6.2

Question 1

What are amines?

Answer 1

Amines are a class of compounds related to ammonia, NH₃. They are organic chemicals where one or more of the hydrogen atoms on ammonia have been replaced by alkyl chains.

Question 2

What are the three types of amine?

Answer 2

• Primary, 1^o - one hydrogen atom has been substituted. The structural formula can be summarised as RNH₂ where R is the alkyl chain.
• Secondary, 2^o - two hydrogen atoms have been substituted. The structural formula can be summarised as RNH₂ where R and R’ are the alkyl chains, which may be the same or different.
• Tertiary, 3^o - all three hydrogen atoms have been substituted. The structural formula can be summarised as RNR’R’’ where R, R’, and R’’ can be the same or different hydrocarbon groups.

Question 3

How do you name amines?

Answer 3

When naming amines, the suffix is always amine. Then alkyl chains must be determined and used to generate the root and prefix of the name.

Question 4

Describe the naming process for primary amines.

Answer 4

Primary → determine root by longest hydrocarbon chain and any prefixes for other groups. Finally, add the suffix amine.

Question 5

Describe how you name a secondary amine.

Answer 5

For secondary amines - determine the root by naming two alkyl chains. Add any prefixes for additional groups and write them in alphabetical order. Finally, add the suffix ‘amine’. As the alkyl groups are attached to the nitrogen atom, secondary amines are often called ‘N-substituted’ and this is given as a prefix to the name.

Question 6

Describe how you name a tertiary amine.

Answer 6

For tertiary amines - determine the root by naming the three alkyl chains. If there is more than one of the same group add the appropriate prefix; di or tri-

Question 7

Define a lewis base.

Answer 7

A lewis base has a lone pair of electrons for donation - ammonia and amines have a lone pair of electrons on the nitrogen atom, making them Lewis bases.

Question 8

Define a brønstead-Lowry base.

Answer 8

A Brønstead-lowry base is a proton acceptor - ammonia and amines can accept protons on the nitrogen atom and are therefore also Brønsted-Lowry bases.

Question 9

Why are amines and ammonia both weak bases?

Answer 9

Amines and ammonia are both weak bases. Using the Brønsted-Lowry model, when an amine reacts with an acid, it accepts a proton. A covalent bond is formed by the nitrogen atom donating its lone pair of electrons to the proton to form a dative covalent (or coordinate) bond.

Question 10

Describe the reactions of amines with dilute inorganic acids.

Answer 10

When HCl_(aq) reacts with a base, a chloride salt and water are produced. When primary amines react with this acid, an alkylammonium salt is made. This is formed by the proton in the acid bein replaced by an alkylammonium ion.

CH₃CH₂NH₃ + HCl → CH₃CH₂NH₂ + Cl^-

Question 11

What is an alkylammonium salt?

Answer 11

An alkylammonium salt is a compound where the hydrogen(s) on an ammonium ion have been substituted by alkyl chains.

Question 12

Why are secondary and tertiary amines also bases?

Answer 12

Secondary and tertiary amines are also bases as they have a lone pair of electrons on the nitrogen atom in their functional group. This also allows them to react with acids and form salts.

Question 13

Draw the structural formula chemical equation for the reaction of ethylamine with dilute hydrochloric acid.

Answer 13

Question 14

How will other strong inorganic acids such as nitric acid react with amines?

Answer 14

Other strong inorganic acids, such as nitric and sulphuric acid will react in a similar way to make a salt.

When nitric acid is used instead of hydrochloric acid the nitrate ion NO₃^- is simply substituted for the chloride ion Cl^- in the balanced chemical equation.

CH₃CH₂NH₂ + HNO₃ + HNO₃ → CH₃CH₂NH₃⁺NO₃^-

Question 15

How will other strong inorganic acids such as sulphuric acid react with amines?

Answer 15

When sulphuric acid is used instead of hydrochloric acid the balanced chemical equation is more complex.

2CH₃CH₂NH₂ + H₂SO₄ → [CH₃CH₂NH₃⁺]₂SO₄^2-

Question 16

Name the salts formed from the reaction of amines with HNO₃, H₂SO₄, and HCl.

