Alkene Hydration and Environmental Impact Revision Notes | A Level AQA Chemistry

1. Why Alkene Hydration Matters

Every time you use hand sanitiser, fuel a car with bioethanol, or encounter an industrial solvent, you are meeting the product of one reaction: the conversion of an alkene into an alcohol. The chemistry behind this is elegant, and the debate about how we should make ethanol at scale touches on some of the most pressing environmental questions of our time.

This lesson covers two things the AQA specification requires you to handle with precision: the electrophilic addition mechanism for alkene hydration, and a genuinely evaluative discussion of biofuel use. Both will appear in exam questions that reward careful reasoning, not just memorised facts.

2. The Mechanism: Electrophilic Addition of Steam to an Alkene

The pi bond electron density lies above and below the plane of the molecule, with the sigma bond axis forming the nodal plane.

Here is the key insight: water is not the electrophile. The acid catalyst provides $\text{H}^+$ ions, and it is this proton that attacks the pi bond first. Water then acts as the nucleophile in the second step, donating a lone pair to the carbocation. This two-step sequence is the heart of the mechanism.

Notice that the acid catalyst is regenerated in Step 3 — this is why only a catalytic amount is needed. The overall equation for the hydration of ethene is:

Worked example: Drawing the Mechanism for Acid-Catalysed Hydration of Propene

Draw the mechanism for the reaction of propene with steam in the presence of an acid catalyst. Identify the major product and explain which carbon the OH group attaches to.

1. Electrophile: $\text{H}^+$ (from acid catalyst) Nucleophile: $\text{H}_2\text{O}$ (lone pair on oxygen) — The acid catalyst provides H+ ions. Water acts as the nucleophile in step 2, not the electrophile.
2. Curly arrow from the pi bond of $\text{CH}_3\text{CH=CH}_2$ to $\text{H}^+$ This gives either $\text{CH}_3\text{CH}^+\text{CH}_3$ (secondary carbocation) or $\text{CH}_3\text{CHCH}_2^+$ (primary carbocation) — The proton can add to either carbon of the double bond. Addition to the terminal CH2 gives a secondary carbocation on the central carbon; addition to the central carbon gives a primary carbocation on the terminal carbon.
3. Secondary carbocation: $\text{CH}_3\overset{+}\text{CH}\text{CH}_3$ is more stable than primary $\text{CH}_3\text{CH}_2\overset{+}\text{CH}_2$ — The secondary carbocation is stabilised by two alkyl groups donating electron density. The more stable carbocation forms preferentially, so H+ adds to the terminal carbon (CH2).
4. Curly arrow from lone pair on O of $\text{H}_2\text{O}$ to the secondary carbocation carbon Forms: $\text{CH}_3\overset{+}\text{CH}(\text{OH}_2)\text{CH}_3$ (oxonium ion) — Water donates a lone pair to the electron-deficient carbocation carbon, forming a bond and creating a positively charged oxygen (oxonium ion).
5. Curly arrow from O-H bond in oxonium ion to $\text{H}^+$ Product: $\text{CH}_3\text{CH(OH)CH}_3$ (propan-2-ol) — Loss of H+ from the oxonium ion regenerates the acid catalyst and gives the alcohol product. The OH group is on carbon 2, confirming Markovnikov addition.

Answer: Major product is propan-2-ol ($\text{CH}_3\text{CH(OH)CH}_3$). The OH group attaches to the secondary carbon because the secondary carbocation intermediate is more stable than the primary alternative.

3. Industrial Production of Ethanol by Hydration of Ethene

The mechanism above describes what happens at the molecular level. In industry, the same reaction is carried out on a massive scale using specific conditions chosen to maximise yield and rate simultaneously. The AQA specification requires you to know these conditions and be able to justify them.

The reaction is reversible, so conditions represent a compromise. A higher temperature increases rate but reduces equilibrium yield; a lower temperature gives a better yield but an impractically slow rate. The chosen conditions (around 300 °C, 60–70 atm) represent the best economic balance between rate and yield.

The conversion per pass through the reactor is relatively low (around 5%), so unreacted ethene is recycled. This continuous process is highly efficient but depends entirely on a non-renewable feedstock: crude oil.

