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An Introduction to Organic Reactions and Their Mechanism

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Title: An Introduction to Organic Reactions and Their Mechanism


1
Chapter 3
  • An Introduction to Organic Reactions and Their
    Mechanism
  • Acids and Bases

2
3.4 Acid Strength
  • Strong acids completely ionized or completely
    dissociated
  • Forward reaction is predominated
  • Most of HA is dissociated
  • Conjugated base is weak and has low attraction
    for proton
  • HA(aq) H2O(l) ? H3O(aq) A-(aq)

3
Acids Strength
  • Weak acids partially ionized or dissolved
  • Reverse reactions is predominated
  • Most of HA is undissociate
  • Conjugated base is strong and has high attraction
    for proton
  • HC2H3O4(aq) H2O(l) H3O(aq)
    C2H3O4-(aq)

4
Acid Strength
5
3.5 The Strength of Acids and Bases Ka and pKa
  • The acidity constant, Ka
  • a quantitative measure of the strength of an acid
    in solution.
  • It is known as the equilibrium constant for a
    chemical reaction known as dissociation in the
    context of acid-base reactions
  • Larger Ka, stronger the acid

6
Acidity and pKa
  • Logarithm scale of Ka with the base of 10
  • pKa - log Ka
  • The larger pKa, the smaller extent of
    dissociation
  • Weaker acids

7
Relative Strength of selected acids and their
conjugated bases
8
Examples
  • An acid (HA) has Ka 10-7, what is its pKa?
  • Another acid (HB) has Ka 5, what is its pKa?
  • Which is the stronger acid?

9
Predicting the Strength of Bases
  • The strength of acids will decide the how weak or
    strong its conjugated bases
  • Stronger the acids, the weaker will be its
    conjugated base
  • After losing a proton, acid will become a
    conjugated base
  • When a base accepts a proton, the resulting
    chemical is called the conjugate acid of that
    original base

10
Acid and its conjugated base
11
Examples
  • The pKa of anilinium ion (C6H5NH3) is equal to
    4.6. On the basis of this fact, decide whether
    aniline (C6H5NH2) is a stronger or weak base than
    methylamine

12
3.6 Predicting the Outcome of Acid-Base Reactions
  • Acid-base reactions always favor the formation of
    the weaker acid and weaker base
  • Equilibrium favor the formation of the most
    stable (lowest potential energy) species
  • Stronger acid stronger base ? weaker base
    weaker acid

13
Example
  • Consider the mixing of an aqueous solution of
    acetic acid, CH3CO2H (see Table 3.1), and NaOH.
    What acid base reaction, if any would take place
  • H2SO4(aq) NH3(aq) ? NH4(aq) HSO4-(aq)

14
3.6 Water Solubility as the Result of Salt
Formation
  • Recall Longer chain of carbon (4 or more) will
    be less soluble
  • Higher molecular weight of carboxylic compounds
    do not dissolve in water
  • Instead, they dissolved in a strong base to form
    a water-soluble salt

15
Water Solubility
  • We can predict that an amine will react with
    aqueous hydrochloric acid

16
Water Solubility
  • Methylamine and most amines of low molecular
    weight are very soluble in water
  • Formed a water-soluble salt from hydrochloric acid

17
3.7 Relationships between structure and Acidity
  • The strength of a Bronsted-Lowry acid depends on
    the extend to which a proton can be separated
    from it and transferred to a base.
  • Removing H ? breaking a bond
  • Making the conjugated base more electronegative
  • Accepting H ? forming a bond
  • Making the conjugated acid less electronegative

18
3.7 Relationships between structure and Acidity
  • Bond strength to proton decreases as we move down
    to column
  • Decreasing effectiveness of orbital overlap
    between the hydrogen 1s orbital and the orbitals
    of successively larger element in the column
  • Less effective the orbital overlap, the weaker
    bond, stronger acid
  • Acidity increases from left to right

19
3.7 Relationships between structure and Acidity
20
3.7 Relationships between structure and Acidity
21
3.7 The Effect of Hybridization
22
3.7 The Effect of Hybridization
  • Electrons of 2s orbtials have lower energy than
    those of 2p orbitals because these electrons are
    much closer to the nucleus
  • Having more s orbitals means that electrons of
    the anion will be lower in energy and more stable

23
3.7 The Effect of Hybridization
  • sp C sH hybridization
  • Contain 50 s character
  • 1s 1p
  • More electronegative spC

24
3.7 The Effect of Hybridization
  • sp2C sH hybridization
  • Contains 33.3 s character
  • 1s 2p
  • Less electronegative than spC

25
3.7 The Effect of Hybridization
  • sp3C-sH hybridization
  • Contains 25 s character
  • 1s 3p
  • Least electronegative C

26
3.7 The Effect of Hybridization
27
3.7 The Effect of Hybridization
  • Relative basicity of the carbanions
  • Ethynide ion is the weakest base
  • The more electronegative carbon, the more stable
    the anion
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