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    Hi !

    I don't get how to determine mesomeric effects in a molecule (every time I think I understood, it turns out that I didn't when doing exercises, ... ). Could someone help me, please ?

    Why has the carbonyl group a -M effect ? According to me, it should be +M, since O has 2 lone pairs, and can therefore give electrons.
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    The O isn't on the ring. It is pulling e- density off the C, which in turn pulls density out of the ring.
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    Thank you ! But why has an hydroxyl group the opposite effect (+M) ? Due to its electronegativity, it should to withdraw electrons from the carbon backbone ...
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    (Original post by icey98)
    Thank you ! But why has an hydroxyl group the opposite effect (+M) ? Due to its electronegativity, it should to withdraw electrons from the carbon backbone ...
    The meso effect is due to the movement of electrons (or electron density if you like) onto orbitals on electronegative elements such as oxygen. To show the meso effect it must be possible to draw alternative structures showing the rearranged electrons.

    The hydroxyl group has an oxygen atom directly attached to the ring with three lone pairs. One of these lone pairs can conjugate down into the ring (or rather between the oxygen and the ring) making a double bond and moving electron density into the ring system, which is particularly intense at positions 2, 4 and 6.

    This link will help

    www.ibchem.com/drop/htm/org/meso_effect.htm
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    Or at least it might if it didn't 404 error.
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    (Original post by Pigster)
    Or at least it might if it didn't 404 error.
    Thanks for the heads-up ...

    http://www.ibchem.com/drop/htm/org/meso_effect.htm
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    Thank you for the link, it is very helpful !
 
 
 

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