Amines and polymers

Question 1

What is a primary amine?

Answer 1

An amine in which the nitrogen atom is attached to one carbon chain.

Question 2

How do we name primary amines?

Answer 2

• Identify name of carbon chain attached to amine group. Name is constructed in the form [Name of carbon chain]amine. E.g. CH₃CH₂NH₂: Ethylamine.
• Ignore the amine group and pretend it is an H atom. Name the compound with omitted amine group. Identify the number of the carbon atom attached to amine group. Name is constructed in the form [Carbon number attached to amine group]-amino[Name of compound without amine group]. E.g. CH₃CH(NH₂)CH₃: 2-aminopropane.

Question 3

What is a secondary amine?

Answer 3

An amine in which the nitrogen atom is attached to two carbon chains.

Question 4

How do we name secondary amines?

Answer 4

1. Identify the longest (alkyl) chain attached to the amine group.
2. Identify the shortest (alkyl) chain attached to the amine group. Add “N-“ as a prefix the the name.
3. Construct the name in the form N-[Name of shortest carbon chain] [Name of longest carbon chain]amine. E.g. CH₃CH₂NHCH₃: N-methyl ethylamine.

Question 5

What is a tertiary amine?

Answer 5

An amine in which the nitrogen atom is attached to 3 carbon chains.

Question 6

Why are amines basic?

Answer 6

• Amines are weak bases, just as ammonia.
• Amines have a lone pair of electrons on the N atom in amine group which are able to accept protons by forming dative covalent bonds with them, making them proton acceptors and thus bases.

Question 7

What is the general fromula for the reaction of an amine with an acid?

Answer 7

Amine +Acid → Amine salt

Question 8

How do we name amine salts?

Answer 8

1. Take the name of the amine with -amine suffix (e.g. ethylamine). Replace -amine suffix with -ammonium suffix to form name of cation.
2. Identify the name of the anion.
3. Construct the name in the form [Name of cation] [Name of anion].

Question 9

What are aliphatic amines?

Answer 9

Amine in which the nitrogen atoms are attached to straight carbon chains only.

Question 10

What are aromatic amines?

Answer 10

Amines in which the nitrogen atoms are attached directly to benzene rings.

Question 11

How are primary aliphatic amines prepared?

Answer 11

Warming a halogenoalkane with the desired alkyl chain with an excess of ammonia in ethanol solvent.

Question 12

What type of reaction occurs during preparation of aliphatic amines?

Answer 12

Nucleophilic substitution.

Question 13

Why is an an excess of ammonia required for the reaction?

Answer 13

To ensure that there are enough ammonia molecules to react with all halogenoalkane molecules in nucleophilic substitution reaction, else the amine groups on the primary amines formed undergo further substitution reactions with to form secondary and eventually tertiary amines.

Question 14

What is the general formula for the preparation of primary aliphatic amines?

Answer 14

Halogenoalkane + Ammonia → Amine + Hydrogen halide

E.g. Chloroethane + Ammonia → Ethyamine + Hydrogen chloride (CH₃CH₂Cl + NH₃ → Ch₃CH₂NH₂ + HCl)

Question 15

How are aromatic amines prepared?

Answer 15

Nitrobenzene and other nitroarenes can be reduced to phenylamines. They are heated with a mixture of tin and concentrated HCl under reflux (both make up reducing agent). The excess HCl in the mixture is then neutralised.

Question 16

What is the equation for the reduction of nitrobenzene?

Answer 16

Nitrobenzene + 6[H] → Phenylamine + 2H₂O

(C₆H₅)NO₂ + 6[H] → (C₆H₅)NH₂ + 2H₂O

Question 17

What are amino acids?

Answer 17

Basic chemical building blocks to polypeptides and proteins, which make up a majority of chemicals found in the body and have many uses; including structural functions such as collagen, or physiological functions such as haemoglobin. There are 20 naturally occuring amino acids (α-amino acids) which are arranged in specific sequences to form polymers called polypeptides that form proteins, these sequences are determined by the DNA found in or body.

Question 18

What is the structure of an α-amino acid?

Answer 18

A central carbon atom is attached to an amine (-NH₂) group, a carboxyl (-COOH) group, a hydrogen atom and a residual (-R) group which can be a variety of structures (usually a carbon chain, except for glycine, which is simple an H).

The general formula for an α-amino acid is:

RCH(NH₂)COOH

Question 19

What is a zwitterion?

