19'3 Physical Properties

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19.3 Physical Properties
?Boiling point

• Amines have higher boiling points than alkanes
  and haloalkanes but generally lower than those of
  alcohols of comparable molecular weight.

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• because the H bond formed between amine
  molecules are weaker than those H bond formed
  between alcohols as N atom is less
  electronegative compared to O atom.

• Therefore, the boiling point of

carboxylic acid gt alcohol gt amine gt carbonyl
compounds/haloalkane gt alkane
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• Table below shows boiling point of amines,
  haloalkane, alkane, alcohol and carboxylic acid
  of comparable molecular weight.

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• Molecules of 1o and 2o amines can form H bonds
  between molecules whereas molecules of 3o amines
  cannot form H bonds to each other.
• As a result, 3o amines generally boil at lower
  temperature than 1o and 2o amines of comparable
  molecular weight.

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• For isomeric amines, 1o amine has higher boiling
  point compared to the 2o amine and 3o amine.
• This is due to the fact that 1o amines can form
  more H bonds between molecules whereas 3o amines
  cannot form H bonds with each other.

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Primary amines (1º)
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Secondary amines (2º)
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Tertiary amines (3º)
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?Solubility

• Like alcohols, all amines including tertiary
  amines, are capable of forming hydrogen bonds
  with water molecules.
• Thus simple amines (amines of fewer than 5
  carbons) are generally water soluble and dissolve
  to form basic aqueous solutions.

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Comparison the solubility between 1,2 3amines
3 hydrogen bonds per a 1 amine molecule
2 hydrogen bonds per a 2 amine molecule
1 hydrogen bond per a 3 amine molecule
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• The solubility

3 lt 2 lt 1
Increasing solubility
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Basicity of Amines

• An amine is a nucleophile (a Lewis base) because
  its lone pair of non-bonding electrons on nitrogen

Ammonium ion
Ammonia
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• Most primary aliphatic amines are stronger bases
  than ammonia.

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Basicity (i) Inductive effect

• An alkyl group is electron donating
• group, and it stabilises the
• alkylammonium ion by dispersing its
• positive charge.

alkylammonium ion
1 Amine
Stabilised by the alkyl group
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• The basicity of amines increases with
  increasing alkyl substitution.

• (CH3)3N gt (CH3)2NH gt CH3NH2 gt NH3

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1 Amine
alkylammonium ion
alkylammonium ion
2 Amine
Strength as a base Methyl, 1, 2
Increasing
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Basicity of Arylamines
Basicity (ii) Resonance effect

• Aromatic amines (e.g., aniline) are weaker bases
  than the corresponding aliphatic and cyclic amines

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Example
Cyclohexylamine pKb 3.36
Aniline pKb 9.42
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• Aromatic amines are less basic.
• The lone pair electrons of nitrogen atom are
  delocalized and overlapped with the aromatic ring
  p electron system and less available for bonding
  to H.
• In resonance terms, arylamines are stabilized
  relative to alkylamines because of the four
  resonance structures.

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Arylamines are stabilised due to the 4 resonance
structures
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Basicity Of Substituted Arylamines

• Substituted arylamines can be either more basic
  or less basic than aniline, depending on the
  substituent.
• Electron-donating substituent which increase the
  reactivity of an aromatic ring toward
  electrophilic substitution, also increase the
  basicity of the corresponding arylamine.

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• Electron-withdrawing substituent which decrease
  ring reactivity toward electrophilic
  substitution, also decrease ring reactivity.
• Table below shows base strength of
• some p-substituted anilines.

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Y- -NH2 H2O ? Y- -NH3 -OH


Stronger base
EDG
EWG
Weaker base