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Medicinal Chemistry > Acid & Bases > Flashcards

Acid & Bases Flashcards

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1
Q

What is the common ion effect?

A

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

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2
Q

What does the presence of the CIE have?

A

Suppresses ionisation of weak acid + base

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3
Q

What is the common ion in these equations?
CH3CONa (s) —> Na+ + CH3CO2-
CH3CO2H ⇌ H+ + CH3CO2-

A

CH3CO2-

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4
Q

What is the effect of having a weak acid + salt in a solution?

A

Ionisation of HA is suppressed by presence of A-
Hydrolysis of A- suppressed by presence of HA
Acid + conjugate base may react with one another

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5
Q

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

A

Weak acid will give up H+
To transform base (OH-) into H2O + conjugate base
= added OH- consumed by reaction = pH changes only slightly

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6
Q

What happens when strong acid added to a buffer?

A

WB react with H+ from SA to form WA

H+ absorbed by A- = pH changes only slightly

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7
Q

What is a buffer?

A

Solution has ability to resist changes in pH upon addition of small amount of acid or base

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8
Q

How do you form a buffer?

A

WB + salt

WA + salt

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9
Q

What are some examples of buffers?

A

CH3COOH/CH3COO-Na+
NaH2PO4/NaHPO4
NH3/NH4Cl

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10
Q

Explain the titration CH3COOH with NaOH

A

Start at low pH + add OH-
Product of [H+][OH-] must main constant
= add OH- = [H+] conc decrease = pH increases
At pKa, [Ac-] + [HAc] are equal
So adding more OH- doesn’t change ratio very much
pH doesn’t change

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11
Q

What is the Henderson-Hasselbach equation?

A

pH = pKa + log conjugate base
———————–
acid

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12
Q

Solution containing 0.1M CH3COOH with 0.15M CH3COONa (pKa 4.76)

A

pH = 4.76 + log(0.15/0.1) = 4.94

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13
Q

Need a buffer with pH of 4.5

A

4.5 = 4.76 + log(base/acid)
log(base/acid) = 4.5-4.76= -0.26
(base/acid) = 10* -0.26 = 0.55
Use 1M CH3COOH with 0.55M CH3COONa

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14
Q

What is the equation used to predict the ability of a buffer to resist change in pH?

A

Change in H+

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15
Q

What does a higher conc mean?

A

Higher buffer capacity

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16
Q

What is involved in buffer capacity?

A

Ratio of [A-]/[HA] - influences pH of solution

Conc of [A-][HA] - control effectiveness of buffer

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17
Q

What does more A- + HA available molecules mean?

A

Less of an effect addition has on pH of system

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18
Q

Why must living systems be buffered?

A

Resist large variations in pH

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19
Q

What are the important buffers?

A

Bicarbonate

Phosphate

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20
Q

What happens if blood pH drops?

A

[H2CO3] increases
By protonation of H2CO3
H2CO3 rapidly loses H2O to form CO2 (aq)
Expelled as CO2 (g)

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21
Q

What happens if blood pH rises?

A

[H2CO3] increases
By deprotonation of H2CO3
Breathing rate = CO2 (g) converted to CO2 (aq)
Then converted to H2CO3 in capillaries of the lungs

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22
Q

How do you select buffer components?

A

Choose acid with pKa close to desired pH
Calculate ratio of base to acid using equ
Use at least 0.1M of lower component
Target = both to be 0.1-1.0M

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23
Q

What is ionisation?

A

Protonation or deprotonation = charged molecules

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24
Q

What are the 3 acidic species if HA dissolved in H2O?

A

HA - may be strong, weak or very weak
H2O - pKa = 15.7
H3O+ - pKa = -1.7

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25
Q

What happens if the acid has pKa value >15.7?

A

H2O stronger acid than HA = NO ionisation

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26
Q

What happens if acid has pKa value 1.7 - 15.7?

A

HA stronger acid than H2O BUT weaker than H3O+ = partial ionisation

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27
Q

What happens if acid has pKa <1.7?

A

HA stronger acid than H3O+ = complete ionisation

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28
Q

What is the strongest acid in aq solution?

A

H3O+

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29
Q

What is the strongest base?

A

OH-

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30
Q

What is the levelling effect?

A

Effect of solvent on property of acids + bases

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31
Q

What is the strength of SA levelled by?

A

Basicity of solvent

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32
Q

What is the strength of SB levelled by?

A

Acidity of solvent

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33
Q

What does the fraction in the acid form depend on?

A

Whether the functional group is acidic or basic

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34
Q

What happens if it was acidic?

A

Formula gives fraction unionised

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35
Q

What happens if it was basic?

A

Formula gives fraction ionised

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36
Q

What is the eqm between unionised + ionised?

A

Acidity constant Ka

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37
Q

What is % ionised equ for acid?

A

1 + 10* (pKa-pH)

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38
Q

What is % ionised equ for base?

A

1 + 10* (pH-pKa)

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39
Q

What is effect of ionisation on antibacterial potency of sulphonamides?
pH 11-7

A

Potency increases

= active species = anion

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40
Q

What is effect of ionisation on antibacterial potency of sulphonamides?
pH 7-3

A

Potency decreases

= only neutral form of compound can transport into cell

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41
Q

What is the pH of stomach?