Answer 19

A dipolar ionic form of an amino acid that is formed by the donation of a hydrogen ion from the carboxyl group to the amino group. Because both charges are present, there is no overall charge.

Question 20

How are zwitterion formed?

Answer 20

Effectively, because the amine group is basic (proton acceptor) and the carboxyl group is acidic (proton donor) on an amino acid, a proton is transferred from the carboxyl group of an amino acid onto its amine group (if allowed, i.e. in aqueous form). This results in a +ve charge being formed on the amine group and a -ve charge being formed on the carboxyl group that cancel out to give a neutral charge.

Question 21

What is the isoelectric point?

Answer 21

The pH value at which the amino acid exists as a zwitterion.

Question 22

What is the significance of the isoelectric point?

Answer 22

There is no net electrical charge at the isoelectric point, this means that zwitterions of an amino acid exist at this pH. Each amino acid has a different isoelectric point due to the influence of different -R groups.

Question 23

Why are amino acids described as amphoteric?

Answer 23

They can take part in both acid and base reactions.

Question 24

How can amino acids act as acids?

Answer 24

When put into an environment with pH greater than that of its isoelectric point, an amino acid acts as an acid. The +ve amine (-NH₃⁺) group on the zwitterion donates protons to the base in solution and reacts to form a negatively charged ion.

E.g. RCH(NH₃⁺)COO^- + OH^- → RCH(NH₂)COO^- + H₂O

Question 25

How can amino acids act as bases?

Answer 25

When put into an environment with pH lesser than that of its isoelectric point, an amino acid acts as a base. the -ve carboxyl (-COO^-) group on the zwitterion accepts protons from the acid in solution and reacts to form a positively charged ion. RCH(NH₃⁺)COO^- + H⁺ → RCh(NH₃⁺)COOH

Question 26

What is a condensation reaction?

Answer 26

A reaaction in which two small molecules react together to form a larger molecule with the elimination of a small molecule such as water.

Question 27

How are polypeptides formed?

Answer 27

When the amine group on one amino acid reacts with the carboxyl group on another in a condensation reaction (omitting a molecule of water), a peptide (amide) bond is formed which joins together the two amino acids to form a dipeptide. This process may repeat as more amino acids are added onto the ends of the growing peptide by condensation reactions, omitting a water molecule per new amino acid joined. This growing chain of amino acids is called a polypeptide and will form the basis of proteins.

Question 28

What is the formula for the formation of a peptide bond?

Answer 28

H₂N-CH(R)-COOH + H₂N-CH(R')-COOH → H₂N-CH(R)-**CO-NH**-CH(R')-COOH + H₂O Note how the -NH₂ on R amino acid could react with -COOH on R' amino acid to produce a structural isomer.

Question 29

What is hydrolysis?

Answer 29

The breaking of bond by reaction with water. Hydrolysis can be catalysed by acid or alkali.

Question 30

What is the process of acid hydrolysis of a polypeptide?

Answer 30

Conditions: Heat under reflux in a solution of concentrated aqueous HCl. Reactants: Polypeptide, H₂O, H⁺ Products: The amine salt form of the amino acid. Equation (Dipeptide equation shown, applies to polypeptides): H₂N-CH(R)-CO-NH-CH(R')-COOH + H₂O + 2H⁺ → H₃N⁺-CH(R)-COOH + H₃N⁺-CH(R')-COOH

Question 31

What is the process of alkaline hydrolysis of polypeptides?

Answer 31

Conditions: Heat under reflux in a solution of aqueous NaOH. Reactants: Polypeptide, OH^-. Products: Carboxyl salt form of the amino acid. Equation (Dipeptide equation shown, applies to polypeptides): H₂N-CH(R)-CO-NH-CH(R')-COOH + 2OH^- → H₂N-CH(R)-COO^- + H₂N-CH(R')-COO^- + H₂O

Question 32

What other types of condensation polymers are there (besides polypeptides)?

Answer 32

- Polyesters. - Polyamides.

Question 33

What is required for the formation of a condensation polymer?

Answer 33

- One type of monomer with two different types of functional groups that are able to react and bond to each other. - Two types of monomers with different functional groups, but with two identical functional groups in each molecule; that are able to react to and bond to each other.

Question 34

How are polyesters formed?

