Worked solutions to textbook questions 1
Chapter 14 From organic molecules to medicines
E1.
List the ailments that can be treated by using acetylsalicylic acid (aspirin).
AE1.
Aspirin is used as
· analgesic or painkiller
· anti-inflammatory, reduces inflammation
· antipyretic, reduces fever
· anticoagulant, reduces blood clotting, prevention heart attack and stroke
Recent research suggests that aspirin can be used to prevent complications arising from diabetes and dementia and as a preventative to bowel cancer.
E2.
Why should someone with stomach ulcers not take aspirin?
AE2.
In the stomach COX-1 (cyclo-oxygenase-1) enzymes produce prostaglandins that protect the stomach lining. COX-2 enzymes are present at the site of an inflammation producing prostaglandins that promote inflammation resulting in pain and fever. Aspirin inhibits the production of both COX-1 and COX-2 enzymes. So while it can treat pain, fever and inflammation it can also cause stomach irritation by also inhibiting those prostaglandins that protect the stomach.
Q1.
a Draw a flow chart showing the reaction pathways for the preparation of aspirin from salicylic acid using:
i ethanoic acid
ii ethanoic anyhdride
b Explain why the pathway using ethanoic anhydride is preferred over the one using ethanoic acid.
c Which pathway could be considered greener because it produces the least amount of waste?
A1.
a i
ii
b The rate and yield is greater using ethanoic anhydride.
c A green approach to chemical synthesis aims to maximise atom economy, the number of reactants atoms incorporated into the desired product. The idea of atom economy was explained in Heinemann Chemistry 1, Chapter 17. Percentage atom economy is given by:
percentage atom economy = × 100
For the reaction pathway using ethanoic acid, percentage atom economy
= × 100
= 91.1%
For ethanoic anhydride pathway, percentage atom economy
= × 100
= 75%
However, since the rate of reaction and the yield are greater using ethanoic anhydride, this is the preferred reaction.
Q3.
Why do the instructions on prescription antibiotics tell you to complete the whole course of tablets?
A3.
At the point when the symptoms of an infection disappear there may still be bacteria present in the body. If you cease taking antibiotics at this stage the bacteria may start multiplying again and the infection could return.
Q4.
Why were antibiotics like penicillin V preferred to penicillin G?
A4.
Penicillin V is more stable in the presence of stomach acid so can be given to the patient in tablet form. Penicillin G is more potent but has to be given in the form of an injection, requiring a visit to a medical clinic.
Q5.
What information about the molecular structure of a drug does each of these analytical techniques provide?
a infrared spectroscopy
b nuclear magnetic resonance spectroscopy
c mass spectroscopy
A5.
IR spectroscopy: Identity of any functional groups that may be present.
Proton NMR: The number of hydrogen environments present in the molecule.
Mass spectroscopy: Molar mass from the molecular ion; structure and identity from fragmentation pattern.
Q6.
Medicinal proteins such as insulin are administered by injection. Suggest why they cannot be given orally as tablets or capsules.
A6.
Since insulin is a protein—if it was given orally the substance would be broken down in the process of digestion before it could be absorbed.
Chapter review
Q7.
Why is the sodium salt of acetylsalicylic acid more soluble than normal aspirin?
A7.
Sodium acetylsalicylic acid is more soluble in water than acetylsalicylic acid because it dissociates into ions (Na+ and the acetylsalicylate anion) which then attract water molecules to form solvation sheaths around them.
Q8.
Briefly explain how aspirin is able to reduce pain and fever.
A8.
Aspirin blocks the action of the enzymes that control the production of prostaglandins – substances that cause the blood vessels to constrict – resulting in heat building up in the body to cause fever. Prostaglandins also make it easier for water to pass from blood capillaries into tissues where it causes swelling and pain.
Q9.
Acetaminophen (paracetamol) is the active agent in some over-the-counter pain killers. It can be synthesised from p-hydroxyaniline (4-hydroxyaminobenzene) and acetic anhydride. An incomplete equation for this reaction is shown in Figure 14.24.
a What by-product is formed in this reaction?
b Determine the atom economy of this reaction.
A9.
a ethanoic acid, CH3COOH. Note the similarity between this reaction and the reaction for the synthesis of aspirin.
b Percentage atom economy = ×100
= × 100
= 58%
Q10.
a The drugs celecoxib and rofecoxib were specifically designed to be COX-2 inhibitors. They do not affect the production of COX-1. Aspirin inhibits both COX-1 and COX-2. What are the benefits of these COX-2 inhibiting drugs over aspirin?
b Find out why rofecoxib, marketed as Vioxx, was withdrawn from use by the pharmaceutical company Merck.
