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    Been doing some mechanisms on forming enols etc form aldehydes and ketones - but have become stuck.

    Why does the electrophile (OH-) take a proton from the alpha carbon as opposed to attacking the carbonyl carbon?

    Thanks?
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    (Original post by tgr141291)
    Been doing some mechanisms on forming enols etc form aldehydes and ketones - but have become stuck.

    Why does the electrophile (OH-) take a proton from the alpha carbon as opposed to attacking the carbonyl carbon?

    Thanks?
    The alpha carbon is more acidic. This can be shown by looking at the potential structures that can be drawn for the conjugate base.


    CH3--CH=O <==> CH3--C-=O

    CH3--CH=O <==> -CH2--CH=O

    the positive charge residing on the alpha carbon (second conjugate base) can be stabilised by the pi electrons from the carbonyl group.

    -CH2--CH=O <--> CH2=CH-O-

    This extra resonance stabilisation makes the second equilibrium more to the RHS than the first. Hence the alpha hydrogen is more easily removed by base.



    Here's a diagram to make things clearer:
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    (Original post by tgr141291)
    Been doing some mechanisms on forming enols etc form aldehydes and ketones - but have become stuck.

    Why does the electrophile (OH-) take a proton from the alpha carbon as opposed to attacking the carbonyl carbon?

    Thanks?
    It can but its reversible, and the product would just collapse to form the carbonyl again since the c=o bond is more favourable

    Is this what u mean (ie why doesn't oh- add to the carbonyl)? Or do u mean why doesn't it deprotonate at the alpha carbon?
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    (Original post by Bloodbath)
    It can but its reversible, and the product would just collapse to form the carbonyl again since the c=o bond is more favourable

    Is this what u mean (ie why doesn't oh- add to the carbonyl)? Or do u mean why doesn't it deprotonate at the alpha carbon?
    Yeah I meant why doesnt the OH- add to the carbonyl.

    Thanks charco and bloodbath
 
 
 
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