Polymers and Synthesis Flashcards
What is the general formula for an alpha-amino acid?
What examiners want: RCH(NH(2))COOH
How I picture it: H(2)N-C(H)(R)-COOH
What is a zwitterion?
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-
What is the isoelectric point?
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
Describe the properties of amino acids at high and low pH
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
Describe the formation of a peptide linkage
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
Describe the acid hydrolysis of proteins
Conditions: 6mol/dm(3) HCl; heat under reflux.
Dipeptide + H(2)O + 2H(+) -> positive forms of each amino acid
Describe the alkaline hydrolysis of proteins
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
Define optical isomers
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
Define chiral carbon
A carbon atom attached to 4 different atoms or group of atoms
Define stereoisomers
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
Describe condensation polymerisation to form a polyester
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).
Describe condensation polymerisation to form a polyamide
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).
What is the difference between condesation and addition polymerisation?
- 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
What are polyesters and polyamides used in?
Fibres in clothing. Kevlar is a polyamide; polyesters can be machine-washed and dried.
Describe the acid hydrolysis of polyesters
Conditions: hot aqueous (H(2)O) acid
Get diol and dicarboxylic acid; acid as a catalyst
Describe the alkaline hydrolysis of polyesters
Conditions: hot aqueous (H(2)O) alkali
Get a di-salt of the carboxylic acid (e.g. -O(-)Na(+)) and a diol
Describe the acid hydrolysis of polyamides
Conditions: hot aqueous (H(2)O) acid
Get dicarboxylic acid and an ammonium salt of the diamine (as in -NH(3)(+))
Describe the alkaline hydrolysis of polyamides
Conditions: hot aqueous (H(2)O) alkali
Get a sodium salt of the dicarboxylic acid and a diamine
How can chemists aid minimisation of plastic waste?
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.
Draw out as much of the flow chart of the reactions of aliphatic functional groups that you can remember.
Have you done it? Check it on page 62 of the A2 chemistry text book.
Draw out as much of the flow chart of the reactions of aromatic functional groups that you can remember.
Have you done it? Hint: there are 2 sections (that aren’t linked). Check it on page 64 of the A2 chemistry text book
Why are pharmaceuticals often only 1 optical isomer?
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.
Explain the differences between producing chiral compounds in biology and in the lab
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.
What are the disadvantages of synthesising a pharmaceutical that is a single optical isomer?
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.
