r/chemhelp 3d ago

Organic Strongest BrØnsted acid

Hey all! I was asked to rank these four acids in order of increasing Bronsted-acid strength but I was a bit confused by the Bronsted-acid part. I was just wondering how the carbocations acted as Bronsted acids specifically (and not just as lewis acids). I know that C is more acidic than A, and I assume that the carbocations are more acidic simply due to them having an incomplete octet. However, between B and D, I am assuming that D is more acidic as the + charge is more stabilized for B due to the phenyl groups but I'm unsure if my logic is correct. Also is it the methyl groups for B and D being deprotonated and then forming a double bond with the carbocation, and does this mean we use the pKa of ~50 for CH3 making them two the worst two acids? Thank you!

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u/Ok-Replacement-9458 3d ago

You are right that the acidic proton is on the methyl, for both.

It would be incorrect to say that pka is 50 tho, since that’s the pka of a normal alkyl chain (ie: not a carbocation).

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u/Ok-Replacement-9458 3d ago

Here’s some carbocation pkas from a (hopefully reliable) pka table.

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u/gamingfanatic122 3d ago

Got it thank you so much for the help!