A

1.5

42
Q

What is aspirin’s pKa?

A

3.5

43
Q

How much is aspirin ionised in the stomach?

A

99%

44
Q

What is the pH of the blood?

A

7.4

45
Q

How much is aspirin ionised in the blood?

A

99%

46
Q

What is the Arrhenius definition of an acid?

A

Any substance which produces H+ in aq solution

47
Q

What is the Arrhenius definition of a base?

A

Any substance which delivers OH- in an aq solution

48
Q

What are cons of Arrhenius Theory?

A

No consideration of solvent effects

NH3 base with no OH?

49
Q

What is the Bronsted-Lowry definition of an acid?

A

Any substance that donates H+

50
Q

What is the Bronsted-Lowry definition of a base?

A

Any substance that accepts H+

51
Q

What does it mean H2O can act as base + acid?

A

Amphoteric

52
Q

What is acidity?

A

Measure of tendency to give up H+

53
Q

What s basicity?

A

Measure of affinity for H+

54
Q

What does a SA have?

A

Strong tendency to give up H+

55
Q

What are cons of Bronsted-Lowry Theory?

A

Works nicely for H2O BUT fails to explain acid-base behaviour in other solvents

56
Q

What is Lewis’ definition of an acid?

A

e- pair acceptor

57
Q

What is Lewis’ definition of a base?

A

e- pair donor

58
Q

What does a Lewis acid + base combine to form?

A

Adduct

59
Q

What are cons of Lewis’ Theory?

A

Lowry acids are Lewis acids, BUT not all Lewis acids comply with Bronsted-Lowry definition?

60
Q

What does Sorenson’s scale measure?

A

[H3O+]

61
Q

How do you calculate pH?

A

-log[H3O+]

62
Q

How do you calculate [H3O+]?

A

10*-pH

63
Q

How do you calculate pOH?

A

-log[OH-]

64
Q

How do you calculate [OH-]?

A

10*-pOH

65
Q

What is Kw equation?

A

Kw = [H3O+][OH-]

66
Q

What does Kw = at RTP?

A

1X10-14

67
Q

What are examples of SA?

A

HCl = hydrogen halides
HClO3, HClO4 = oxyacids of halides
H2SO4, HNO3, H3PO4 = other oxyacids

68
Q

What are examples of SB?

A

NaOH, KOH, CsOH = alkali metal hydroxides

Ca(OH)2, Sr(OH)2 = alkali earth metal hydroxides

69
Q

Do SA + SB completely ionise in aq solutions?

A

YES

70
Q

What are examples of WA?

A

HCOOH
CH3COOH
HF
HCN

71
Q

What are examples of WB?

A

NH3
H2O
N(CH3)3
NH4OH

72
Q

Do WA partially ionise in aq solutions?

A

YES

73
Q

How do you calculate pKa?

A

-log(Ka)

74
Q

How do you calculate Ka?

A

10*-pKa

75
Q

What pKa value will a SA have?

A

Lower pKa

76
Q

What pKa value will a WA have?

A

Higher pKa

77
Q

Do WB partially ionise in aq solutions?

A

YES

78
Q

What does eqm favour in aq solutions with WB?

A

Reactants

79
Q

How do you calculate pKb?

A

-log(Kb)

80
Q

How do you calculate Kb?

A

10*-pKb

81
Q

What Kb value does a stronger base have?

A

Higher Kb

82
Q

What Kb value does a weaker base have?

A

Lower Kb

83
Q

What will a stronger base have?

A

Weaker conjugate acid

84
Q

What will a stronger base have?

A

Higher pKa value

85
Q

What does the pKa of an acid + pKb of conjugate base related to?

A

Related to Kw

86
Q

Does a base have a pKa?

A

NO
Refers to its conjugate acid
eg. NH3 = NH4+

87
Q

What is pH?

A

Level of H+ in solution

88
Q

What is pKa?

A

Degree at which acid dissociates in solution

89
Q

What is the equation for Ka?

A 
 [C]
90
Q

What is the equation for the [H3O+] for weak acids?

A

√Ka X C

91
Q

What is the equation for Kb?

A 
[C]
92
Q

What is the equation for the [OH-] for weak bases?

A

√Kb X C

93
Q

How does electronegativity effect pKa?

A

As H bond becomes more polarised, H becomes more positive
= bond easier to break
More electronegative = stronger acid

94
Q

How does anion size effect pKa?

A

Size + H held more loosely = bond easier to break

Larger size stabilises anion

95
Q

What happens to acidity of halogens as atom size increases?

A

Acidity increases

96
Q

How does resonance effect pKa?

A

Delocalisation of negative charge on conjugate base will stabilise anion = stronger acid

97
Q

What is the resonance form?

A

Stabilises negative charge

98
Q

What is resonance hybrid?

A

Actual structure

99
Q

How does hybridization effect pKa?

A

Affects H bonded to atom

Higher % of s-character of hybrid orbital = L.P held closer to nucleus = more stable

100
Q

How does inductive effect effect pKa?

A

Halogens ore electronegative
Pulls bonding e- towards itself
Pulls e- through sigma bond
Stabilises conjugate base through decrease of e- density around O2

Medicinal Chemistry (19 decks)

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