CHEM 331
Problem Set #3: Substitutent Effects and LFERs
1. a) Interpret the following σ values in terms of the electronic character of each group. Discuss the inductive
and resonance effects of each (σI and σR) using benzoic acid derivatives as examples, showing resonance
where appropriate. Substituent σmeta σ
p
ara
-OH 0.13 -0.38
-COCH3 0.38 0.50
-S(CH3)2
+ 1.0 0.90
b) The picryl (2,4,6-trinitrophenyl) substituent, C6H2(NO2)3 has values of σmeta 0.43, σpara 0.41. What
conclusion may be drawn regarding the resonance effect of this substituent and the configuration of the two
aromatic rings in 4-(2,4,6-trinitrophenyl)benzoic acid?
2. The pKa’s and reaction constants (susceptibility factors) ρ for the acid dissociation of substituted 1-
naphthylamines and quinolines are given below.
a) Explain why the quinoline is much more susceptible to electronic substituent effects than 1-
naphthylamine.
b) Predict the fraction of each compound present in the protonated form at pH = 6.00
NH2
X
N
X
1-naphthylamine quinoline
pKa = 3.85 pKa = 4.88
ρ = 2.81 ρ = 5.90
3. a) Estimate the pKa values of 4-methyl-2,5-dinitrophenol and 3,4,5-trimethylaniline using the data given in
Tables 1 and 2 (attached) at 25oC.
b) The structure of the herbicide sulcotrion is shown below and is found to have a pKa of 3.13. Identify the
acidic proton and explain the unusual acidity. Sketch a labeled plot of fractional abundance of the acid and
basic forms over a pH range of 2 – 12.
SO
O
ClOO
O
sulcotrion
4. The pKa values of m- and p-monosubstituted benzoic acids in 50% aqueous ethanol correlate with σ, and
have a ρ value of 1.60. The pKa of benzoic acid in this system is 5.71.The pKa values of some 4-X-3,5-
dimethylbenzoic acids in this medium are shown below. Use these results to examine and comment on the
applicability of additivity of σ values.
X N(CH3)2 NH2 Cl Br CN COOCH3NO2
pKa 6.23 6.88 5.59 5.55 4.90 5.44 4.91
PS 3 2010
