Many drugs are optically active compounds. Often it is only one enantiomer that has the desired therapeutic effect. Thalidomide, a drug used to treat morning sickness in pregnant women in the 1960s is an example of this.
The two enantiomers of Thalidomide
The R isomer (pictured on the right above) acts as a sedative while the S isomer (pictured on the left) has teratogenic properties that resulted in thousands of children being born with deformities.
Thalidomide was produced and sold as a racemic mixture of the two enantiomers which resulted in the birth defects. Even if it was produced in a pure enantiomeric form, this wouldn’t have solved the problem as there is evidence that the enantiomers can convert under physiological conditions. Mass producing pure enantiomeric forms of drugs can be expensive and low yielding.
Taxol is another drug that has a complex level of chirality. In total, Taxol has 11 asymmetric carbon atoms. Taxol is used to treat cancer and is a naturally occurring substance that is found in the bark of Yew trees.
The structure of Taxol
Obtaining Taxol from its natural source is extremely low yielding and causes environmental damage as it destroys the trees. Synthesising the drug from scratch in the laboratory would require a large multistep process that would also prove low yielding and extremely costly.
As such, a compromise is found and the drug is produced by a semi-synthetic route starting with a material found in the needles of the Yew tree.
This is a more advantageous method as harvesting the needles of the tree does not cause as much environmental damage and significantly lessens the number of steps needed to manufacture the final product. In addition, the overall yield of Taxol is improved compared to extraction from the bark.
The challenge in the synthesis is creating the correctly oriented chiral centres. This is done through a method known as asymmetric synthesis.
Asymmetric synthesis involves using a chiral auxiliary attached to the molecule to force through steric hindrancethe predominance of one enantiomer over another.
Consider the achiral molecule below. It is possible to make two enantiomers by adding an amino group to carbon 2.
If you want to favour the production of one enantiomer over the other, attaching a chiral auxiliary molecule to force the addition of the amino group by blocking the path of attack from one side of the molecule. Below is an example of one such chiral auxiliary.
Notice how the addition of the chiral auxiliary hinders attack from one side of the molecule. This doesn’t eliminate the production of the unwanted enantiomer but greatly decreases it.
Once the reaction is complete, the chiral auxiliary can be removed for reuse and the final product isolated.
✍️ In your notebook, draw an annotated diagram to represent the ideas on the structure of the atom from each of the following scientists:
Structure of the Atom
What does an atom mainly consist of?
✍️ To get an idea of the proportions within an atom, look at the following video. After watching it, write a brief summary of the main points in your notebook.
✍️ Examine table 4 in your data booklet. How much more mass does a proton or neutron have compared to an electron?
Take a look at the figure below describing nuclear symbols.
What is the difference between the mass number and the atomic number?
✍️ For each of the three examples above, write down the number of protons, electrons and neutrons represented by the notation.
Below is a simulation that can be used to practice this notation. Choose the game option and try all the puzzles. Don’t move to the next puzzle until you get 5 out of 5!
Table 3 in your data booklet has a version of the electromagnetic spectrum.
✍️ Using Table 3 and the graphic above, describe the relationships between colour, wavelength, frequency and energy across the spectrum. Make a summary in your notebook.
Find an example of:
an emission spectrum
a line spectrum
an absorption spectrum
What are the key differences and similarities?
Which of the above were we looking at when we viewed the purple light given off from the hydrogen sample through the diffraction grating in our last class?
Watch the two videos below on the emission spectrum of hydrogen and read the section in your text. Make a summary for discussion in class tomorrow.
Oxidation and reduction can be considered in terms of:
oxygen gain or hydrogen loss,
electron transfer OR
change in oxidation number
Consider the following reaction:
Zn(s) + Cu2+(aq) –> Zn2+(aq) + Cu(s)
Take a look below for a model of what is happening on an atomic level.
✍️ Looking at the change in charges, which species is gaining electrons and which species is losing electrons?
✍️ Define the terms oxidising agent and reducing agent. Identify which is which in the above reaction.
