CHEM 109A- Zhang
CLAS W10
Acid Strength “Effects”/Trends for predicting pKas & relative Acid/Base Strength
You can use the following effects to explain/compare acid (and base) strengths
Element Effects (depend on the atom that the acidic H is bound to)
EN (electronegativity) – use when elements are related horizontally/in same period
As EN of element increases the acid strength increases (more polar bond is
weaker and H is more likely to be donated/pulled off).
HF > H2O > NH3
EN of atom in bold 4.0
3.5
3.0 from T10.3 in text
EN = ENatom-ENH 1.9
1.4
0.9 measure of bond polarity (larger diff = more polar)
pKa of acid
3.2
15.7 36 smaller pKa = stronger acid
Size/BDE (bond dissociation energy) – use when elements are related vertically/in same family
As BDE (energy required to break the bond) decreases the acid strength increases
(the weaker the bond, the easier it is to donate/pulled off a H atom). Can think of
this as a size effect b/c know the periodic trend for size & larger the atom, the
longer and weaker the bond.
HI > HBr > HCl > HF
Atomic radius atom in bold 133
114
99
64 pm from F12.38 Zumdahl
Approx bond length
170
151
136
101 pm, atomic radius H = 37 pm
Approx BDE
71
87
103
135 kcal/mol
pKa
-10
-9
-7
3.2
Hybridization – use when hybridization of the element attached to the H changes
EN of atom decreases with increase in hybridization (sp (50% s) > sp2 (33% s) >
sp3 (25% s)) because electrons are in orbitals with progressively less s-character
and therefore are a little farther from the nucleus.
H
C
H
C
C
H
H
ethyne/acetylene
Hybridization of atom in bold
pKa
H
C
C
H
H
ethane
sp2
44
sp
25
C
H
H
ethene
H
H
H
sp3
> 60
Structure Effects (depend on the structure of the molecule)
Inductive Effect – use when an electronegative atom is attached to the molecule
An electronegative atom/electron withdrawing atom creates a bond dipole @ a
distance from the atom attached to the acidic H. This dipole can stabilize (Zhang
prefers the through bond explanation) the anion formed more or less depending on
its distance from the locus of negative charge (closer is more stabilizing than
farther away). Any stabilization of the conjugate base (weaker conjugate base)
makes the acid stronger.
Change in EN atom – more EN atom = more acidic
O
OH
Cl
EN atom
pKa
Cl
2.84
O
O
OH
OH
>
Br
Br
2.99
O
>
OH
OH
>
O(H)*shares EN w/H
4.22
H
H
4.82
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CHEM 109A- Zhang
CLAS W10
Acid Strength “Effects”/Trends for predicting pKas & relative Acid/Base Strength
Change in distance from acidic H – closer to acidic H = more acidic
O
Cl
O
O
O
OH
>
Cl
pKa
2.86
OH
>
4.05
Cl
OH
>
OH
4.53
4.82
Change in # of EN atom – more EN atoms = more acidic
Cl
Cl
OH
OH
Cl
>
Cl
>
Cl
# of EN atom 3
pKa
12.24
Cl
OH
2
12.31
1
12.86
>
OH
0
16.0
Resonance Effect – use when can drawn resonance structures for the conjugate base
The more valid (obey octet rule) resonance structures you can draw for the
conjugate base the more stable it is (less basic) and the stronger the acid.
O
OH
O
OH
>
pKa
4.2
OH
4.8
>
>
10.0
OH
16.0
What other effect might be present in the first two structures listed above?
Also know how to use these effects/trends to compare base strength
Remember, the stronger the acid, the weaker the conjugate base
H
H
H
EX. Circle the stronger base amide ion Nor hydroxide ion
Might want to first answer: Which is the stronger acid?
-O
It is a good idea to know the following pKa values, but Zhang will provide pKa values
Hydronium ion
-1.7 = -2
Acetic acid
4.8 = 5
Ammonium ion
9.4 = 9
Methanol (MeOH) 15.5 = 15
Water
15.7 = 16
Ammonia
36
Acid form (HA) or base form (A-) in solution?
Depends on pH of solution and pKa of HA: pKa = pH - log([A-]/[HA])
pH < pKa
pH = pKa
pH > pKa
more acidic than pKa
less acidic/more basic than pKa
[HA] or [A-] = 10pH-pKa
HA form
[HA] = [A-], 50/50
A- form
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