Baeyer's Strain theory.pptx

BPAHRM

1. Stability of cycloalkanes

2. Baeyer’s Strain Theory Sache – Mohr Theory Coulson and Moffitt’s Modification

3. BAEYER’S STRAIN THEORY

4. Adolf Von Baeyer, 1885 Nobel prize – 1905 This theory explains the relative stability of the first few cycloalkanes, ie , the ring opening tendencies of cyclopropane and cyclobutane etc. When carbon is bonded to four other atoms, the angle between any pair of bonds is a tetrahedral angle, 109.5 . But the ring of cyclopropane is a triangle, with three angles of 60 , and the ring of cyclobutane is a square with four angles of 90 . Therefore, one pair of bonds to each carbon cannot assume the tetrahedral angle, but must be compressed to 60 or 90 to fit the geometry of the ring.

5. Baeyer postulated that, any deviation of bond angles from the normal tetrahedral value would impose a condition of internal strain on the ring, and hence to be unstable compared with molecules in which the bond angles are tetrahedral. He also assumed that, all cycloalkanes were planar and thus calculated the angles through which each of the valency bond was deflected from the normal direction in the formation of the various rings. This is called angle strain. Cyclopropane and cyclobutane undergo ring opening reactions since these relieve the strain and yield the more stable open-chain compounds. Because the deviation of the bond angle in cyclopropane (109.5 – 60 = 49.5 ) is greater than in cyclobutane (109.5 – 90 = 19.5 ). Cyclopropane is more highly strained, more unstable, and more prone to undergo ring-opening reactions than in cyclobutane .

6. The angles of a regular pentagon (108 ) are very close to the tetrahedral angle (109.5 ), and hence cyclopentane should be virtually free of angle strain. The angles of a regular hexagon (120 ) are larger than tetrahedral angle. So the angle strain in cyclohexane is higher than cyclopentane. The angle strain is maximum in the case of cyclopropane. So it is expected to open-up to release the strain within it. So cyclopropane undergo ring opening reactions with Br 2 , HBr and H 2 (Ni) to give open-chain addition products.

7. Cyclobutane is less reactive than cyclopropane because of the less angle strain compared to cyclopropane. The cyclopropane have least strain and should be most stable. The strain increases continuously with increase in the number of carbon atoms in the ring. Cyclohexane and higher members are found to be quite stable. They do not undergo ring opening reactions.

8. (109.28   Angle strain,

9. Concluded to main 3 postulates: The rings are flat. Preparation of higher cycloalkanes is difficult because they are less stable. Larger rings would be very highly strained, as their bond angles would be very different from the optimum 109.5 .

10. Heats of combustion and relative stabilities Heat of combustion is the quantity of heat evolved when one mole of a compound is burned to carbon dioxide and water. It is used to determine the stability of cycloalkanes. For open-chain alkanes, each methylene group, -CH2- , contributes very close to 157.4 kcal/mol to the heat of combustion.

11. For cyclopropane, the heat of combustion per –CH2- group is 9 kcal higher than the open chain value of 157.4. For cyclobutane , it is 7 kcal higher than open chain value. If cyclopropane and cyclobutane evolve more energy per –CH2- group than an open chain compound, it can mean only that they contain more energy per –CH2- group. So they are less stable than open-chain compounds, and tend to undergo ring-opening reactions, related to this instability.

12. Limitations of Baeyer’s strain theory According to Baeyer, rings larger than cyclopentane and cyclohexane should be unstable, and hence also should have high heats of combustion, relative instability. Heat of combustion should increase steadily with ring size. From the values we can see, none of the rings larger than four carbons, does the heat of combustion per –CH2- deviate not much from the open – chain value of 157.4 For Baeyer’s most stable compound, cyclopentane, value is deviated as 1.3 kcal per –CH2- or 6.5 kcal for the molecule.

13. Rings containing 7 to 11 carbons, have about the same value as cyclopentane, and when we reach rings of twelve carbons or more, heats of combustion are indistinguishable from the open chain values. Then none of the ring is appreciably less stable than open-chain compounds, and the larger ones are completely free of strain. Once they have been synthesized, these large ring cycloalkanes show little tendency to undergo the ring-opening reactions characteristic of cyclopropane and cyclobutane .

14. The angles that Baeyer used for each ring were based on the assumption that the rings were flat. For example, the angles of a regular hexagon are 120 , and for a regular decagon are 144 . But the cyclohexane ring is not a regular hexagon and the cyclodecane ring is not a regular decagon. These rings are not flat, but are puckered. So that each bond angle of carbon can be 109.5 . A three membered ring must be planar, since three points define a plane. A four membered ring need not be planar, but puckering here would increase strain (angle).

15. A five membered ring need not be planar, but in this case, a planar arrangement would permit the bond angles to have nearly the tetrahedral value. All rings larger than this are puckered.

16. Baeyer’s second false assumption: The fact that a compound is difficult to synthesize does not necessarily mean that it is unstable. The closing of ring requires two ends of a chain brought close enough to each other for a bond to form. The larger the ring one wishes to synthesize, the longer must be the chain from which it is made. The likelihood of the two ends of the chain approaching each other is less. Under these conditions, the end of the one chain is more probably to encounter the end of a different chain, and thus yield an entirely different product.

17. Reactions are carried out in highly dilute solutions where collisions between two different chains are difficult, under these conditions, the ring closing reactions, although slow, is the principal one. Five and six membered rings are large enough to be free of angle strain and small enough that the ring closure is probably occur.

18. Orbital picture of angle strain

19. Factors affecting stability of conformations Angle strain Tortional strain Van der waals strain ( steric strain) Dipole-dipole interactions (hydrogen bond)