Stabilities theory
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Baeyer's strain theory proposed that deviations from the ideal tetrahedral bond angle of 109.5° cause instability in cycloalkane rings. It explained stability trends for the first few cycloalkanes but failed for larger rings. Sachse and Mohr proposed that larger rings avoid strain by being non-planar, with carbons in different planes allowing 109.5° angles. Coulson and Moffitt further explained stability using molecular orbital theory, that overlap of carbon orbitals is maximized at 109.5°, so deviation causes weaker bonds and more reactivity. This accurately describes stability trends, including cyclopropane being most reactive due to smallest angles limiting orbital overlap.
Stabilities theory
- 2. Baeyer’s strain theory limitation of Baeyer’s strain theory
- 3. Baeyer’s Strain Theory Baeyer’s theory is based upon some assumptions that are helpful to understand instability of cycloalkane ring systems. 1. All ring systems are planar. Deviation from normal tetrahedral angles results in to instable cycloalkanes. 2. The large ring systems involve negative strain hence do not exists. 3. The bond angles in cyclohexane and higher cycloalkanes (cycloheptane, cyclooctane, cyclononane, etc.) are not larger than 109.5⁰ because the carbon rings of those compounds are not planar (flat). Adolf Baeyer (1885). Proposed a theory to explain the relative stability of the first few cycloalkanes on the fact that the normal angle between any pair of bonds of carbon atom is 109⁰ 28' (or 109.5⁰) in tetrahedral geometry (methane molecule).
- 4. Baeyer postulated that any deviation of bond angles from the normal tetrahedral value would impose a condition of internal strain on the ring. This is called Angle Strain, which determined the stability of the ring. Baeyer proposed that the optimum overlap of atomic orbitals is achieved for bond angel of 109.5⁰ (for carbon atom, in a molecule if it is sp3 hybridized and orbital overlap is maximum). In short, it is ideal bond angle for alkane compounds that produces maximum bond strength and stable molecule.
- 5. Fig.1: Orbital overlap between (A) Propane (B) Cyclopropane. Maximum overlap occurs in propane For an open chain compound (propane), when carbon is connected to other two carbon atoms, it is sp3 hybridized and utilize these hybrid orbitals to form the bonds (strong sigma bonds) (Fig.1). In cyclopropane, the carbon atom don’t use these hybrid orbitals to form the bonds hence, the bond (bent bond) is weaker as compare to usual carbon-carbon bond. This is described as Angle Strain. If bond angles deviate from the ideal then the ring produce strain. Higher strain produces higher instability, increased reactivity and increases heat of combustion. In simple higher the deviation lesser the instability. Baeyer observed different bond angles for different cycloalkanes and also observed some different properties and stability. On this basis, he proposed angle strain theory. The theory explains reactivity and stability of cycloalkanes.
- 6. Table 1: Angle strain of first few cycloalkanes Compound Bond angle Deviation Angle strain Cyclopropane 60⁰ 49.5⁰ 24.75⁰ Cyclobutane 90⁰ 19.5⁰ 9.75⁰ Cyclopentane 108⁰ 1.5⁰ 0.75⁰ Cyclohexane 120⁰ 10.5⁰ -5.27⁰ The ring of cyclopropane is triangle. All the three angles are of 600 in place of 109.50 (normal bond angle for carbon atom) this implies that the normal tetrahedral angle between two bonds is compressed to 600 and that each of the two bonds involved is pulled in by 24.75o (Table 1).
- 7. The value 24.75o then represents the angle strain or the deviation through which each bond bends from the normal tetrahedral direction. In same manner, cyclobutane is square and the bond angles are of 90o in place of 109.5o (normal bond angle for carbon atom) to adjust them into square ring system (angle strain 9.75o)
- 8. The deviation for cyclopropane and cyclobutane ring systems then normal tetrahedral angle will produce strain in ring. The ring strain will make them unstable as compare to molecules having tetrahedral bond angle. According to Baeyer cyclopropane should be highly strained molecule and most unstable compare to cyclobutane. The triangle ring therefore, be expected to open up on slightest provocation and thus releasing the strain within it. This is actually so cyclopropane undergoes ring opening reactions with Br2, HBr and H2 to give open chain addition products. Cyclopentane (angle strain 0.75o ) consider to be under least strain and should be most stable. Thus it is not surprising that it does not undergo ring-opening reactions. The angle strain in cyclohexane is higher than that in case of cyclopentane. This strain increase continuously with increase in the number of carbon atoms in the ring. According to the theory, cyclohexane and the higher cycloalkanes should become increasingly unstable and hence more reactive. Contrary to this prediction, cyclohexane and the higher members are found to be quite stable, they do not undergo addition reaction instead they react by substitution reaction. Thus the theory satisfactorily accounts for the first three cycloalkanes, but it is not valid for cyclohexane and higher members.
