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Part 2CHM1C3
Organic Acids and Bases
R
O
O
H HO
H
HO
H
HR
O
O
R
O
O
Acid BaseConjugate
AcidConjugate
Base
Content of Part 2
Definition of Bronsted acids and bases
Definition of conjugate acids and bases
Ka
pKa
Typical pKa values
Eplaining differences in acidity: Resonance Effects
Eplaining differences in acidity: Inductive Effects
– Learning Objectives Part 2 –
Organic Acids and Bases
After completing PART 2 of this course you should have an understanding of, and be able to demonstrate, the
following terms, ideas and methods.
(i) You should be able to show the equilibrium between an organic acid in water with its conjugate
base and the hydroxonium ion.
(ii) You should know what Ka equals with respect to this equilbrium.
(iii) You should know the relationship between Ka and pKa.
(v) You should understand that the smaller the pKa or the more negative the pKa the stronger is the
acid.
(vi) By consideration of resonance structures of structurally related organic acids you should be able to
make an assessment of which structure is likely to be the most acidic.
(vii) By consideration of inductive effects in structurally related organic acids you should be able to
make an assessment of which structure is likely to be the most acidic.
CHM1C3– Introduction to Chemical Reactivity of Organic
Compounds–
Bronsted Acids and Bronsted Bases
Bronsted Acid: A Bronsted acid (HA) is a compound which acts as a proton donor.
Bronted Base: A Bronsted Base (B:) is a compound which acts as a proton acceptor.
HA + B: A + BH
BronstedAcid
BronstedBase
ConjugateBase
ConjugateAcid
proton acceptor
proton donor
proton acceptor
proton donor
Examples of Bronsted Acids and Bronsted Bases
AH + B: A + BH
BronstedAcid
BronstedBase
ConjugateBase
ConjugateAcid
H3O + NH3: H2O: + NH4
CH3CO2H + CH3O CH3CO2 + CH3OH
H2SO4 + H2O: HSO4 + H3O
Quantifying the Equilibrium: Ka
The dissociation of an acid, HA, in water may be represented as
HA + H2O: A + H3O
The water is acting as the base.
Furthermore, the water is acting as the solvent and is in huge excess.
Ka =
HA
A H3O
The degree of ionisation is quantified by the equilibrium constant…
Values of Ka
[1] Very strong acid
Almost complete ionization Ka =
HA
A H3O LargeSmall
large number
Approaches infinity
[2] Very weak acid
No perceptible ionization
Ka =
HA
A H3O SmallLarge
small number
Approaches zero
The pKa
Very weak acid
low ionization
Very strong acid
high ionization
= -Log10 Ka
Ka pKa % Dissociation
@ 1 mM
1 x 10 3 -3 99.9
1 x 101 -1 92
1 x 100 0 62
1 x 10-1 1 27
1 x 10 -11 11 0.0003
HA + H2O: A + H3O
pKa = -Log10H A
A H3O
Some Heteroatom pKa Valuesi.e. atoms attached to acidic protons other than carbon
Acid pKa
HBr -8
HCl -7
H2SO4 -3
HNO3 -1.4
HF 3.18
CF3CO2H 0.23
CCl3CO2H 0.66
NCCH2CO2H 2.47
HCO2H 3.75
Acid pKa
PhCO2H 4.20
CH3CO2H 4.76
(CH3)3CCO2H 5.03
4-nitrophenol 7.15
2-nitrophenol 7.23
3-nitrophenol 8.36
Phenol 10.00
C2H5SH 10.6
CF3CH2OH 12.4
Acid pKa
CH3OH 15.5
C2H5OH 15.9
STRONG ACID
WEAK ACID
Resonance Effects
and
Acidity
Explaining the Differences in Acidity: Resonance Effects
H3C
O H HO
H
HO
H
HH3C
O
HHHH
4.76
15.5
pKa
This resonance process imparts stability on the anionic structure (see Part 1 of the course)
Thus, carboxylate anion is more stable than the alkoxide anion.
Lone pairs of electrons adjacent to double bonds are able to delocalise through a process referred to as resonance.
H3C
O
O
H HO
H
HO
H
HH3C
O
O
H3C
O
OH3C
O
O
pKa = 7.23
pKa = 8.36
Explaining the Differences in Acidity: Resonance Effects
OHN
O
O
H2O
ON
O
O
H3O
OH
NO
O
H2O
O
NO
O
H3O
Stronger Acid
Weaker Acid
3-Nitrophenol
O
NO
O
O
NO
O
O
NO
O
O
NOH
OO
NO
O
Lone pair delocalised into -system of the aromatic ring
Lone pair delocalised into -system of the nitro group
2-Nitrophenol
It is not possible for the lone pair to be positioned on the carbon atom adjacent to the nitrogen atom.
O
N
O
O
O
N
O
O
O
N
O
O
O
N
O
O
Lone pair delocalised into -system of the aromatic ring
Therefore, there is one less resonance structure in this case, and this anion is subsequently less stable, and more difficult to form from its protonated form.
