@DrBSK Chemistry

  methylcyclohexane conformations: 1) Methylcyclohexane is cyclohexane in which one hydrogen atom is replaced with a methyl group substituent.  2) Methylcyclohexane can adopt two basic chair conformations:  one in which the methyl group is axial, and one in which it is equatorial .  3) The most stable conformation of methylcyclohexane is the  chair conformation in which the methyl group is equatorial. 4) The alternative chair conformation, in which the methyl group is axial, is 7.3 kJ/mol higher in energy.  5) This difference corresponds to a equatorial:axial conformer ratio of 19:1 at  25 °C.

  In chair conformation, the adjacent hydrogen atoms on all the neighboring carbon atoms are staggered and the force of repulsion in them is the minimum. It also does not have any angle strain. So it is most stable. The half chair form is least stable as it has maximum strain as five carbon atoms are in one plane and one carbon atom is out of plane. The boat conformation has steric strain due to interaction between two flagpole hydrogen and torsional strain is also present. It also does not have any angle strain. So it is less stable than a chair and twist boat. Twist boat has less steric strain and torsional strain then the boat conformation due to which flagpole hydrogen atoms move away from each other and reduce the strain so it is more stable than boat conformation.

 Conformations of  Cyclohexane 1) Rings larger than cyclopentane would have angle strain if they were planar. However, this strain, together with the eclipsing strain inherent in a planar structure, can be relieved by puckering the ring.  2) Cyclohexane is a good example of a carbocyclic system that virtually eliminates eclipsing and angle strain by adopting non-planar conformations. 3)  Cycloheptane and cyclooctane have greater strain than cyclohexane, in large part due to transannular crowding (steric hindrance by groups on opposite sides of the ring). 4) Cyclohexane is rapidly rotating between the two most stable conformations known as the chair conformations in what is called the "Chair Flip" shown below. 5)   Several other notable cyclohexane conformations occur during the transition from one chair conformer to the other-  the boat, the twist, and the half-chair.   6) T he relative energies of the conformations is a direct reflection of their...

 

  Semester-3   Unit-2    Fundamentals of Organic Chemistry Baeyer Strain Theory Adolf von Baeyer (a Nobel prize winner) proposed a theory in 1885 to explain the relative stability of the first few cycloalkane. The theory based on following facts: 1. Cyclo-alkanes are saturated compounds. So, all the carbons should have normal tetrahedral angle of 109 o 28 1 (or 109 o  5 1 ). 2. Any deviation of bond angles from the normal tetrahedral value would, impose a condition of internal strain on the ring called bond angle strain or Beayer strain 3. Higher value of angle strain of cycloalkane shows instability. As instability increases reactivity also increase. 4. If all the rings in cycloalkanes assumed to be planar, the bond angle strain for each ring can be calculated below  . 5. According the above table smaller ring compounds cyclo propane and cyclobutane suffer large distortion and believed to be the cause of grater instability and enhanced r...