Unit 5

cycloalkane

1. Cyclo alkanes : Strain Theory PREPARED BY- Dr.PRIYANKA

2. Energy profile of the interconversion of cyclohexane conformations - Chair conformation: In this conformation, all bonds are in staggered form, make molecule most stable puckered conformation. - Twist boat or skew boat conformation: This conformation exists in between chair and boat conformation and more stable than boat but less stable than chair conformation. - Half chair conformation: This is highly unstable and strained conformation of Cyclohexane . - Boat conformation: This is less stable conformation than chair form by 6.8 kcal/mole is due to flagpole interaction of hydrogen atoms.

3. Baeyer postulated that any deviation of bond angles from the normal tetrahedral value would impose a condition of internal strain on the ring. Baeyer’s Strain Theory

4. Baeyer’s Strain Theory The bond angle is of 1090 28' (or 109.50) for carbon atom in tetrahedral geometry (methane molecule). Baeyer proposed “any deviation of bond angle from ideal bond angle value (109.50) will produce a strain in molecule. Higher the deviation lesser the instability.

5. These assumptions are helpful to understand instability of cycloalkane ring systems. - The ring of cyclopropane is triangle. All the three angles are of 600 in place of 109.50 (normal bond angle for carbon atom) to adjust them into triangle ring system. - In same manner, cyclobutane is square and the bond angles are of 900 in place of 109.50 (normal bond angle for carbon atom) to adjust them into square ring system. Cyclo alkanes : Strain Theory

6. As a result of this, the strain is more in cyclopropane as compare to cyclobutane . It will make cyclopropane less stable than cyclobutane . So, cyclopropane easily undergoes ring opening reaction as compare to cyclobutane . - According to Baeyer, the relative order of stability for some common cycloalkanes is as under.

7. Limitation of Baeyer’s strain theory Successfulness of Baeyer’s angle strain theory - Baeyer rightly proposed that cyclopropane and cyclobutane are flat molecule and having angle of 60˚ and 90˚ those are much deviated from the ideal tetrahedral value of 109.50 hence these ring systems are unstable and easily undergo ring opening reactions. There is much angle strain in cyclopropane and cyclobutane . - Baeyer also proposed that Cyclopentane is not need to be planar but it is planar as in that condition the angle is much near to ideal tetrahedral angle.

8. Orbital Picture of Angle Strain - As we known that the bond formation effective atomic orbital overlap is required. - For carbon atom, in a molecule if it is sp3 hybridized then the bond angle is 109.50 and orbital overlap is maximum. - This is the reason for highly stable compounds. - For an open chain compound, when carbon is connected to other two carbon atoms, it is sp3 hybridized and utilize these hybrid orbitals to form the bonds. - In cyclopropane , the bond angle is 600 instead of 109.50. - The carbon Atom don’t use sp3 hybridized orbital hybridized to form the bond - Hence, the bond is weaker as compare to usual carbon-carbon bond

9. Coulson -Moffitt Model (Bent bond and Banana bond) - Bent bond One of the first bent bond theories for cyclopropane was the so-called Coulson -Moffitt Model - A bent bond, also known as a banana bond, is a type of covalent chemical bond with a geometry somewhat reminiscent 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.

10. Sachse Mohr’s theory (Theory of strainless rings) - Baeyer suggested that both large and small polymethylene rings should be strained, 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 puckered conformations both of which are completely free from strain. These are called the Chair Form and Boat Form

11. 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.

12. Strain means deformation of bond length and bond angle from their ideal values under the influence of internal force. - This gives rise to an instability within a structure with higher internal energy. - Strain is term used for the energy associated with a system due to its geometry. - When a bond length or bond angle is distorted, potential energy is stored in the molecule. this potential energy that is stored in the molecules is referred to as the strain or the strain energy. - Type of Strain: ( i ) Torsional Strain or Eclipsed Strain (ii) Angle Strain (iii) Steric Strain or Van- der Walls Strain

13. 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 Steric strain can also be referred to as steric hindrance, which is related to Van Der Waals repulsion, or the strain that occurs as the distance between two atoms or substituents decreases. Steric strain occurs when multiple substituents are close together, and cannot be separated by rotation. Angle Strain: Strain caused when the actual bond angles in the atoms of the molecule differ from the ideal bond angles of their geometry.