Chapter 22 Amines

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Chapter 22Amines
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22.1Amine Nomenclature
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Classification of Amines

• Alkylamine
• N attached to alkyl group
• Arylamine
• N attached to aryl group
• Primary, secondary, or tertiary
• determined by number of carbon atoms directly
  attached to nitrogen

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Nomenclature of Primary Alkylamines (RNH2)

• Two IUPAC styles
• 1) analogous to alcohols replace -e ending by
  -amine
• 2) name alkyl group and attach -amine as a suffix

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Examples some primary alkylamines
(RNH2 one carbon directly attached to N)
ethylamine or ethanamine
CH3CH2NH2
cyclohexylamine orcyclohexanamine
1-methylbutylamine or2-pentanamine
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Nomenclature of Primary Arylamines (ArNH2)

• Name as derivatives of aniline.

p-fluoroaniline
5-bromo-2-ethylaniline
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Amino groups as substituents

• amino groups rank below OH groups and higher
  oxidation states of carbon
• in such cases name the amino group as a
  substituent

HOCH2CH2NH2
p-aminobenzaldehyde
2-aminoethanol
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Secondary and Tertiary Amines

• Name as N-substituted derivatives of parent
  primary amine.
• (N is a locant-it is not alphabetized, but is
  treated the same way as a numerical locant)
• Parent amine is one with longest carbonchain.

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Examples
CH3NHCH2CH3
N-methylethylamine
4-chloro-N-ethyl-3-nitroaniline
N,N-dimethylcycloheptylamine
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Ammonium Salts

• A nitrogen with four substituents is
  positivelycharged and is named as a derivative
  of ammonium ion (NH4).

CH3NH3
methylammoniumchloride
N-ethyl-N-methylcyclopentylammoniumtrifluoroaceta
te
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Ammonium Salts

• When all four atoms attached to N are carbon,the
  ion is called a quaternary ammonium ion andsalts
  that contain it are called quaternary ammonium
  salts.

benzyltrimethylammonium iodide
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22.2Structure and Bonding
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Alkylamines
147 pm
112
106
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Alkylamines
Most prominent feature is high electrostaticpoten
tial at nitrogen. Reactivity of nitrogen
lonepair dominates properties of amines.
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Geometry at N
Compare geometry at N of methylamine,
aniline,and formamide.
H
H
H
sp3
sp2
NH2
C
NH2
C
O
H

• Pyramidal geometry at sp3-hybridized N in
  methylamine.
• Planar geometry at sp2-hybridized N in formamide.

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Geometry at N
Compare geometry at N of methylamine,
aniline,and formamide.
sp3
sp2

• Pyramidal geometry at sp3-hybridized N in
  methylamine.
• Planar geometry at sp2-hybridized N in formamide.

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Geometry at N
Angle that the CN bond makes with bisector
ofHNH angle is a measure of geometry at N.
sp3
sp2
180
125

• Note this is not the same as the HNH bond
  angle

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Geometry at N
Angle that the CN bond makes with bisector
ofHNH angle is a measure of geometry at N.
sp3
sp2
180
125
142.5
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Geometry at N
Geometry at N in aniline is pyramidal closer
tomethylamine than to formamide.
142.5
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Geometry at N

• Hybridization of N in aniline lies between sp3
  and sp2.
• Lone pair of N can be delocalized into ring best
  if N is sp2 and lone pair is in a p orbital.
• Lone pair bound most strongly by N if pair is in
  an sp3 orbital of N, rather than p.
• Actual hybridization is a compromise that
  maximizesbinding of lone pair.

142.5
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Electrostatic Potential Maps of Aniline
Nonplanar geometry at N. Region of
highestnegative potential is at N.
Planar geometry at N. High negative potential
shared by N and ring.
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22.3Physical Properties
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Physical Properties

• Amines are more polar and have higher boiling
  points than alkanes but are less polar
  andhave lower boiling points than alcohols.

CH3CH2CH3
CH3CH2NH2
CH3CH2OH
dipolemoment (?)
0 D
1.2 D
1.7 D
boiling point
-42C
17C
78C
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Physical Properties
CH3CH2NHCH3
CH3CH2CH2NH2
(CH3)3N
boilingpoint
50C
34C
3C

• Boiling points of isomeric amines decrease
  ingoing from primary to secondary to tertiary
  amines.
• Primary amines have two hydrogens on N capable
  of being involved in intermolecular hydrogen
  bonding. Secondary amines have one. Tertiary
  amines cannot be involved in intermolecular
  hydrogen bonds.

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22.4Basicity 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 920)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 920)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 920)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


H2N

Strongeracid
When anilinium ion loses a proton, theresulting
lone pair is delocalized into the ring.
Weakeracid
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Decreased basicity of arylamines


H2N

Aniline is a weaker base because its lone pair is
more strongly held.
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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
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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).

X pKa of conjugate acid H 4.6 CH3 5.3
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Effect of Substituents on Basicity of Arylamines

• 2. Electron withdrawing groups, especially
  ortho and/or para to amine group, decrease
  basicity and can have a large effect.

X pKa of conjugate acid H 4.6 CF3 3.5O2N 1.0
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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
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Heterocyclic Amines
is more basic than
imidazole
pyridine
pKa of conjugate acid 7.0
pKa of conjugate acid 5.2
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Imidazole

• Which nitrogen is protonated in imidazole?

H
H

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Imidazole

• Protonation in the direction shown gives a
  stabilized ion.

H


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22.5Tetraalkylammonium 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
(aqueous)
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Mechanism

CH3CH2CH2CH2Br
CN
(in butyl bromide)