Dehydration of ethanol to form ethene

Dehydration of ethanol to form ethene

Demonstration and Class practical

Ethanol vapour is dehydrated by passing over a heated catalyst to produce ethene gas. This is collected over water and tested for typical properties of an unsaturated hydrocarbon.
 

Lesson organisation

The experiment can be performed as a demonstration or as a class experiment according to circumstances. The main risk to be considered in making the choice is the reliability of the students involved handling very hot glassware and manipulating the apparatus for safe gas collection over water, while avoiding suck-back of cold water into the hot tube.

A demonstration will take about 10 - 15 minutes, with a few more minutes for testing the gases. A class experiment takes longer, probably about 30 - 40 minutes.

Chemicals

Ethanol (IDA, Industrial Denatured Alcohol) (HIGHLY FLAMMABLE, HARMFUL), 2 - 3 cm³ (provided in small bottles)

Mineral wool, sufficient for a loosely-packed wad to absorb the liquid at the bottom of the test-tube

Pumice stone or porous pot (unglazed), a supply of small fragments - about pea-sized

Bromine water, 0.02 M (HARMFUL), about 5 cm³ - diluted to pale orange-brown

Acidified potassium manganate(VII) (potassium permanganate) solution, about 0.01 M, about
5 cm³ (Note 4)

Apparatus

The teacher and/or each working group will require:

Eye protection

Safety screens (for demonstration)

Boiling tube (Note 1)

Rubber bung, 1 hole – to fit boiling tube

Delivery tube (see diagram)

Bunsen valve (Note 2)

Test-tubes, 6

Corks or bungs, to fit test tubes, 6

Dropping pipette

Bunsen burner

Heat resistant mat

Test-tube rack

Glass or plastic trough, small (Note 3)

Wooden splint

Retort stand, boss and clamp 

Health & Safety and Technical notes

Ethanol (IDA, Industrial Denatured Alcohol) (HGIHLY FLAMMABLE, HARMFUL) - see CLEAPSS Hazcard.

Bromine water (HARMFUL) - see CLEAPSS Hazcard. 

Potassium manganate(VII) (potassium permanganate) solution, about 0.01 M  - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

Dilute sulfuric acid, 0.1 M - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

The assembled apparatus needs to look like this:m

1 A large, 150 x 25 mm, hard glass (borosilicate) test-tube.

2 The purpose of the Bunsen valve (see diagram below) is to prevent suck-back of water into the hot tube when heating is stopped and the gas inside the apparatus contracts on cooling. The Bunsen valve is constructed from a short length (about 3 cm) of clean, unused, soft rubber tubing, with one end stoppered with a short length of glass rod. A 1 cm long slit is carefully cut into the middle of the rubber tubing with a scalpel. The resulting assembly is fitted onto the lower end of the delivery tube. Note that the effectiveness of the home-made valve is variable, and for class use a supply of spares should be provided.

3 The trough needs to be small enough to match the scale of gas collection – the large traditional trough which used to be used with gas jars is not appropriate. For demonstration or class use square, clear, plastic sandwich boxes make excellent troughs.

4 The potassium manganate(VII) solution is made by dissolving solid potassium manganate(VII) in 0.1 mM sulfuric acid. 1 dm³ of stock solution is made by dissolving 1.6 g of potassium manganate(VII) crystals in 1 dm³ of 0.1 M sulfuric acid.

For a demonstration, the class and teacher should be protected by safety screens in case of unexpected suck-back causing the hot tube to shatter on a demonstration bench.

