AQA A level Chemistry 3.3.11: Amines

What is a primary amine? (2)

- A primary amine has one carbon atom bonded to the nitrogen atom

- Represented as RNH₂

Provide an example of a primary amine. (1)

CH₃NH₂

What is a secondary amine? (2)

- A secondary amine has two carbon atoms bonded to the nitrogen atom

- Represented as R₂NH

Provide an example of a secondary amine. (1)

CH₃CH₂NHCH₃

What is a tertiary amine? (2)

- A tertiary amine has three carbon atoms bonded to the nitrogen atom

- Represented as R₃N

Provide an example of a tertiary amine. (1)

(CH₃)₃N

What is a quaternary ammonium salt? (2)

- A quaternary ammonium salt has four carbon atoms bonded to the nitrogen atom

- Represented as R₄N⁺

Provide an example of a quaternary ammonium salt. (1)

(CH₃)₄N⁺

What property allows primary, secondary, and tertiary amines to act as bases? (1)

The nitrogen atom has a lone pair, which can accept a proton, allowing them to act as bases

Why can tertiary amines act as nucleophiles? (1)

Tertiary amines can donate their lone pair of electrons to a δ+ carbon, acting as nucleophiles

Why do quaternary ammonium salts not react? (1)

Quaternary ammonium salts have no lone pair on the nitrogen atom, so they do not react

Why are short-chain amines soluble in water? (1)

Short-chain amines are soluble in water because they form hydrogen bonds with water molecules

What is the shape of an amine molecule and why? (2)

- Amines are pyramidal in shape

- Due to three bonding pairs and one lone pair of electrons

What is the shape of quaternary ammonium salts? (1)

Quaternary ammonium salts are tetrahedral in shape.

Why do amines generally have lower boiling points than alcohols with the same carbon chain length? (2)

- Amines have weaker hydrogen bonds compared to alcohols

- Resulting in lower boiling points

What are the two methods for preparation of aliphatic amines? (2)

1. Nucleophilic substitution of haloalkanes (please see and memorise all the flashcards from 3.1.3 on amine reactions)

2. Reduction of a nitrile

What is the primary disadvantage of method 1 for producing aliphatic amines? (2)

- A mixture of amine products is formed due to the product also acting as a nucleophile

- Resulting in a low yield of primary amine

Draw the basic structure of quaternary ammonium salts (3)

What are amines used for in the manufacture of synthetic materials? (1)

Amines are used in the manufacture of nylon, dyes, and drugs

What are quaternary ammonium salts used for? (1)

Used as cationic surfactants in fabric softening and hair products

How do quaternary ammonium salts function in hair conditioners? (2)

1. They attract to the negative charges on wet hair surfaces

2. Forming a coating that prevents static electricity and flyaway hair.

Why are quaternary ammonium salts effective as fabric softeners? (1)

They keep the fabric surface smooth by preventing the build-up of static electricity

What is the general equation for the reduction of a nitrile to an amine? (1)

RCN + 2H₂ → RCH₂NH₂

Which catalyst is used in the reduction of nitriles to amines? (1)

Nickel catalyst

What is the 2-step method for producing an amine from a haloalkane? (2)

1. RBr + KCN → RCN + KBr (nucleophilic substitution in aqueous ethanol)

2. RCN + 2H₂ → RCH₂NH₂ (catalytic hydrogenation)

Why does the 2-step method for producing amines yield a purer product compared to the 1-step method? (1)

The 2-step method increases the length of the carbon chain and reduces the formation of by-products

What is the product of the reaction between bromoethane and potassium cyanide followed by hydrogen gas and nickel catalyst? (1)

Propylamine (CH₃CH₂CH₂NH₂)

What are aromatic amines? (1)

Aromatic amines, also called arenes, contain a benzene ring

How are aromatic amines formed? (2)

- By the reduction of nitrobenzene

- Using HCl and tin as the catalyst.

Write the balanced equation for the reduction of nitrobenzene to aromatic amines. (1)

C6H5NO2 + 6[H] → C6H5NH2 + 2H2O

What happens to the product of nitrobenzene reduction in the presence of HCl? (1)

- The aromatic amine reacts with HCl

- To form a salt, C6H5NH3Cl

How is the free aromatic amine liberated from its salt? (2)

- By adding sodium hydroxide (NaOH)

- Resulting in C6H5NH2 + H2O + NaCl

Write the balanced equation for the liberation of free aromatic amine from its salt. (1)

C6H5NH3Cl + NaOH → C6H5NH2 + H2O + NaCl

How do amines and ammonia behave as bases in water? (2)

- They partially dissociate in water.

- For example: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻

What happens when ammonia reacts as a base with a proton? (1)

NH₃ + H⁺ → NH₄⁺

What is the reaction of methylamine reacting as a base with H⁺? (1)

CH₃NH₂ + H⁺ → CH₃NH₃⁺

What is the reaction of dimethylamine with H⁺? (1)

(CH₃)₂NH + H⁺ → (CH₃)₂NH₂⁺

What happens when trimethylamine reacts with H⁺? (1)

(CH₃)₃N + H⁺ → (CH₃)₃NH⁺

What is the reaction of phenylamine with H⁺? (1)

C₆H₅NH₂ + H⁺ → C₆H₅NH₃⁺

How does the availability of the lone pair affect the strength of amines as bases? (2)

- The strength of amines as bases increases

- With the availability of the lone pair on nitrogen for protonation

Arrange the following in increasing base strength: aromatic amines, ammonia, aliphatic amines. (1)

Aromatic amines < Ammonia < Aliphatic amines

Why are aromatic amines weaker bases? (3)

- The lone pair on nitrogen overlaps with the delocalised ring.

- This makes the lone pair on nitrogen less available.

- This is called the negative inductive effect.

Why are aliphatic amines stronger bases? (3)

- The R group pushes electrons towards the nitrogen atom.

- This makes the lone pair on nitrogen more available.

- This is called the positive inductive effect