2, 6 -dimethyl-4-nitrophenol is more acidic than 3, 5-dimethl-4-nitrophenol.because due to the steric effect at methyl group in 3, 5- dimethyl - 4 - nitrophenol - NO2 group come out of plane of benzen ring and due to this -NO2 group nolnges conjugation with benzen ring. It means there is no -R effect of -NO2 there is only -I effect but in 2, 6 - dimethyl-4 - nitrophenol. -NO2 group in conjugation with benzen ring, it means there is -R aswellas -I effect of -NO2 group because at that 2, 6 -dimethyl - 4- nitrophenol is more acidic than 3, 5-dimethyl - 4-nitrophenol.