Question

Please give the proper reason with details in this question?

Answer 1

Well, in a dehydration reaction, remember that the `beta` hydrogen is removed along with the adjacent `"OH"`, but the base that acts actually approaches the `beta` hydrogen first, and the electrons flow towards the `"OH"`.

`(3)` is quite hard; it is problematic in the same way as the right-hand `beta` hydrogen in `(2)`, combined with having no alkane `beta` hydrogens to the right of `"OH"`. It would either generate `"C"="C"="C"` or a triple bond.

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So the one that is hardest to dehydrate is the one that `(i)` does not have a `beta` hydrogen, and/or `(ii)` would generate `"C"="C"="C"` rather than conjugated `pi` bonds.

• `(1)` could work just fine to generate a `pi` bond adjacent to the `"OH"`; there are three `beta` hydrogens to its right.

• `(2)` is possible, but the two `beta` hydrogens to the left of the `"OH"` is preferentially chosen since forming `"C"="C"="C"` (by choosing the right-hand `beta` hydrogen) is not as easy as forming conjugated `pi` bonds.

• `(4)` is arguably the easiest, because it has four adjacent `beta` hydrogens, two on each side of the `"OH"`.