Organic 11: Amines

Question 1

What are amines derived from?

Answer 1

Ammonia
At least one hydrogen atom is replaced by an alkyl or aryl group

Question 2

What is an alkyl group?

Answer 2

An alkane that has lost a hydrogen
E.g. a methyl group (CH3)

Question 3

What is an aryl group?

Answer 3

An aromatic compound

Question 4

Why are amines useful?

Answer 4

They are very reactive
So are commonly used as intermediates in organic synthesis

Question 5

What is a primary amine?

Answer 5

An amine where one hydrogen is replaced by an alkyl or aryl group

Question 6

What is a secondary amine?

Answer 6

An amine where two hydrogen atoms are replaced by alkyl or aryl groups

Question 7

What is a tertiary amine?

Answer 7

An amine where all three hydrogens have been replaced by an alkyl or aryl group

Question 8

What is the general formula for amines?

Answer 8

RNH2
Where R is an alkyl or aryl group

Question 9

How do you name amines?

Answer 9

The prefix represents the alkyl or aryl group, and the suffix -amine represents the amine.
E.g. methylamine

Question 10

Describe the shape of ammonia.

Answer 10

Pyramidal
Bond angle of 107

Question 11

Why is the bond angle of ammonia different to that of a normal tetrahedron?

Answer 11

The lone pair of electrons on the nitrogen repels more than the bonding pair of electrons, reducing the bond angle.

Question 12

Why are the boiling points of amines lower than the boiling points of their comparable alcohols (e.g. methyl amine and methanol)?

Answer 12

Primary amines can form hydrogen bonds to each other using their NH2 groups
Alcohols can form hydrogen bonds to each other using their OH groups
Nitrogen is less electronegative than oxygen so forms weaker hydrogen bonds
Which need less energy to break, so lower boiling point

Question 13

What state are amines at room temperature?

Answer 13

Shorter chain amines (such as methyl and ethyl amine) are gases at room temperature
Longer chain amines are volatile liquids (rotting fish and rotting animal flesh smell of di- and triamines which are produced when proteins decompose)

Question 14

Describe the solubility of amines.

Answer 14

Short chain amines (e.g. up to four carbon chain length) are very soluble in water and in alcohols because they form hydrogen bonds

Question 15

Why are phenyl amines not soluble?

Answer 15

The benzene ring is not soluble as it cannot form hydrogen bonds as C-H bonds are non-polar
Hydrogen bonds also form more readily between molecules with polar groups like oxygen, fluorine or nitrogen

Question 16

Why are amines so reactive?

Answer 16

Due to the lone pair on the nitrogen
It may be used to form a bond with a H+ ion when the amine acts as a base
Or form a bond with an electron-deficient carbon atom, when the amine is acting as a nucleophile

Question 17

Why are amines classed as Bronsted-Lowry bases?

Answer 17

Because they can accept protons

Question 18

How do amines react with acids?

Answer 18

Amines can react with acids to form salts

Question 19

Describe and give an equation for the reaction of ethylamine with hydrochloric acid.

Answer 19

Ethylamine reacts with dilute hydrochloric acid to form ethyl ammonium chloride:
C2H5NH2 + H+ + Cl- –> [C2H5NH3]+ + Cl-
Ethyl ammonium chloride is an ionic compound that will crystallise as the water evaporates

Question 20

Describe how phenylamine reacts with hydrochloric acid.

Answer 20

Dissolves in excess hydrochloric acid
Forms soluble ionic salt: phenyl ammonium chloride

Question 21

How would you regenerate phenylamine from phenyl ammonium chloride?

Answer 21

Add a very strong base like sodium hydroxide
This removes the proton from the salt and regenerates the insoluble amine
As well as water and a chloride ion

Question 22

Give the order of base strength for amines.

Answer 22

Tertiary amine > Secondary amine > Primary amine > Ammonia > Aromatic amines

Question 23

Why are primary alkylamines stronger bases than ammonia?

Answer 23

An inductive effect is created as the alkyl groups release their electrons towards the nitrogen atom, increasing the electron density on the nitrogen atom, which makes it more attractive to protons
So acts as a better proton acceptor (base)

Question 24

Why are secondary alkylamines stronger bases than primary alkylamines and ammonia?

Answer 24

They have twice as many inductive effects due to the two alkyl groups so have greater electron density on the nitrogen
So it will be a better proton acceptor

Question 25

Why are tertiary alkylamines stronger bases than secondary alkyl amines?

