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					Acids and Bases

4 Lectures

Essential Reference
Sykes, Mechanisms in Organic Chemistry, Chapter 3




                                                    1
       Acid–Base Concepts

Arrhenius Acid: A substance which dissociates to
form hydrogen ions (H+) in solution.

           HA(aq)          H+(aq) + A–(aq)

Arrhenius Base: A substance that dissociates in, or
reacts with water to form hydroxide ions (OH–).

       MOH(aq)  M+(aq) + OH–(aq)
         Acid–Base Concepts

• Brønsted–Lowry Acid: Substance that can donate H+
• Brønsted–Lowry Base: Substance that can accept H+




•   Chemical species whose formulas differ only by one
    proton are said to be conjugate acid–base pairs.
Acid–Base Concepts
Acid–Base Concepts
          Acid–Base Concepts

•   A Lewis Acid is an electron-pair acceptor. These
    are generally cations and neutral molecules with
    vacant valence orbitals, such as Al3+, Cu2+, H+, BF3.

•   A Lewis Base is an electron-pair donor. These are
    generally anions and neutral molecules with
    available pairs of electrons, such as H2O, NH3, O2–.

•   The bond formed is called a coordinate bond.
                             F                     CH3
        Cl

                                                   C           Cl    Zn       Cl
        Al                   B
                                            H 3C         CH3
  Cl          Cl       F           F

                   Boron trifluoride    Carbocation
  Aluminium                                                     Zinc chloride
  trichloride
   Lewis Acids – accept a pair of electrons
                                                                O
  H3C        NH2
                                   O


methylamine
                             Diethylether                  Acetone


 Lewis Bases – can donate a pair of electrons
                                                                          7
Acid–Base Concepts
              Dissociation of Water

      H       H       H       H     H       H       H
          O       +       O             O       +       O


                                        H


•   This equilibrium gives us the ion product constant
    for water
• Kw =        Kc = [H+][OH–] = 1.0 x 10–14

•   If we know either [H+] or [OH–] then we can
    determine the other quantity.
            Dissociation of Water

•   The concentration of OH– ions in a certain household
    ammonia cleaning solution is 0.0025 M. Calculate the
    concentration of H+ ions.


•   Calculate the concentration of OH– ions in a HCl
    solution whose hydrogen ion concentration is 1.3 M.
           pH – A Measure of Acidity

•   The pH of a solution is the negative logarithm of the
    hydrogen ion concentration (in mol/L).

                          pH = –log [H+]
                         pH + pOH = 14

    Acidic solutions:   [H+] > 1.0 x 10–7 M,   pH < 7.00
    Basic solutions:    [H+] < 1.0 x 10–7 M,   pH > 7.00
    Neutral solutions: [H+] = 1.0 x 10–7 M,    pH = 7.00
The pH Scale
          pH – A Measure of Acidity

•   Nitric acid (HNO3) is used in the production of
    fertilizer, dyes, drugs, and explosives. Calculate the
    pH of a HNO3 solution having a hydrogen ion
    concentration of 0.76 M.

•   The pH of a certain orange juice is 3.33. Calculate
    the H+ ion concentration.

•   The OH– ion concentration of a blood sample is
    2.5 x 10–7 M. What is the pH of the blood?
pH – A Measure of Acidity
          Strength of Acids and Bases

•   Strong acids and bases: are strong electrolytes
    that are assumed to ionise completely in water.

•   Weak acids and bases: are weak electrolytes that
    ionise only to a limited extent in water.

•   Solutions of weak acids and bases contain ionised
    and non-ionised species.
                              Strength of Acids and Bases

                           ACID    CONJ. BASE                              ACID    CONJ. BASE
                           HClO4     ClO4–                                 HSO4–    SO42–
Increasing Acid Strength




                                                Increasing Acid Strength
                           HI        I–                                    HF       F–
                           HBr       Br –                                  HNO2     NO2 –
                           HCl       Cl –                                  HCOOH    HCOO –
                           H2SO4     HSO4 –                                NH4+     NH3
                           HNO3      NO3 –                                 HCN      CN –
                           H3O+      H2O                                   H2O      OH –
                           HSO4–     SO42–                                 NH3      NH2 –
         Acid Ionization Constants

