Turn on thread page Beta
    • Thread Starter
    Offline

    1
    ReputationRep:
    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?
    • Community Assistant
    • Study Helper
    Offline

    17
    ReputationRep:
    Community Assistant
    Study Helper
    (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:
    Offline

    0
    ReputationRep:
    (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?
    • Thread Starter
    Offline

    1
    ReputationRep:
    (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
 
 
 

University open days

  1. University of Bradford
    University-wide Postgraduate
    Wed, 25 Jul '18
  2. University of Buckingham
    Psychology Taster Tutorial Undergraduate
    Wed, 25 Jul '18
  3. Bournemouth University
    Clearing Campus Visit Undergraduate
    Wed, 1 Aug '18
Poll
How are you feeling in the run-up to Results Day 2018?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.