Wine, beer, champagne, or maybe a shot of something stronger - many of us have a favourite drink. Alcoholic beverages remain a popular part of our lives not just because of their taste, but thanks to the weird and wonderful effects they have on the body. The molecule responsible for the feeling of joy and relaxation associated with drinking is ethanol. But have you ever considered how it is produced?
- This article is about the production of ethanol.
- We'll first define ethanol before learning about the two ways by which it is produced: the fermentation of glucose, and the hydration of ethene.
- This will involve considering the reactions' conditions, equation, method, and byproducts. We'll also compare their advantages and disadvantages.
- We'll finish by looking at the use of ethanol as a biofuel.
What is ethanol?
Ethanol (C2H5OH; structural formula CH3CH2OH) is an organic molecule with the hydroxyl functional group (-OH). It belongs to the alcohol homologous series. In particular, ethanol is a primary alcohol with two carbon atoms, containing just one hydroxyl group. Its structure is shown below.
Fig. 1: The structure of ethanol, with its hydroxyl group highlighted.StudySmarter Originals
Ethanol has many uses in our daily lives. We don't just find it in the wide variety of alcoholic drinks available in shops, bars, and restaurants, but in disinfectants, solvents, and as a fuel additive. Believe it or not, ethanol is also used as an antidote to highly-dangerous methanol poisoning! As a result, the production of ethanol is important to understand. Let's now consider the two main ways ethanol is produced industrially.
Production of ethanol process
There are two main ways in which ethanol is produced:
- The fermentation of glucose.
- The hydration of ethene.
The two processes have a few key differences in terms of their reactants, conditions, and byproducts. They also both have their advantages and disadvantages. We'll start with an overview of the two methods before comparing them in more detail.
Production of ethanol: Fermentation of glucose
Ethanol can be produced through the fermentation of glucose. In this process, specific strains of yeast convert glucose (C6H12O6) from plant carbohydrates into ethanol inside a fermenter with set environmental conditions. Most alcoholic beverages made in breweries undergo this fermentation process.
Fermentation process
The plant carbohydrates, the starting materials in the production of ethanol, typically come from crops such as sugar cane or sugar beet. The yeast, which could be a species such as Saccharomyces cerevisiae, contains enzymes responsible for converting the glucose in plant carbohydrates into ethanol via anaerobic respiration. The reaction also produces carbon dioxide (CO2).
Anaerobic respiration is a process in which glucose is broken down to generate energy (ATP) in the absence of oxygen.
Ethanol is toxic to yeast in high concentrations. Consequently, the yeast dies out when the solution reaches levels of about 15% ethanol. This means that the production of ethanol by fermentation is a batch process. Thanks to ethanol's boiling point, which is lower than that of water, the ethanol can then be separated from the rest of the solution by fractional distillation. You should also note that fermentation has a slow rate of reaction, especially when compared to the hydration of ethene (which we'll look at in just a second).
Fermentation word equation
The chemical equation for fermentation is shown below, alongside the word equation.
$$C_{6}H_{12}O_{6}\rightarrow 2C_{2}H_{5}OH+2CO_{2}$$
$$Glucose \rightarrow ethanol + carbon\ dioxide$$
Fermentation starting materials and by-products
We mentioned that fermentation requires plant carbohydrates. These frequently come from crops such as sugar beet or corn. Crops are quick and easy to grow, and often just the discarded part of the plant (such as the stem or husk) is used in fermentation, leaving the edible portion for human or animal consumption.
You can see from the chemical equation that fermentation releases carbon dioxide (CO2), a greenhouse gas. This may seem harmful from an environmental point of view. However, fermentation itself is technically carbon neutral - all of the carbon released was taken in via photosynthesis during the crop's lifetime, and we'll support this statement with chemical equations later on in the article.
Fermentation doesn't break down all of the plant material and so we are left with a residual byproduct. The leftover matter makes a more-than-suitable cattle feed.