•   Acid Ionization Constant: the equilibrium
    constant for the ionization of an acid.
    HA(aq) + H2O(l)          H3O+(aq) + A–(aq)
•   Or simply:   HA(aq)      H+(aq) + A–(aq)

                                 
                        [H ][A ]
                   Ka 
                          [HA]
       Acid Ionization Constants


ACID                   Ka           CONJ. BASE
HF                   7.1 x 10 –4     F–
HNO2                 4.5 x 10 –4     NO2 –
C9H8O4 (aspirin)     3.0 x 10 –4     C9H7O4 –
HCO2H (formic)       1.7 x 10 –4     HCO2 –
C6H8O6 (ascorbic)    8.0 x 10 –5     C6H7O6 –
C6H5CO2H (benzoic)   6.5 x 10 –5     C6H5CO2 –
CH3CO2H (acetic)     1.8 x 10 –5     CH3CO2 –
HCN                  4.9 x 10 –10    CN –
C6H5OH (phenol)      1.3 x 10 –10    C6H5O –
pKa’s AND ACID STRENGTH




                          19
DEFINITION OF pKa                (A concise way to state the
                                 strength of an acid.)


       HA + H2O                H3O+ + A-
       [H3O +] [A-]
Ka =                       Log Ka = Log [H3O+] + Log [A-]/[HA]

          [HA]
                                         [A ]                  
 pKa = - log Ka           pH  pKa  Log
                                         [HA]
           strong acids          weak acids

pKa      0     2      4    6         8        10      12        14

                                                               20
 Ka           10-2        10-6             10-10               10-14
  The Henderson-Hasselbalch Equation

   For any acid HA, the relationship between the pKa,
  the concentrations existing at equilibrium and the
  solution pH is given by:

                          [A ]      
           pH  pKa  Log
                          [HA]
When the concentration [A-] = concentration [AH],
the pKa = pH
                                                    21
Consider the Dissociation of Acetic Acid(pKa = 4.76)
      Assume 0.1 eq base has been added to a fully
               protonated solution of acetic acid
  • The Henderson-Hasselbalch equation can be used
    to calculate the pH of the solution:
                                                          [A  ]
  HA + H2O                                 pH  pKa  Log
                          A + H3O                         [HA]

    [A-] = 0.1 and [AH] = 0.9


   pH  pKa  log               [ 0.1]
                                [ 0.9 ]

   pH  4.76  (0.95)
                                          pH  3.81                22
  Consider the Dissociation of Acetic Acid
                          Another case....
• What happens if exactly 0.5 eq of base is added to a
  solution of the fully protonated acetic acid?

• With 0.5 eq OH¯ added:                [A-] = 0.5 and [AH] = 0.5

                 [A  ]
  pH  pKa  Log
                 [HA]


  pH  pKa  log           [ 0.5]
                           [ 0.5]


 pH  4.76  log1                     pH  4.76
                                                                23
  Consider the Dissociation of Acetic Acid
                    A final case to consider....
 • What is the pH if 0.9 eq of base is added to a solution of
   the fully protonated acid?
 • With 0.9 eq OH¯ added:
                                                         [A  ]
   [A-] = 0.9 and [AH] = 1                pH  pKa  Log
                                                         [HA]

pH  pKa  log      [0.9]
                    [0.1]


 pH  pKa  log9

pH  4.76  0.95                      pH  5.71
                                                              24
Acetic Acid pKa
4.76




                                         25
                  Acetic Acid pKa 4.76
Glycine




                    26
          Glycine
Titration of Glycine




                       27
                   ACID STRENGTH
        HA     +   H2O            H3O+           +      A-

 In water, all acids form hydronium ion the important factor of
 difference is the conjugate base.

The difference between a strong acid and a weak acid
is in the stability of the conjugate base.

