CHEM 331
1.
The values of σ meta findings.
for –OH and –OCH
3
are positive, whereas the values for σ para
are negative. Explain these
2.
The pK a
’s and reaction constants (susceptibility factors) ρ for the acid dissociation of substituted 1naphthylamines and quinolines are given below. a) Explain why the quinoline is much more susceptible to electronic substituent effects than 1naphthylamine. b) Predict the fraction of each compound present in the protonated form at pH = 6.00
NH
2
N
X X
1-naphthylamine pK a
= 3.85
ρ = 2.81 quinoline pK a
= 4.88
ρ = 5.90
3.
Estimate pK a
values of 4-methyl-2,5-dinitrophenol and 3,4,5-trimethylaniline and use Excel to present the fractional abundance of each acid and conjugate base over a pH range of 2 – 12 sing the data given in Tables
1 and 2 (attached) at 25 o C.
4. The structure of the herbicide sulcotrion has is hown below and is found to have a pK a
of 3.13. Draw the structure of the conjugate base and explain the unusual acidity.
O O Cl
O
O S sulcotrion
O
5.
The relative rates of alkaline hydrolysis of substituted benzamides in water at 100 ° C are as follows below.
Demonstrate the applicability of the Hammett equation to this reaction, calculate the ρ value, and comment on any deviations from the correlation.
Substituent Relative
Rate
Substituent Relative
Rate
Substituent Relative
Rate m -I 2.60 -NO
2
5.60 p -OCH
3
0.49 p -I 1.69 H 1.00 -NH m -Br 2.97 m -CH
3
0.83 p -NH
2
2
0.93
0.20 p -Br 1.91 p -CH
3
0.65 m -OH 0.19
PS 3 2008

6.
For the following mechanisms: a) Explain the value of the reaction (susceptibility) constant in terms of the Hammond postulate. b) Specify which σ values are used ( σ , σ + , σ −
, or σ ortho phenols ) in obtaining the susceptibility constant and indicate if the reaction centre is in direct resonance with the substituents. Illustrate with an example. i) hydrolysis of substituted benzyl chlorides; ρ = − 4.45
Cl OH
+
H
2
O slow
X X ii) addition of cyanide ion to substituted benzaldehydes; ρ = 2.55
O H
-
O H
CN
H O
CN -
X
H
CN slow
X X X
7. a) The hydrolysis of a series of ethyl benzoates by hydroxide ion in 85% aqueous ethanol has been investigated. A Hammett plot of the second order rate constants ( k
B
) gave a reaction constant ρ = 2.56.
Calculate how much faster ethyl 4-nitrobenzoate will undergo base catalyzed hydrolysis compared to ethyl benzoate under similar conditions.
O O
O O
O
2
N
X b) The base catalyzed hydrolysis of phenyl N-phenyl carbamates occurs by the elimination of PhO group as the rate determining step. Estimate the second order rate constant, k
B carbamate at 25 o C using the k
B carbamates and the Hammett relation.
for 3,4,5-trichlorophenyl N-phenyl
values given in the Table below for other substituted phenyl N-phenyl
H
N O k
B
NH
2
-O
O
+
OH-
+ CO
2
+
X
X log k
B
(
X k
B
(
H
)
)
=
Σ
i p p p
X
-CH
-OCH
-Cl
3
3 k
B
(M -1 s -1 ) X k
3.0 x 10
2.5 x 10
4.2 x 10
1
1
2 m -Cl m -NO p
-NO
2
2
B
(M -1 s -1 )
1.8 x 10 3
1.3 x 10
2.7 x 10
4
5
8.
For the following Hammett plot and reaction below, two reaction (susceptibility) constants ( ρ ) are obtained, one for EDG and another for EWGs. Explain the magnitude and sign of each of the two reaction constants and provide mechanism/s consistent with these observations.
X
CH
2
OTs
CH
3
C O
O
TsO
X
CH
2
O C
O
CH
3
PS 3 2008

Table 1: Hammett constants for some common substituents
Substituent
σ meta
σ para
σ
-
σ +
CH
3
Ph (C
6
H
5
σ o phenols
0.23 0.68
Br 0.39 0.26
I 0.35
OH
OCH
3
NO
2
CO
2
CH
3
OCOCH
3
NH
2
N(CH
3
)
2
Note: σ
- and σ + apply to para substituted groups only
-1.3
Table 2: Reaction and acidity constants for aromatic acids in water at 25 o
C
Acid
Benzoic acid
ρ
1.00 pK aH
4.19
Phenoxy acetic acid
2-chlorophenoxy acetic acid
Conjugate acid of aniline
0.30
0.30
2.89
3.17
3.05
4.63
PS 3 2008