Answer 16

1. HCl = alkylammonium chloride
2. HNO₃ = alkylammonium nitrate
3. Sulphuric acid = alkylammonium sulphate

Question 17

How may the molecular ratio of amine to acid change in the reaction between an amine and an acid.

Answer 17

When an amine reacts with sulphuric acid two molecules of amine are required for every molecule of sulphuric acid. This is because the alkylammonium sulphate produced contains two alkylammonium ions for every one sulphate ion. Alkylammonium ions have a single positive charge whilst a sulphate ion has a double negative charge.

Question 18

Describe the preparation of aliphatic amines.

Answer 18

Using a sealed test tube, a haloalkane, ammonia and ethanol are heated together to make an amine. Reflux cannot be used as the ammonia is so volatile it would escape out of the condenser rather than react.

A haloalkane, such as 1-chloropropane, will undergo nucleophilic substitution in a two-stage process to form a primary amine.

Question 19

Give the two stages of forming the aliphatic amine, 1-chloropropane.

Answer 19

• Stage 1: The ammonia reacts with the haloalkane to make an ammonium salt.

CH₃CH₂CH₂Cl + NH₃ → CH₃CH₂CH₂NH₃ Cl

• Stage 2: An additional ammonia molecule reacts to form the propylamine product and ammonium chloride salt.

CH₃CH₂CH₂NH₃Cl + NH₃ ⇔ CH₃CH₂CH₂NH₂ + NH₄Cl

(as this is a reversible reaction the addition of excess ammonia will drive the reaction to the right and increase the yield of the desired primary amine product)

Question 20

During the preparation of aliphatic amines, additional substitution of the hydrogen atoms on the nitrogen atom can occur. What products could be made in this reaction?

Answer 20

• In this reaction, initially, N-dipropylamine, a secondary amine, is made.

CH₃CH₂CH₂Cl + (CH₃CH₂CH₂)₂NH ⇔ (CH₃CH₂CH₂)_{<span>3</span>}N+HCl

• Further substitution would produce N-tripropylamine, a tertiary amine.

CH₃CH₂CH₂Cl + (CH₃CH₂CH₂)₂NH₂ ⇔ (CH₃CH₂CH₂)₃N + HCl

• The final stage is a quaternary ammonium salt, where each hydrogen on the ammonium ion has been replaced with an alkyl chain.

CH₃CH₂CH₂Cl + (CH₃CH₂CH₂)₃N ⇔ (CH₃CH₂CH₂)₄N⁺Cl^-

• These further substitution reactions can occur because amines have lone pairs of electrons on the nitrogen atom that can act as a nucleophile.

Question 21

How does the ratio of reactants used in the preparation of an aliphatic amine affect the type of salt produced?

Answer 21

When preparing amines by this method there is always a mixture of the products produced. As the second stage of the mechanism is an equilibrium reaction, using excess ammonia favours the primary amine and excess haloalkane favours the quaternary ammonium salt.

Question 22

How are aromatic amines prepared?

Answer 22

• Nitroarenes, such as nitrobenzene, can be reduced to produce an amine. The reducing agent is made in situ by using a mixture of tin and concentrated HCl. The reaction occurs at 100^oC.
• After about half an hour, a strong alkali, such as sodium hydroxide, is added. This undergoes a neutralisation reaction to remove the excess hydrochloric acid and produce the amine. Separating the aromatic amine is a multi-stage process tat includes steam distillation, solvent extraction and further distillation.

Question 23

Describe the class of compounds, amino acids.

Answer 23

Amino acids are a class of compounds with two functional groups: a carboxylic acid, -COOH, and an amine group, -NH₂. When both functional groups are attached to the same carbon atom, the compound is called a σ-amino acid. This group of organic chemicals has the general formula RCH(NH₂​)COOH.

Question 24

Why are proteins a useful molecule?

Answer 24

Proteins are an essential nutrient for all animals as they are used to make, among other things, hormones, muscles and enzymes. All proteins are polymer chains made up of amino acid monomers bonded together. In human biochemistry there are around 20 σ-amino acids.

Question 25

Why are amino acids amphoteric?