Answer 25

More inductive effects of alkyl groups to increase electron density on the nitrogen atom to make it a better proton acceptor

Question 26

Why are aromatic amines weaker bases than ammonia?

Answer 26

The lone pair on the nitrogen is partially delocalised into the benzene ring
Which reduces the electron density on the nitrogen atom
Making it a poor proton acceptor

Question 27

How are primary aliphatic amines produced from halogenoalkanes? (Describe the reaction and conditions)

Answer 27

Nucleophilic substitution with ammonia

Conditions:
Excess concentrated ammonia solution
Ethanolic
Under pressure

Reaction:
Lone pair on ammonia is attracted to delta positive carbon atom on C-X bond
The electron pair in C-X moves to the X atom, making the halide ion a leaving group
Then, one hydrogen on the NH3 group must leave to form a neutral amine (arrow from N-H bond to N)
A second ammonia molecule reacts with the H+ to form an NH4+ ion

Products
Primary amine
Ammonium ion (NH4+)

Question 28

Name the steps involved in forming a primary amine from a halogenoalkane via nitriles.

Answer 28

Step 1: Nucleophilic substitution of halogenoalkane to produce nitrile

Step 2: Reduction of nitrile to form primary amine

Question 29

How are nitriles prepared from halogenoalkanes? (Describe the reaction and conditions)

Answer 29

Nucleophilic substitution with cyanide

Conditions:
Warm
Aqueous, alcoholic solution of KCN (Potassium cyanide)

Reaction:
The lone pair on the cyanide ion is attracted to the delta positive carbon atom on the C-X bond
The electron pair on the C-X bond moves to the X (halogen), so the halide ion is the leaving group

Products
Nitrile compound
Halide ion

Question 30

How are primary amines prepared from nitriles?

Answer 30

Nitriles can be reduced to primary amines using a nickel/hydrogen catalyst.
R-CN + 2H2 –> R-CH2NH2

Question 31

Where is benzene naturally derived from?

Answer 31

Crude oil

Question 32

What is the organic synthesis route to make phenylamine from benzene?

Answer 32

Benzene –> Nitrobenzene –> Phenylamine

Step 1: Nitration (electrophilic substitution)
Step 2: Reduction

Question 33

How is the electrophile formed for the electrophilic substitution of benzene?

Answer 33

HNO3 + 2H2SO4 –> 2HSO42- + NO2+ + H3O+
(NO2+ is the electrophile)

Question 34

Describe the mechanism for electrophilic substitution of benzene using a nitronium ion.

Answer 34

Conditions:
Concentrated nitric and sulphuric acid (to produce electrophile)

Reaction:
A bond forms between carbon atom and electrophile using electrons from the delocalised system (very electron dense aromatic ring is attracted to positive electrophile)
The carbon must then lost a H+ with the electron in the C-H bond returning to the delocalised system to regain stability

Products:
Nitro compound (i.e. nitrobenzene)
H+

Question 35

How is phenylamine produced from nitrobenzene?

Answer 35

Reduction

Conditions:
Tin and hydrochloric acid as the reducing agent (Sn/HCl)

Reaction:
The tin and hydrochloric acid react to form hydrogen, which reduces the nitrobenzene by removing oxygen atoms on the NO2 group and replacing them with hydrogen

Nitrobenzene + 6[H] –> Phenylamine + 2H2O

Question 36

Why are amines better at reacting with halogenoalkanes than ammonia is?

Answer 36

Because amines have a lone pair on the nitrogen atom which has a higher electron density than on ammonia due to the inductive forces of electrons from the alkyl group

Question 37

What is a quaternary ammonium salt?

Answer 37

An organic salt containing a positively charged nitrogen atom (ammonium ion) bonded to four organic groups
It does not have a hydrogen bonded to it
R4N+

Question 38

What are amines used for?

Answer 38

Manufacture of synthetic materials like nylon, dyes, and drugs

Question 39

What are quaternary ammonium salts used for and why?

Answer 39

Manufacture of hair and fabric conditioners (cationic surfactants)

They have a long hydrocarbon chain and a positively charged organic group, so form cations
Wet hair and fabric pick up negative charges on their surfaces, so positive charges of cations attract them to the surface and form a coating that prevents static electricity
This keeps the surface of the fabric (or hair) smooth

In aqueous solution, their ions cluster with the charged ends in the water and the hydrocarbon tails on the surface

Question 40

How do primary amines react with acyl chlorides or acid anhydrides?

Answer 40

In addition-elimination reactions
Producing N-substituted amides

The amine adds on to the acyl chloride and eliminates HCl

This is a useful reaction to form polymers like nylon