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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: scienceguy on July 24, 2014, 12:43:18 AM

Title: Protonation of alcohol or carbonyl group
Post by: scienceguy on July 24, 2014, 12:43:18 AM

What is more easily protonated? The oxygen in an alcohol group or the oxygen in a carbonyl group? I tried looking this up in March's Advanced Organic Chemistry but couldn't find it.

Title: Re: Protonation of alcohol or carbonyl group
Post by: salteen on July 24, 2014, 02:58:11 AM

The Evans and Bordwell pKa tables are excellent references which should answer your question definitively. But you can also predict the answer from first principles. How do the electronic properties of a carbonyl oxygen and an alcohol oxygen differ, and what effect would this have on their stabilities when protonated?

Title: Re: Protonation of alcohol or carbonyl group
Post by: mjc123 on July 24, 2014, 05:22:12 AM

I expect March, like most textbooks, has been through a few changes since I was a student, but my second edition has a whole chapter on acids and bases, with a big table of pKas. Of course in this case you have to look up the pKa of the conjugate acid. It gives R₂C=OH⁺ as having a typical pKa of -7, while RCH₂OH₂⁺ is -2. What does that tell you?

Title: Re: Protonation of alcohol or carbonyl group
Post by: scienceguy on July 24, 2014, 12:55:55 PM

Of course! pKa! Thx!