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What I do know: E2 reactions are concerted and have an anti-coplanar transition state, where the leaving group and the hydrogen being abstracted have a dihedral angle of 180 degrees.

What I don't understand: A does not have a halogen that acts as a leaving group. B has two halogens (bromine and fluorine). Thus, how do I determine the substrate(s), if any, that will undergo E2?

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  • $\begingroup$ A has a mesylate that is every bit as good a leaving group as a halogen. In B is there a proton with a dihedral angle of 180 to a halogen? $\endgroup$
    – Waylander
    Nov 17, 2022 at 8:33
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    $\begingroup$ I would say that it's possible thath both of them could undergo an E2 step. A has the mesylate group that acts like an halogen. B in one of its conformation (may be not the stablest one) has an hydrogen atom in anti-coplanar. To choose between F and Br: Br is a better LG than F. To remove an F atom you need very strong conditions $\endgroup$
    – Andrea Pellegrini
    Nov 17, 2022 at 8:43
  • $\begingroup$ Be careful with the CIP rules in determining the E/Z assignment. While Br is the better leaving group, configurationally, does it matter whether HBr or HF is lost? $\endgroup$
    – user55119
    Nov 17, 2022 at 19:46

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