Chemistry - 3.3.11: Amines

Amines

Derivatives of ammonia in which one or more of the hydrogen atoms in the ammonia molecule have been replaced by alkyl or aryl groups

What are amines useful for and why?

As intermediates in synthesis - they are very reactive compounds

What do the terms primary, secondary and tertiary refer to for amines?

The number of substituents (R groups) on the nitrogen atom

Primary amines general formula

RNH₂

Secondary amines general formula

RR’NH

Tertiary amines general formula

RR’R”N

How do you name amines?

Use “-amine” as the suffix + name substituents in alphabetical order

What shape do amines have around the nitrogen atom?

Trigonal pyramidal - bond angle of 107 degrees

Describe the boiling points of amines

High boiling points due to hydrogen bonds (but lower than alcohols as N less electronegative than O)

Describe the solubilities of amines (3)

1. Primary amines up to 4C in length are very soluble in water and alcohols due to hydrogen bonds

2. Most soluble in less polar solvents

3. Phenylamine not very soluble in water due to the benzene ring as cannot form hydrogen bonds

Are amines reactive?

Yes as they have a lone pair of electrons, so can be used to form bonds with H⁺ ions or electron-deficient carbon atoms

What can amines act as?

Nucleophiles or weak bases

Why will phenylamine dissolve in excess HCl despite being relatively insoluble?

It forms a soluble ionic salt phenylammonium chloride

What happens when amines react with acids?

They gain a H⁺ from the acid to form an ammonium salt

What happens when phenylammonium chloride reacts with strong bases?

The strong base removes the proton from the salt, regenerating phenylamine

Inductive effect

The electron-releasing effect of alkyl groups

Why are amines with more alkyl groups bonded to the N atom stronger bases? (2)

1. Inductive effect: the alkyl groups release electrons away from the alkyl group and towards the nitrogen atom

2. This increases the electron density on the nitrogen atom, making it a better electron pair donor

How is the inductive effect represented by?

An arrow

Why are amines with aryl groups bonded directly to the N atom weaker bases? (2)

1. The electrons are delocalised into the benzene ring (as they overlap)

2. So the lone pair of electrons on N are less available

Order ammonia, primary aliphatic and primary aromatic amines based base strength (ascending order)

Primary aromatic amines < ammonia < primary aliphatic amines

Why can amines act as nucleophiles?

The lone pair of electrons can attack positively charged carbon atoms

How might primary amines be prepared? (2)

By the nucleophilic substitution of halogenoalkanes by ammonia or by the reduction of nitriles

What happens in the nucleophilic substitution of halogenoalkanes by ammonia and why?

1. First, the halide ion is substituted by ammonia - primary amine

2. The primary amine is a nucleophile so also substitutes the halide ion in another halogenoalkane, forming a secondary amine

3. The secondary amine takes part in nucleophilic substitution, forming a tertiary amine

4. The tertiary amine takes part in nucleophilic substitution, forming a quaternary ammonium salt

Why is it not efficient to prepare amines by the nucleophilic substitution of halogenoalkanes by ammonia?

It produces a mixture of primary, secondary and tertiary amines as well as a quaternary ammonium salt, so the products would need to be separated by fractional distillation

What can you do when preparing amines by the nucleophilic substitution of halogenoalkanes by ammonia to improve the yield of primary amines?

Use an excess of ammonia

How are amines prepared by the reduction of nitriles? (2)

1. Nucleophilic substitution of a halogenoalkane by CN^-, forming a nitrile

2. Nitrile is reduced to primary amines with a nickel / hydrogen catalyst

What are aromatic amines used in?

The manufacture of dyes

How are aromatic amines prepared? (2)

1. Nitration: benzene reacts with nitric acid and sulfuric acid to produce nitrobenzene

2. Nitrobenzene is reduced to phenylamine by tin and HCl

Write out the equation for the reduction of nitrobenzene

Nitrobenzene+ 6[H] → Phenylamine + 2H₂O

What catalyst is needed for the reduction of nitrobenzene?

Tin and HCl

When preparing phenylamine by the reduction of nitrobenzene, why might NaOH be added?

The reaction is carried out in HCl so phenylammonium chloride is formed - NaOH is added to liberate the free amine

How do amides form from amines?

In nucleophilic addition-elimination reactions with acyl chlorides and acid anhydrides

What forms in nucleophilic addition-elimination reactions between amines and acyl chlorides / acid anhydrides?

N-substituted amides

What are amines used in?

The manufacture of synthetic materials like nylon, dyes and drugs

What are quaternary ammonium compounds used as?

Cationic surfactants

What happens when amines react with water? Give the equation and the colour change.

RNH₂ + H₂O ⇌ RNH₃⁺ + OH^-

Colour change from green to blue