PKa of 2-nitrophenol vs 3-nitrophenol

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SUMMARY

2-nitrophenol has a pKa of 7.17, while 3-nitrophenol has a higher pKa of 8.28. The difference in acidity is attributed to the position of the nitro group (-NO2) relative to the hydroxyl group (-OH). In 3-nitrophenol, the nitro group is further from the hydroxyl group, allowing for better delocalization of the negative charge in the conjugate base, thus stabilizing it more effectively than in 2-nitrophenol. This stabilization results in 3-nitrophenol being less acidic than 2-nitrophenol.

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Homework Statement


I am given the structures for 2-nitrophenol and 3-nitrophenol, and asked which one has the higher pKa and why.

Homework Equations


pKa=-log(Ka)

The Attempt at a Solution


I looked up the pKas in a reference and found the pKa of 2-nitrophenol to be 7.17 while the pKa of 3-nitrophenol to be 8.28. My question is why, though. The structures are very similar. I was wondering if the electronegative -NO2 (being father away from the O- than in 2-nitrophenol) would stabilize the conjugate base of 3-nitrophenol, by delocalizing the negative charge, making it more willing to give up the proton. However, the opposite seems to be the case.

Any hints?
Peter
 
Physics news on Phys.org
Thanks Borek. I didn't see that one.
 

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