A10.
a The COX-2 inhibitors only works on the enzymes that are present at the site of inflammation and not on those present in the stomach. Hence it does not cause stomach irritation or bleeding.
b Vioxx was withdrawn in 2004 because of concern that it has adverse side effects and may lead to the development of heart disease or stroke.
Q11.
From the description of the use of the Australian native plant with the Aboriginal name of bunaangu, what active ingredient might be present to give it these medicinal properties?
‘You only eat the sap of the bunaangu. It has a bitter taste. You boil it up to drink. It helps toothache and a bad chest.’
A11.
From the description of the bitter taste of bunaangu and the symptoms that it helps to relieve, it seems as though the plant might be a source of salicin or possibly salicylic acid.
Q12.
As well as aspirin, other analgesics exist such as acetaminophen, ibuprofen, naproxen sodium, ketoprofen. Use the Web to find out:
a the names under which they are sold
b their side effects
c their chemical structure
d how they are synthesised
A12.
The website ‘Aspirin Adventures’ provides an easy to read summary of various pain killers. http://www.chemheritage.org/educationalservices/pharm/asp/asp00.htm
Q13.
Codeine, morphine and heroin are drugs derived from the opium poppy. The molecular structure of these drugs is shown in Figure 14.25. Identify the part of each molecular structure that is common to all three drugs. How does the structure of these molecules differ?
Figure 14.25
Molecular structures of codeine, morphine and heroin. Caution—it is not possible to convert codeine into morphine or heroin by any simple laboratory process.
A13.
Different functional groups are attached to the benzene rings in each structure. Small changes to the molecular structure can result in significant changes to chemical or biochemical activity.
Q14.
Suggest some advantages and disadvantages of using genetically modified plants to produce therapeutic proteins.
A14.
Advantages: convenient source of the medicinal protein that can be grown rather than produced via chemical synthesis. Could be included in the diet rather than having to be taken as a medication or could be used to supplement inadequate food supplies.
Disadvantages: the effects of adding a medicinal protein to food has not been subjected to long-term studies. Also the modified gene may be transferred to other plants and result in unwanted harmful effects.
Q15.
Use the Web to find out about the diseases that are treated by the following medicinal proteins:
a growth hormone
b interferon
c monoclonal antibodies
A15.
a Growth hormone is used to treat children who fail to grow at a normal rate due to growth hormone deficiency.
b Interferon is used to treat Hepatitis C.
c Specific monoclonal antibodies can be used to treat certain types of cancer of the kidney, leukaemia or lymphoma. Cytotoxic (cancer killing) substances are joined to the antibody and the resulting complex is given to the patient. The antibody, with the cytotoxic agent attached, then binds only to the cancer cells and eliminates them. Sometimes just the antibody itself is sufficient to trigger the body’s normal defence mechanism and stimulate it to eliminate the cancer cells.
Q16.
Illustrate, using aspirin as an example, how biochemists modify the structure of drugs to eliminate undesirable side effects.
A16.
Salicylic acid was used as a pain killer but found was to found to cause bleeding of the stomach walls. Its structure was modified by reaction salicylic acid (aspirin) with ethanoic acid to form the ester, acetylsalicylic acid which while an effective pain killer did not have this side effect. A soluble form of aspirin was developed by converting the carboxylic acid functional group to a sodium salt.
Q17.
Explain why pharmaceutical companies manufacture drugs such as aspirin using chemical reactions, while biotechnological processes are used to produce drugs such as insulin.
A17.
The active ingredients in drugs such as aspirin consist of organic molecules that can be made via a chemical pathway from some fairly simple feedstock. However substances such as insulin are proteins that are made by the body. It would be very difficult to manufacture such a large molecule via a chemical pathway. Instead, they are made by adapting the structure of a related protein obtained from a biological source.
Unit 3 Area of Study 2 review
Multiple-choice questions
Q1.
The smallest molecule in which homologous series must contain two carbon atoms?
A an alkane
B an alkanol
C an alkene
D an alkanoic acid
A1.
C. There is a double bond between two carbon atoms in alkenes. The first members of the other homologous series, methane, methanol and methanoic acid all contain 1carbon atom.
Q2.
The systematic name for CH3CH2CH2CH(CH3)2 is:
A 1,1-dimethylbutane
B 2-methylpentane
C 2-methylpentene
D propyldimethylmethane
A2.
