Why is equatorial position more stable

Very interesting! Imagine looking along the C-1 to C-2 bond which is coplanar with the C-4 to C-5 bond. Note that in the conformation where methyl is axial, there is a gauche interaction between the axial methyl group and C This is absent in the conformation where methyl is equatorial. This gauche interaction is an example of van der Waals strain, which is what makes the axial conformer higher in energy.

There is actually a second gauche interaction if you look along C-1 to C This gauche interaction is with C Bottom line: in two unequal conformations of a cyclohexane ring, the conformation where steric interactions are minimized will be favoured. Since this ratio of conformers represents a system at equilibrium, we can actually use it to calculate the difference in energy of these two conformers using the following equation:.

For methylcyclohexane at room temperature K the ratio of equatorial to axial conformers translates to an energy difference of 1. Since there are two gauche interactions, and the strain energy is 1. Now this opens up all kinds of questions. If a methyl group CH 3 leads to an energy difference of 1. Or a Cl? Or OH? Or tert-butyl? Would this change the equilibrium? In the molecule above, the CH 2 groups at C-3 and C-5 have been replaced by oxygen.

Since there are no longer any significant diaxial interactions between the methyl group and substitutents on the ring, there is no significant energy difference between the equatorial and axial conformations of this molecule.

This is a topic commonly taught to undergraduates in Organic Chemistry, and goes along with the discussion on A- values. Substituents in cyclohexane can take two positions, axial and equatorial, and the preferred conformation is dictated by stereoelectronic effects. Dear sir, I willbe very thankful if you kindly explain me the most stable conformation of the given molecule. When I look down the 1 to 2 carbon and 5 to 4 carbon for the axial position I am getting exactly the Newman projection you have, however, for the equatorial position Newman when I look down the 1 to 2 carbon and 4 to 5 carbon I am not getting the same Newman projection.

Your email address will not be published. When you feel the need, look around! It provides templates for various 6-ring chair structures from the Templates menu; choose Rings. There are templates for simple chairs, without substituents e.

In either case, you can add, delete, or change things as you wish. Various kinds of stereo bonds wedges and bars are available by clicking the left-side tool button that is just below the regular C-C single bond button. It may have a wedge shown on it, but this will vary depending on how it has been used. To choose a type of stereo bond, click on the button and hold the mouse click; a new menu will appear to the right of the button. A basic chair structure is provided on the default template bar that is shown.

More options are available by choosing the Rings template. See my page Symyx Draw for a general guide for getting started with this program. The free drawing program ChemSketch provides similar templates and tools. See my page ChemSketch for a general guide for getting started with this program. If you want to draw chair structures by hand and if you are going on in organic chemistry, you should … Be careful.

The precise zigs and zags, and the angles of substituents are all important. Your textbook may offer you some hints for how to draw chairs. A short item in the Journal of Chemical Education offers a nice trick, showing how the chair can be thought of as consisting of an M and a W. When the methyl group is in the equatorial position this strain is not present which makes the equatorial conformer more stable and favored in the ring flip equilibrium.

Actually, 1,3-diaxial steric strain is directly related to the steric strain created in the gauche conformer of butane discussed in Section: When butane is in the gauche conformation 3. When looking at the a Newman projection of axial methylcyclohexane the methyl group is at a 60 o dihedral angle with the ring carbon in the rear.

This creates roughly the same amount of steric strain as the gauche conformer of butante. Given that there is actually two such interactions in axial methylcyclohexane, it makes sense that there is 2 3. The Newman projection of equatorial methylcyclohexane shows no such interactions and is therefore more stable.

Newman projections of methyl cyclohexane and butane showing similarity of 1,3-diaxial and gauche interactions. Strain values for other cyclohexane substituents can also be considered. The relative steric hindrance experienced by different substituent groups oriented in an axial versus equatorial location on cyclohexane determined the amount of strain generated. The strain generated can be used to evaluate the relative tendency of substituents to exist in an equatorial or axial location. Looking at the energy values in this table, it is clear that as the size of the substituent increases, the 1,3-diaxial energy tends to increase, also.

Note that it is the size and not the molecular weight of the group that is important. Table 4. Indicate axial and equatorial positions. Flipped chair now looks like this. It may help to use molecular model to answer this question.

This leads to less of a strain on the molecule. The table above states that each interaction accounts for 1. The total strain in equatorial bromocyclohexane will be 2 1. After plugging in the calculated value for K, X can be solved algebraically.

This forces the ethyl substituent to have only have 1,3- diaxial interactions between hydrogens, which only provides a slight difference to a methyl group. Steven Farmer Sonoma State University.

Objectives After completing this section, you should be able to Draw the chair conformation of cyclohexane, with axial and equatorial hydrogen atoms clearly shown and identified.