p-Nitrophenol and 2,6-dimethyl-4-nitrophenol both have a pKa of 7.15 but 3,5-dimethyl-4-nitrophenol has pKa of 8.25. Why 3,5-dimethyl-4-nitrophenol is much less acidic is to be explained. Concept introduction: Resonance delocalization occurs only when the molecule is planar facilitating the overlapping of the orbitals. If the substituent groups present prevent co-planarity of the molecule then resonance is not possible. pKa = -log Ka, where Ka is the dissociation constant of the acid. Thus a stronger acid has a low pKa value. To explain: Why 3,5-dimethyl-4-nitrophenol is much less acidic with pKa of 8.25 when compared to p-nitrophenol and 2,6-dimethyl-4-nitrophenol both of which have a pKa of 7.15.

Question

Part VII. The H-NMR of a compound with molecular formula C5 H 10 O2 is given below. Find the following: (a) The no. of protons corresponding to each signal in the spectra (6) Give the structure of the compound and assign the signals to each proton in the compound. a 70.2 Integration Values C5H10O2 b 47.7 C 46.5 d 69.5 3.6 3.5 3.4 3.3 3.2 3.1 3.0 2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.0 Chemical Shift (ppm) 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8

Answer 1

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Chapter 17.SE, Problem 64AP

Interpretation Introduction

Interpretation:

p-Nitrophenol and 2,6-dimethyl-4-nitrophenol both have a pKa of 7.15 but 3,5-dimethyl-4-nitrophenol has pKa of 8.25. Why 3,5-dimethyl-4-nitrophenol is much less acidic is to be explained.

Concept introduction:

Resonance delocalization occurs only when the molecule is planar facilitating the overlapping of the orbitals. If the substituent groups present prevent co-planarity of the molecule then resonance is not possible.

pKa = -log Ka, where Ka is the dissociation constant of the acid. Thus a stronger acid has a low pKa value.

To explain:

Why 3,5-dimethyl-4-nitrophenol is much less acidic with pKa of 8.25 when compared to p-nitrophenol and 2,6-dimethyl-4-nitrophenol both of which have a pKa of 7.15.

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