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Reactions:-�Just because unstability and due to holding the character of angle strain they (lower cycloalkane like cyclopropane and cyclobutane) are more suseptible to ring opening reactions.��

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��Halogenation – Photo-halogenation � Catalytic-halogenation��Catalytic Hydrogenation��Effect of Heat��Reaction with hydrogen halides��

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Halogenation – Photo-halogenation � Catalytic-halogenation

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�Catalytic Hydrogenation�Effect of Heat

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Reaction with hydrogen halides�

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Baeyer’s Strain Theory

Baeyer’s theory is based upon some assumptions that are helpful to understand instability of cycloalkane ring systems.

All ring systems are planar. Deviation from normal tetrahedral angles results in to instable cycloalkanes.

The large ring systems involve negative strain hence do not exists.

.

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Adolf Baeyer (1885). Proposed a theory to explain the relative stability of the first few cycloalkanes on the fact that the normal angle between any pair of bonds of carbon atom is 109⁰ 28' (or 109.5⁰) in tetrahedral geometry (methane molecule).

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Baeyer postulated that any deviation of bond angles from the normal tetrahedral value would impose a condition of internal strain on the ring. This is called Angle Strain, which determined the stability of the ring.

Baeyer proposed that the optimum overlap of atomic orbitals is achieved for bond angel of 109.5⁰ (for carbon atom, in a molecule if it is sp³ hybridized and orbital overlap is maximum).

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In short, it is ideal bond angle for alkane compounds that produces maximum bond strength and stable molecule.

Fig.1: Orbital overlap between (A) Propane (B) Cyclopropane. Maximum overlap occurs in propane

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If bond angles deviate from the ideal then the ring produce strain. Higher strain produces higher instability, increased reactivity and increases heat of combustion. In simple higher the deviation lesser the instability.

According to Baeyer, the relative order of stability for some common cycloalkanes is as under

Cyclopentane > Cyclobutane > Cyclopropane

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Compound	Bond angle	Deviation	Angle strain
Cyclopropane	60⁰	49.5⁰	24.75⁰
Cyclobutane	90⁰	19.5⁰	9.75⁰
Cyclopentane	108⁰	1.5⁰	0.75⁰
Cyclohexane	120⁰	10.5⁰	-5.27⁰

�

Table 1: Angale strain of first few cycloalkanes

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Limitations of Baeyer’s angle strain theory

Baeyer was not able to explain the effect of angle strain in larger ring systems.
According to Baeyer Cyclopentane should be much stable than cyclohexane but practically it is reversed.
Larger ring systems are not possible according to Baeyer as they have negative strain but they exist and much stable.
Larger ring systems are not planar but wrinkled (puckered) to eliminate angle strain.

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Coulson-Moffitt Model or Concept of Maximum Overlap of Carbon Orbitals (Bent bond/Banana bond Theory)

A bent bond, also known as a banana bond, is a type of covalent chemical bond with geometry somewhat indicative of a banana.

The term itself is a general representation of electron density or configuration resembling a similar "bent" structure within small ring molecules, such as cyclopropane (C₃H₆) or as a representation of double or triple bonds within a compound that is an alternative to the sigma and pi bond model.

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Note that sigma bond (strong bond) involves end-on-overlap and pi bond involves side-long overlap. Bent bond is intermediate between sigma and pi bonding. The overlap being neither end-on nor side-long. This makes the overlap less efficient than sigma overlap and the cyclopropane C-C bonds weaker than normal C-C sigma bonds

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Coulson-Moffits proposed that the smallest carbon valency angle which can be 104⁰. Therefore, suggested that in cyclopropane only a partial overlap of the sp3 hybrid orbitals occurs, so C-C-C bond angle is decreased slightly from 109.5⁰ to 104⁰ and the H-C-H angle increased to 120⁰ compare to normal tetrahedral angle.

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This decrease in overlap results in weakening of the bond and therefore partially explains the instability of cyclopropane.
In case of cyclobutane, also has bent bonds but the decrease in overlap is less than in the case of cyclopropane.
Cyclobutane is, therefore more stable (not participate in ring-opening reactions) and less reactive than cyclopropane.

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Sachse-Mohr Theory (Theory of Strainless Rings)

Sachse Mohr’s theory proposed that higher member ring can become free from strain if all the ring carbons are not forced into one plane. They exhibit in two non-planar ‘folded’ or ‘puckered’ conformations both of which are completely free from strain. These are strainless as the carbon atoms lie in different planes and the normal valency angle (109.5⁰) is retained.

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These are called the ‘Chair’ Form or the ‘Z’ Form and the ‘Boat’ Form or the ‘C’ Form because of their shapes.

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The chair conformation is the most stable conformation of cyclohexane.

In chair cyclohexane there are two types of positions, axial and equatorial. The axial positions point perpendicular to the plane of the ring, whereas the equatorial positions are around the plane of the ring. Mohr (1918) further elaborated this theory and applied it to compounds with the two rings fused together.