13.8 Amines and amides

What are amines and how are they named?

  • Amines are the organic versions of ammonia
  • An alkyl group takes the places of one, two or three of the hydrogens
  • They are called primary, secondary and tertiary amines, respectively

 

  • Smaller (lower) primary amines are called things like methylamine (CH3–NH2) and ethylamine (CH3–CH2–NH2).

 

  • Simple secondary and tertiary amines are also easy to name.  Dimethylamine is CH3–NH–CH3 and trimethylamine is CH3–N(CH3)–CH3

 

  • Bigger amines have names beginning with amino.  For example,

CH3–CH(NH) –CH2–CH2–CH3  is called 2-aminopentane.

 

  • Amines with low RMM are gases or volatile liquids
  • Volatile amines have strong smells like ammonia
  • Ethylamine and trimethylamine smell like decaying fish
  • 1,4-diaminobutane and 1,5-diaminopentane are called putrescine and cadaverine because they are given off when flesh rots

 

Properties of amines

  • Amines are similar to ammonia but the alkyl groups modify their properties
  • Most of their behaviour can be explained by the lone pair of electrons on the N

 

The lone pair of electrons explains why amines and ammonia are:

·        very soluble in water

·        a base

·        a Ligand

·        a nucleophile

 

Solubility of amines

  • Amines form hydrogen bonds with water
  • Small amines are soluble in water
  • Larger amines are less soluble because their long alkyl groups disrupt the hydrogen bonding in water

 

Amines as bases

  • The lone pair on the N can take part in dative covalent bonding
  • An amine can donate a pair of electrons to an H+
  • By generously donating two electrons, the amine is an H+ acceptor and acts as a base
  • The amine in question becomes an alkylammonium ion, with one extra hydrogen than normal and a +1 positive charge
  • If the amine has nicked a proton from water, an OH- is left over, causing the solution to be alkaline
  • Solutions of amines are alkaline
  • Amines react with acids (usually the oxonium ion H3O+) to form alkylammonium ions.  Since the lone pair is playing the dative covalent bonding game, it can’t interact with everything else so much.  Therefore, alkylammonium ions lose their smell
  • The result is, adding acids to amines takes away their smell

 

Amines as ligands

  • The lone pair on the N can take part in dative covalent bonding
  • An amine can donate a pair of electrons to an H+

Problems for 13.8

1 Name the amines with the following structures.

a         CH3–CH2–NH2                                           Ethylamine

b        CH3–N(CH3)–H                           Dimethylamine (there are 2 isomers)

c         CH3–CH(CH3)–NH2                                          2-methylethylamine x 2-aminopropane

d        CH3–N(CH3)–CH2–CH3                               Ethyldimethylamine

e         cyclohexane–NH2                                           Cyclohexylamine

 

I never got round to doing the rest sorry!

This is not acceptable young man.  See me.