Question

Amines are basic in nature. The pK_b value is a measure of the basic strength of an amine. Lower the value of pK_b, more basic is the amine. The effect of substituent on the basic strength of amines in aqueous solution was determined using titrations. The substituent “X” replaced \[\ce{^”-CH2^“}\] group in piperidine (compound 1) and propylamine \[\ce{CH3CH2CH2NH2}\], (compound 2).

Compound 1: 

Compound 2: \[\ce{HXCH2CH2NH2}\]

The experimental data is tabulated below:

Substituent “X”	Electro-negativity of X	Substituted piperidine compound	pKa	Substituted propylamine compound	pKa
\[\ce{CH2}\]	2.55		11.13	\[\ce{CH3CH2CH2NH2}\]	10.67
\[\ce{NH}\]	3.12		9.81	\[\ce{NH2CH2CH2NH2}\]	10.08
\[\ce{O}\]	3.44		8.36	\[\ce{HOCH2CH2NH2}\]	9.45
\[\ce{CH3CON}\]	3.6		7.94	\[\ce{CH3CONHCH2CH2NH2}\]	9.28
\[\ce{C6H5CON}\]	3.7		7.78	\[\ce{C6H5CONHCH2CH2NH2}\]	______

Study the above data and answer the following questions:

a. Plot a graph between the electro-negativity of the substituent vs. the pK_b value of the corresponding substituted propyl amine (given that pK_a + pK_b = 14). Is there any relation between the electro-negativity of the substituent and its basic strength?  

b. The electronegativity of the substituent \[\ce{^“C6H5CON^“}\] is 3.7; what is the expected pK_a value of compound \[\ce{C6H5CONHCH2CH2NH2}\]?

1. 9.9
2. 9.5
3. 9.3
4. 9.1

c. The pK_a value of the substituted piperidine formed with the substituent “X” is found to be 8.28. What is the expected electronegativity of “X”?

1. 3.5
2. 3.4
3. 3.8
4. 3.1

OR

What is the most suitable pK_a value of the substituted propylamine formed with substituent “X” with electronegativity 3.0?

1. 10.67
2. 10.08
3. 10.15
4. 11.10

Answer 1

1. 
   
   The pK_b increases with an increase in the electronegativity of the substituent; therefore, the basic strength decreases with an increase in the electronegativity of the substituent
2. 9.1
3. 3.5
   OR
   10.15