AQA Chem4: Amines

Revision notes on amines from the organic section of AQA Chem4

HideShow resource information
  • Created by: anna
  • Created on: 05-06-13 16:37

Amines

Their functional group is NH2 and their ending is -amine.

 

Classification of amines:

  • it's similar to classifying alcohols and haloalkanes
  • primary amines are bonded to one R group e.g. R-NH2
  • secondary amines are bonded to two R groups e.g. R-NHR
  • tertiary amines are bonded to three R groups

Characteristics:

  • amines are polar with the N being negative
  • the boiling points of amines are lower than their comparable alcohols as they have weaker hydrogen bonds as the O in alcohols have a bigger difference in electronegativity
  • amines can act as bases
1 of 6

Basicity of Amines

  • amines act as both Bronsted-Lowry bases (proton acceptors) and Lewis bases (electron pair donor)
  • the basicity of an amine is related to the availability of the lone pair of electrons on the N atom
  • Kb is the measure of basicity of an amine and can be quoted as pKb
  • a strong base has a large Kb value and a small pKb value
  • the inductive effect - alkyl groups push electrons in the C-N bond towards the N increasing the electron density on the N atom this means the electrons are more available to donate so the base is stronger
  • this means methylamine is stronger than ammonia and dimethylamine is stronger than methylamine
  • phenylamine is a weak base due to the ring of stabilising delocalised electrons and because the lone pair is not fully available to donate
2 of 6

More characteristics

Solubility of Amines:

  • primary and secondary amines are soluble in water and form hydrogen bonds with it
  • tertiary amines are insoluble as there are no H atoms bonded to the N so no hydrogen bonds can be formed with the water
  • the higher the Mr of the amine the lower the solubility

Amines with Acids:

  • amines dissolve readily in acids making an alkyl ammonium salt
  • R-NH2 + H+ --> R-NH3+
3 of 6

Preparation of Amines

1) A haloalkane with ammonia:

  • R-Hal + 2NH3 --> R-NH2 + NH4Hal
  • this method is rarely used as a mixture of primary, secondary and tertiary amines and quaternary ammonium salts is formed
  • have to use an excess of ammonia

2) Reduction of Nitriles:

  • R-CN + 2H2 --> R-CH2NH2
  • catalytic hydrogenation using either H2 with Ni catalyst or NaBH4

3) Reduction of a nitro group (forms aromatic amines):

  • e.g. C6H5NO2 + 3H2 --> C6H5NH2 + 2H2O
  • in industry it's carried out in the presence of an Ni catalyst
  • in the lab the H2 is obtained from the reaction HCl with Sn or Fe
4 of 6

Reactions of Amines

Reaction with haloalkanes:

  • the mechanism is nucleophilic substitution
  • R-NH2 + R1-Hal --> RNHR1 + H-Hal
  • in excess of the the haloalkane further substitution takes place forming a tertiary amine from the secondary amine
  • if yet further substitution happens a quaternary ammonium salt is formed
  • no further substitution happens after this as the quaternary ammonium salt is unable to act as a nucleophile as there is no lone pair of electrons on the N

Reaction with acyl chlorides:

  • amines react with acyl chlorides to form substituted amides
  • the mechanism is nucleophilic addition-elimination
  • R-NH2 + R-COCl --> R-CONHR + HCl
5 of 6

Uses of Amines and Quaternary Ammonium Salts

There are many uses of amines and quaternary ammonium salts:

  • amines can be used to make polymers such as polyurathane (lycra) and polyamides (used in carpets)
  • aromatic amines are used in the production of synthetic dyes
  • quaternary ammonium salts are used as cationic surfactants which are used in things such as fabric and hair conditioners
6 of 6

Comments

No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all Amines and azo dyes resources »