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I am dealing with some basic reactions in organic chemistry. One of these is the acid reflux of alcohols. Something I came across was hydride shifts. I know how and why they occur, but I don't understand why they are called 1,2 hydride or methyl shifts. What do these numbers indicate? I initially thought it would be the position of the carbon atoms involved in the shift, in the principal chain, but that was probably wrong because a shift occurring at the 3rd carbon atom was also called a 1,2 shift. Help?

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    $\begingroup$ 1,2 means adjacent carbon atoms (not necessarily locant numbers on the principal chain) $\endgroup$ – Aniruddha Deb Nov 4 '20 at 11:26
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I believe that in the examples where a 1,2 hydride shift was mentioned, the hydrogen atom would have 'shifted' between two adjacent carbon atoms. I think the 1,2 refers to the fact that the carbon atoms involved in the shift are adjacent.

A 1,3 shift would mean a the hydrogen atom shifts to the second carbon from the original carbon atom it was bonded to, if you get what I mean.

The same applies to methyl shifts.

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