MO6 Acids

Question 1

Define an acid

Answer 1

• An acid is a PROTON (H+) DONOR.
• When it donates a protonated it forms the conjugate base.
• a highly polarised Z-H bond (Z=polar atom eg. O, N, S)

Question 2

Explain what pKa measures

Answer 2

• PKa measures acid strength which is a combined measure of how easily a proton is lost and how stable the conjugate base is.
  -A higher pKa = smaller Ka = weaker acid, less readily giving up its protons.
• a measure of where the equilibrium lies

Question 3

What does pH indicate?

Answer 3

The concentration of hydrogen ions in solution

Question 4

Why is knowing the percentage ionisation important?

Answer 4

Ionised VS unionised compounds have different physics chemical properties, and there is a range of pH in different body compartments (blood VS stomach) eg.
- to passively diffuse through a membrane=unionised
- protein binding and intermolecular interactions=ionised
- solubility (dissolving and circulation in blood plasma to move around the body)=ionised

Question 5

If you are dealing with acids at pH 7, what happens when the pKa is 5?

Answer 5

%ionisation = 99%, conjugate base is dominant

Question 6

If you’re dealing with acids at pH 7 what happens when the pKa is 7?

Answer 6

%Ionisation= 50%, 1:1 mixture of acid and conjugate base

Question 7

If you’re dealing with acids at pH 7 what happens when the pKa is 8?

Answer 7

%Ionisation is 9.1%, acid is dominant

Question 8

For a carboxylic acid what is the range of pKa values and why are CA acids?

Answer 8

PKa values range from 3-5.
CA’s are acidic because of the polar RZ-H bond (C-O-H) and stabilised conjugate base (through resonance)

Question 9

What does a carbooxylic acid become when a strong base is reacted with it?

Answer 9

A CARBOXYLATE (COO-) which is a salt usually with an inorganic counterion (a cation of Na, K or Mg). The reaction removes a proton form the CA (as it is an acid, acids donate) creating a -vely charged ion that combines with a +vely charged cation.

Question 10

What does a sulfonamide look like

Answer 10

N(H)-S-2(=O)

Question 11

What makes sulfonamides acidic?

Answer 11

• polar N-H bond
  -conjugate base is resonance stabilised

Question 12

What is a bioisostere?

Answer 12

A chemical functional group which can replace another chemical group without significantly affecting the biological activity of a drug.

Question 13

What are some common acidic groups

Answer 13

• CA
  -sulfonamide
  -tetrazole (bioisostere of CA)
  -enols (Thiol, phenol)