Winter 2013 Final 8B

[nicoleclarke_lec1M]

Why would both substiuent groups be in equatorial positions in the chair cyclohexane conformation in a cis isomer? I thought we had learned in lecture that cis isomers are always (a,e) or (e,a) meaning one is equatorial and one is axial. Please someone explain?

[Selina_Han_3I]

What you recalled in lecture is correct; however, in lecture, we discussed cis isomers with substituents attached to carbons 1 and 4. In cis-1-chloro-3-ethylcyclohexane, the substituents are connected to carbons 1 and 3. Cis means that both substituents will be on the same side, and therefore pointing in the same direction (both pointing up or both pointing down). So, in the diagram, carbons 1 and 3 have axial up and equitorial down positions. The chlorine and the ethyl substituents will point up in the axial position, rather than point down in the equitorial position to reduce steric strain.

[Brian Devlin 3B]

How would you describe trans-1-chloro-3-ethylcyclohexane?

[nicoleclarke_lec1M]

the trans molecule would have one in the equatorial position and one in the axial position. Since the ethyl substituent take priority, it will be in the equatorial position.