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Why new double bond or triple bond creates to the center always? When pentan-2-ol reacts with $\ce{Al2O3}$ ∆ the double bond creates to the 2nd carbon. Can't it create on the 1st carbon?

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2 Answers 2

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It can and will form a double bond to the 1st carbon, however it will much more likely form one to the second carbon.

You will get a mixture of both 1-pentene and both cis- and trans-2-pentene. The major component will be the trans-2-pentene because that is the most stable compound. Remember, more substituents on a double bond mean the bond is more stabilized for a couple reasons. Steric interferences mean that trans- is more stable as well.

Since the product is more stable, the transition state to form that product is more stable and therefore easiest to form, generally.

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You can eliminate pentan-2-ol to both pent-1-ene and pent-2-ene (and to both stereoisomers of the latter).

To a certain degree, the reagents you use determine which product you get. If you turn the alcohol into a leaving group and then add a strong, sterically demanding base, the pent-1-ene content will increase. If you add a small, mild base, you will observe more pent-2-ene. Careful tuning of reaction conditions often allows for these differences in reactivity.

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