Answer 25

The carboxylic acid functional group is a weak acid that will partially ionise in water. The nitrogen atom on the amine group has a lone pair of electrons and can act as a base. This means that amino acids are amphoteric (they can act can act as an acid and a base), as their carboxylic acid group can react with bases and their amine group can react with acids.

Question 26

How do σ-amino acids form zwitterions?

Answer 26

σ-amino acids have the two functional groups, -COOH and -NH₂, attached to the same carbon atom. σ-amino acids can form zwitterions, where their two functional groups exchange a proton and make an internal salt. The carboxylic acid donates a proton to the amino group. As the two charges cancel each other out, the resultant molecule has no overall charge.

Question 27

Describe how an amino acid is affected by changes in the pH.

Answer 27

The isoelectric point is when there is no net electrical charge due to each zwitterion having an internal balance of charge. By changing the pH you alter the amino acid so that only one of its functional groups is charged. This means at a low pH, where there is a lot of H⁺, the carboxylic acid functional group becomes -COOH and only the amine group is charged. However, at high pH, the amine group becomes -NH₂ and only the carboxylic acid group is charged.

Question 28

Draw a zwitterion in a low and high pH.

Answer 28

Question 29

The carboxylic acid functional group is a weak acid and partially ionises in solution. This functional group will undergo all the common reactions of an organic acid what reactants?

Answer 29

• Metal oxides
• Alkalis
• Carbonates
• Alcohol

Question 30

Describe the reaction between metal oxides and the carboxylic acid functional group of amino acids.

Answer 30

Metal oxides - metal oxides are bases, so a neutralisation reaction occurs. The hydrogen atom on the carboxylic acid group is exchanged for a metal ion to produce a metal salt. Water is also produced in this reaction.

Question 31

Describe the reaction between alkalis and the carboxylic acid functional group of amino acids.

Answer 31

alkalis - alkalis are soluble bases, so a neutralisation reaction occurs. The reaction produces a salt and water.

Question 32

Describe the reaction between carbonates and the carboxylic acid functional group of amino acids.

Answer 32

Carbonates - this neutralisation reaction produces a salt, water and carbon dioxide gas. As a gas is evolved, effervescence is observed.

Question 33

Describe the reaction between an alcohol and the carboxylic acid functional group of amino acids.

Answer 33

Alcohol - this reaction produces an ester and releases a molecule of water.

Question 34

How does the amine functional group act to the reduce the change of pH?

Answer 34

The amine functional group can act as a base due to the lone pair of electrons on the nitrogen atom. When an acid is added to an amino acid, the amine group accepts a proton. The result is an ammonium salt.

Question 35

Define an amide.

Answer 35

An amide is a class of compound with a functional group made of an acyl group, which is directly attached to an amine.

Question 36

What are amides?

Answer 36

Amides are a class of compounds with an acyl group (RC(O)-) attached to an amine group (-NH₂). Organic primary amides have the general formula RC(O)NH₂ where R is a hydrocarbon chain or a hydrogen atom.

Question 37

What molecules are amides closely related to?

Answer 37

Amides are related to carboxylic acids, where the hydroxyl group has been substituted with an amine group.

They are also closely related to ammonia, where one or more of the hydrogen atoms have been substituted with an acyl group.

Question 38

Amides can be classified based on the groups attached to the nitrogen atom. What are the different classes of amide?

Answer 38

1. Primary amides - the nitrogen atom has two hydrogen atoms and one acyl group attached.
2. Secondary amides - the nitrogen atom has one hydrogen atom, one acyl group and one alkyl group attached.
3. Tertiary amides - the nitrogen atom has one hydrogen atoms attached.
4. Polyamides

Question 39

Describe a polyamide.

Answer 39

Polyamides - this is a type of condensation polymer; a very long-chain molecule, with a repeating pattern of atoms. They are made from a reaction between a carboxylic acid and an amine and contain a number of secondary amide groups.

Question 40

How do you name a primary amide?

Answer 40

When naming primary amides, the suffix is ‘amide’ and the root is generated by the number of carbon atoms in the parent hydrocarbon chain which has the functional group. To number prefixes, the functional group is considered to be on carbon atom 1.