- 9. According to Baeyer, the relative order of stability for some common cycloalkanes is as under- Cyclopentane > Cyclobutane > Cyclopropane Limitations of Baeyer’s angle strain theory Baeyer was not able to explain the effect of angle strain in larger ring systems. According to Baeyer Cyclopentane should be much stable than cyclohexane but practically it is reversed. Larger ring systems are not possible according to Baeyer as they have negative strain but they exist and much stable. Larger ring systems are not planar but wrinkled (puckered) to eliminate angle strain.
- 10. Sachse-Mohr Theory (Theory of Strainless Rings)
- 11. According to Baeyer, the carbon atoms of a ring are all in the same plane. The strain was calculated on the assumption that cyclic rings are planer. The strain is minimum for cyclopentane and goes on increasing as the size of the ring increases. Hence, members higher than cyclopentane should be increasingly unstable but Hermann Sachse, soon pointed out that large rings need not be strained, because the carbons need not be coplanar. Sachse Mohr’s theory proposed that higher member ring can become free from strain if all the ring carbons are not forced into one plane. They exhibit in two non-planar ‘folded’ or ‘puckered’ conformations both of which are completely free from strain. These are strainless as the carbon atoms lie in different planes and the normal valency angle (109.5⁰) is retained. These are called the ‘Chair’ Form or the ‘Z’ Form and the ‘Boat’ Form or the ‘C’ Form because of their shapes.
- 12. The chair conformation is the most stable conformation of cyclohexane. In chair cyclohexane there are two types of positions, axial and equatorial. The axial positions point perpendicular to the plane of the ring, whereas the equatorial positions are around the plane of the ring. Mohr (1918) further elaborated this theory and applied it to compounds with the two rings fused together.
- 13. Coulson-Moffitt Model or Concept of Maximum Overlap of Carbon Orbitals (Bent bond/Banana bond Theory)
- 14. A bent bond, also known as a banana bond, is a type of covalent chemical bond with geometry somewhat indicative of a banana. The term itself is a general representation of electron density or configuration resembling a similar "bent" structure within small ring molecules, such as cyclopropane (C3H6) or as a representation of double or triple bonds within a compound that is an alternative to the sigma and pi bond model.
- 15. It explains the relative stability of cycloalkanes in term of MO theory as follows- We know that a covalent bond between two atoms is formed by the overlap of orbitals of the atoms involved. The greater the extent of overlap the stronger is the bond formed. The atomic orbitals overlap to the maximum extent if they overlap along their axes. As the axes of Sp3 orbitals are at angles of 109028’ to each other, the C-C bond will have their maximum strength if the C-C- C bond angles have a value of 109028’. Cyclopropane has C-C-C bond angles of 600, Cyclobutane has C-C-C bond angles have a value of 900. The higher cycloalkanes and alkanes have C-C-C bond angles of 109028’.
- 16. As shown in above picture. The small bond angle of cyclopropane indicates that the overlap of sp3 orbital of carbon is less than the overlap of sp3 orbital of carbon in alkanes (Eg. Propane). The bond angles of cyclopropane are less than the bond angles of cyclobutane, which in turn are less than the bond angles of higher cycloalkanes of n-alkanes. Therefore, the overlap of orbitals in cyclopropane is less than that in cyclobutane, which in turn in less that that in higher cycloalkanes or n- alkanes. The overlap of sp3 orbitals of carbons in cyclopentane, higher cycloalkanes or n-alkanes is maximum because in these cases it is possible for the sp3 orbitals along their axes, the bond angles being approx. equal to 109028’.
- 17. This implies that the C-C bonds in cyclopropane are weaker than the C-C bonds of cyclobutane, which in turn are weaker than the C-C bonds in higher cycloalkanes and n- alkanes. Cyclopropane undergoes ring- opening reactions very readily. On other hand Cyclobutane ring only under drastic condition undergoes ring- opening reaction. Cyclopentane and higher members do not undergo ring opening reaction and behaves very much like the alkanes.
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