                                                      WEAK ACID
   E                                        A-        has a strong conjugate
   N                                                  Base, higher E
   E
   R                                        A-
   G                           ionisation            STRONG ACID
   Y                           easier                has a weak conjugate
            HA                                       Base, lower E
                                                                        28
FACTORS THAT INCREASE ACIDITY

   STABILISATION OF A CONJUGATE BASE




                                       29
We will study the factors that lead STABILISATION
to lower energy (stabilisation) in   FACTORS
the conjugate base.                 1. Mesomeric effect
                              A
                                       2. Electronegativity
                                       3. Atom size
             stabilisation             4. Hybridisation
                               A
                                       5. Inductive effect
                                       6. Charge

                                       7. Solvation effect
                Stabilisation of the
HA              conjugate base         8. Steric effect
                makes the acid
                stronger.                                 30
Aspirin               Paracetamol                    Morphine
NSAID                 analgesic                      Analgesic

                                   Amide                 Phenol
                                   pKa 0                 pKa 10
     Carboxylic
     Acid pKa 3




                                           Alcohol           Amine
                  Phenol pKa 9.5           pKa 16            basic
                                                                  31
Mesomeric Effect



                   32
           MESOMERIC EFFECTS
increasing quality
                           pKa Values
of resonance
               O              18        Alcohol
           R       H



                   O
                       H        10      Phenol



               O
                               4        Carboxylic acid
       R
               O H                                   33
 MESOMERIC EFFECT
                         charge is better                 O
                         on oxygen
                                            H3C       C
  H3C CH2
                   H3C CH2 O                              O       localised

                                                          resonance
                                                          lowers energy
                                                          of the ion          1/
                                                                                2
      -H                  -H                                              O
                                                              H3C     C
                                   -H                                     O   1/
                                                                                   2
H3C CH2 H
                 H3C CH2 O        H               O
       lower energy conjugate         H3C    C                    delocalised
       base is easier to form                     O       H
ACETATE ION
                                           O
                                 H3C   C
                                           O
           O
                            -H
H3C    C
           O   H

                                            O
 acetic acid
                                 H3C    C
                                            O

 Delocalisation of charge
 lowers the energy.
                                 acetate ion
   DELOCALISATION
                                   .
        p system                  ..
   H                      .      O ..
                                       ..
  H C                    C                  ..
                                       O    ..
     H
all atoms in the carboxyl group are sp2 hybridised
    OH                  OH               OH               OH




H2O
                       PHENOL

      O                  O                 O                 O




                                                                  37
H         non-equivalent structures charge on carbon and oxygen
    PHENOXIDE ION
O                       O

                            Not all canonical
                            structures are equal.
                            Why?


    notice the
    alternate pattern
                                              
                                           O
O                       O                         
                                   

                                          


                                         SUMMARY
  UNEQUAL CONTRIBUTIONS OF CANONICAL STRUCTURES

MINOR              charge on
CONTRIBUTORS       carbon
                               O    O      O


                                                  E
           O         O                            N
                                                  E
                                                  R
                                                  G
                                                  Y


charge on oxygen

       MAJOR CONTRIBUTORS

                                   RESONANCE
                                   HYBRID
HYBRID ORBITAL PICTURE OF PHENOLATE ION



           H         .              .         H
           .                                  .         ..
                                                          ..
  H                  .               .                  O ..

            H                                 H

      Electrons are distributed throughout the pi system,
      but the oxygen atom has a larger share than the
      carbon atoms.
CYCLOHEXANOL ANION - NO RESONANCE


                         H
      O
        -       -O



               no pi system
            charge is localised
    H                      Acetic Acid                      H
    O                                                       O H
H                                                       H
                   O                            O
        H3C                      H3C
                   O H                          O



                   O                              O
        H3C                         H3C
                   O H                            O
                                equivalent structures
        Charge separated
                                charge on oxygen's          42
        structure
We will study the factors that lead STABILISATION
to lower energy (stabilisation) in   FACTORS
the conjugate base.                 1. Mesomeric effect
                              A
                                       2. Electronegativity
                                       3. Atom size
             stabilisation             4. Hybridisation
                               A
                                       5. Inductive effect
                                       6. Charge

                                       7. Solvation effect
                Stabilisation of the
HA              conjugate base         8. Steric effect
                makes the acid
                stronger.                                 43
ELECTRONEGATIVITY VALUES
H         C              N        O     F
2.1       2.5            3.0      3.5   4.0