Procedure

Assemble the apparatus within easy reach of a Bunsen flame.

a Half fill the trough with water and fill the test-tubes with water, leaving them submerged in the trough. Bungs for the test-tubes can be placed upside-down in the trough, so that the tubes filled with gas can just be pressed onto them before they are removed to a rack.

b Place a wad of mineral wool in the bottom of the boiling tube so that it fills the bottom to a depth of about 1 cm without packing too firmly. Using a dropping pipette, carefully drop about 2 cm³ of ethanol into the mineral wool so that it soaks in. It should be possible to invert the tube without any significant amount of liquid draining out.

c Clamp the boiling tube at the neck end so that the mouth is tilted slightly upwards. Fill most of the tube with pieces of pumice stone or broken porous pot. This will ensure maximum contact time between the ethanol vapour and the hot catalyst. Fit the delivery tube so that it dips into the water in the trough. Fit a Bunsen valve, if desired.

d Heat the pumice stone or porous pot strongly with the tip of medium Bunsen flame for several seconds until thoroughly hot. Avoid heating the tube too close to the rubber stopper. Then flick the flame quickly onto the mineral wool for a few seconds to vaporise some of the ethanol. Then return the flame to the catalyst.

e When a steady stream of gas bubbles is established, collect four to six tubes full of gas by holding them over the Bunsen valve. It is better to have an assistant to manipulate the gas collection tubes, changing and sealing them as they are filled. Take care not to lift the water-filled tubes out of the water when moving them, to avoid letting air into them. Seal the full tubes by pressing them down on the bungs, then place them in a rack.

f Continue with the Bunsen burner heating the mineral wool for about one second out of every ten and the pumice stone for the other nine. To avoid the risk of suck-back, do not remove the Bunsen burner flame from the heated tube while the gas is being collected. If suck-back becomes unavoidable, quickly remove the delivery tube from the water by lifting the whole apparatus using the clamp stand.

g When six tubes of gas have been collected, or gas production ceases, remove the delivery tube from the water by lifting the clamp stand, and then stop heating. Keep the Bunsen flame away from the end of the delivery tube.

h Test the tubes of collected gas as follows:

• Pass the first two tubes (mainly air anyway) round the class so that the students can cautiously smell the gas.
• Uncork the last tube collected, and hold a lighted spill in its mouth to ignite the gas. It will burn with a yellow flame.
• To another test-tube of gas, add about 1 cm depth of bromine water. Re-seal it and shake well. If the pale orange bromine water is decolourised, the gas contains an unsaturated hydrocarbon.
• Add about 1 cm depth of acidified potassium manganate(VII) solution to another test-tube of gas. Re-seal and shake well. If the purple solution is decolorised, possibly leaving a brown coloration of manganese dioxide, the gas contains an unsaturated hydrocarbon.

The results of the last two tests are characteristic of unsaturated hydrocarbons, whose molecules contain carbon-carbon double bonds.

Teaching notes

This experiment would form part of the teaching and learning sequence for the chemistry of hydrocarbons and alcohols. The teacher will need to decide:

• how much the students should know about hydrocarbons before this lesson
• if they are familiar with hydrocarbons, whether they should be familiar with the potassium manganate(VII) and bromine water tests for a carbon-carbon double bond before the lesson, or should be introduced to these tests by means of this experiment.

For the dehydration of ethanol, the only likely product is ethene. Ball and stick molecular models will be useful for modelling both the dehydration and the tests. 

The ethanol dehydration is represented by:

CH₃CH₂ → CH OH(g)₂=CH₂(g) + H₂O(g)

The experiment can be linked to the industrial applications of such reactions; see the web-links below.

Bromine in non-aqueous solution adds across the double bond of an alkene, to give for example 1,2-dibromoethane, CH₂Br–CH₂Br (a suspected carcinogen). However, in aqueous solution the main product is a bromoalkanol, such as 2-bromoethanol, CH₂Br–CH₂OH .

Acidified manganate(VII) cleaves C=C double bonds to form two carbonyl compounds (ketones and aldehydes), while alkaline permanganate produces a diol such as ethan-1,2-diol, CH₂OH–CH₂OH.

Note that in industry the term ‘dehydration of ethanol’ refers to the removal of water from ethanol solutions produced by fermentation, not to the reaction investigated here. Industrially some ethanol is made by the reverse reaction, the addition of water to ethene. However, a really important dehydration of an alcohol is the dehydration of 1-phenylethanol to produce phenylethene, better known as styrene, the monomer for polystyrene.