Question 41

How do you name secondary amides?

Answer 41

When naming secondary amides, the suffix is ‘amide’ and the root is generated by the number of carbon atoms in the parent hydrocarbon chain which has the functional group. The alkyl chain is attached to the nitrogen, so it is called an N-substituted compound.

Question 42

Define optical isomers.

Answer 42

Optical isomers are molecules which are non-superimposable mirror images of each other. They have the same chemical properties but interact with polarised light differently.

Question 43

Define a chiral carbon.

Answer 43

A chiral carbon has four different groups attached to it.

Question 44

Explain the effect of optical isomerism.

Answer 44

• Optical isomerism is another type of stereoisomerism (stereoisomers are a class of isomers that have a different arrangement of atoms in space). The isomers are non-superimposable mirror images of each other, known as ‘enantiomers’.
• This type of isomerism is called optical isomerism as each enantiomer interacts with light in a different way. Although both enantiomers have the same chemical properties and similar physical properties, they may have different biological properties.

Question 45

Give an example of the effect of optical isomerism.

Answer 45

One example is limonene, (IUPAC name: 1-methyl-4-(1-methylethenyl)-cyclohexene). It is a chiral hydrocarbon that is naturally found in citrus fruit. One enantiomer smells strongly of oranges whilst the other enantiomer smells of pine.

Question 46

How are enantiomers similar and different to each other?

Answer 46

Remember that enantiomers are chemically identical, they just interact with biological systems and light differently.

Question 47

How can different enantiomers be scripted?

Answer 47

Each enantiomer can be given a prefix to denote its effect on a plane of polarised light. If the enantiomer rotates the plane of polarised light in the clockwise direction it is given the prefix ‘+’. The prefix ‘-‘ is given if the rotation is in the anticlockwise direction. This can only be determined by experiment and can not be deduced from the structure.

Question 48

When talking about enantiomers what is a racemic mixture?

Answer 48

A 50:50 mixture of enantiomers is called a racemic mixture or a racemate. A racemic mixture will have no effect on polarised light as the rotations of the two isomers cancel each other out.

Question 49

What is required for optical isomers to be formed from an organic compound?

Answer 49

For optical isomers to be formed in an organic compound, there must be a carbon atom with four different groups attached. This carbon is known as the chiral centre and is often shown with a ‘*’ on diagrams of the molecule. It is also possible for inorganic atoms such as nitrogen to be chiral centres.

Question 50

Describe the lines of symmetry in chiral molecules.

Answer 50

Chiral molecules do not have a plane of symmetry due to the asymmetric, or chiral, carbon. Some molecules have more than one chiral centre. Each will be a carbon atom with four different groups attached.

Question 51

Define a repeat unit.

Answer 51

A repeat unit is the arrangement of atoms that occurs many times in a polymer.

Question 52

Define condensation polymerisation.

Answer 52

Condensation polymerisation is the chemical reaction to form a long-chain molecule by the elimination of a small molecule, such as water.

Question 53

What are the two types of polymerisation?

Answer 53

Addition polymerisation and condensation polymerisation.

Question 54

What is addition polymerisation?

Answer 54

Addition polymerisation - where monomers have at least one C=C, which breaks and joins the other monomers together. There is only one product from this type of polymerisation. Common addition polymers include polyethene, made from many ethene monomers.

Question 55

What molecules react to form a polymer in condensation polymerisation?

Answer 55

Two different monomers, with different functional groups, react to form a polymer and release another small molecule, often water. Common condensation polymers include nylon and polyester.

Question 56

How do you classify polymers?

Answer 56

• Monomers containing a C=C bond must form an addition polymer.
• If the monomer contains two different functional groups or more than one monomer is present, each with different functional groups, then a condensation polymer is formed.

(you do not need to recall the structures of synthetic polyesters and polyamides or their monomers.)

Two worked examples on page 160-161.

Question 57

What are polyesters?

Answer 57

Polyesters are a class of condensation polymer made by the chemical reaction of a dicarboxylic acid (-COOH) and a diol (-OH).

Question 58

How are polyesters formed?