          Si             P        S     Cl
          1.9            2.2      2.5   3.0
                              F
                                        Br
           increase                     2.8
      periodic chart trends
                                        I
                                        2.5   44
  EFFECT OF ELECTRONEGATIVITY
increasing
electronegativity          pKa Values    Conjugate base
                                            H
           C         2.5   CH4      45     H C
                                            H
                                           H
            N       3.0    NH3     35        N
                                           H


            O       3.5    H2O    18     H O


            F       4.0    HF      3        F
                                                      45
We will study the factors that lead STABILISATION
to lower energy (stabilisation) in   FACTORS
the conjugate base.                 1. Mesomeric effect
                              A
                                       2. Electronegativity
                                       3. Atom size
             stabilisation             4. Hybridisation
                               A
                                       5. Inductive effect
                                       6. Charge

                                       7. Solvation effect
                Stabilisation of the
HA              conjugate base         8. Steric effect
                makes the acid
                stronger.                                 46
       EFFECT OF ATOMIC SIZE
increasing
atom size          pKa Values


             HF         3.5     F-      0.136 nm




             HCl         -7     Cl -     0.181 nm



                         -9      Br -    0.195 nm
             HBr



                        -10      I-       0.216 nm
             HI
                                               47
We will study the factors that lead STABILISATION
to lower energy (stabilisation) in   FACTORS
the conjugate base.                 1. Mesomeric effect
                              A
                                       2. Electronegativity
                                       3. Atom size
             stabilisation             4. Hybridisation
                               A
                                       5. Inductive effect
                                       6. Charge

                                       7. Solvation effect
                Stabilisation of the
HA              conjugate base         8. Steric effect
                makes the acid
                stronger.                                 48
EFFECT OF HYBRIDISATION
 sp3 carbon           sp2 carbon               sp carbon
 25% s 75%p           33% s 67%p               50% s 50%p

                       H           H
 H            H
                  H
                           C   C
     C   C                             H        C       C          H
 H H          H        H           H
                                           electrons have lower
                                           energy in sp hybrid -
                                           closer to nucleus



                                                             49
        H       H       O                                      O

H       C       C                        sp3
                                                               O   H
        H       H       O       H
                                                   Cycylohexane
Propanoic acid 4.8                                 Carboxylic acid 4.9
H           H       O
                                                               O
    C       C
                                         sp2
H                   O       H
                                                               O   H


    Propenoic acid 4.25                             Benzoic acid 4.2
                    O

H   C       C                       sp

                    O       H       Propynoic acid 1.84                50
We will study the factors that lead STABILISATION
to lower energy (stabilisation) in   FACTORS
the conjugate base.                 1. Mesomeric effect
                              A
                                       2. Electronegativity
                                       3. Atom size
             stabilisation             4. Hybridisation
                               A
                                       5. Inductive effect
                                       6. Charge

                                       7. Solvation effect
                Stabilisation of the
HA              conjugate base         8. Steric effect
                makes the acid
                stronger.                                    51
   TYPES OF INDUCTIVE EFFECTS
     ELECTRON                           ELECTRON
     WITHDRAWING                        DONATING
     GROUPS                             GROUPS


       -          +                      +        -
       Cl           C                CH3              C

      F, Cl, Br, N, O                   R, CH3, B, Si
   electronegative elements     alkyl groups and elements
   take electron density        less electronegative than carbon
   from carbon                  donate electron density to carbon

These electron withdrawing and donating groups work through
the sigma bond system, unlike the similarly named MESOMERIC
                                                                    52
EFFECT work through the p system.
 INDUCTIVE EFFECTS HALOACIDS
Chlorine helps                                       O
to stabilise -CO2-                   -         +
by withdrawing
electrons
                                    Cl           C
                                                     O
                                                O
                                 
         Cl           C-       C           C
                            +
                                                O
       The effect diminishes with distance
         - it carries for about 3 bonds.                 53
        INDUCTIVE EFFECT IN ACETIC ACIDS

