AQA A Level Chemistry复习笔记7.5.1 Preparation of Amines

Amines

• Amines can be thought of as derivates of ammonia, in which one or more of the hydrogens is replaced by an alkyl or aryl group
• The number of substituted hydrogens is the basis of classifying amines

Classification of amines

• Notice the classification is not the same as in alcohols and haloalkanes, where the designation primary, secondary and tertiary is based on the substituents on the carbon atom rather than the nitrogen atom
• If the R group is an alkyl group (methyl, ethyl, etc) then then it is an aliphatic amine; if it is an aryl group (benzene ring or phenyl) then it is an aromatic amine
• Aliphatic and aromatic amines share similar chemical reactions and the aryl group can strongly influence the chemistry and reactivity of the amine group

Naming Amines

• Amines can be named using common names or IUPAC systematic names
• The common way to name amines is to use the alkyl (or aryl) prefix followed by -amine
• The IUPAC systematic name uses the numbered prefix amino- followed by the alkane (or aromatic) stem

Nomenclature of aliphatic and aromatic amines table

Preparing Amines

• Primary amines can be prepared from different reactions including:
  • The reaction of halogenoalkanes with ammonia
  • The reduction of nitriles

   

Reaction of halogenoalkanes with ammonia

• This is a nucleophilic substitution reaction in which the nitrogen lone pair in ammonia acts as a nucleophile and replaces the halogen in the halogenoalkane
• When a halogenoalkane is reacted with excess, hot ethanolic ammonia under pressure a primary amine is formed

Formation of primary amine

Reduction of nitriles

• Nitriles contain a -CN functional group which can be reduced to an -NH2 group
• The nitrile vapour and hydrogen gas are passed over a nickel catalyst or LiAlH4 in dry ether can be used to form a primary amine

Nitriles can be reduced with LiAlH4 or H2 and Ni catalyst

• Phenylamine is an organic compound consisting of a benzene ring and an amine (NH2) functional group
• It can be produced in a three-step synthesis reaction followed by the separation of phenylamine from the reaction mixture
  • Step 1- Benzene undergoes nitration with concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4) at 25 to 60 oC to form nitrobenzene
  • Step 2 - Nitrobenzene is reduced with hot tin (Sn) and concentrated hydrochloric acid (HCl) under reflux to form an acidic mixture that contains the organic product C6H5N+H3
  • Step 3 - Sodium hydroxide (NaOH) is added to the acidic reaction mixture to form phenylamine
  • Step 4 - The phenylamine is separated from the reaction mixture by steam distillation

   

The overall reaction of formation of phenylamine from benzene

Multi-step synthesis of phenylamine from benzene

Dye Manufacture

• Aromatic amines are used to make a class of synthetic dyes called azo dyes
• Azo dyes contain an azo functional group, R-N=N-R, in which the R groups can be alkyl, aryl or a mixture of both

• The synthesis takes place in three steps, beginning with the creation of nitrous acid in situ
  • In situ literally means on site and is a term used to describe reagents that are made in the reaction vessel, usually because they are too unstable to exist outside of the reaction conditions

   

• The nitrous acid reacts with the phenylamine to create the intermediate benzenediazonium ion
• The third step is called the coupling reaction where the benzenediazonium ion reacts with another aromatic compound to produce an azo dye
• By carefully varying the aromatic compound it is possible to make a wide variety of azo dyes

Summary table showing the steps in azo dye production