Organic - Amines

Question 1

What are amines?

Answer 1

Derivatives of ammonia in which one or more of the hydrogen atoms in the ammonia molecule have been replaced by alkyl or aryl groups.

Question 2

What are amines useful for?

Answer 2

Amines are very reactive compounds, so they are useful as intermediates in synthesis - the making of new molecules.

Question 3

What are the different types of amines?

Answer 3

NH3: ammonia
RNH2: primary amine
R2NH: secondary amine
R3N: tertiary amine
R4NCl+: quaternary ammonium salt (not technically an amine, e.g. NH4Cl)

Question 4

What is the shape of primary amines?

Answer 4

Ammonia is a pyramidal molecule with bond angles of approx. 107 degrees. The angles of a perfect tetrahedron are 109.5 degrees. The difference is caused by the lone pair, which repels more than the bonding pairs of electrons in the N-H bonds. Amines keep this basic shape.

Question 5

What is the boiling point of primary amines?

Answer 5

Amines are polar. Primary amines can hydrogen bond to one another using their -NH2 groups, however the boiling points of amines are lower than those of comparable alcohols because nitrogen is less electronegative than oxygen (so the hydrogen bonds are not as strong). No hydrogen bonding between tertiary amine molecules.

Shorter chain amines such as methylamine and ethylamine are gases at room temperature, and those with slightly longer chains are volatile liquids. They have fishy smells. Rotting fish and rotting animal flesh smell of di- and triamines, produced when proteins decompose.

Question 6

What is the solubility of primary amines?

Answer 6

Primary amines with chain lengths up to about four carbon atoms are very soluble both in water and in alcohols because they form hydrogen bonds with these solvents. Most amines are also soluble in less polar solvents. Phenylamine is not very soluble in water due to the benzene ring, which cannot form hydrogen bonds.

Question 7

How do amines react?

Answer 7

Amines have a lone pair of electrons and this is important in the way they react. The lone pair may be used to form a bond with:

• a H+ ion, when the amine is acting as a base
• an electron-deficient carbon atom, when the amine is acting as a nucleophile

Question 8

What type of bases are amines?

Answer 8

Amines are Bronsted-Lowry bases (proton acceptors). They act as weak bases because the lone pair on the nitrogen atom can readily accept a proton.

RNH2 + H2O ->

Question 9

What are the reactions of amines as bases?

Answer 9

• Amines react with acids to form salts.
• The products are ionic compounds that will crystallise as the water evaporates.
• If a strong base is added, it removes the proton from the soluble ionic salt and regenerates the insoluble amine.

Question 10

What does the strength of a base depend on?

Answer 10

How readily the nitrogen lone pair can accept a proton. The higher the electron density of the nitrogen lone pair, the better it will be able to accept H+, the stronger the base. Both ammonia and amines have a lone pair of electrons that attract a proton.

ethylamine > ammonia > phenylamine

order of base strength:
secondary amine > tertiary amine > primary amine > ammonia > aromatic amine

Question 11

What is the inductive effect?

Answer 11

Alkyl groups release electrons away from the alkyl group and towards the nitrogen atom. This is called the inductive effect and is shown by an arrow.

The inductive effect of the alkyl group increases the electron density on the nitrogen atom (better than hydrogen atoms) and therefore makes it a better electron pair donor (i.e. more attractive to protons). The more alkyl groups, the higher the electron density of the lone pair on the nitrogen, the stronger the base. So primary alkylamines are stronger bases than ammonia.

Question 12

What is the base strength of secondary alkylamines?

Answer 12

Secondary alkylamines have two inductive effects and are therefore stronger bases than primary alkylamines.

Question 13

What is the base strength of tertiary alkylamines?

Answer 13

Tertiary alkylamines are not stronger bases than secondary ones because they are less soluble in water, reducing the expected base strength.

Question 14

Why are arylamines/aromatic amines weaker bases than ammonia?

Answer 14

Aryl groups withdraw electrons from the nitrogen atom because the lone pair of electrons overlaps with the delocalised system on the benzene ring. This means it’s less available to combine to the H+.

The nitrogen becomes a weaker electron pair donor (as there’s a reduction in electron density) and therefore less attractive to protons, so arylamines are weaker bases than ammonia.

Question 15

What are aromatic amines?

Answer 15

they have nitrogen joined directly to the benzene ring

Question 16

What is aliphatic?

