Alcohols as fuels
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:
Corn Biofuel Dangerously Oversold – New Scientist
Can Ethanol from Corn be Made Sustainable – Scientific American
The complete combustion of ethanol is as follows:
C2H6O(g) + 3O2(g) –> 2CO2(g) + 3H2O(g)
✍️ 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
- 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:
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
- esters are often fragrant and many have fruity smells
✍️ Write the equation (using structural formula for all organic compounds) between ethanol and butanoic acid. Name the ester produced.
Reaction Pathways (HL only)
So far we have talked about alkanes, alkenes and alcohols. We have also made halogenalkanes, aldehydes, ketones, carboxylic acids and esters in our discussions.
✍️ Discuss at your table how you could make ethanoic acid from ethene. What reagents would you need and under what conditions (heat, reflux, distillation) would you use at each step?
✍️ Construct a map that connects the types of compounds we have discussed so far. Over the arrows, put the conditions and reagents needed for the reactions.