Organic Chemistry

1
Organic Chemistry
William H. Brown Christopher S. Foote
2
Acids and Bases
Chapter 4
3
Brønsted-Lowry Definitions

• Acid a proton donor
• Base a proton acceptor

4
Conjugate Acids bases

• Conjugate base the species formed from an acid
  when it donates a proton to a base
• Conjugate acid the species formed from a base
  when it accepts a proton from an acid

5
Conjugate Acids Bases

• Which is the favored site of protonation?

6
Conjugate Acids Bases

• Which is the favored site of protonation?

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Strong Acids and Bases

• Strong acid completely ionized in aqueous
  solution. Examples are
• HCl, HBr, HI, HNO3, HClO4, and H2SO4
• Strong base completely ionized in aqueous
  solution. Examples are
• LiOH, NaOH, KOH, Ca(OH)2, and Ba(OH)2

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Acids Base Strengths

• Acetic acid is a weak acid
• it is incompletely ionized in aqueous solution

9
Weak Acids and Bases

• The equation for the ionization of a weak acid,
  HA, in water and the acid ionization constant,
  Ka, for this equilibrium are

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Weak Acids and Bases
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Weak Acids and Bases

• Equilibrium favors reaction of the stronger acid
  and stronger base to give the weaker acid and
  weaker base

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Structure and Acidity

• A. Electronegativity of the atom bearing the
  negative charge
• within a period, the greater the
  electronegativity of the atom bearing the
  negative charge, the more strongly its electrons
  are held, the more stable the anion A-, and the
  greater the acidity of the acid HA

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Structure and Acidity

• B. Size of the atom bearing the negative charge
• within a column of the Periodic Table, acidity is
  related to the size of the the atom bearing the
  negative charge
• atomic size increases from top to bottom of a
  column
• the larger the atom bearing the negative charge,
  the greater its stability, and the greater the
  acidity of HA

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Structure and Acidity

• C. Resonance Delocalization of Charge in A-
• for compounds with the same functional group, the
  more stable the anion A-, the stronger the acid
  HA
• compare an alcohol and a carboxylic acid
• ionization of the O-H bond of an alcohol gives an
  anion for which there is no resonance
  stabilization

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Structure and Acidity

• ionization of a carboxylic acid gives a
  resonance-stabilized anion
• a carboxylic acid is a stronger acid than an
  alcohol

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Structure and Acidity

• D. Electron-withdrawing inductive effect
• the polarization of electron density of a
  covalent bond due to the electronegativity of an
  adjacent covalent bond

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Structure and Acidity

• stabilization of an anion by the inductive effect
  falls off rapidly with increasing distance of the
  electronegative atom from the site of the
  negative charge

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Structure and Acidity

• E. Hybridization
• for anions differing only in the hybridization of
  the charged atom, the greater the of s
  character to the hybrid orbital of the charged
  atom, the more stable the anion, and the greater
  the acidity of HA

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Lewis Acids and Bases

• Lewis acid any molecule of ion that can form a
  new covalent bond by accepting a pair of
  elections
• Lewis base any molecule of ion that can form a
  new covalent bond by donating a pair of elections

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Lewis Acids and Bases
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Prob 4.8

• For each conjugate acid-base pair, identify
  the first species as an acid or base and the
  second as its conjugate base or acid.

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Prob 4.9

• Complete a net ionic equation for each
  proton-transfer reaction. Label the original acid
  and its conjugate base.

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Prob 4.10

• Complete a net ionic equation for each
  proton-transfer reaction. Label the original acid
  and its conjugate base.

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Prob 4.11

• Write a structural formula for the conjugate
  acid formed by treating each molecule or ion with
  HCl.

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Prob 4.13

• Account for the greater stability of the
  anion derived from acetone compared with the
  anion derived from ethane.

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Prob 4.16

• Arrange the compounds in each set in order of
  increasing acid strength.

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Prob 4.17

• Arrange the compounds in each set in order of
  increasing base strength.

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Prob 4.20

• Write an equation for the reaction of acetic
  acid, pKa 4.76, with each base. Which equilibria
  lie toward the left? Which lie toward the right?

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Prob 4.21

• Write an equation for the reaction of
  ethanol, CH3CH2OH, pKa 15.9 with each base. Which
  equilibria lie toward the left? Which lie toward
  the right?

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Prob 4.22

• Benzoic acid, C6H5COOH, pKa 4.19, is only
  slightly soluble in water but its salt,
  C6H5COO-Na, is quite soluble in water. In which
  solutions will benzoic acid dissolve more readily
  than in water?

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Prob 4.23

• 4-Methylphenol, CH3C6H4OH, pKa 10.26, is only
  slightly soluble in water, but its sodium salt,
  CH3C6H4O-Na , is quite soluble in water. In
  which solutions will 4-methylphenol dissolve?

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Prob 4.24

• For an acid-base reaction, one way to
  determine the predominant species at equilibrium
  is to say that the reaction arrows points to the
  acid with the higher value of pKa. Explain why
  this rule works.

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Prob 4.25

• Will acetylene react with sodium hydride
  according to the following equation to form a
  salt and hydrogen?

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Prob 4.26

• Using the values of pKa given in Table 4.1,
  calculate the equilibrium constant, Keq, for this
  acid-base reaction.

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Prob 4.28

• Complete the equation for each Lewis
  acid-base reaction.

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Prob 4.29

• For each reaction, label the Lewis acid and
  the Lewis base, and use curved arrows to show the
  flow of electrons in each reaction.

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Prob 4.30

• 2,4-Pentanedione is a considerably stronger
  acid than acetone. Write a structural formula for
  each conjugate base, and account for the greater
  stability of the conjugate base from
  2,4-pentanedione.

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Prob 4.32

• In which equation does the sec-butyl cation
  react as a Lewis acid? In which equation does it
  react as a Brønsted-Lowry acid? Write Lewis
  structures for the reactants and products, and
  show by the use of curved arrows how each
  reaction occurs.

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Prob 4.34

• Write equations for the reaction of each
  compound with H2SO4, a strong protic acid.

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Prob 4.36

• Write a structural formula for the conjugate
  base formed when each compound is treated with
  one mole of a base stronger than the compounds
  conjugate base.

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Prob 4.39

• For each pair of molecules or ions, select
  the stronger base and write its Lewis structure.

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Prob 4.43

• When imidazole is dissolved in water, proton
  transfer to it gives a cation. Is the cation
  better represented by A or B?

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Acids and Bases

• End Chapter 4