Answer 34

Reactants: One type of monomer if monomer is a hydroxycarboxylic acid. Two types of monomers; one being a dicarboxylic acid, the other a dialcohol. Products: Polyester, H₂O. Equation (hydroxycarboxylic acid): n HO-R-COOH → -[-O-R-CO-]-_n + n H₂O Equation (dialcohol + dicarboxylic acid): n HO-R-OH + n HOOC-R'-COOH → -[-O-R-O-OC-R'-CO-]-_n + 2n H₂O Ester bonds form between the hydroxyl (-OH) and carboxyl (-COOH) groups on the monomers, joining them together to form a polymer.

Question 35

What are examples and uses of polyesters?

Answer 35

- Terylene is a polyester formed by the condensation polymerisation of ethane-1,2-diol and benzene-1,4-dicarboxylic acid. It is often blended with cotton and used to make clothing and bedding. - Poly(lactic acid) is a polyester formed by the condensation polymerisation of lactic acid (2-hydroxypropanoic acid). It is used to make biodegradable plastics in bags and disposable cutlery.

Question 36

How are polyamides formed?

Answer 36

Reactants: One type of monomer if monomer is an amino acid. Two types of monomers; one being a dicarboxylic acid, the other a diamine. Products: Polyamide, H₂O. Equation (diamine + dicarboxylic acid): n H₂N-R-NH₂ + n HOOC-R'-COOH → -[-HN-R-NH-OC-R'-CO-]-n + 2n H2O Amide bonds form between the amine (-NH₂) and carboxyl (-COOH) groups on the monomers, joining them together to form a polymer.

Question 37

What are examples and uses of polyamides?

Answer 37

- Nylon-6,6 is a polyamide formed from the condensation polymerisation of 1,6-diaminohexane and hexane-1,4-dioic acid. It is commonly used in clothing as well as for rope making due to its great strength and lightness. - Kevlar is a polyamide formed from the condensation polymerisation of benzene-1,4-diamine and benzene-1,4-dioic acid. It is used mostly for protective clothing and bulletproof vests due to its lightness and strength.

Question 38

What is the process of acid hydrolysis of polyesters?

Answer 38

Conditions: Heated under reflux in aqueous HCl (catalyst). Reactants: Polyester, H₂O. Products: Hydroxycarboxylic acid/ dialcohol + dicarboxylic acid. Equations: - [-O-R-CO-]-_n + n H₂O → n HO-R-COOH - [-O-R-O-OC-R'-CO-]-_n + 2n H₂O → n HO-R-OH + n HOOC-R'-COOH

Question 39

What is the process of alkaline hydrolysis of polyesters?

Answer 39

Conditions: Heated under reflux in aqueous NaOH. Reactants: Polyester, OH^-. Products: Hydroxycarbonyl salt/ dialcohol + dicarboxylate salt. Equations: - [-O-R-CO-]-n + n OH^- → n HO-R-COO^- - [-O-R-O-OC-R'-CO-]-n + 2n OH^- → n HO-R-OH + n ^-OOC-R'-COO^-

Question 40

What is the process of acid hydrolysis of polyamides?

Answer 40

Conditions: Heated under reflux with aqueous HCl. Reactants: Polyamide/polypeptide, H⁺, H₂O. Products: Amine salt form of amino acid/ diammonium salt + dicarboxylic acid. Equation: -[-HN-R-NH-OC-R'-CO-]-n + 2n H₂O + 2n H⁺ → n H₃N⁺-R-NH₃⁺ + n HOOC-R'-COOH

Question 41

What is the process of alkaline hydrolysis of polyamides?

Answer 41

Conditions: Heated under reflux with aqueous NaOH. Reactants: Polyamide/polypeptide, OH^-. Products: Carboxylate salt form of amino acid/ diamine + dicarboxylate salt. Equation: -[-HN-R-NH-OC-R'-CO-]-_n + 2n OH^- → n H₂N-R-NH₂ + n ^-OOC-R'-COO^-

Question 42

What are the differences between addition polymers and condensation polymers?

Answer 42

_Functional groups involved_ Addition polymerisation: Alkene (C=C). Condensation polymerisation: Amine (-NH₂) and carboxyl (-COOH)/ alcohol (-OH) and carboxyl (-COOH). _Type of reaction_ Addition polymerisation: Addition reactions. Condensation polymerisation: Condensation reactions. _Number of monomers involved_ Addition polymerisation: 1 Condensation polymerisation: 1 or 2 _Products formed_ Addition polymerisation: Poly(alkene). Condensation polymerisation: (Polyester or polyamide) + H₂O. _Type of backbone:_ Addition polymerisation: C-C backbone. Condensation polymerisation: C-C backbone with amide (CO-NH) groups or ester (CO-O) groups at regular intervals between the once monomers.