Baeyer, Adolf (1885) proposed a theory to clarify the relative stability of the first few cycloalkanes based on the fact that in tetrahedral geometry, the normal angle between any pair of carbon atom bonds is 109.28' (or 109.50)(methane molecule).
In tetrahedral geometry, the bond angle for carbon atoms is 109.28' (or 109.50)(methane molecule).
Baeyer discovered that various cycloalkanes have different bond angles, as well as different properties and stability.
He proposed angle strain theory based on this.
The theory describes cycloalkane reactivity and stability.
The optimum overlap of atomic orbitals, according to Baeyer, is achieved for a bond angle of 109.50. In a nutshell, it's the best bond angle for alkanes.
The most efficient and desirable overlap of atomic orbitals results in the highest bond strength and the most stable molecule.
Rings cause strain when bond angles deviate from the ideal.
The higher the pressure, the more unstable the system.
Higher strain results in increased reactivity and combustion heat.
According to Baeyer, "any deviation of bond angle from the ideal bond angle value (109.50) creates a strain in the molecule." The lower the variance, the less unstable the situation.
Baeyer's Strain Theory Chemistry is Cycloalkanes are Founded on the Following Assumptions:
Planar rings are used in all ring structures. Unstable cycloalkanes arise from deviations from standard tetrahedral angles.
Negative strain is present in large ring structures, but they do not exist.
Since the carbon rings of cyclohexane and higher cycloalkanes (cycloheptane, cyclooctane, cyclononane, etc.) are puckered rather than planar (flat), their bond angles are not greater than 109.50.
These assumptions are useful in understanding cycloalkane ring system instability.
Baeyer Strain Theory in Cycloalkanes
When carbon is bound to two other carbon atoms in an open-chain compound (propane), it is s sp₃ hybridized, and these hybrid orbitals are used to form bonds (strong sigma bonds). Since the carbon atoms in cyclopropane do not use these hybrid orbitals to form bonds, the bond (bent bond) is weaker than a typical carbon-carbon bond. This is known as angle strain.
The ring produces strain when bond angles deviate from the ideal. Higher strain results in increased volatility, reactivity, and heat of combustion. Simply put, the lower the deviation, the lower the instability.
Baeyer discovered that various cycloalkanes have different bond angles, as well as different properties and stability. He proposed angle strain theory based on this. The theory describes cycloalkane reactivity and stability.
The cyclopropane ring is a triangle. The standard tetrahedral angle between two bonds is compressed to 600 and each of the two bonds involved is pulled in by 24.75⁰ so all three angles are 600 instead of 109.50 (normal bond angle for carbon atom). The angle strain, or deviation, of each bond from the usual tetrahedral direction, is defined by the value 24.75⁰.
Similarly, cyclobutane is square, with bond angles of 90⁰ instead of 109.5⁰ (normal bond angle for carbon atom) to make the ring system square (angle strain 9.75⁰)
For cyclopropane and cyclobutane ring systems, a deviation from the usual tetrahedral angle causes ring pressure. In comparison to molecules with a tetrahedral bond angle, the ring strain will make them unstable. In comparison to cyclobutane, Baeyer believes that cyclopropane should be a highly stressed and unstable compound. As a consequence, the triangle ring can be expected to open up at the slightest provocation, releasing the tension within it. This is valid since cyclopropane undergoes Br₂ ring-opening reactions.
Cyclopentane (angle strain 0.75⁰) is considered to be the least stressed and the most stable. It is not surprising, then, that it has no ring-opening reactions.
In cyclohexane, the angle strain is greater than in cyclopentane. If the number of carbon atoms in the ring increases, the strain increases as well. Theoretically, cyclohexane and higher cycloalkanes should become more unstable and reactive as time goes on. In contrast to this prediction, cyclohexane and its higher members are found to be very stable, undergoing substitution rather than additional reactions. As a result, the hypothesis adequately accounts for the first three.
Cyclopentane > Cyclobutane > Cyclopropane
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Limitations of Baeyer Strain Theory in Cycloalkanes
In larger ring structures, Baeyer was unable to describe the impact of angle pressure.
According to Baeyer, cyclopentane should be much more stable than cyclohexane, but the situation is the opposite.
According to Baeyer, larger ring structures are not feasible due to negative pressure, but they do exist and are very stable.
To remove angle pressure, larger ring structures are wrinkled (puckered) rather than planar.
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 unfavourable 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.