Jun 15, 2017
90
16
Status
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

2+ Year Member
Feb 13, 2017
166
118
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! :)
 
OP
D
Jun 15, 2017
90
16
Status
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

2+ Year Member
Feb 13, 2017
166
118
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