Bayers strain theory

it deals with strains, bayers strain theory and its limitation

1. Reaction of cycloalkane

2. Reaction of cyclo propane

3. Reaction of cyclobutane

4. Reaction of Cyclo hexene

5. Stability of cyclo alkanes

6. STRAIN Deformation of a bond length and bond angles from their ideal values under the influence of internal forces.

7. It gives rise to instability within in a structure with high internal energy. When a bond angle or length is distorted the potential energy is stored. The stored potential energy in the molecule is referred as strain or strain energy.

8. Types of Strain Torsional Strain Angle strain Steric Strain

9. Torsional Strain Torsional strain  or eclipsing  strain  is the increase in potential energy of a molecule due to repulsion between electrons in bonds that do not share an atom.

10. Angle strain Angle strain  is the increase in potential energy of a molecule due to bond  angles  deviating from the ideal values.  

11. Steric strain   Steric strain  (van der Waals  strain ): An increase in molecular potential energy ( strain ) caused when atoms or groups separated by at least four covalent bonds are forced closer than their van der Waals radii allow.

12. A dolf Von Baeyer ( of the University of Munich) In 1885 proposed a theory to explain the relative stability of cycloalkanes.

13. His theory based on the fact that the normal angle between any pair of bonds of a carbon atom is 109◦C 28’.

14. Baeyer postulated that any deviation of bond angles from the normal tetrahedral value would impose a condition of internal strain on the ring .

15. He assumed that all cycloalkanes are planar. He calculated the angles through which each of the valence bond was deflected from the normal direction in the formation of the various ring . This is called angle strain. Angle strain determines the stability of the ring.

16. Cyclopropane

17. Cyclopropane Three carbon atoms occupy the corners of an equilateral triangle. Cyclopropane has C-C-C bond angle of 60◦C Normal tetrahedral angle is 109◦ 28’ between any two bond is compressed to 60◦.

18. Cyclopropane

19. Cyclopropane Each of the bond is pulled in by 24 ◦ 44’ Whether the angle strain is positive or negative, its magnitude determines the extent of strain in the ring. Angle of strain is more in cyclopropane.

20. According to Baeyer strain theory , cyclo propane should be highly strained molecule and consequently most unstable. It is expected to open up on slightest provocation and thus releasing the strain within it. Thus cyclo propane undergoes ring opening reaction.

21. Cyclo butane

22. cyclopentane

23. Cyclopentane Angle of strain is minimum in cyclo pentane. It is under least strain and more stable. It does not undergo ring opening reaction. The angle of regular pentagan 108 are very close to the tetrahedral angle 109 5 .Hence cyclopentane is virtually free of angle strain.

24. Baeyer proposed [incorrectly] there should be a certain amount of strain in cyclohexane , further he suggested [incorrectly]that as one proceed to cycloheptane , cyclooctane , the deviation of the bond angle is progressively larger and the molecules would become progressively more strained.

25. Baeyer considered the rings smaller or larger than cyclopentane and cyclohexane were unstable. Because of instability that the 3 and 4 membered ring underwent ring opening reaction. It was because of this instability greater difficulty was encountered in the synthesis of larger ring.

26. ANGLE STRAIN IN CYCLO ALKANE COMPOUND BOND ANGLE ANGLE STRAIN CYCLO PROPANE 60◦ 24◦44’ CYCLO BUTANE 90◦ 9◦ 44’ CYCLO PENTANE 108◦ 0.44’ CYCLO HEXANE 120◦ -5◦ 16’

27. Heat Combustion The quantity of heat evolved when one mole of compound is burned to CO2 and water. Heat of combustion support of Baeyer’s proposal that rings smaller or larger than cyclopentane and cyclohexane are unstable.

28. Ring Size Heat of Combustion 3 166.6 4 164 5 158.7 6 157.4 7 158.3 14 157.2 Open chain compound 157.4

29. Cyclohexane angle of strain increases with increases in the number of carbon atom in the ring. According to baeyer’s theory cyclohexane and higher alkane should be unstable and reactive. Contrary to this prediction, cyclohexane and the higher members are found to be quite stable.

30. They donot undergo ring opening reactions. Instead they resemble open chain alkanes in reactivity i.e it undergoes substitution. But bayers strain theory satisfactorily account for cyclo propane, cyclobutane and cyclopentane . But it is not valid for cyclohexane and higher member.

31. Limitation of B aeyer’s strain theory Bayer was not able to explain the effect of angle strain in larger ring systems. According to Baeyer cyclo pentane should be much stable than cyclohexane but practically it is reversed. Larger ring systems are not possible according to B aeyer as they have negative strain but they exist and much stable Larger rings are not planar but puckered to eliminate angle strain.

32. Thank you