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Ive been struggling to solve this dehydrohalogenataion excercise. How does the E2 mechanism work in this case and how is it affected by the stereoselectivity and antiperiplanar position? How do I get to the transition state and how does the chair conformations for the cyclohexane ring look like?

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    $\begingroup$ It will help to write the cyclohexane ring in the chair configuration. What does the transition state look like, and how to get there? $\endgroup$ – Abel Friedman Nov 29 '14 at 3:10
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    $\begingroup$ Welcome to Chemistry.se! Enjoy the features of this site by taking the tour. Additional information can be obtained in the help center. In accordance with our homework policy, I would like you to add some more of your thoughts towards the solution of the problem (Maybe start with @AbelFriedman's suggestion). You can simply edit your question. $\endgroup$ – Martin - マーチン Nov 29 '14 at 6:15
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As you noted, the chlorine and the other group being eliminated must be antiperiplanar. In the case of the chair form of cyclohexane, this means that the chlorine and other group being eliminated must be trans-diaxial. In the figure below I've drawn the 2 possible conformations (interconverted by a chair flip) for your deuterated cyclohexane. Only in the top conformation is the chlorine axial, and the hydrogen on the adjacent (non-methylated) carbon is also positioned trans and axial. Only this hydrogen can be eliminated as it is the only one that is axial and antiperiplanar to the chlorine. Therefore, the only product produced when this material is treated with base and an E2 occurs, is the pictured 1-deuterio, 3-methylcyclohexene. The hydrogen on the methylated carbon cannot be eliminated since it is never axial when the chlorine is axial.

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    $\begingroup$ The hardest part for me is to visualize the different conformations of the cyclohexane chair, it takes time and lots of practice. Thank you so much for your answer, it really cleared up all my doubts on this topic! $\endgroup$ – Angelik Nov 29 '14 at 3:34
  • $\begingroup$ Your welcome! It is extremely helpful to build a model until you become proficient in the visualization step. If the answer was helpful, please mark it as accepted. $\endgroup$ – ron Nov 29 '14 at 3:37

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