most acidic proton

dentalwhiz

Full Member
Jun 15, 2017
90
16
  1. Pre-Dental
    In a given molecule, a ketone is given on one end, an aldehyde on the other. The answer states that the H on the methyl carbon on the ketone is more acidic than the H on the aldehyde. Doesn't proton acidity increase as the 's' character of the bond increases? This leads me to believe that the H on the aldehyde is more acidic- am I missing something here? I know that resonance plays a role in proton acidity, but don't they both have equal roles in this scenario? Any clarification would be great
     

    themuffinman11

    Full Member
    2+ Year Member
    Feb 14, 2017
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    130
      When looking for the most acidic proton, always think how stable the conjugate base would be (so the molecule with the proton removed). Carbons that are Alpha (next to a carbonyl group) will have acidic protons because of resonance with the carbonyl. This is why the H on the methyl carbon on the ketone is more acidic. Removing the H from the aldehyde carbon would be very bad as a negative sp2 carbon is very unstable. Let me know if you need any clarification! :)
       

      dentalwhiz

      Full Member
      Jun 15, 2017
      90
      16
      1. Pre-Dental
        I just did another problem where you have to decide which proton would be most acidic. An aldehyde H is compared to a H on a methyl carbon (CH3-CH-C=OH) adjacent to it. It states that the aldehyde H would be more acidic. I'm so confused... I thought I had a solid grasp on these types of questions but I guess not... Doesn't the logic you just stated not apply in this situation?
         

        themuffinman11

        Full Member
        2+ Year Member
        Feb 14, 2017
        167
        130
          I just did another problem where you have to decide which proton would be most acidic. An aldehyde H is compared to a H on a methyl carbon (CH3-CH-C=OH) adjacent to it. It states that the aldehyde H would be more acidic. I'm so confused... I thought I had a solid grasp on these types of questions but I guess not... Doesn't the logic you just stated not apply in this situation?

          Can you draw the structures out in both questions (or take a picture of each question) so I can just double check it
           
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