Anion Binding

Question 1

Why bind anions?

Answer 1

Chemical - act as catalysts and bases
Environmental - some pose pollution problems
Biological
Medical

Question 2

What are the challenges with binding anions?

Answer 2

Large - require receptors with large enough cavities
Many different geometries, so complementary binding site required
High free energy of solvation, so anion hosts must compete more effectively with surrounding medium
Many anions exist in narrow pH window
Negatively charged - receptors can deploy positive charges to strengthen anion coordination

Question 3

What are the classes of anion receptor?

Answer 3

Electrostatic
Hydrogen bonding
Lewis acid

Question 4

What is an example of combining non-covalent interactions?

Answer 4

Electrostatically assisted hydrogen bonds

Question 5

What is the problem with polyaza? What can be used instead?

Answer 5

Operate in water but only at low pH - may protonate anion

Guanidinium - pKa 3 units higher so may work at higher pH

Question 6

What factors should be considered when looking at anion binding data?

Answer 6

1) Geometric complementarity of host and guest

2) Basicity of anion

Question 7

What is the trend of pKa of the halides?

Answer 7

Decreases down the group. Fluorine is very electronegative so forms strong hydrogen bonds between HF - this is a weak acid

Question 8

What is Paulings Rule?

Answer 8

pKa = 8 - 5p

p is the number of oxo groups

Question 9

What is stronger - halogen or hydrogen bonding?

Answer 9

Halogen