Answer 16

not aromatic

Question 17

What are quaternary ammonium salts?

Answer 17

Contain quaternary ammonium ions and are related to amines, but they are not amines and do not possess a lone pair of electrons on the nitrogen.

Question 18

Why do amines act as nucleophiles?

Answer 18

The lone pair of electrons from an amine will attack positively charged carbon atoms. So amines, like ammonia, will act as nucleophiles.

Question 19

How do you prepare amines with the reaction of ammonia with halogenoalkanes?

Answer 19

Primary aliphatic amines are produced when halogenoalkanes are reacted with ammonia. There is nucleophilic substitution of the halide by NH2.

However, the primary amine produced is also a nucleophile and this will react with the halogenoalkane and produce a secondary amine.

The secondary amine will react to give a tertiary amine. This in turn will react to produce a quaternary ammonium salt.

Therefore one mole of halogenoalkane reacts with two moles of ammonia producing a primary amine and an ammonium salt.

Question 20

Why is the reaction of ammonia with halogenoalkanes not very efficient?

Answer 20

A mixture of primary, secondary, and tertiary amines and a quaternary ammonium salt is produced. This means that this is not a very efficient way of preparing an amine, though the products may be separated by fractional distillation. A large excess of concentrated ammonia gives a better yield of primary amine.

Question 21

What conditions are required for the preparation of primary aliphatic amines by the reaction of ammonia with halogenoalkanes?

Answer 21

Halogenoalkanes are reacted with an excess of concentrated ammonia dissolved in ethanol at pressure in a sealed container. The mechanism is nucleophilic substitution. It’s the easiest method but the least pure.

Question 22

How do you prepare amines by reduction of nitrile compounds?

Answer 22

Via hydrogenation. Primary alkyl amines can be prepared from halogenoalkanes in a two-step process:

Step 1 - Halogenoalkanes react with a cyanide ion in aqueous ethanol. The cyanide ion replaces the halide ion by nucleophilic substitution to form a nitrile.

Step 2: Nitriles contain the functional group -CN. They can be reduced to primary amines, e.g. with a nickel/hydrogen catalyst.

Question 23

Why is the reduction of nitriles more efficient/common?

Answer 23

This gives a purer product than a bromoalkane and ammonia because only the primary amine can be formed. The carbon chain of the product is one carbon atom longer than the starting material.

Question 24

How do you prepare primary aromatic amines?

Answer 24

These are usually prepared by the reduction of nitro compounds (e.g. nitrobenzene) using tin or iron and concentrated hydrochloric acid. This would actually form the ammonium salt and so NaOH is added to give the amine.

C6H6NO2 + 6[H] –(Fe or Sn and HCl followed by NaOH)–> C6H6NH2 + 2H2O

C6H6NH2 is used to make dyes

C6H6N+H3Cl-
phenylammonium chloride

Question 25

What is the positive inductive effect?

Answer 25

electrons are being donated

Question 26

What is the negative inductive effect?

Answer 26

electrons are being withdrawn

Question 27

Why are primary amines stronger bases than ammonia?

Answer 27

R group donates electron density towards the nitrogen lone pair, making the availability of the lone pair on the nitrogen greater and so it is a stronger base than ammonia.

Question 28

How can you make phenylamine from benzene?

Answer 28

Benzene is reacted with a mixture of concentrated nitric and concentrated sulphuric acid (catalyst). This produces nitrobenzene and water.

Nitrobenzene is reduced to phenylamine, using tin or iron and hydrochloric acid as the reducing agent. The tin/iron and hydrochloric acid react to form hydrogen, which reduces the nitrobenzene by removing oxygen atoms of the NO2 group and replacing them with hydrogen atoms.

Since the reaction is carried out in hydrochloric acid, the salt C6H5NH3+Cl- is formed and sodium hydroxide is added to liberate the free amine:

C6H5NH3+Cl- + NaOH -> C6H5NH2 + H2O + NaCl

Question 29

How are amides forms from amines?

Answer 29

Amines will react with acid chlorides and acid anhydrides. These are nucleophilic substitution reactions (sometimes called addition-elimination reactions) and the products are N-substituted amides.

The amine adds on to the acid chloride and then HCl is eliminated. The reaction is useful in forming polymers such as nylon.

