22.4 Measures of Amine Basicity

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22.4Measures of Amine Basicity
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Measures of Basicity

• The basicity of amines may be measured by
• 1) Kb
• 2) pKb
• 3) Ka of conjugate acid
• 4) pKa of conjugate acid

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Basicity Constant (Kb) and pKb

• Kb is the equilibrium constant for the reaction

R3NHHO
Kb
R3N
and
pKb
- log Kb
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Ka and pKa of Conjugate Acid

• Ka is the equilibrium constant for the
  dissociation of the conjugate acid of the amine

R3NH
Ka
R3NH
and
pKa
- log Ka
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Relationships between acidity and basicity
constants
Ka Kb 10-14
pKa pKb 14
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22.5Basicity of Amines
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Effect of Structure on Basicity

• 1. Alkylamines are slightly stronger bases than
  ammonia.

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Table 22.1 (page 866)Basicity of Amines in
Aqueous Solution

• Amine Conj. Acid pKa
• NH3 NH4 9.3
• CH3CH2NH2 CH3CH2NH3 10.8

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Table 22.1 (page 866)Basicity of Amines in
Aqueous Solution

• Amine Conj. Acid pKa
• NH3 NH4 9.3
• CH3CH2NH2 CH3CH2NH3 10.8

CH3CH2NH3 is a weaker acid than NH4therefore,
CH3CH2NH2 is a stronger base than NH3.
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Effect of Structure on Basicity

• 1. Alkylamines are slightly stronger bases than
  ammonia.
• 2. Alkylamines differ very little in basicity.

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Table 22.1 (page 866)Basicity of Amines in
Aqueous Solution

• Amine Conj. Acid pKa
• NH3 NH4 9.3
• CH3CH2NH2 CH3CH2NH3 10.8
• (CH3CH2)2NH (CH3CH2)2NH2 11.1
• (CH3CH2)3N (CH3CH2)3NH 10.8

Notice that the difference separating a
primary,secondary, and tertiary amine is only
0.3 pK units.
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Effect of Structure on Basicity

• 1. Alkylamines are slightly stronger bases than
  ammonia.
• 2. Alkylamines differ very little in basicity.
• 3. Arylamines are much weaker bases
  than ammonia.

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Table 22.1 (page 866)Basicity of Amines in
Aqueous Solution

• Amine Conj. Acid pKa
• NH3 NH4 9.3
• CH3CH2NH2 CH3CH2NH3 10.8
• (CH3CH2)2NH (CH3CH2)2NH2 11.1
• (CH3CH2)3N (CH3CH2)3NH 10.8
• C6H5NH2 C6H5NH3 4.6

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Decreased basicity of arylamines



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Decreased basicity of arylamines

• Aniline (reactant) is stabilized by conjugation
  of nitrogen lone pair with ring p system.
• This stabilization is lost on protonation.

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Decreased basicity of arylamines

• Increasing delocalization makes diphenylamine a
  weaker base than aniline, and triphenylamine a
  weaker base than diphenylamine.

C6H5NH2
(C6H5)2NH
(C6H5)3N
3.8 x 10-10
6 x 10-14
10-19
Kb
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Effect of Substituents on Basicity of Arylamines

• 1. Alkyl groups on the ring increase basicity,
  but only slightly (less than 1 pK unit).

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Basicity of Arylamines

• X pKb pKa
• H 9.4 4.6
• CH3 8.7 5.3

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Effect of Substituents on Basicity of Arylamines

• 1. Alkyl groups on the ring increase basicity,
  but only slightly (less than 1 pK unit).
• 2. Electron withdrawing groups, especially
  ortho and/or para to amine group, decrease
  basicity and can have a large effect.

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Basicity of Arylamines

• X pKb pKa
• H 9.4 4.6
• CH3 8.7 5.3
• CF3 11.5 2.5
• O2N 13.0 1.0

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p-Nitroaniline
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p-Nitroaniline

• Lone pair on amine nitrogen is conjugated with
  p-nitro groupmore delocalized than in aniline
  itself. Delocalization lost on protonation.

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Effect is Cumulative

• Aniline is 3800 times more basic
  thanp-nitroaniline.
• Aniline is 1,000,000,000 times more basic than
  2,4-dinitroaniline.

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Heterocyclic Amines
is more basic than
piperidine
pyridine
Kb 1.6 x 10-3
Kb 1.4 x 10-9
(an alkylamine)
(resembles anarylamine inbasicity)
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Heterocyclic Amines
is more basic than
imidazole
pyridine
Kb 1 x 10-7
Kb 1.4 x 10-9
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Imidazole

• Which nitrogen is protonated in imidazole?

H
H

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Imidazole

• Which nitrogen is protonated in imidazole?

H
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Imidazole

• Protonation in the direction shown gives a
  stabilized ion.

H


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22.6Tetraalkylammonium Saltsas Phase-Transfer
Catalysts
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Phase-Transfer Catalysis

• Phase-transfer agents promote the solubility
  ofionic substances in nonpolar solvents.
  Theytransfer the ionic substance from an
  aqueousphase to a non-aqueous one.
• Phase-transfer agents increase the rates
  ofreactions involving anions. The anion is
  relativelyunsolvated and very reactive in
  nonpolar mediacompared to water or alcohols.

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Phase-Transfer Catalysis
Quaternary ammonium salts are phase-transfercatal
ysts. They are soluble in nonpolar solvents.

Cl
Methyltrioctylammonium chloride
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Phase-Transfer Catalysis
Quaternary ammonium salts are phase-transfercatal
ysts. They are soluble in nonpolar solvents.
Cl
Benzyltriethylammonium chloride
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Example
The SN2 reaction of sodium cyanide with
butylbromide occurs much faster when
benzyl-triethylammonium chloride is present than
whenit is not.

CH3CH2CH2CH2Br
NaCN
benzyltriethylammonium chloride

CH3CH2CH2CH2CN
NaBr
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Mechanism

CN
Cl
(aqueous)
(aqueous)
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Mechanism

CN
Cl
(aqueous)
(aqueous)

Cl
CN
(aqueous)
(aqueous)
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Mechanism
CN
(aqueous)
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Mechanism
CN
(in butyl bromide)
CN
(aqueous)
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Mechanism

CH3CH2CH2CH2Br
CN
(in butyl bromide)
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Mechanism

CH3CH2CH2CH2Br
CN
(in butyl bromide)

Br
CH3CH2CH2CH2CN
(in butyl bromide)