reactions at the benzylic positions...?

Question

when warmed in dilute sulfuric acid, 1-phenyl-1,2-propanediol undergoes dehydration and rearrangement to give 2-phenylpropanal.
why do we get at the end the 2-phenylpropanal and not the 1-phenyl-1-propanone?

explanation please,

thanx

There is no mistake SOME BODY,
The question says:" 1-phenyl-1-propanone"....In addition, this is not my Homework, and you are not supposed to answer either...

Answer 1

You must have made a mistake in copying the question when looking for someone to do your homework for you, because there is no such thing as 1-phenyl-propanone.

However, assuming the most likely mistake you made, the reason is because the intermediate is a carbocation on the 2-position of the propane part. You get there by protonating the OH in the 2-position and then just making water as a leaving group. Then the phenyl group will literally just jump to it (if you're drawing a mechanism start the arrow at the bond between the propane and the phenyl, and draw it to the carbocation, that is the middle position in the propane).

If you can't figure the rest out from there you'll probably fail the test even if we do your homework for you.

Answer 2

the hydroxy group on the second position will most likel be protonated because the carbocation intermediate that will be formed is more stable.
the other product will not be formed because there is a C-H bond on the #1 C which cannot be broken to give rise to a ketone. "somebody" was partly right. 1-phenyl-1-propanone should be named ethylphenylketone instead...