    H     O            H     O               H    O            H         O
I                Br                    Cl                  F
    H     O H          H     O H             H    O H          H         O H
    3.2                2.9                  2.8                    2.6



          Cl     O                H    O                       H         O
    Cl                       H                        Cl
          Cl     O H              H    O H                     H         O H

           O.7                   4.8                               2.8


                                                                             54
                  O     INDUCTIVE EFFECTS
                           4.8
                  O H
              O

              O H          4.5
Cl
          O                       distance


          O H
                            4.0
     Cl


     Cl       O
                            2.8
              O H                            55
               INDUCTIVE EFFECTS
               O
       H             3.75
               O H

           O                      H3C
                                        CH2
  H3 C               4.75         H2C         O
           O H              4.8    H2C
                                              O H

H3 C       O         4.87
 H2C
           O H


                                                  56
We will study the factors that lead STABILISATION
to lower energy (stabilisation) in   FACTORS
the conjugate base.                 1. Mesomeric effect
                              A
                                       2. Electronegativity
                                       3. Atom size
             stabilisation             4. Hybridisation
                               A
                                       5. Inductive effect
                                       6. Charge

                                       7. Solvation effect
                Stabilisation of the
HA              conjugate base         8. Steric effect
                makes the acid
                stronger.                                 57
SOLVATION LOWERS ENERGY
 Cl- (g) + n H2O               Cl- (aq) + HEAT (DH)

 Solvation is a type of weak bonding.
 Energy is released when an ion is solvated.
 This lowers the energy of the ion.


                     Cl- (g)                   O H
   E
   N
         + n H2O                       O HH H O
   E
   R                                   H       Cl-   H
   G
                     Cl- (aq)
                                           H         H
   Y                                       H O HH O
                                             H O
                                                         58
                                        solvated ion
WATER IS A POLAR SOLVENT

    It can solvate both cations and anions



                                  CATIONS
                                    -
                        +      -
                                 O -
                        H
     Polar bonds in
     water make it a     +        H +
     polar molecule.
                       ANIONS


                                             59
              SIZE AND SOLVATION
             CH 3     COOH        4.75                    CH3
       CH3CH2         COOH        4.87             CH3    C COOH             5.0
                                  4.8                     CH3
   CH 3CH2CH 2        COOH

  Notice that these are all similar            ….but this one has a larger pKa


  This is probably a solvation effect.
                                                                steric hindrance
  Solvation lowers the energy of the ion.                 H
                                                          O H     H O
Stronger                H O
                        H O                                              H     H
                                                                 O
Acid          H
              H
                      O
                      O
                            H
                            H    H
                                 H       The bulky               C   -       H O
              O H
              O H
                                         t -butyl group              O H
                      C -
                                H O
                                H O
                      C
                          O H
                          O H            is not as well              H O
                  O H
                  O H                    solvated.        O H
unbranched                H O
                          H O                                        Weaker
                  H
                  H
                                                          H          Acid          60
We will study the factors that lead STABILISATION
to lower energy (stabilisation) in   FACTORS
the conjugate base.                 1. Mesomeric effect
                              A
                                       2. Electronegativity
                                       3. Atom size
             stabilisation             4. Hybridisation
                               A
                                       5. Inductive effect
                                       6. Charge

                                       7. Solvation effect
                Stabilisation of the
HA              conjugate base         8. Steric effect
                makes the acid
                stronger.                                    61
          CONJUGATE ACIDS
    weak base = strong conjugate acid
 The conjugate acid of a weak base will be a strong acid.


   strong base = weak conjugate acid
The conjugate acid of a strong base will be a weak acid.

The strongest bases usually carry a negative charge.

Weaker bases are usually neutral molecules with an unshared pair of
electrons.

                                                                  62
COMPARATIVE TRENDS




                     63
              GENERALISATION
    MAJOR EFFECTS
          Electronegativity
          Size                                    These five can
          Hybridisation                           cause big changes
          Resonance                               in the pKa.
          Highly-(+)-charged atom


    MINOR EFFECT                                   Smaller changes,
                                                   unless several add
           Inductive Effects                       together.


