Daley amp Daley Chapter 5 Acidbase theory 51 Acids amp bases overview amp basics 52 Acid amp base strength 53 Equilibrium acidbase reactions 54 The leveling effect of solvents ID: 641144
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Slide1
CHE2060 Lecture 5: Acid-base chemistry
Daley &
Daley
Chapter 5
:Acid-base theory
5.1 Acids & bases: overview & basics
5.2 Acid & base strength
5.3 Equilibrium acid-base reactions
5.4 The leveling effect of solvents
5.5 Estimation of acidity by conceptual knowledge
5.6 Classes of organic acids & bases
5.7 Functional groups: acid-base natureSlide2
The ‘leveling’ effect
Solvents may hijack intended reactionsSlide3
Remember solvents?
Karty
, p.304
Solvents
are compounds that are:Present in large amounts (larger than solvents);Used to create a soluble environment for solute reactants; andSometimes plays a role in solute reactions.
Everyday example
: water is the solvent for extraction of caffeine and other
coffee or tea compounds from beans or leaves.
Ideally
, solvents either have no impact on reactions or actually enhance them.
However
,
if solvents are not selected carefully they can interfere with – or hijack – our intended reactions.Slide4
So, what’s ‘leveling’?
Karty
, p.304
Leveling
is a reaction of the solvent with one of the reactants. This reactionuses up some or most of that reactant and restricts the intended reaction.Acids & bases are often impacted by leveling because they react with a wide variety of solvents, like water
.
pka
~ - 1.7
pka
~ - 7
So
HCl
is a powerful acid
until
it is diluted in water.
Once
HCl
reacts with water, the only acid remaining is hydronium which is 1.99E5 times weaker than
HCl
.
Water ‘levels’ (or reduces or restricts) the strength of
HCl
.Slide5
Base strength can also be leveled by solvents.
Karty
, p.304
Deprotonated dimethyl amine is a base. Note its two free electron pairs.
pKa 38
pKa
15.7
Deprotonated dimethyl amine is a powerful base. Its conjugate acid, dimethyl amine, has a
pKa
of 38!
For bases, the equilibrium is pushed
towards the acid with the higher
pKa
.
The strength of deprotonated dimethyl amine pushes equilibrium towards products by a factor of 2.oE22!
However, if water is used as a solvent when deprotonated dimethyl amine is used as a base, water will act as an acid, react with and ‘neutralize’ the powerful base.
Reaction with the water solvent produces hydroxide as the remaining base.
And hydroxide is a weaker base than deprotonated
dimethylamine
.Slide6
Conclusions about leveling
Karty
, p.304
If
water is used as the solvent, no acid stronger than hydronium, and no base weaker than hydroxide, can exist to appreciable extents. For
other solvents
:
The
strongest acid
that can exist in solution, to appreciable concentrations,
is the protonated solvent
.
The
strongest base
that can exist in solution, to appreciable concentration,
is the deprotonated solvent
.
Take-home message?
Solvents must be chosen carefully!Slide7
Example: wise choice of solvent
Karty
, p.304
If you want to use deprotonated dimethyl amine as a base, diethyl ether is a better solvent than water.
Use
pka
values to show that diethyl ether is a better solvent!
pKa
45
pKa
38
For bases, the equilibrium is pushed towards the acid with the higher
pKa
, so when deprotonated dimethyl amine is placed in diethyl ether solvent the two barely react.Slide8
Example: you choose the solvent!
Karty
, p.304
Would each of these solvents be appropriate for the reactant?
Why?Reactant:
Deprotonated
ethelyne
(acetylene) is a base.
For bases, the equilibrium is pushed towards the acid with the higher
pKa
.
To preserve the ‘baseness’ of the reactant we want to push equilibrium back towards it; to the left.
So use solvents that are
stronger
acids than the conjugate acid of the reactant.
DMSO (
dimethylsulfoxide
) and diethyl ether have higher
pKa
values then acetylene.
Solvents:
pKa
25 for its conjugated acid
pKa
: 15.7 16 17 35 45