6.2.1 Amines

What are amines?

Amines are organic compounds derived from ammonia, where one or more hydrogen has been replaced by a carbon chain or ring

What should be noted when drawing the structure of an amine?

The lone pair on the N should be shown

What is an aliphatic amine?

The nitrogen atom is attached to at least one straight or branched carbon chain

• Simplest one is methylamine (CH₃NH₃)

What is an aromatic amine?

The nitrogen atom is attached to an aromatic ring

• Simplest is phenylamine (C₆H₅NH₂)

How do we name primary amines?

• Adding the suffix -amine to the name of the alkyl chain

• Where a primary amine contains an amine group on any carbon except carbon-1, the amine is named by using amino- and a number to indicate its position

How do we name secondary or tertiary amines?

• Di- or tri- are used to indicate number of alkyl groups attached

What is an N-substituted derivative?

When two or more different groups are attached to a nitrogen atom, the compound is named as a N-substituted derivative of the larger group

Show the structure of a primary amine

Show the structure of a secondary amine

Show the structure of a tertiary amine

Name this molecule

Methylamine

Name this molecule

Dimethylamine

Name this molecule

Propylamine

Name this molecule

Trimethylamine

Name this molecule

2-aminobutane

Name this molecule:

CH₃NHCH₃CH₂CH₃

N-methylpropylamine

Name this molecule:
CH₃N(CH₂CH₃)CH₂CH₂CH₃

N-ethyl-N-methylpropylamine

What is the geometric shape and bond angle of an amine?

• Pyramidal shape due to extra repulsion of the lone pair

• 107° bond angle

What is the boiling point of amines like and why?

• Amines can form hydrogen bonds so have high boiling points

What is the solubility of amines like and why?

Amines are soluble if short-chained

• As the carbon chain length increases, the molecule becomes more non-polar

How do amines behave like bases?

The lone pair of electrons on the nitrogen atom can accept a proton to form a dative covalent bond between the electron lone pair and the proton

Show how ethylamine acts as a base to form an ion

Which factor affects the base strength of amines? Fill in the blanks:

The ____ of the ____ ____

• Alkyl groups have a _____ ______ effect so they push ______ toward the ____

• The ‘push’ increases with the _____ of the group, which increases the ______ ______ of the nitrogen, making it more ‘______’

• This increases the ability of the _____ _____ on the nitrogen to attract a _______

The size of the alkyl group

• Alkyl groups have a positive inductive effect so they push electrons toward the amine

• The ‘push’ increases with the size of the group, which increases the charge density of the nitrogen, making it more ‘negative’

• This increases the ability of the lone pair on the nitrogen to attract a proton

Why is phenylamine a weaker base than ammonia?

Resonance:

• The lone pair on the amine is drawn into the delocalised ring

• This reduces the ability for the lone pair to attract a proton

Show the reaction behind ammonia and HCl

NH₃ + HCl → NH₄⁺Cl^-

Give the reaction behind ethylamine and HCl

• ethylamine + HCl → ethylammonium chloride

• CH₃CH₂NH₂ + HCl → CH₃CH₂NH₃⁺Cl^-

Show the reaction behind ethylamine and sulfuric acid

• ethylamine + sulfuric acid → ethylammonium sulfate

• 2CH₃CH₂NH₂ + H₂SO₄ → （CH₃CH₂NH₃⁺)₂SO₄^2-

Give the reaction between phenylamine and nitric acid

• Phenylamine + nitric acid → phenylammonium nitrate

• C₆H₅NH₂ + HNO₃ →C₆H₅NH₃⁺NO₃^-

Explain in brief the preparation of aliphatic amines

Haloalkanes are warmed gently with excess ammonia with ethanol as a solvent

• Reaction performed in a sealed flask

Explain how primary amines are formed:

1. Ammonia has a _____ _____ ____ ______ on the ______ atom, which allows ammonia to act as a ______ in a _____ reaction with a ______

2. This produces an ______ __

3. _____ _____ is then added to generate the ____ from the salt

1. Ammonia has a lone pair of electrons on the nitrogen atom, which allows ammonia to act as a nucleophile in a substitution reaction with a haloalkane

2. This produces an ammonium salt

3. Aqueous alkali is then added to generate the amine from the salt

Give the equations for the formation of propylamine from 1-chloropropane

1. Salt formation

   • CH₃CH₂CH₂Cl + NH₃ → CH₃CH₂CH₂NH₃⁺Cl^-

   • 1-chloropropane + ammonia → propylammonium chloride salt

2. Amine formation

   • CH₃CH₂CH₂NH₃⁺Cl^- + NaOH → CH₃CH₂CH₂NH₂ + NaCl + H₂O

   • Propylammonium chlordide + sodium hydroxide → propylamine + sodium chloride + water

Give the essential reactions for the formation of aliphatic amines

Ethanol as a solvent

• Prevents any substitution of the haloalkane by water to produce alcohols

Excess ammonia

• Reduces further substitution of the amine group to form secondary and tertiary amines

Why is the reaction for formation of primary amines different from secondary and tertiary amines?

The product from the formation of primary amines still contains a lone pair of electrons on the nitrogen atom that can react further with a haloalkane to form a secondary amine, and then a tertiary amine

Show how dipropylamine is prepared from propylamine

Making an ammonium salt

• CH₃CH₂CH₂Cl + CH₃CH₂CH₂NH₂ → (CH₃CH₂CH₂)₂⁺Cl^-

• 1-chloropropane + propylamine → dipropylammonium chloride salt

React with alkali to obtain product

• (CH₃CH₂CH₂)₂NH₂⁺Cl^- + NaOH → (CH₃CH₂CH₂)₂NH + NaCl + H₂O

• DIpropylammonium chloride + sodium hydroxide → dipropylamine + sodium chloride + water

Give the conditions needed for preparing aromatic amines

• Tin catalyst

• Reflux (reduction)

• Concentrated HCl

How is phenylamine (C₆H₅NH₂) made?

1. Reduction of nitrobenzene

   • Nitrobenzene is heated under reflux with tin and hydrochloric acid to form the ammonium salt, phenylammonium chloride)

   • Tin and HCl act as a reducing agent

2. Reaction with excess sodium hydroxide

   • Reaction with sodium hydroxide forms the aromatic amine, phenylamine

Show the mechanism for the formation of a primary amine (e.g. ethylamine)