Methylcyclohexane Newman projection

Conformations of methylcyclohexane. Plat ring perspective, 0 CHAIR CONFORMATIONS, AND Q NEWMAN PROJECTION. 

The two chair conformations of methylcyclohexane interconvert at room temperature, so the one that is lower in energy predominates. Careful measurements have shown that the chair with the methyl group in an equatorial position is the most stable conformation. It is about 7.6 kJ/mol (1.8 kcal/mol) lower in energy than the conformation with the methyl group in an axial position. Both of these chair conformations are lower in energy than any boat conformation. We can show how the 7.6 kJ energy difference between the axial and equatorial positions arises by examining molecular models and Newman projections of the two conformations. First, make a model of methylcyclohexane and use it to follow this discussion. 

Draw a Newman projection, similar to Figure 3-25, down the Cl —C6 bond in the equatorial conformation of methylcyclohexane. Show that the equatorial methyl group is also anti to C5. (Using your models will help.)... 

Let us make a model of the conformation of methylcyclohexane with axial methyl. If we hold it so that we can sight along the Ci—C2 bond, we see something like this, represented by a Newman projection ... 

If we use a Newman projection that lets us view down two carbon-carbon bonds at the same time to compare the two conformations of methylcyclohexane (Figure 3.22), we see that the axial methyl and a carbon-carbon bond on an adjacent carbon atom are gauche with respect to each other. We can also describe the interaction of the axial methyl with the third carbon atom away from the point of attachment of the methyl group in a different way. 

Aspects of cyclohexane conformational analysis. A. Interconverting chair forms of cyclohexane, with axial and equatorial locations labeled. In the left structure letters x are axial while letters y are equatorial. Note that the chair flip moves axial substituents to the equatorial position and vice versa. B. Newman projections down the C1-C2/ C5-C4 bonds of methylcyclohexane. In the axial form, there is a gauche butane interaction between the methyl and C3. C. Views of cyclohexane, equatorial methylcyclohexane, and axial methylcyclohexane. Note that chair cyclohexane is a relatively disk-shaped molecule, and an equatorial methyl does little to disrupt this shape. In contrast, an axial substituent puts a "kink" into the structure. Also evident is the steric interaction between one methyl hydrogen and the two axial hydrogens on C3 and C5. 

WORKED PROBLEM 5.9 Draw the Newman projection of methylcyclohexane looking from the other adjacent methylene group toward the methyl-bearing carbon. Be sure that you see the second gauche methyl—ring interaction. Next, compare this gauche interaction to that ingaett e-butane. Are the two exactly the same ... 

A Newman projection of the conformation of methylcyclohexane with the equatorial methyl group made from the same perspective reveals none of these gauche interactions—this arrangement resembles the anti form of butane (Fig. 5.29). 

The strain caused by a 1,3-diaxial interaction in methylcyclohexane is the same as the strain caused by the close proximity of the hydrogen atoms of methyl groups in the gauche form of butane (Section 4.9). Recall that the interaction in gauche-butane (called, for convenience, a gauche interaction) causes gauche-butane to be less stable than anti-butane by 3.8 kJ mol The following Newman projections will help you to see that the two steric interactions are the same. In the second... 

Problem 43. Using Newman-type projections, show the gauche methyl-methylene interactions for the conformational isomer of methylcyclohexane with the methyl group axial. 

Build a model of methylcyclohexane, and use the model to complete the following Newman 1 projections of methylcyclohexane in the chair conformation ... 

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