Question 30

What is the economic importance of amines?

Answer 30

Amines are used in the manufacture of synthetic materials such as nylon and polyurethane, dyes, and drugs.

Quaternary ammonium compounds are used industrially in the manufacture of hair and fabric conditioners. They have a long hydrocarbon chain and a positively charged organic group, so the form cations.

Both wet fabric and wet hair pick up negative charges on their surfaces. So the positive charges of the cations attract them to the wet surface and form a coating that prevents the build-up of static electricity. This keeps the surface of the fabric smooth (in fabric conditioner) and prevents flyaway hair in hair conditioners.

They are called cationic surfactants because in aqueous solution, the ions cluster with their charged ends in the water and their hydrocarbon tails on the surface.

Question 31

What are cationic surfactants?

Answer 31

Quaternary ammonium salts with some long chain alkyl groups are used as cationic surfactants. These molecules are complexes with a positive and negative end. This makes them good conditioners as the two ends are attracted to different substances, preventing static from building up on surfaces. Cationic surfactants are therefore useful in industry.

Question 32

How are quaternary ammonium salts formed?

Answer 32

• NH3 reacts with halogenoalkanes to form primary amines initially.
• The overall reaction is a nucleophilic substitution, with ammonia acting as a nucleophile through the lone pair on the nitrogen atom attacking the electron deficient delta+carbon atom of the carbon-halogen bond.
• However, as well as ammonia, the amines produced also have a lone pair on the nitrogen atom, so they can react with halogenalkanes. (In fact, the product has a greater electron density on the nitrogen lone pair and is even better at reaction with halogenalkane than the original molecule).
• This means that the hydrogen’s on the nitrogen can be successfully replaced by the R group from the haloalkanes, right through to quaternary ammonium salts.

Question 33

What happens if a large excess of ammonia or of the haloalkane is used in the nucleophilic substitution reaction?

Answer 33

• if a large excess of ammonia is used, then a primary amine is the main product
• if a large excess of the haloalkane is used, then the quaternary ammonium salt is the main product

Question 34

What do surfactants consist of?

Answer 34

Soap and detergent molecules are called surfactants (helps oil and water mix) and consist of:

• an ionic “head” -> this will dissolve in water and is known as being hydrophilic
• a long alkyl chain (“tail”) -> this dissolves grease (grease is not soluble in water) and is known as being hydrophobic

Question 35

How are micelles formed?

Answer 35

Micelles are formed as the soap/detergent molecules surround the dirt/grease molecules, with the hydrophobic tails in the centre dissolving the grease, and the hydrophilic heads around the outside dissolving in the water.

Question 36

Why do fabric softeners often contain cationic surfactants?

Answer 36

as the surface of most materials is negatively charged

Question 37

What are the disadvantages of preparing amines by the reaction of NH3 with halogenoalkanes and by reduction of nitrile compounds?

Answer 37

By the reaction of NH3 with halogenoalkanes:

• further substitution likely
• impure product

By reduction of nitrile compounds:

• toxic/poisonous KCN
• expensive LiAlH4
• lower yield because 2 steps
• H2 is flammable

Question 38

How can you produce only the primary amine?

Answer 38

The multiple number of possible substitutions means that a mixture of products are produced. Therefore the reaction has low efficiency and reaction conditions have to be changed so that only a single substitution occurs. Ammonia can be added in excess in order to achieve only the primary amine, or the mixture of products can be separated using fractional distillation.

Question 39

Give an example of the negative inductive effect.

Answer 39

Benzene rings - draw electron density away from the nitrogen making it ‘less available’.

Question 40

Give an example of the positive inductive effect.

Answer 40

Alkyl groups - push electron density towards the nitrogen making it ‘more available’. More alkyl groups means more ‘pushing’.

Question 41

How do amines undergo nucleophilic substitution reactions?

Answer 41

Amines can act as nucleophiles because the lone electron pair is attracted to partially positive regions on other molecules. This means amines can substitute halides on halogenoalkanes to form primary, secondary or tertiary amines and quaternary ammonium salts.

Question 42

How do amines undergo nucleophilic addition-elimination reactions?

Answer 42

Amines can also undergo nucleophilic addition-elimination reactions with acyl chlorides to produce amides and N-substituted amides.

The same reaction mechanism can also occur with acid anhydrides to produce an amide and a carboxylic acid.