How can we rationalize the acidities of the isomeric nitrophenols?

1 Answer
Jan 24, 2018

As a chemist, as a physical scientist, you should interrogate the data....

Explanation:

And so we gots isomeric #C_6H_4(NO_2)(OH)#...

#"p-nitrophenol"# has #pK_a=7.16#

#"m-nitrophenol"# has #pK_a=8.36#

#"o-nitrophenol"# has #pK_a=7.23#

(....and please note, that these data should have been SUPPLIED with the question. You are not expected to remember them....you ARE expected to interpret them.)

And on these data, #"m-nitrophenol"# is a weaker acid than the other isomers. Why? Well, we note that the nitro group contains a quaternized nitrogen with a FORMAL positive charge on the nitrogen centre...#-stackrel(+)N(=O)O^(-)#. Meta-substitution does not support delocalization of the negative charge onto nitrogen, whereas ortho and para substitution of the phenol does...you should try it out and convince yourself of the phenomenon....

Steric effects are unlikely to influence this trend, and these relative acidities likely represent a mesomeric, i.e. an electronic effect.