AQA Chem4: 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

• 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

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+

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

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

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