To work out which species is being oxidised and which is being reduced, it is possible to assign oxidation numbers. Below is a short summary of the rules for assigning oxidation numbers. Alternatively, click here.
✍️ For each of the following, work out the oxidation number of the underlined element
You can use oxidation number to decide which species is oxidised and which species is reduced in a reaction.
Oxidation number increases = species is oxidised
Oxidation number decreases = species is reduced
In our next lesson, we will carry out a redox titration. In order to practice the calculation and to understand the process, try a few of the problems found at the link below. Working is given for you if you get stuck but try to do the problems before looking through the answers!
Watch the slide show to see the different steps and colour changes occurring when you use the Winkler method to determine oxygen concentration. Try to identify which equation is happening when and what species the colours are due to!
✍️ Read chapter 9.1 of your text book. Use this and any other sources to add anything important to your notes to make a complete summary of this section.
2. Electrochemical Cells
An electrochemical cell or voltaic cell can be created to generate electricity. This is how batteries work. Above is a simplified diagram of the Daniel Cell which is formed using two half cells – copper and zinc.
✍️ Sketch the diagram in your notebook and then add the following to the diagram:
the 1/2 cell reaction for each cell
the direction of electron flow through the wires
the direction of ions moving in the salt bridge
the overall cell reaction
The above cell can be simplified using cell diagram convention. It would look like this:
Zn(s)⎮Zn2+(aq) ⎮⎮ Cu2+(aq)⎮Cu(s)
The rules for this type of shorthand is to always put the anode before the cathode and to separate the two solutions by the salt bridge (⎮⎮).
3. Electrolytic Cells
Electrolysis of a molten salt
Watch the video on the electrolysis of molten lead bromide. This was the demonstration you saw in class. Make sure you understand what is happening at the anode and the cathode.
✍️ Predict the products when you electrolyse the following molten salts. State what you get at the anode and the cathode.
a) zinc chloride
b) aluminium oxide
✍️ Draw a diagram for the electrolysis of zinc chloride. Include the cathode and anode, the direction of electron flow and the half equations that occur at each electrode. Use the example given in class as a guide.
Electrolysis of Aqueous Solutions (HL)
When you have a molten salt, there is only one possible half reaction able to occur at each electrode. Once you add water by electrolysing a solution of that salt, you add another species. Water can be oxidised and/or reduced. You can find these 1/2 equations in your data booklet.
Possible reactions for a solution of a general salt (MA) being electrolysed:
CathodeM+or H2O is reduced AnodeA–or H2O is oxidised
The equation with most positive E⦵ is reduced/oxidised in preference.
Electrolysis of Water
Below is the apparatus that can be used to demonstrate the hydrolysis of water.
Think about the following questions:
Dilute acid or sodium hydroxide is used instead of pure water. Why?
The demo is done with dilute sodium hydroxide solution. Write a list of all the species present in the solution.
What are the possible reactions at the cathode?
What are the possible reactions at the anode?Watch the video of the electrolysis of water using a dilute sodium hydroxide solution.
After observing the demonstration, determine which reaction was taking place at each electrode.
Write the overall reaction for the electrolysis of water.
Write a summary in your notes in your own words about the electrolysis of water.
Effects of Concentration and Nature of Electrodes on the Products of Electrolysis
Consider the electrolysis of a dilute solution of sodium chloride using graphite electrodes.
✍️ What species are present in solution?
✍️ What possible reactions can occur at the a) anode and b) cathode? Make a prediction of which reaction you think is most likely to occur.
A dilute solution of sodium chloride with 5 mL of universal indicator was electrolysed for a few seconds. Below is the before and after pictures of the apparatus.
✍️ From your observations, what products were formed at the anode and cathode?
✍️ What would happen if a concentrated solution of sodium chloride was electrolysed instead?
Nature of the electrode
Consider the electrolysis of a dilute copper sulfate solution with a) inert graphite electrodes and b) copper electrodes.
✍️ For the two situations, predict what the products would be at both the anode and the cathode. Remember that if you are using a reactive metal like copper as the electrode, then that can also participate in the electrolysis.