Fermentation conditions
Effective fermentation requires certain conditions. These are shown below alongside the reasoning behind them.
| Condition | Reasoning |
| The temperature is kept at 35 oC | Maximises product yield. A lower temperature decreases the rate of reaction whilst a higher temperature denatures the enzymes involved in anaerobic respiration. |
| Oxygen is excluded from the reactor (anaerobic conditions) | Prevents ethanol from oxidising into ethanoic acid. |
| Yeast | Provide enzymes to catalyse the reaction. |
Production of ethanol: Hydration of ethene
Now that we've finished looking at the chemistry of fermentation, let's see how we can produce ethanol via the hydration of ethene. This is another method of alcohol production used in industry.
Hydration process
The term hydration gives you a clue about this way of ethanol production. If you picture a reaction where water is involved, you're on the right track!
Hydration is an example of an electrophilic addition reaction. Essentially, it involves adding steam to ethene in the presence of a phosphoric (V) acid catalyst, resulting in ethanol. Ethene (CH2CH2) is the simplest alkene, an unsaturated hydrocarbon with a C=C double bond. Adding steam to ethene turns this alkene into a saturated molecule and adds the hydroxyl (-OH) group to one of its carbon atoms. The reaction is reversible, but we can alter the conditions to increase the yield considerably.
Fig. 2: Ethene to ethanol. Both molecules are shown with their functional groups highlighted.StudySmarter Originals
We separate the ethanol from the unreacted gases by cooling the mixture. Although a small amount of steam condenses as well, the ethanol collected is essentially pure. Fractional distillation takes care of the steam impurities. Any unreacted gases are continuously recycled over the catalyst to achieve an overall conversion rate of around 95%. You should also note that this process has a fast rate of reaction.
Hydration equation
Here's the chemical equation for the hydration of ethene:
$$C_{2}H_{4(g)} + H_{2}O_{(g)} \rightleftharpoons C_{2}H_{5}OH_{(g)} $$ ΔHº = -45 kJ mol-1
Note the use of the equilibrium arrow (⇌). It shows us that this reaction is reversible and exists in a state of dynamic equilibrium.
Pay attention to the wording of exam questions. For example, we've used molecular formulae in the equation above, but a question might ask for structural formulae. Here's the equation, rewritten using structural formulae:
CH2CH2(g) + H2O(g) ⇌ CH3CH2OH(g)
Hydration mechanism
Some exam boards require you to understand the mechanism for ethene hydration. If yours does, don't worry - we've got you covered.
As we mentioned, the hydration of ethene is an example of an electrophilic addition reaction. It has a similar mechanism to many of the other electrophilic addition processes we discuss in the article Reactions of Alkenes, so we'd recommend checking those out first if you are unfamiliar with this sort of reaction.
Here's how it works.
- In the first step, one of phosphoric acid's hydrogen atoms is attracted to the high electron density of ethene's C=C double bond. The hydrogen adds on to ethene, forming a positive carbocation and a negative dihydrogenphosphate(V) ion (H2PO4-).
- The carbocation is attacked by a lone pair of electrons from a water molecule. This forms an intermediate ion with a positive -OH2+ group.
- The negative dihydrogenphosphate(V) ion formed in the first step attacks one of the hydrogen atoms from the intermediate ion's -OH2+ group. The hydrogen adds on to the dihydrogenphosphate ion, regenerating the catalyst and leaving us with ethanol (C2H5OH).
You can simplify the mechanism by removing the negative dihydrogenphosphate ion, as shown below. We instead use just H+ to represent the acid catalyst.
Fig. 3: The simplified mechanism for the hydration of ethene.StudySmarter Originals
Hydration starting materials and by-products
The chemical equation above tells us that hydration doesn't produce any by-products. This might make it look like an environmentally-friendly process, but we must consider the bigger picture. Hydration requires ethene, which is produced by cracking hydrocarbons found in crude oil. Crude oil is non-renewable - it is a finite resource, meaning that it can't be replaced naturally at the rate that we are using it. Therefore, producing ethanol by hydrating ethene isn't sustainable.
