60°, resulting in significant angle strain and making the molecule highly unstable and reactive.
3. Which cycloalkane has the highest angle strain according to Baeyer's theory?
Cyclopropane has the highest angle strain. Its triangular structure forces the bond angles to be 60°, which is the largest deviation from the ideal tetrahedral angle of 109.5°. This severe compression of bond angles results in weak, 'bent' bonds and makes cyclopropane the most strained and reactive among the simple cycloalkanes. Cyclobutane, with 90° angles, has the next highest strain.
4. Why is Baeyer's Strain Theory not applicable to larger cycloalkanes like cyclohexane?
Baeyer's Strain Theory fails for larger rings because it was based on a critical, incorrect assumption: that all cycloalkane rings are planar (flat). In reality, cycloalkanes with six or more carbon atoms, like cyclohexane, adopt puckered, non-planar conformations to relieve strain. For instance, cyclohexane exists in stable 'chair' and 'boat' forms where the bond angles are very close to the ideal 109.5°, making it almost free of angle strain and highly stable.
5. How does angle strain affect the chemical reactivity of a cycloalkane?
A higher angle strain leads to lower stability and higher chemical reactivity. The energy stored in the strained ring is released during a reaction. This is why highly strained molecules like cyclopropane and cyclobutane readily undergo ring-opening reactions, similar to alkenes. In contrast, stable, strain-free molecules like cyclohexane do not undergo ring-opening and instead undergo substitution reactions, which are characteristic of unreactive alkanes.
6. How did the Sachse-Mohr theory of 'strainless rings' improve upon Baeyer's theory?
The Sachse-Mohr theory corrected the main flaw in Baeyer's theory by proposing that rings with six or more carbons are not planar. It stated that these rings can pucker or fold to allow the carbon atoms to maintain the ideal tetrahedral bond angle of 109.5°. By adopting these non-planar conformations, the rings become 'strainless' (free of angle strain). This successfully explained the observed high stability of cyclohexane and other larger rings, which Baeyer's theory could not.
7. What are other types of strain in cycloalkanes besides angle strain?
Besides angle strain, other important types of strain affect the stability of cycloalkanes. These include:
- Torsional Strain (Pitzer Strain): This is the strain caused by the repulsion between electrons in eclipsing bonds on adjacent carbon atoms. It is significant in planar conformations, such as that of cyclobutane.
- Steric Strain (van der Waals Strain): This arises from the repulsion between non-bonded atoms or groups that are forced into close proximity. It is particularly important in substituted cycloalkanes or in less stable conformations like the 'boat' form of cyclohexane.