Determining the mass of products produced during electrolysis
The amount of products produced depends on the following factors:
the voltage applied and therefore the current in the circuit
the time the solution was electrolysed for
the charge on the ion
A dilute solution of copper sulfate was electrolysed for 20 minutes using a current of 1.50 A. We can predict the mass of copper metal formed at the cathode. Consider the following half-equation:
Cu2+(aq) + 2e– –> Cu(s)
From this equation, if we know how many moles of electrons we have used in the circuit, then we can find how many moles of copper metal and therefore what mass of metal was deposited.
Step one, is to use the following equation to work out how much charge was supplied in the circuit.
Q = It
Q = charge measured in Coulombs (C)
I = current measured in Amperes (A) or Joules/second (J s-1)
t = time measured in seconds (s)
If the current supplied was 1.50 A and the time was 20 minutes, then the charge can be calculated thus:
Q = It
= 1.50 x 20 x 60
= 1800 C
Step two, is to work out how many moles of electrons have that much charge. There is a constant in your data booklet called the Faraday’s Constant (F). It is the amount of charge carried by 1 mole of electrons and the value is 9.65 x 104 C mol-1.
Therefore we can use the following equation:
n(e–) = Q / F
n(e–) = number of moles of electrons measured in moles (mol)
Q = charge measured in Coulombs (C)
F = Faraday’s constant measured in Coulombs per mole (C mol-1) and found in your data booklet.
Calculating the number of moles of electrons is as follows:
n(e–) = Q / F
= 1800 / 9.65 x 104
= 1.9 x 10-2 moles
Step three, uses the mole ratio from the half equation above to determine the number of moles of copper deposited.
n(Cu) = 1/2 x n(e–)
= 1/2 x 1.9 x 10-2
= 9.3 x10-3 moles
Step four, converts moles to mass!
m(Cu) = n(Cu) x Ar(Cu)
= 9.3 x 10-3 x 63.55
= 0.59 g
Work through the problems found here. Your answers may be a little different from those supplied as they use a slightly different value for Faraday’s constant and molar volume of an ideal gas. You should use the values given by the IB.
Depending on whether the halogenoalkane is primary, secondary or tertiary, depends on the mechanism for this reaction.
Primary halogenoalkanes tend to react via a SN2 mechanism.
Tertiary halogenoalkanes tend to react via a SN1 mechanism.
Secondary halogenoalkanes use either and you can’t predict which one.
Examine the two mechanisms. They are written for any halogenoalkane and any nucleophile.
Key to the mechanisms:
L = leaving group. This is the halogen F, Cl, Br or I.
Nu– = nucleophile. This could be OH– or any other species with a lone pair.
Now have a look at the following animation. Here is the link if you want to see the original.
There is more than one type of mechanism here so choose unimolecular nucleophilic substitution for SN1 and bimolecular nucleophilic substituion for SN2.
✍️ After examining the mechanisms and the animation, try and answer the following questions:
What does S and N stand for in the notation of the mechanism (SN2)?
The numbers 1 and 2 stand for the molecularity of the mechanism. What does this mean?
Define the terms unimolecular and bimolecular.
Which mechanism has a carbocation intermediate? Identify it.
Which mechanism forms a transition state?
What are the coloured arrows trying to indicate in the SN1 mechanism?
Conditions for the Reactions
SN1 reactions are best conducted using protic, polar solvents. SN2 reactions are best conducted using aprotic, polar solvents.
Polar, aprotic solvents include:
Polar, protic solvents include:
propan-1-ol and propan-2-ol
✍️ Draw the structures for the two groups of solvents.
✍️ What is the difference between an aprotic and a protic solvent?
Rate of Reaction
Examine the two mechanisms again.
✍️ Write a rate equation for each mechanism.
How does the type of halogen affect the rate of reaction?
✍️ Using your data booklet fill in the electronegativities and bond enthalpies for the difference carbon-halogen bonds.
C-X Electronegativity Bond enthalpy
✍️ What trends to you notice? Discuss these with your table.
✍️ Which halogenoalkane would a nucleophile be most attracted to?
Despite the polarity of the bonds, the most important factor in determining rate is bond strength.
