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enter image description here

I'm confused about how this chair conformation was constructed and why that particular Cl leaves. Why is one of the Cl's drawn axial while the others are drawn equatorial (it seems like the others could have been drawn axial as well)? And, to reiterate, why is it that Cl that's favored to leave over the other two?

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In cyclohexane, the equatorial position is energetically favored over the axial position. So the lowest energy conformer is the one where the most substituents are in equatorial position. (Or rather: Where you minimize the energy according to the A Value).

The axial Cl is favored as leaving group because of the elimination reaction mechanism. Normally, we see $\beta_{1,2}$ elimination in an anti-fashion. Which is what is proposed by your image.

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  • $\begingroup$ Can you explain more clearly why that Cl leaves as opposed to the others? $\endgroup$ – Billy Joel Dec 12 '13 at 9:39
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    $\begingroup$ As said, bimolecular elimination "likes" anti-periplanar arrangement of leaving groups. And the axial chlorine is the only one, for which is this possible. The other two would need synclinal, or syn-periplanar, which is not that favored. $\endgroup$ – ssavec Dec 12 '13 at 9:45
  • $\begingroup$ What does anti-perpiplanar mean? $\endgroup$ – Billy Joel Dec 12 '13 at 9:51
  • $\begingroup$ @BillyJoel, it means there's roughly a 150-180 degree angle between the beta-hydrogen being abstracted by the base and the chlorine leaving group. See the IUPAC gold book entry on torsion angle. $\endgroup$ – Greg E. Dec 12 '13 at 13:04
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The chair conformation is the most stable due to the following reasons:

  • The carbons in cyclohexane have sp3 hybridization and hence they tend to have an angle of 109.5 degrees. The chair conformation allows it to have the closest possible angle which would be 110.9 degrees.Due to this the strain reduces and results in a lowered energy. The two chair conformations have the lowest total energy, and are therefore the most stable.

  • Since cyclohexane's planar form results into bond angle being 120 degrees (which means a higher deviation from 109.5). Hence the energy of the planar form is higher and it always has the tendency to convert to chair form.

Due to steric hindrance in the axial location, substituent groups prefer to be equatorial and that chair conformer predominates in the equilibrium.

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