Considr the following rings

1. 4 membered ring


2. 3 membered ring


3. 5 membered ring

since each and every carbon atom is sp3 hybridised, the bond angle is 109 degree 28'.

In structure 1, the angle is nearly 90 degree. So to attain more stabilility it has to increase the bond angle by 19 degrees 28'.

In 2, it is from 60   to 109 degree 28'.

In 3, it is from 108 to 109 degree 28'.

So there is a ring strain in each of them.

The order of ring strain is (lowest to highest ) :-

3<1<2. hence the order of stability is 3>1>2. hence according to bayers strain theory, the 5 membered rings are most stable.

Angle of strain = ½ (internal angle of the ring - 109 degree 28).

Hope it helps.

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