Answer 58

When the carboxylic acid group on one monomer reacts with the alcohol group on the other monomer, an ester link (R-COO-R’) is formed between the two molecules. This happens many times and produces a long-chain molecule, which is called a polyester.

Question 59

How may only one monomer be used to make a polyester?

Answer 59

It is possible to have a molecule that contains both an alcohol group and a carboxylic acid group. This allows just one monomer to be used to make the polyester.

Question 60

State the monomer and monomer structure for PET.

Answer 60

Question 61

Give the polymer name and structure for lactic acid.

Answer 61

Question 62

What are polyamides?

Answer 62

A class of condensation polymer made by the chemical reaction of a dicarboxylic acid (-COOH) and diamine (-NH₂).

Question 63

How are polyamides formed?

Answer 63

When the carboxylic acid group on one monomer reacts with the amine group on the other monomer, an amide link (R-CONH-R’) is formed between the two molecules. This happens between many molecules and produces a long-chain molecule called a polyamide.

Question 64

How can the polyamide be made using just one monomer?

Answer 64

It is possible to have a molecule that contains both an amine group and a carboxylic acid group. This allows just one monomer to be used to make the polyamide.

Question 65

Draw the general equation for an amino acid reacting to make a polyamide.

Answer 65

Question 66

Give the monomer and monomer structure for the polyamide Nylon.

Answer 66

Question 67

Give the polymer name and state the mechanism for the polyamide monomer benzene.

Answer 67

Question 68

What is the hydrolysis of condensation monomers?

Answer 68

Hydrolysis is a chemical reaction in which water breaks bonds. For condensation polymers, hydrolysis is the reverse reaction of polymerisation - causing the polymer chains to break down into their constituent monomers.

Question 69

Describe the hydrolysis of polyesters and polyamides.

Answer 69

Hydrolysis of polyesters and polyamides with water, however, has a very slow rate of reaction. This is why condensation polymers, such as nylon, do not degrade when it rains. However, the reaction can be achieved in acidic or basic conditions and the rate can be increased further by heating the reaction mixture.

Question 70

Describe the acid hydrolysis of polyesters.

Answer 70

When a polyester is reacted with a strong aqueous acid, the reaction produces a diol and a dicarboxylic acid. The acid hydrolysis of polyester has a slow rate of reaction.

Question 71

Describe the alkali hydrolysis of polyesters.

Answer 71

When a polyester is reacted with hot sodium hydroxide solution, the reaction rate produces the diol and the salt of the dicarboxylic acid.

Question 72

Draw the molecule produced by the acid hydrolysis of this polyester. (H⁺/H₂​O)

Answer 72

Question 73

Draw the molecule produced by the alkali hydrolysis of this polyester. (NaOH/H₂O)

Answer 73

Question 74

Explain the hydrolysis of polyamides.

Answer 74

Polyamides will undergo hydrolysis in acidic and basic conditions. The rate of reaction of acid hydrolysis is much faster than basic hydrolysis for polyamides. Acids can easily hydrolyse nylon and cause holes in clothing, so you must be careful wen working with them.

Question 75

Describe the acid hydrolysis of polyamides.

Answer 75

When a polyamide is reacted with a strong aqueous acid, the reaction produces a diammonium salt and a dicarboxylic acid.

Question 76

Describe the alkali hydrolysis of polyamides.

Answer 76

When a polyamide is reacted is reacted with hot sodium hydroxide solution, the reaction produces the diamine and the salt of the dicarboxylic acid.

Question 77

What molecule is produced by the acid hydrolysis of this polyamide?

Answer 77

Question 78

What molecule is produced by the alkali hydrolysis of this polyamide?

Answer 78

Question 79

How can the hydrolysis of polymers pave the way for more environmentally friendly plastics?

Answer 79

Controlling the rate of hydrolysis of condensation polymers has allowed the development of degradable polymers. Many of these polymers have ester links that can undergo hydrolysis and cause the breakdown of the polymer chains.

To make these polymers more environmentally friendly, there has been a lot of research into finding monomers from renewable sources rather than monomers derived from the non-renewable raw material, crude oil. One example is poly(lactic acid) a polyester made from lactic acid extracted from the renewable resource, corn starch.