What is the difference between axial and equatorial positions of cyclohexane

An asymmetric carbon is often a chiral stereogenic center, since interchanging any two substituent groups converts one enantiomer to the other. However, care must be taken when evaluating bridged structures in which bridgehead carbons are asymmetric. This caveat will be illustrated by Clicking Here. Alkenes having two different groups on each double bond carbon e. Chiral stereogenic axes or planes may be present in a molecular configuration, as in the case of allenes , but these are less common than chiral centers and will not be discussed here.

For additional information about chiral axes and planes Click Here. Structural formulas for eight organic compounds are displayed in the frame below. Some of these structures are chiral and some are achiral. First, try to identify all chiral stereogenic centers. Formulas having no chiral centers are necessarily achiral. Formulas having one chiral center are always chiral; and if two or more chiral centers are present in a given structure it is likely to be chiral, but in special cases, to be discussed later, may be achiral.

Once you have made your selections of chiral centers, check them by pressing the "Show Stereogenic Centers" button. The chiral centers will be identified by red dots. Structures F and G are achiral. The former has a plane of symmetry passing through the chlorine atom and bisecting the opposite carbon-carbon bond. The similar structure of compound E does not have such a symmetry plane, and the carbon bonded to the chlorine is a chiral center the two ring segments connecting this carbon are not identical.

Structure G is essentially flat. All the carbons except that of the methyl group are sp 2 hybridized, and therefore trigonal-planar in configuration. Remember, all chiral structures may exist as a pair of enantiomers. Other configurational stereoisomers are possible if more than one stereogenic center is present in a structure. Identifying and distinguishing enantiomers is inherently difficult, since their physical and chemical properties are largely identical.

Fortunately, a nearly two hundred year old discovery by the French physicist Jean-Baptiste Biot has made this task much easier. This discovery disclosed that the right- and left-handed enantiomers of a chiral compound perturb plane-polarized light in opposite ways. This perturbation is unique to chiral molecules, and has been termed optical activity. Plane-polarized light is created by passing ordinary light through a polarizing device, which may be as simple as a lens taken from polarizing sun-glasses.

Such devices transmit selectively only that component of a light beam having electrical and magnetic field vectors oscillating in a single plane. The plane of polarization can be determined by an instrument called a polarimeter , shown in the diagram below. Monochromatic single wavelength light, is polarized by a fixed polarizer next to the light source. A sample cell holder is located in line with the light beam, followed by a movable polarizer the analyzer and an eyepiece through which the light intensity can be observed.

In modern instruments an electronic light detector takes the place of the human eye. This site may be examined by Clicking Here. Chemists use polarimeters to investigate the influence of compounds in the sample cell on plane polarized light. Samples composed only of achiral molecules e. The prefixes dextro and levo come from the Latin dexter , meaning right, and laevus , for left, and are abbreviated d and l respectively.

If equal quantities of each enantiomer are examined , using the same sample cell, then the magnitude of the rotations will be the same, with one being positive and the other negative.

To be absolutely certain whether an observed rotation is positive or negative it is often necessary to make a second measurement using a different amount or concentration of the sample. Since it is not always possible to obtain or use samples of exactly the same size, the observed rotation is usually corrected to compensate for variations in sample quantity and cell length. Compounds that rotate the plane of polarized light are termed optically active.

Each enantiomer of a stereoisomeric pair is optically active and has an equal but opposite-in-sign specific rotation. Specific rotations are useful in that they are experimentally determined constants that characterize and identify pure enantiomers.

For example, the lactic acid and carvone enantiomers discussed earlier have the following specific rotations. A mixture of enantiomers has no observable optical activity. When chiral compounds are created from achiral compounds, the products are racemic unless a single enantiomer of a chiral co-reactant or catalyst is involved in the reaction.

The addition of HBr to either cis- or transbutene is an example of racemic product formation the chiral center is colored red in the following equation. The axial bonds will either face towards you or away.

These will alternate with each axial bond. The first axial bond will be coming towards with the next going away. There will be three of each type. The terms cis and trans in regards to the stereochemistry of a ring are not directly linked to the terms axial and equatorial. It is very common to confuse the two. It typically best not to try and directly inter convert the two naming systems. Axial vs. Equatorial Substituents When a substituent is added to cyclohexane, the ring flip allows for two distinctly different conformations.

Solution Due to the large number of bonds in cyclohexane it is common to only draw in the relevant ones leaving off the hydrogens unless they are involved in a reaction or are important for analysis. Questions Q4. Solutions S4. 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. We see cyclohexane drawn in 2 ways: Both can be used to draw the exact same molecule, but they are simply different ways of representing it.

Chair flipping Chairs can change conformations through a process called chair flipping, creating 2 conformations for the same chair. Keep it Simple. One the other hand, all axial substituents point either straight up or straight down. Draw the following structure in its most stable chair conformation: Answer.

First, arbitrarily number the carbons. This numbering has nothing to do with naming the molecule, but it is only used to help keep track of where the substituents are in relation to one another. We then draw a regular chair conformation and a chair conformation in its flipped formed. We now add substituents to each. At each carbon on the cyclohexane, there is a one substituent that points up and one that points down, which is something we will utilize in this step.

If the substituent is a wedge on the 2-D cyclohexane, then place the substituent so it is going upward on the chair at the corresponding carbon e. If it is a dash , then place the substituent so it is facing downward on the corresponding carbon. Do this for each chair shown above: Both of these answers would be correct if we just had to convert the 2-D to the 3-D structure; however, questions often ask for the most stable structure.