4. Fermentation vs Hydration: Comparing Two Routes to Ethanol

Ethanol can be produced by two fundamentally different routes: hydration of ethene (a petrochemical process) and fermentation of sugars (a biological process). Understanding the comparison between these routes is essential for the environmental discussion that follows.

The fermentation equation is: $\text{C}_6\text{H}_{12}\text{O}_6\text{(aq)} \rightarrow 2\text{C}_2\text{H}_5\text{OH(aq)} + 2\text{CO}_2\text{(g)}$. Notice that carbon dioxide is a by-product, which reduces the atom economy compared to hydration. However, the carbon in that $\text{CO}_2$ was originally absorbed from the atmosphere by the plant during photosynthesis, so the net addition to atmospheric $\text{CO}_2$ is theoretically close to zero — a key argument for biofuels.

5. Environmental and Ethical Issues in Biofuel Decision Making

The AQA specification asks you to discuss environmental and ethical issues linked to biofuel use. This is a genuine evaluation task — there is no single correct answer, and the examiner expects you to present evidence on multiple sides before reaching a reasoned conclusion.

The central argument for bioethanol as a fuel is carbon neutrality. Plants absorb $\text{CO}_2$ during photosynthesis to build the sugars that are later fermented. When the ethanol is burned, that same $\text{CO}_2$ is released. In principle, the cycle is closed and no net $\text{CO}_2$ is added to the atmosphere. This contrasts sharply with burning fossil fuels, which releases carbon that has been locked underground for millions of years.

However, the reality is more complicated. Growing, harvesting, transporting, and processing biofuel crops all require energy — often from fossil fuels. Fertiliser production releases nitrous oxide ($\text{N}_2\text{O}$), a greenhouse gas far more potent than $\text{CO}_2$. When these lifecycle emissions are included, the carbon savings from some biofuels are considerably smaller than the headline figure suggests.

• Arguments FOR biofuel use: reduces dependence on finite fossil fuels; lower net CO2 emissions over the fuel lifecycle (when managed well); supports rural economies and agricultural employment; existing combustion engine infrastructure can be adapted
• Arguments AGAINST biofuel use: land used for fuel crops competes with food production, raising food prices and threatening food security; deforestation to create farmland can release more CO2 than the biofuel saves; monoculture farming reduces biodiversity; water-intensive crops strain water supplies in vulnerable regions
• Ethical dimensions: wealthy nations consuming biofuels may drive up food prices in poorer nations; indigenous land rights may be threatened by large-scale biofuel plantations; intergenerational equity — are we using land sustainably for future generations?

The food versus fuel debate is particularly sharp. In 2007–2008, a global food price crisis was partly attributed to the diversion of maize and sugar crops into bioethanol production in the United States and Brazil. This illustrates how a decision that appears environmentally beneficial in one context can have serious humanitarian consequences in another.

Second-generation biofuels attempt to address these concerns by using non-food plant material — agricultural waste, wood chips, or dedicated energy crops grown on marginal land. These avoid the food competition problem but face technical challenges in breaking down cellulose efficiently. Third-generation biofuels from algae are promising but not yet commercially viable at scale.

6. Exam Technique and Extended Response Practice

AQA questions on this topic tend to fall into two categories: mechanism questions (which reward precise arrow-drawing and correct identification of intermediates) and evaluate/discuss questions (which reward structured argument with a conclusion). The skills are different, and it is worth practising both explicitly.

For mechanism questions, the mark scheme typically awards one mark per correct curly arrow, one mark for identifying the carbocation intermediate, and one mark for the correct product. For a three-step mechanism, that can be five or six marks — all lost if you skip the intermediate or draw arrows incorrectly.

For evaluate questions on biofuels, the mark scheme rewards: identifying a relevant point, supporting it with chemical or contextual evidence, and reaching a conclusion that weighs the evidence. A response that lists points without connecting them to evidence, or that fails to reach a conclusion, will not access the top mark band.

Hydration is an addition process: the pi bond is broken and atoms add across the double bond. Environmental impact then depends on the source of feedstocks and the conditions needed to run the process.

Keep the denominator language precise in atom-economy reasoning: the comparison is based on the mass of desired product relative to the total mass of reactants.