The central goal of this unit is to help you identify the environmental and structural factors that can be used to control chemical reactions.

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Unit 6How do we control chemical change?
In order to control chemical processes, we need
to consider two important factors
STRUCTURE ReactantsProducts
CONDITIONS T and PSolvent
The central goal of this unit is to help you
identify the environmental and structural factors
that can be used to control chemical reactions.
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Unit 6How do we control chemical change?
FOUR MAIN MODULES
Recognizing interactions between reacting
molecules.
M1. Characterizing Interactions
.
Exploring the influence of external factors.
M2. Changing the Environment
Analyzing the effect of charge stability.
Evaluating the impact of electronic and steric
effects.
M4. Selecting the Reactants
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Context
To illustrate the power of chemical ideas and
models in controlling chemical change, we will
focus our attention in understanding the
synthesis and properties of drugs and the way
they interact inside our body.
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The Problem
Drugs are chemical compounds that interact with a
biological system to produce a biological
response.
It is thus of central importance to know how to
control their synthesis as well as their chemical
behavior inside our body.
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Unit 6 How do we control chemical change?
Module 1 Characterizing Interactions
Central goal To characterize the nature and
effect of interactions between polar molecules
and water.
d-
d
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The Challenge
Drugs interact with a variety of systems in our
body in the stomach, in the bloodstream, inside
cells. Drug action and delivery depends on the
nature and effect of these interactions.
Recognizing potential interactions between
substances is crucial in controlling chemical
reactions.
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The Setting
The structure that drugs adopt inside our body
depends on their interactions with substances
such as water, lipids, proteins, and nucleic
acids.
How do drugs react with water? How can we predict
the outcome?
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Polar Water
Water molecules are polar and many chemical
reactions in which these molecules participate
are driven by interactions with their positive
and negative charge centers.
H2O
d
d-
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Liquid Water
Proton transfer even occurs between two water
molecules
H2O (l) H2O(l) H3O(aq) OH-(aq)
Auto-ionization is not thermodynamically favored,
though. In pure liquid water H3O OH-
1.0 x 10-7 M at 25 oC
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Waters Auto-Ionization
Given that H2O does not change much by
auto-ionization, we can take it as a constant and
include it as part of the auto-ionization
equilibrium constant
H2O (l) H2O(l) H3O(aq) OH-(aq)
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pH and pOH
It is common to express and monitor the
concentrations of H3O and OH- in water solutions
using pH or pOH values
pH -log H3O pOH -logOH-
Given that Kw H3OOH- 1 x 10-14 ALWAYS
in water solutions
pH pOH 14 in water solutions.
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Water as a Reactant
Many substances react with water via proton
transfer and alter the concentration of H3O and
OH- ions in solution (change the pH).
pH scale
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Lets Think
The pH of Milk of Magnesia is close to 10.5
that of our stomach is close to 2.0.
How many times larger is H3O in our stomach
compared to OH- in Milk of Magnesia?
What would you expect to happen when equal
amounts of these two solutions are mixed?
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Acid-Base Reactions
Those substances that react with water via proton
transfer are classified as Brønsted-Lowry acids
or bases
B-L AcidsProton Donors(Increase H3O)
HCl(g) H2O(l)
B-L BasesProton Acceptors(Increase OH-)
NH3(g) H2O(l)
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Lets Think
Predict the outcome of the reaction of these
drugs with water. Determine whether they behave
as acids or bases in this solvent.
H2O(l)
Ephedrine
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Conjugate Species
In these reactions, new acids and bases are
formed
HCl(g) H2O(l) ? Cl (aq)
H3O(aq)
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Lets Think
The solubility and activity of most drugs
strongly depends on their acid-base properties.
Identify all of the conjugate pairs in these
acid-base equilibria involving common drugs?
C4H4N2O3(aq) H2O(l) C4H3N2O3 (aq)
H3O(aq)
C17H19NO3(aq) C6H8O6(aq) C17H20NO3(aq)
C6H7O6(aq)
C9H8O4(aq) C6H7O7(aq) C9H7O4 (aq)
C6H8O7(aq)
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Strength
The extent to which an acid or base reacts with
water determines its strength.
HCl is a strong acid.
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Strength

• Weak acids are much less than 100 ionized in
  water.

Similar ideas can be applied to evaluate the
strength of basic substances.
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Strength
In general, we can use the value of these
dissociation constants to judge acid/base
strength.
HA(aq) H2O(l) A (aq) H3O(aq)
If we take, Ka x Kb for a conjugate pair, we see
that
The larger the value of Ka (the stronger the
acid), the smaller Kb (the weaker the conjugate
base).
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Lets Think
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pKa and pKb
It is common to express the strength of an acid
or a base using these quantities
pKa - log (Ka) pKb - log (Kb)
For a conjugate acid/base pair pKa pKb 14
The larger the value of pKa, the weaker the
acid.The larger the value of pKb, the weaker the
base.
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Let's apply!
Assess what you know
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Drugs, Drugs, Drugs
Over 70 of the drugs in the market have
acid-base properties. Of these, close to 60 are
bases, around 25 are acids, and the rest are
amphoteric.
The strongest acids tend to be carboxylic acids
(pka 4) the weakest tend to be barbiturates.
pKa 8
Most of the basic drugs contain amine groups. pKa
9
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Drugs Behavior
The analysis of the acid-base properties of drugs
is crucial to understand and control their
behavior in our body.
This determines which will be absorbed faster.
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Predict
Which of these two drugs will be absorbed at a
faster rate through stomach cell membranes (made
of lipids)?
MorphineC17H20NO3
AspirinC9H8O4
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Drugs Behavior
Ionic forms are more likely to be soluble in
water and insoluble in cell membranes, made of
non-polar lipids. Thus, charged species tend to
be absorbed at a slower rate.
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Infer
Which proton are these drugs likely to transfer
to water? Write the chemical equation of their
reaction with water.
Tylenol
Aspirin
All of these analgesics are acidic.
Ibuprofen
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Identify
pKa 9.1
Which is stronger, the acid form of each
analgesic or its conjugate base?
Tylenol
Aspirin
pKa 4.5
Ibuprofen
pKa 3.5
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Discuss with a partner what are the main
differences between a weak acid and a strong base.
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Characterizing Interactions
Summary
Many substances react with water via proton
transfer and alter the concentration of H3O and
OH- ions in solution (change the pH).
Brønsted-Lowry AcidsProton Donors(Increase H3O)
Brønsted-Lowry BasesProton Acceptors(Increase
OH-)
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Dissociation
Although acids and bases change H3O and OH-
in water, the concentration of these ions is
related by
Kw H3OOH- 1 x 10-14 in water at 25 oC.
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For next class, Investigate how changes in
concentration and temperature affect acid-base
reactions. How does chemical equilibrium shift in
an acid-base reaction when the concentration of
reactants or products are changed?