Hydration conditions
Because the hydration of ethene is reversible, the reaction's conditions are vital when it comes to determining yield. The table below summarises the conditions used in ethene hydration and the reasoning behind them.
| Condition | Reason |
| Temperature of 300 °C | The forward reaction is exothermic. This means that a lower temperature favours the forward reaction. However, too low of a temperature slows the rate of reaction and so a compromise temperature is used. |
| Pressure of 60 - 70 atm | The forward reaction produces fewer moles of gas. This means that a higher pressure favours the forward reaction. However, too high of a pressure is expensive to maintain and often causes the ethene to polymerise into poly(ethene). Therefore, a compromise pressure is used. |
| Phosphoric acid catalyst | The catalyst speeds up the rate of reaction, increasing yield. |
| Excess of ethene | An excess of the reactants favours the forward reaction. However, using an excess of steam dilutes the catalyst and negates its effect, so ethene is chosen instead. |
Comparing methods of ethanol production
Deciding on the best way to produce ethanol isn't an easy task. There are advantages and disadvantages to both methods that must all be taken into consideration. The table below summarises the differences between ethanol production via the fermentation of glucose and the hydration of ethene.
| Fermentation | Hydration |
| Starting materials | Plant carbohydrates | Ethene, steam |
| Catalyst | Enzymes from yeast | Phosphoric acid |
| Conditions | Low temperature (35 °C), anaerobic | Relatively high temperature (300 °C), high pressure (60 -70 atm) |
| Rate of reaction | Slow | Fast |
| Continuous/batch processing | Batch | Continuous |
| Sustainability of starting materials | Renewable | Non-renewable |
| Purity of final product | Impure | Essentially pure |
Despite hydration's improved product purity and faster rate of reaction, fermentation is used more widely for ethanol production. This is because of its lower costs and environmental impact.
Production of ethanol: Using ethanol as a biofuel
In the last 50 years or so, ethanol has established itself as a handy alternative to petrol or diesel. It burns just like any other fuel and is a useful way of powering our vehicles. For example, E10 petrol (containing up to 10% ethanol) is the standard fuel-grade in the UK, whilst in France you can find E85 petrol stations. Brazil even owns an extensive fleet of vehicles that run on 100% ethanol!
The ethanol used in this way is produced using fermentation, and so is a type of biofuel.
Biofuels are renewable fuels derived in a short period of time from biomass, which simply means any organic living matter.
Like all biofuels, so-called bioethanol is derived from renewable resources. On the other hand, petrol and diesel come from crude oil, which we've already learned is non-renewable. Other popular fuel sources, like coal and gas, are also non-renewable - they are types of fossil fuels. The renewability of biofuels gives them an advantage over coal, gas, diesel, and petrol, and is partly why they are increasing in popularity so rapidly.
However, there are negatives to using biofuels, including bioethanol. We'll now discuss both the pros and cons of their use.
Advantages of biofuels
Biofuels manage to one-up traditional fossil fuels or crude oil derivatives in many ways. Here are a few:
- Biofuels are renewable. They are made from fast-growing plant or animal matter - often from crops that can be planted and harvested in less than a year. Further crops can then be replanted in the same area in subsequent years. On the other hand, crude oil and fossil fuels take millennia to form and so are non-renewable.
- Biofuels have a lower carbon footprint than traditional fuels. All carbon-based fuels (including biofuels) release carbon dioxide (CO2) when burned. However, biofuels are theoretically carbon-neutral because all the carbon they release was absorbed from the atmosphere during the original plant's lifespan.
- Biofuels are cheap, easy to process, and can be locally made. Sourcing biofuels doesn't require drilling in deep-sea mines or cutting down forests to access coal deposits.
- Biofuels can be made from waste material, such as crop stalks or husks, that would otherwise be discarded.
- Biofuels release fewer additional harmful pollutants, such as sulphur dioxide and nitrous oxides, which are associated with the combustion of traditional fuels.
Let's use bioethanol as an example of the carbon-neutrality of biofuels. We want to prove that the number of moles of carbon dioxide taken in during its lifecycle - from formation to combustion - is the same as the number of moles of carbon dioxide released.
We know that bioethanol is made via fermentation - in particular, the fermentation of glucose from plant carbohydrates. Glucose (C6H12O6) is made by plants in the process of photosynthesis, which turns carbon dioxide (CO2) and water (H2O) into glucose and oxygen (O2). Producing one mole of glucose uses six moles of carbon dioxide. Here's the equation:
$$6CO_{2} + 6H_{2}O \rightarrow C_{6}H_{12}O_{6} + 6O_{2}$$
Any plant lovers out there? Head over to Photosynthesis for more about this topic.
