Compare predicted angle strain against that calculated from actual compounds

•	The table below shows both the predicted angle strain (in planar rings) and the angle strain calculated from actual compounds.

•	Complete the table by entering the angle strain in cyclohexane calculated from actual compounds. Angle strain = average internal angle across all 5 structures - 109.5. Your answer should be close to zero.

 

No. Atoms in Ring	Measure of Angle Strain (internal angle - 109.5)
	Predicted (in planar rings)	Calculated (crystal structure data)
3	49.5	49.5
4	19.5	21
5	1.5	6
6	10.5
7	19	6.5
8	25.5	7

 

•	Add the angle strain data calculated from actual compounds to the graph we plotted previously. This will allow us to easily compare the predicted angle strain against that observed in actual compounds.

 

Plot comparing predicted angle strain in planar rings against that calculated from actual compounds

 

•	Compare the two data series. What conclusions can we draw?

•	The observed angle strain is greatest in cyclopropane. Angle strain then decrease rapidly with ring size but reaches a minimum for cyclohexane, not cyclopentane (as predicted for the planar structure). Angle strain then increases again but not nearly as quickly as predicted.

•	Why are six-membered rings essentially free of angle strain? and why is there some angle strain in five membered rings even though the bond angles in the planar structure are almost 109.5 degrees?

•	The answer, at least to the first question, is that rings in actual compounds are not planar, they can adopt many different conformations just a easily as acyclic compounds do. To help answer the second question, lets look at some ring conformations in more detail.