B. It may assist you to draw a full structural formula. This molecule is an alkane as there are no multiple bonds, therefore the name ends in ane There are 5 carbon atoms in the longest chain name must end in pentane. There is a methyl group on the second carbon atom from the end. This is represented as 2-methyl. The full name is 2methylpentane.
Q3.
Which compound would be expected to have the highest boiling point?
A CH3CH2CH2OH
B CH3CH2CH2CH3
C CH3CH2CH2Cl
D CH3CH2CH3
A3.
A. The alkanols have higher boiling points as the strength of the intermolecular forces are greater. The strength of hydrogen bonding in alkanols is greater than those in chloroalkanes. Only dispersion forces act between alkane molecules.
Q4.
Which of the following statements are true of the homologous series of primary alkanols?
I The members differ by one CH2 unit.
II They are all strong bases.
III They can be oxidised to form carboxylic acids.
A I and II
B II and III
C I and III
D I, II and III
A4.
C. Members of a homologous series differ by CH2. Alkanols can be oxidised to carboxylic acids by strong oxidants such as acidified permanganate or dichromate.
Q5.
What is the product formed by the reaction of CH2CH2 with Br2?
A CH2BrCH2Br
B CH2CHBr
C CH3CH2Br
D CHBrCHBr
A5.
A. Ethene, CH2CH2, will undergo an addition reaction with bromine. The double bond is broken and a bromine atom bonds to each carbon atom to form 1,2-dibromethane.
Q6.
When ethanol is heated under reflux with an acidified solution of potassium dichromate (VI), the final product has the formula:
A CH3CH2CH2OH
B CH3COOCH3
C CH3COOH
D CH3CH2CH3
A6.
C. Ethanol is oxidised to ethanoic acid by an acidified potassium dichromate solution.
Q7.
Consider the following reaction pathway:
I II III
ethene ® chloroethane ® ethanol ® ethanoic acid
The reactions that occur in steps I, II and III of the pathway are:
A cracking, addition, hydrolysis
B chlorination, substitution, addition
C addition, hydrolysis, oxidation
D addition, reduction, hydrolysis
A7.
C. Ethene undergoes an addition reaction with HCl to form chloroethane. This undergoes a hydrolysis reaction with water or NaOH to produce ethanol. Ethanol is oxidised to ethanoic acid by an acidified potassium dichromate solution.
Q8.
Acetylsalicylic acid could be formed from salicylic acid by warming it with:
(Chemically inactive hydrogen atoms have been omitted for clarity.)
A ethanol
B methanol
C ethanoic acid
D methanoic acid
A8.
C. The –OH group on salicylic acid undergoes an esterification reaction with ethanoic acid to form acetylsalicylic acid (aspirin).
Q9.
Consider the following molecular structures of substances that all function as drugs. (Chemically inactive hydrogen atoms have been omitted for clarity.)
The substances that are expected to have similar pharmaceutical effects are:
A I and II
B II and III
C I and III
D I, II and III
A9.
C. Compounds I and III have similar molecular structures and would be expected to have similar pharmacological effects.
Q10
The ester methyl ethanoate could be made by reacting together:
A CH3CH2OH and CH3COOH
B CH3CH2OH and HCOOH
C CH3OH and CH3CH2COOH
D CH3OH and CH3COOH
A10.
D. Methyl ethanoate is formed by an esterification reaction between methanol, CH3OH and ethanoic acid, CH3COOH.
Q11.
The correct systematic name for this substance is
A 2,4-dimethylhexane
B 2-methyl-4-ethylpentane
C 2-ethyl-4-methylpentane
D 1,1,3-trimethylpentane
A11.
A. The substance is an alkane. The longest chain contains 6 carbon atoms. There are methyl side chains on the 2nd and 4th carbon atoms.
Q12.
A carboxylic acid with five carbon atoms per molecule could have the formula:
A C5H11COOH
B CH3(CH2)4COOH
C CH3CH2CH2CH2COOH
D C3H7COOCH3
A12.
C. The carboxylic acid having five carbon atoms is pentanoic acid.
Q13.
Which statement about compounds I and II is not true?
A The 13C NMR spectrum of compound I has three peaks, and that of compoundII has two peaks.
B The low resolution 1H NMR spectrum of I has four peaks, and that of II has three peaks.
C The mass spectrum of both will show a peak at a mass-to-charge ratio of 60.
D The fingerprint region of the IR spectra will be identical for both compounds.
A13.
D. The compounds shown are propan-1-ol and propan-2-ol. They are isomers. The fingerprint region of the IR spectra of these compounds would be different as their structures are different.