Please give the proper reason with details in this question?

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1 Answer
Jan 11, 2018

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.


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".