Please give the proper reason with details in this question?

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Please give the proper reason with details in this question?

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Answer 1 · 2018-01-11T23:31:49

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

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

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

$(1)$ $(1)$ could work just fine to generate a $π$ $pi$ bond adjacent to the $OH$ $"OH"$ ; there are three $β$ $beta$ hydrogens to its right.
$(2)$ $(2)$ is possible, but the two $β$ $beta$ hydrogens to the left of the $OH$ $"OH"$ is preferentially chosen since forming $C = C = C$ $"C"="C"="C"$ (by choosing the right-hand $β$ $beta$ hydrogen) is not as easy as forming conjugated $π$ $pi$ bonds.
$(4)$ $(4)$ is arguably the easiest, because it has four adjacent $β$ $beta$ hydrogens, two on each side of the $OH$ $"OH"$ .

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