Polymers and Synthesis

Question 1

What is the general formula for an alpha-amino acid?

Answer 1

What examiners want: RCH(NH(2))COOH

How I picture it: H(2)N-C(H)(R)-COOH

Question 2

What is a zwitterion?

Answer 2

A dipolar ionic form of an amino acid that is formed by the donation of an H(+) from the COOH to the NH(2) group. No overall charge is present. End up with:
+H(3)N-C(H)(R)-COO-

Question 3

What is the isoelectric point?

Answer 3

The pH value at which the amino acid exists as a zwitterion. Where there is no net electrical charge. Each amino acid has an individual isoelectric point, that isn’t necessarily pH 7. A lot are closer to pH 6, though some are much higher/lower

Question 4

Describe the properties of amino acids at high and low pH

Answer 4

At a pH that is lower (more acidic) than the isoelectric point, the amino acid acts as a base, accepting protons. The proton is accepted onto the COO- group that has formed because of the zwitterion. A positively charged ion is formed
At a pH that is higher (more alkaline) than the isoelectric point, the amino acid acts as an acid, donating protons. The proton is donated from the NH(3)+ group that has formed because of the zwitterion. A negatively charged ion is formed

Question 5

Describe the formation of a peptide linkage

Answer 5

The COOH group on one amino acid joins to the NH(2) group on another, eliminating water (condensation reaction) and forming a peptide linkage:
-C(O)-N(H)-
where the O is double bonded to the C
Can form 2 different dipeptides from the same 2 amino acids; depends which ends join
Can form a polypeptide by adding subsequent amino acids to the exposed NH(2) and COOH groups on each end

Question 6

Describe the acid hydrolysis of proteins

Answer 6

Conditions: 6mol/dm(3) HCl; heat under reflux.

Dipeptide + H(2)O + 2H(+) -> positive forms of each amino acid

Question 7

Describe the alkaline hydrolysis of proteins

Answer 7

Conditions: aqueous sodium hydroxide; just above 100degC
Protein + NaOH -> 2 smaller proteins, ending in -NH(2) and -COO(-)Na(+)
Dipeptide + 2NaOH -> 2 amino acids + H2O

Question 8

Define optical isomers

Answer 8

Optical isomers, or enantiomers are stereoisomers that are non-superimposable mirror images of each other. One occasion of occurrence is when there are 4 different groups on a carbon atom

Question 9

Define chiral carbon

Answer 9

A carbon atom attached to 4 different atoms or group of atoms

Question 10

Define stereoisomers

Answer 10

Species with the same structural formula but with a different arrangement of atoms in space. Examples of this are isomers that exhibit E/Z isomerism or optical isomerism

Question 11

Describe condensation polymerisation to form a polyester

Answer 11

Monomers have a -COOH group and an -OH group. These groups react together to form an ester link, with the elimination of water (OH from COOH and H from OH). Polyesters come in 2 forms: those made from 2 monomers (a dicarboxylic acid and a diol) and those made from 1 monomer (a hydroxycarboxylic acid monomer - an OH and a COOH group).

Question 12

Describe condensation polymerisation to form a polyamide

Answer 12

Monomers have a -COOH group and an -NH(2) group. These groups react together to form an amide link (like a peptide bond), with the elimination of water. Polyamides come in 2 forms: those made from 2 monomers (a dicarboxylic acid and a diamine) and those made from one monomer (an amino acid; proteins and polypeptides are examples of this kind of polyamide).

Question 13

What is the difference between condesation and addition polymerisation?

Answer 13

• number of functional groups needed (1 for A; 2 for C)
• main chain is continuous Cs in A (C contains Os or Ns)
• only need 1 monomer for A, often need 2 for C
• no other products for A, also get water in C

Question 14

What are polyesters and polyamides used in?

Answer 14

Fibres in clothing. Kevlar is a polyamide; polyesters can be machine-washed and dried.

Question 15

Describe the acid hydrolysis of polyesters

Answer 15

Conditions: hot aqueous (H(2)O) acid

Get diol and dicarboxylic acid; acid as a catalyst

Question 16

Describe the alkaline hydrolysis of polyesters

Answer 16

Conditions: hot aqueous (H(2)O) alkali

Get a di-salt of the carboxylic acid (e.g. -O(-)Na(+)) and a diol

Question 17

Describe the acid hydrolysis of polyamides

Answer 17

Conditions: hot aqueous (H(2)O) acid

Get dicarboxylic acid and an ammonium salt of the diamine (as in -NH(3)(+))

Question 18

Describe the alkaline hydrolysis of polyamides

Answer 18

Conditions: hot aqueous (H(2)O) alkali

Get a sodium salt of the dicarboxylic acid and a diamine

Question 19

How can chemists aid minimisation of plastic waste?

Answer 19

They can develop degradable polymers. An example is those similar to poly(lactic acid). They contain bonds that can be hydrolysed, e.g. at an ester/amide link.
Photodegradable polymers can also be made: a C=O bond can absorb radiation and break, fracturing the polymer chain.

Question 20

Draw out as much of the flow chart of the reactions of aliphatic functional groups that you can remember.

Answer 20

Have you done it? Check it on page 62 of the A2 chemistry text book.

Question 21

Draw out as much of the flow chart of the reactions of aromatic functional groups that you can remember.

Answer 21

Have you done it? Hint: there are 2 sections (that aren’t linked). Check it on page 64 of the A2 chemistry text book

Question 22

Why are pharmaceuticals often only 1 optical isomer?

Answer 22

Each optical isomer often has a different effect on the body, with one aiding it (alleviating symptoms) whilst the other being potentially harmful. Therefore, the synthesis of pharmaceuticals often requires only 1 optical isomer to be produced. By producing only the required optical isomer, drug doses can also be reduced.

Question 23

Explain the differences between producing chiral compounds in biology and in the lab

Answer 23

In the lab, the product will be a mixture of the 2 optical isomers that will then have to be separated. In biology, enzymes ensure that only one isomer is formed, as the other will not fit into the active site of the enzyme.

Question 24

What are the disadvantages of synthesising a pharmaceutical that is a single optical isomer?

Answer 24

Increases costs, as separation techniques are time consuming and expensive. Techniques used are enzymes, electrophoresis and chromatography. The non-active isomer has to be disposed of.

Question 25

What are the advantages of synthesising a pharmaceutical that is a single optical isomer?

Answer 25

Reduces possible side effects caused by the non-active isomer and improves pharmacological activity. Dosage can be reduced.

Question 26

How are pharmaceuticals synthesised in the modern era to always give a single optical isomer?

Answer 26

• enzymes are used as biological catalysts (or bacteria) to ensure only 1 isomer is produced
• chiral pool synthesis: using a naturally occurring chiral molecule (will only be in 1 form) within the synthetic route, leading to 1 isomer (e.g. using alpha amino acids and sugars)
• use chemical chiral synthesis/chiral catalysts: using transition metal complexes to produce chiral catalysts that transfer their chirality to the single isomer product.