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Medicinal Chemistry > Acid Base Theory > Flashcards

Acid Base Theory Flashcards

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1
Q

Why is the ionisation state important?

A

It is important for the absorption of drugs.

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2
Q

How does the pH in different body fluids influence the ionisation state of drugs?

A
  • Drugs with basic side chains may be protonated and become positively charged
  • Drugs with acidic side chains may be deprotonated and become negatively charged.
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3
Q

Based on the Arrhenius theory define an acid.

A

Any substance which produces a hydrogen ion in aqueous solution i.e. a proton donor.

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4
Q

Based on the Arrhenius theory define base.

A

Any substance which delivers a hydroxide ion in an aqueous solution i.e. a hydroxyl donor.

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5
Q

What are the limitations of the Arrhenius theory?

A
  • No consideration of solvent effects
  • Ammonia is a base with no hydroxyl group
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6
Q

Based on the bronsted-lowry theory define an acid.

A

Any substance that donates a H+

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7
Q

Based on the bronsted-lowry theory define base.

A

Any substance that accepts a H+

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8
Q

Why is water described as amphoteric?

A

It can act as both acid and base.

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9
Q

Define acidity

A

A measure of the tendency of a compound to give up H+

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10
Q

Define basicity

A

A measure of a compound’s affinity for a H+

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11
Q

What does bronsted-lowry theory fail to explain?

A
  • Acid-base behaviour in other solvents
  • Proton-less compounds like AlCl3
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12
Q

Based on the Lewis theory define acid.

A

Electron pair acceptor

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13
Q

Based on the Lewis theory define base.

A

Electron pair donor

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14
Q

Give the equation to calculate pH.

A

pH = -log[H3O+]

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15
Q

Give the equation to calculate pOH.

A

pOH = -log[OH-]

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16
Q

Describe aqueous solutions of weak acids.

A
  • Weak acids only partially ionise in aqueous solutions
  • Equilibrium is setup favouring reactants
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17
Q

Give the equation for pKa

A

pKa = -logKa

18
Q

What is meant by the common ion effect?

A

The shift in equilibrium caused by the addition of a compound having an ion in common with the dissolved substance.

19
Q

Describe the presence of a common ion.

A

Suppresses the ionisation of a weak acid or a weak base.

20
Q

What is the effect of having a weak acid HA and its salt in solution?

A
  • Ionisation of HA is suppressed by the presence of A-
  • Hydrolysis of A- is suppressed by the presence of HA
  • The acid and conjugate base may react with one another
21
Q

What happens when a strong base is added to a buffer?

A
  • The weak acid HA will give up its H+ to transform the base into water and the conjugate base
  • OH- is being consumed so pH only changes slightly.
22
Q

What happens if a strong acid is added to a buffer?

A
  • The weak base will react with the H+ from the strong acid to form the weak acid.
  • The H+ gets absorbed by the A- instead instead of reacting with water to form H3O+ so the pH changes only slightly
23
Q

What is a buffer solution?

A

A solution which has the ability to resist changes inn pH upon the addition of small amounts of either acid or base.

24
Q

What do buffers contain?

A

Either a weak acid and its salt or a weak base and its salt.

25
Q

How does a buffer work?

A
  • At the pKa HA = A- so the system is able to absorb the addition of OH- or H+
  • Adding OH- near the pH where HA = A- then HA releases H+ to offset the OH- added so ratio of HA to A- does not change much
  • Adding H+ the A- can absorb H+ to form HA
26
Q

State the Henderson-Hasselbalch equation.

A

pH = pKa + log([base]/[acid])

27
Q

What is the goal of a buffer?

A
  • To keep the pH of a solution within a narrow range.
28
Q

What is the importance of the ratio of [A-]/[HA]?

A

It influences the pH of a solution.

29
Q

What is the importance of the concentrations of [A-] [HA]?

A

Controls the effectiveness of a buffer.

30
Q

Describe the effect of more A- and HA molecules.

A
  • The less of an effect addition of a strong acid or base will have on the pH of a system
31
Q

The amount of strong base required to change the pH by one unit depends on what?

A
  • The concentration of the components.
  • Higher concentration gives a higher beta.
32
Q

What happens to blood pH when you hyperventilate?

A

If blood pH drops due to metabolic production of H+ then [H2CO3] increases by protonation of HCO3-, H2CO3 rapidly loses water to form CO2(aq) which is expelled as CO2(g).

33
Q

What happens to blood pH when you hypoventilate?

A

If the blood pH rises [HCO3-] increases by deprotonation of H2CO3 then breathing changes and CO2(g) is converted to CO2(aq) and then to H2CO3 in the capillaries in the lungs.

34
Q

What is meant by the ionisation of drugs?

A

Protonation or deprotonation resulting in charged molecules.

35
Q

The acidity or basicity of a compound plays a major role in controlling what?

A
  • Absorption and transport to site of action
  • Solubility, bioavailability, absorption and cell penetration, plasma binding, volume of distribution
  • Binding of a compound at its site of action
  • Un-ionised form involved in hydrogen bonding
  • Ionised form influences strength of salt bridges or H bonds
  • Elimination of compound
  • Biliary and renal excretion
  • CYP P450 metabolism
36
Q

Which form of the drug diffuses into the bloodstream?

A

The unionised form

37
Q

What is the strongest acid you can have in an aqueous solution?

A

H3O+

38
Q

What is the strongest base you can have in aqueous solution?

A

OH-

39
Q

What is meant by levelling effect?

A
  • The effect of solvent on the property of acids and bases
  • Strength of strong acid is levelled by the basicity of the solvent
  • Strength of a strong base is levelled by the acidity of the solvent
40
Q

Give the equation to find pKa.

A

pKa = pH - log[A-]/[HA]
therefore, [base]/[acid] = 10^(pH-pKa)

Medicinal Chemistry (21 decks)

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