Dear Ajay
Baeyer strain theory or strain theory explains specific behaviour of chemical compounds in terms of bond angle strain.
It was proposed by Adolf von Baeyer in 1885 to account for the unusual chemical reactivity in ring-opening reactions of cyclopropanes and cyclobutanes where this angle strain is relieved.
The four valences of the carbon atom point in the directions connecting the center of the sphere to the corners of the tetrahedron, forming an angle of 109°28' with one another. The direction of attachment can undergo alteration, but a strain is generated increasing with the size of the deflection.
If one now tries, most significantly only with models, to connect a large number of carbon atoms without constraint, that is in the directions of the tetrahedron axes or of the wires, one obtains either a zig-zag line or a 5-membered ring, which is obvious since the angle in a regular pentagon is 108°, only slightly different from the 109°28' angle between the valences. To make a larger or smaller ring one must bend the wires, that is a strain would result
 
All the best.                                                           
AKASH GOYAL
AskiitiansExpert-IIT Delhi
 
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