Give the decreasing order or reactivity for the elimination with strong bases of `RX(X=F,XI,Br, `and `I). `
`b.` How is this information important to decide between `E2` and `E1cB`?

Explanation

Whenever `R.D.` transition step involves breaking of the `(C-X)` bond, the rate is directly related to the leavvbility of `X^(Θ)`, which in turn is related to its basicity. The weakest base is the bast leaving group and is in the following order`:`
`I^(Θ) Br^(Θ)gtCI^(Θ)gtF^(Θ)`
The order of reactivity is `RIgtRBrgtRCIgtRF`.
`b.` In `EIcB`, the expulsion of `C^(Θ)` occurs in the fast second step and the rate of the overall reaction is independent of the leavability of `X`.
Therefore, all `RX` react with the same rate.
In `E2` the expulsion of `X^(Θ)` occurs in the `R.D.S.` and the nature of `X` influences the rate.