Q17-64E - Organic Chemistry (Mcmurry)

Question

p-Nitrophenol and 2,6-dimethyl-4-nitrophenol both have pKa 5 7.15, but 3,5-dimethyl-4-nitrophenol has pKa 5 8.25. Why is 3,5-dimethyl-4- nitrophenol so much less acidic?

Step-by-Step Solution

Answer

In order for this delocalization to be possible the nitro group must be coplanar with the aromatic ring so that the pi orbitals are aligned. This alignment is still possible with the 2,6-dimethyl-4-nitrophenol but with 3,5-dimethyl-4-nitrophenol there is a problem.

However, the methyl groups of 3,5-dimethyl-4-nitrophenol interfere with the nitro group by adopting a coplanar configuration due to steric clashing. Thus, the nitro group cannot easily stabilize the resulting phenolate. So, 3,5-dimethyl-4- nitrophenol is so much less acidic.

Step1:Understand the increase or decrease of acidy

In order to understand the increase or decrease of acidy, an analysis of the resulting conjugate base must be made. p-Nitrophenol has an enhanced acidity over phenol (pKa = 10) due to the delocalization of the negative charge to the nitro group as seen n the following resonance structures.

Step2: Why is 3,5-dimethyl-4- nitrophenol so much less acidic?

In order for this delocalization to be possible, the nitro group must be coplanar with the aromatic ring so that the pi orbitals are aligned. This alignment is still possible with the 2,6-dimethyl-4-nitrophenol but with 3,5-dimethyl-4-nitrophenol there is a problem.

However, the methyl groups of 3,5-dimethyl-4-nitrophenol interfere with the nitro group by adopting a coplanar configuration due to steric clashing. Thus, the nitro group cannot easily stabilize the resulting phenolate. So, 3,5-dimethyl-4- nitrophenol is so much less acidic.