✍️ Knowing this, rank the halogenoalkaness in order from fastest to slowest for reaction with a nucleophile.
Everything you need to know about these two mechanisms is summarised on this sheet here.
✍️ Before trying the review questions, read the relevant section in your text and annotate your notes with any extra important information.
The use of ethanol as a fuel is growing around the world. It is hailed as a more environmentally friendly fuel than fossil fuel because the carbon dioxide released from burning the fuel was what the crop absorbed whilst it was growing meaning that no new carbon dioxide has been added to the atmosphere.
Can you see a problem with this logic? Take a look at the cycle of ethanol production and use below. How ‘green’ is ethanol as a fuel?
Production and Use of Ethanol as a fuel
There has been a lot written about ethanol as an alternative fuel. If you’re interested, here are a couple of articles with more information:
✍️ Write equations for the complete combustion of methanol, propanol and butanol.
Oxidation of Alcohols
1. Primary, Secondary and Tertiary Alcohols
✍️ Draw the structures and name all the alcohols with molecular formula C4H10O.
✍️ Classify these into primary, secondary and tertiary alcohols.
2. Common Oxidising Agents
In the next unit (Topic 9/19 of your syllabus) we will discuss these in more detail. However, for now, we will look at two reagents that are used for oxidising alcohols:
acidified potassium permanganate (VII) KMnO4 OR
acidified sodium dichromate (VI) Na2Cr2O7
Either of these two reagents can be used. It is important to learn what their colours before and after reaction.
3. An Experiment
A student decided to look at what types of alcohols were able to be oxidised. She decided to use the following alcohols:
✍️ Draw the full structural formula for each of the alcohols above.
✍️ Classify them as either primary, secondary or tertiary.
She decided to try reacting the alcohols with acidified sodium dichromate(VI) in one trial and acidified potassium permanganate(VII) in the other. She set up the two trials as shown below with these reagents.
Acidified potassium permanganate(VII) BEFORE reaction with alcohols.
Acidified sodium dichromate(VI) BEFORE reaction with alcohols
Into the wells, she put two drops of the following alcohols:
A1 Ethanol A2 Propan-1-ol A3 Propan-2-ol A4 2-methylpropan-2-ol
and B1 or B2 – no alcohol as this was the control
After 15 minutes, she observed the following changes.
Acidified sodium dichromate(VI) AFTER reaction with alcohols
Acidified potassium permanganate(VII) AFTER reaction with alcohols
✍️ From her results, which types of alcohols (primary, secondary and/or tertiary) undergo oxidation?
✍️ The tray with the potassium permanganate(VII) showed a reaction but a brown precipitate formed in the wells. What is this?
For now we aren’t going to worry about trying to balance these redox equations but instead just focus on what happens to the alcohol.
This will depend on whether the alcohol is primary secondary or tertiary. Below is a diagram representing the different possibilities for the oxidation of alcohols.
✍️ After examining the chart, what were the products of the reactions in each of the wells A1, A2, A3 and A4?
4. Techniques for Oxidising Alcohols
Heating under reflux
Techniques for heating and recovering products in organic chemistry.
✍️ Using the above chart, what would be the products when the following are oxidised under the conditions specified:
Butan-1-ol is reacted with stoichiometrically equivalent amounts of acidified potassium permanganate (VII) and the product is removed by distillation as it is formed.
Methanol is reacted with excess acidified sodium dichromate (VI) and heated under reflux before the product is removed by distillation.
Butan-2-ol is reacted with excess acidified potassium dichromate (VI) and heated under reflux before the product is removed by distillation.
Methylpropan-2-ol is heated under reflux with excess potassium permanganate (VII).
Esterification is a type of condensation reaction where an alcohol and a carboxylic acid are combined to form an ester.
Some important points to note are:
this is a reversible reaction so a 100% yield is impossible to obtain
reaction requires heat
reaction requires an acid catalyst usually in the form of concentrated sulfuric acid
This concludes the material for standard level. You should now read the section in your text book (10.2) which is relevant to alkenes and addition polymerisation and add any thing else you find important to your notes.