The effects are purposely not listed in an exact order of importance
from top to bottom; it is not really possible to establish an exact order.   64
NITROPHENOLS




               65
                        2-Nitrophenol
H
    O       O             H2O    O      O       O   O
            N                           N           N
                O                           O           O

                                        H3O


                                  O     O       O   O
        O       O
                                        N           N
                N                           O           O
                    O




                                                        66
                2-Nitrophenol
H
    O   O
                      H2O       O   O
        N                                   H3O
            O                       N
                                        O




                                O   O
                                    N
                                        O


                                             67
                3-Nitrophenol
H
    O                             O
                    H2O                       H3O

            O                             O
        N                             N
        O                             O



O                     O               O


            O                 O                   O
        N                 N                   N
        O                 O                   O   68
 H                             O           O
     O
                     H2O



     N                         N           N
O        O                 O       O   O       O

4-Nitrophenol              H3O

                 O             O           O




                 N             N           N
             O       O     O       O   O       O   69
H
    O                   O
                                    O
            H2O



    N                   N
O       O           O       O       N
                                O       O
                     H3O



              4-Nitrophenol


                                        70
A
       Ortho             NITROPHENOLS
       Meta     Placing a nitro group on the benzene ring
Para            of a phenol increases its acidity.


               The effect is largest when the nitro group
               is placed in an ortho or a para position on
               the ring, and considerably smaller for the meta
               position.

               Multiple nitro groups at the ortho and para
               positions can increase the pKa of a phenol
               to the point that it becomes a very strong
               acid.
                                                             71
            pKa VALUES OF NITROPHENOLS
H           H                       H                 H
    O           O       O               O                 O
                        N
                            O
                                                  O
                                            N
pKa 10
            I   M       pKa 7.6
                                            O
                                                      O
                                                          N
                                                              O
    H
                                             I
                                                          M
        O       O                                     I
                N
                    O                   pKa 8.6
                                                      pKa 7.1

        N
    O       O               pKa 4
                                                                  72
Warfarin
Inhibits the clotting action of blood – through an action on vitamin K-derived
clotting factors.
Prescribed – elderly deep vein thrombosis or pulmonary embolism
Used in the UK as a sodium salt – acidic drug – the presence of an acidic
hydrogen may not be obvious




                                                                           73
SUBSTITUTED BENZOIC ACIDS




                            74
pKa VALUES OF SUBSTITUTED BENZOIC ACIDS

                                    Ortho    Meta     Para
                O         Me       4.7      4.46      4.3

R               OH        OMe      4.8      4.2       5.0
                          Cl       2.9      3.8       4.0
    R = H 4.2
                          NO2      2.2      3.5       3.0
 Electron-donating groups have only a small effect.


 Electron-withdrawing groups considerably increase
 the strength of the acid, especially when placed in the
 ortho position.                                             75
           2-Nitrobenzoic acid
HO   O                      O        O
         O       H2O                     O         H3O
         N                               N
           O                               O




                                 O       O
                                             O
                                             N
                                               O

                                                    76
HO   O                     O   O
                 H2O
                                           H3O
             O                         O
         N                         N
         O                         O

     3-Nitrobenzoic acid
                           O   O
 O   O


                                       O
             O                     N
         N
                                   O
         O                                  77
             3-Nitrobenzoic acid

HO       O                 O       O         O       O

                  H2O



     N                         N                 N
 O       O                 O       O         O       O




                                       H3O
                                                     78
SUMMARY




          79
ELECTRON-WITHDRAWING EFFECTS
STRENGTHEN ACIDS

    O                    O                               O
R
                                          O          R S O
    O                    O
                                                         O


Any effect that “bleeds” electron density away from
the negatively-charged end of the conjugate base will
stabilise (lower the energy) of the conjugate base and
therefore make the acid stronger.

                                                             80
Conversely …..

ELECTRON-DONATING EFFECTS
WEAKEN ACIDS
                                                          O
     O                   O
                                          O        R S O
R
     O                   O                                O


Any effect that “pushes” extra electron density toward
the negatively-charged end of the conjugate base will
destabilise (increase the energy) of the conjugate base
and make the acid weaker.
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Description: Learn about pKa here.