The glucose produced in photosynthesis is then fermented into ethanol. We learned the equation earlier:
$$C_{6}H_{12}O_{6} \rightarrow 2C_{2}H_{5}OH + 2CO_{2} $$
This produces two moles of carbon dioxide.
When we burn ethanol as a fuel, we produce water and more carbon dioxide:
$$C_{2}H_{5}OH + 2O_{2} \rightarrow 2CO_{2} + 3H_{2}O$$
But note that fermenting one mole of glucose produces two moles of ethanol. Overall, burning the ethanol produced from one mole of glucose results in four moles of carbon dioxide:
$$2C_{2}H_{5}OH + 4O_{2} \rightarrow 4CO_{2} + 6H_{2}O$$
Let's add up the moles of carbon dioxide taken in during photosynthesis, and those released during the production and combustion of ethanol.
- We took in six moles of carbon dioxide during photosynthesis.
- We released two moles of carbon dioxide when producing ethanol in fermentation.
- We released four moles of carbon dioxide when burning ethanol in combustion.
Overall, the number of moles of carbon dioxide taken in during the life cycle of bioethanol is the same as the number of moles of carbon dioxide released. Bioethanol, like all biofuels, is carbon neutral.
Disadvantages of biofuels
For your exams, you could be expected to argue both for and against the use of biofuels. At the very least, it is important to understand why some people aren't convinced by their use. Here are some of the disadvantages of biofuels.
- Although biofuels are carbon neutral themselves, they require energy to harvest, process, and transport. This energy frequently comes from non-renewable sources and thus releases carbon dioxide into the atmosphere. However, the same could be said about traditional fuels - they too require specific processing, handling, and transportation.
- Growing the plant matter used to produce biofuels needs a large water input.
- Growing plant matter for biofuels also takes up a lot of land. It is argued that this land could be used more efficiently to grow crops for human or animal consumption, and that making space for biofuel plantations is a cause of deforestation and habitat loss.
- Government incentives to produce biofuels have led to a decrease in agriculture in certain countries, resulting in food shortages and climbing grocery bills.
Biofuels - good or bad?
It is easy to look at the list of downsides to biofuels that we shared above and instantly write them off as a well-meaning but ineffective way of reducing carbon dioxide emissions. However, it is important to put things in perspective.
For example, land use of biofuels is a big issue as we struggle to find room for all the crops needed to feed our growing population. But there are other ways of using land more efficiently. 77% of all agricultural land on Earth is used to rear livestock or grow crops for their feed, yet animal products only account for 18% of our calories. If we switched over to a more plant-based diet, we would easily be able to feed a larger population. The American Journal of Clinical Nutrition reckons that a meat-eater's diet requires 17 times more land than a vegetarian's - think about that if you want to reduce your ecological footprint!
Furthermore, biofuels don't have to replace crops for human consumption. Plants destined to become biofuels can be grown on areas of land otherwise unsuitable for cultivation. We can also make biofuels out of waste products. For example, the fuel biogas is a mixture of gases, including methane, that is produced by anaerobically digesting manure and sewage. These are materials that would else be discarded - turning them into fuel can be a great way to save resources.
Production of Ethanol - Key takeaways
Ethanol is an alcohol with the chemical formula C2H5OH. It is found in solvents, fuels, and alcoholic drinks.
Ethanol is primarily produced in two ways: the fermentation of glucose and the hydration of ethene.
In fermentation, enzymes from yeast turn glucose from plant carbohydrates into ethanol and carbon dioxide under anaerobic conditions.
In hydration, ethene is reacted with steam and a phosphoric acid catalyst under high temperatures and pressures to form ethanol.
Fermentation and hydration both have advantages and disadvantages. When producing ethanol, factors like the rate of reaction, the renewability of the starting materials, and the purity of the final product must be considered.
Ethanol produced via fermentation is an example of a biofuel: A fuel derived from living matter.
Advantages of biofuels include their sustainability, carbon neutrality, low costs, and lack of additional pollutants.
Disadvantages of biofuels include their processing and transport needs, their water and land use, and their negative impact on the food system.
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