Investigate angle strain in planar rings.

•	Ideally the sp3 hybridised carbon atoms of fully saturated carbocyclic rings would adopt bond angles of 109.5 degrees. However, in planar rings the internal bond angles will depend on the number of atoms in the ring.

 

•	The table below gives values for the internal angles for the regular planar polygons shown above and an indication of angle strain per carbon atom due to deviation from the ideal sp3 angle of 109.5 degrees.

 

No. atoms in ring	Internal angle in planar ring (degrees)	Measure of angle strain abs(internal angle - 109.5)
3	60	49.5
4	90	19.5
5	108	1.5
6	120
7	128.5
8	135

 

•	Complete the table above by calculating the angle strain (angle stain = internal angle - 109.5) for 6- to 8-membered rings.

•	This data is best presented as a graph. Plot the predicted angle strain against ring size (for three- to eight-membered planar rings). What conclusions can you draw?

•

Strain is predicted to be largest for three-membered rings but rapidly decreases and reaches a minimum for a five-membered ring. In fact, a planar five-membered ring is predicted to be virtually free of angle strain. After the minimum at five, angle strain increases steadily as the rings get larger.