Stabilities – Baeyer’s Strain Theory, Limitation of Baeyer’s Strain Theory

In 1885, a German chemist, Adolf von Baeyer, a Nobel laureate, formulated a theory that described why the first few cycloalkanes were relatively stable. He derived this theory from the following information. Saturated compounds are cycloalkanes. All carbon atoms should have 109.50 tetrahedral angles. An angle strain would occur if bond angles deviated from the normal tetrahedral angle. Angle strain decreases the stability of the compound as its value increases. According to him, all cycloalkanes are flat and coplanar, meaning that they are two-dimensional and exist on one plane.

• Planar rings are all available. Tetrahedral angles that deviate from the norm produce instable cycloalkanes.
• Therefore, the large ring systems do not exist due to negative strain.
• As a result of nonplanar carbon rings in cyclohexane and higher cycloalkanes (cycloheptane, cyclooctane, etc.), bond angles in these compounds are smaller than 109.50.

Based on the fact that any pair of carbon atoms has a normal angle of 1090 28' (or 109.50) in tetrahedral geometry (methane molecule), the relative stability of the first few cycloalkanes is explained.

Cyclopropane

An equilateral triangle is formed by the three carbon atoms found in cyclopropane. C-C-C bonds in cyclopropane have a 60° angle. As a result, the bond angles between two bonds are compressed to 60° and each is pulled in by 1/2(109°28'-60°)=24°44'. 24°44' then represents the angle of deviation from the normal tetrahedral direction through which each bond bends.

Cyclo-butane

Four carbon atoms are arranged in a square; they occupy the corners. This means the C—C—C bonds have an angle of 90°, & the angle strain on each bond is 1/2 (109°28'-90°) = 9°44'.

Cyclo-pentane

An ordinary pentagon consists of five carbon atoms at its corners. Cyclopentane has a bond angle of 108° between its C-C-C bonds. 1/2(109°28'- 108°) = 00 44' is the angle strain which each bond fills.

Cyclo-hexane

Each carbon atom resides in the four corners of the hexagon in cyclohexane. C-C-C bonds in cyclohexane have an angle of 120°. A deviation of 1/2(109°28'- 1200)= -5°16' will also occur in the case of cyclo-heptane from the normal tetrahedral angle. Cyclooctane has a polarity of -12°46'.

Derivation

• C—C bonds must be compressed in order to satisfy the geometry of the ring, as indicated by the + symbol. As implied by the - sign, the C-C bonds need to be widened to conform to the ring geometry. A ring's strain is determined by the magnitude of the angle strain, regardless of whether it is positive or negative.
• As with cyclopropane, there is the most deviation from the normal tetrahedra angle. This makes it most unstable and most likely to rupture upon slightest provocation, thus letting go of all the tension within it.
• For ease of cyclopentane, there is a minimum deviation from the standard tetrahedral angle. Therefore, cyclopentane should be most stable because it is under least strain.
• It is expected that cyclohexane and higher cycloalkanes will become increasingly unstable and therefore more reactive in accordance with Baeyer Strain Theory.

According to Baeyer, some common cycloalkanes are arranged in the following order of stability:

Cyclopropane < Cyclobutane < Cyclopentane

Limitations

• Despite the fact that Bayer assumed cycloalkanes were planar, the theory has limits.
• The angle strain in the Larger Ring System could not be explained by Bayer.
• Although Baeyer predicted Cyclopentane would be more stable than Cyclohexane, this is actually reversed in practice.
• In Baeyer's view, larger ring systems are inconceivable since they are subject to negative strains, but they do exist and are much more stable. There is not a planar edge on larger ring systems, but a pucker that allows angle strain to be eliminated.

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