Reactions of Alkenes with Steam
A concise revision guide to the reaction of alkenes with steam, including the conversion of ethene into ethanol, the use of concentrated phosphoric acid and why the reaction is carried out industrially rather than in a school laboratory.
The Overall Reaction
Alkenes react with steam, H2O(g), to form alcohols. This reaction is called hydration because water is added across the carbon-carbon double bond.
For example, ethene reacts with steam to form ethanol.
The reaction converts the alkene functional group into an alcohol functional group. The C=C double bond opens, one hydrogen atom adds to one carbon atom, and an -OH group adds to the other carbon atom.
Key idea: Steam adds across the C=C double bond, so the alkene becomes an alcohol.
The displayed equation shows water added across the C=C double bond of ethene to produce ethanol.
Conditions for Hydration
The hydration of ethene is carried out using steam and a catalyst of concentrated H3PO4, which is phosphoric acid.
The reaction also needs a high temperature, usually around 300 to 600 °C, and a high pressure of about 70 atm.
These conditions are much more severe than the conditions normally used in a school laboratory, so the reaction is treated as an industrial process.
Exam focus: Include both the reagent and the conditions: steam, concentrated H3PO4, high temperature and high pressure.
Why This Is an Addition Reaction
This reaction is an addition reaction because atoms are added across the C=C double bond.
The alkene starts as an unsaturated molecule. During hydration, the double bond opens and the atoms from water are added to the two carbon atoms from the original double bond.
In the hydration of ethene, this produces only one main product: ethanol.
Hydration: An addition reaction in which water, usually as steam in this industrial process, adds across a carbon-carbon double bond to form an alcohol.
Remember: For steam, the functional group change is alkene to alcohol. This is different from hydrogenation, which converts an alkene into an alkane.
Industrial Advantages and Exam Mistakes
The process is preferred industrially because no waste products are formed, giving the reaction a high atom economy.
Producing only one main product also makes separation simpler and cheaper, reducing overall processing costs.
A common exam error is to describe the reaction as a normal laboratory preparation. The very high pressure makes the process unsuitable for typical laboratory use.
Exam focus: Link the industrial scale to the high pressure, and link the efficiency to no waste products and easier product separation.
Check Your Understanding
Use these short activities to check the key ideas before moving on to the next alkene reaction.
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Some ionic radii are shown.
| Ion | Ionic radius / nm |
|---|---|
| Na+ | 0.102 |
| K+ | 0.138 |
| F− | 0.133 |
| Cl− | 0.180 |
Which compound has the strongest ionic bonding?
Explain why the metallic bonding in magnesium is much stronger than that in sodium.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.