3-Nitrophenol
O
H H2OO
H3O
O O
H
H2OO O
H3O
pKa = 20
pKa = 9
Stronger Acid
Weaker Acid
Explaining the Differences in Acidity: Resonance Effects
O
O O O OOO
An EnolateDr Cox’s Lecture Course
two resonance structures
three resonance structures
O
Less stable anion
More stable anion
Inductive Effects
and
Acidity
Same Hydroxonium
Ion:
Protonated water
Nature of anion is different
F3C
O
O
H HO
H
HO
H
HF3C
O
O
H
O
O
H HO
H
HO
H
HH
O
O
Ph
O
O
H HO
H
HO
H
HPh
O
O
H3C
O
O
H HO
H
HO
H
HH3C
O
O
0.23
3.75
4.20
4.76
5.03
pKa
Explaining the Differences in Acidity: Inductive Effects
O
O
H HO
H
HO
H
H
H3C
H3C
H3CO
O
R
O
O
R
O
O
R
O
O
This resonance is the same for all the acids above.
Thus, the R groups are influencing the stability of the carboxylate anion
R affects CO2-
H3C
O
O
CF3 is a strong electron withdrawing group (-I group) and is pulling electron density away from the carboxylate, i.e. reducing the charge on the carboxylate, and thus stabilising it, in a relative sense.
R = CF3 this is the strongest acid. CF3 = -I Inductive Group
Therefore,
F3C
O
O
Is the most stable anion.
R = CH3 this is a weaker acid. CH3 = +I Inductive Group
Therefore,
Is a less stable anion.
CH3 is an electron donating group (+I group) and is pushing extra electron density onto the carboxylate, i.e. increasing the charge on the carboxylate, and thus destabilising it, in a relative sense.
Some Carbon Atom pKa Valuesi.e. carbon atoms attached to acidic protons
Acid pKa
CH3C(O)CH2C(O)CH3 9
CH3NO2 10.2
CH2(C≡N)2 11.2
Cyclopentadiene 16.0
PhC(O)CH3 19.0
CH3C(O)CH3 20
PhC≡CH 21
CH3C≡N 25
HC≡CH 26
VERY WEAK ACID
NOT REALLY AN ACID!
Acid pKa
(Ph)3CH 31.5
PhCH3 41
Ph-H 43
CH4 48
Cyclohexane 51
– Summary Sheet Part 2 –
Organic Acids and Bases
A Bronsted acid is a compound which can donate a proton (H+). Once the proton has been donated the resulting structure
is referred to as the conjugate base.
A Bronsted base is a compound which can accept proton. Once the proton has been accepted the resulting structure is
referred to as the conjugate acid.
Any acid/base reaction is, in principle, an equilibrium process. The equilibrium can be quantified by considering the
degree of ionisation of an acid dissolved in water, where the water acts as the Bronsted base. This quantification is
referred to as the pKa and is equal to the –log Ka, where Ka is equal to the equilibrium concentration of the conjugate base
multiplied by the equilibrium concentration of the hydroxonium ion divided by the equilibrium concentration of the
Bronsted acid.
Consideration of inductive and resonance effects on the conjugate base between structurally related compounds allows a
qualitative assessment of the order of acidity. The more delocalised the lone pair of electrons (formed from deprotonation
of the acid) the more stable the conjugate base. If the conjugate base is stabilised, the easier it will be formed, and thus
the stronger the Bronsted acid will be.
CHM1C3– Introduction to Chemical Reactivity of Organic
Compounds–
http://classes.yale.edu/chem220a/studyaids/pKa.html
http://www.chromatography.co.uk/TECHNIQS/Other/buffers.htm
http://home.planet.nl/~skok/techniques/laboratory/pka_pkb.html
http://www.wiu.edu/users/mftkv/Chem331/acidstrength.htm
http://www.geocities.com/le_chatelier_uk/pka.html (interesting if you have audio!)
http://www.chem.wisc.edu/areas/reich/pkatable/ (pKas in DMSO as solvent)
http://www.agsci.ubc.ca/courses/fnh/410/protein/1_13.htm (pKas of aminoacids)
http://classes.yale.edu/chem220a/studyaids/pKa.html
http://www.chem.umd.edu/courses/chem231fribush/3-Chapter2-3.pdf
www for further pKa information
Question 1: Acids and Bases
Rationalise why acid A is a stronger acid than acid B.
A, pKa = 11.2 B, pKa = 25
N
H
N
N
H
Answer 1: Acids and Bases
Rationalise why acid A is a stronger acid than acid B.
A, pKa = 11.2 B, pKa = 25
-H++H+
N
N
N
H
N
N C
NN
CN
N
H
-H++H+
N
N C
Most stable anion, as charge more delocalised over three resonance structures, compared to 2 in the conjugate base of B. Therefore, A is most acidic
Question 2: Acids and BasesA and B are two structurally related phenols. Identify the one which you think will be the most acidic.
A B
OH
N
OH
N
Answer 2: Acids and BasesA and B are two structurally related benzoic acids. Identify the one which you think will be the most acidic.
A B
OH
N
OH
NO
N
O
N
O
N
O
N
O
N
O
N
O
N
O
C
N
O
N
Two establish which is the strongest acid we need to consider the conjugate base resonance structures. We will be able to establish which has the most resonance structures, and is therfore the most stable conjugate base and therefore the most easiest to form.
4 Resonance Structures
5 Resonance Structures
Most Acidic
Question 3: Acids and BasesA and B are two structurally related phenols. Identify the one which you think will be the most acidic.
A B
OH
O
OH
O
Answer 3: Acids and BasesA and B are two structurally related phenols. Identify the one which you think will be the most acidic.
A B
OH
O
OH
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Two establish which is the strongest acid we need to consider the conjugate base resonance structures. We will be able to establish which has the most resonance structures, and is therefore the most stable conjugate base, and thus the easiest to form.
4 Resonance Structures
5 Resonance Structures
Most Acidic