The hydration of alkenes
A demonstration of the hydration of alkenes. Students react hex-1-ene, CH3CH2CH2CH2CH=CH2, with concentrated sulfuric acid, followed by water, to make hexan-2-ol, CH3CH2CH2CH2CH(OH)CH3. The hexan-2-ol is then distilled as a teacher demonstration and the product tested.
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Lesson organisation
This is a useful practical to illustrate the addition reactions of alkenes. It also teaches the preparation and purification of an organic liquid.
The student experiment and the teacher part of the experiment will fit well into a forty-five minute lesson. The hexan-2-ol can then be allowed to dry overnight and the distillation and tests carried out in another 45 minute lesson.
Apparatus and chemicals
Each group of students will need:
Goggles
Disposable nitrile gloves
Beaker (250 cm3)
Measuring cylinder (5 cm3 or 10 cm3), 2
Boiling tubes, 2
Glass stirring rod
Ice, a few lumps
Hex-1-ene (Irritant, Highly flammable), 5 cm3 (see note 1)
Sulfuric acid, 75% (Corrosive), 5 cm3 (see note 2)
For the distillation the teacher will need:
Eye protection
Beaker (250 cm3)
Separating funnel (250 cm3)
Conical flask (250 cm3)
Filter funnel
Filter paper
Distillation apparatus, with thermometer (0-200° or 0-250°C) (see diagram)
Test-tube rack
Test-tubes, with corks, 8
Forceps
Scalpel
Distilled or deionised water, 10 cm3
Anhydrous sodium carbonate (Irritant), 25 g
Bromine water, 0.01 mol dm-3 (Harmful at this concentration), about 1 cm3 (see note 3)
Acidified potassium manganate(VII) solution, about 0.001 mol dm-3 (Low hazard at this concentration), about 1 cm3 (see note 4)
Sodium (Highly flammable, Corrosive), a small piece no bigger than 2 mm (see note 5)
Technical notes
Hex-1-ene (Irritant, Highly flammable) Refer to CLEAPSS Hazcard 45C.
Sulfuric acid (Corrosive) Refer to CLEAPSS Hazcard 98A and Recipe card 69
Anhydrous sodium carbonate (Irritant) Refer to CLEAPSS Hazcard 95A.
Bromine water (Harmful at concentration used) Refer to CLEAPSS Hazcard 15B and Recipe Card 28
Potassium manganate(VII) solution (Low hazard at concentration used) Refer to CLEAPSS Hazcard 81 and Recipe Card 56.
Sodium (Highly flammable, Corrosive) Refer to CLEAPSS Hazcard 88.
1 Hex-1-ene is a slightly unusual reagent which may have to be ordered specially.
2 75% sulfuric acid can be purchased, but not from main school suppliers, eliminating the need for dilution from the more common 95% ('concentrated') sulfuric acid. However, this is likely to be a more expensive route because of minimum order and delivery charges.
3 Bromine water can also be purchased as the diluted solution.
4 The potassium manganate(VII) is best purchased as a 0.02 mol dm–3 solution as making it up from the solid is difficult. For this purpose it should be diluted to a pale pink colour with dilute (1 mol dm–3) sulfuric acid (Irritant)
5 Using forceps, remove a piece of sodium from the oil, and place it on a tile. Ensure that conditions are dry. Using a scalpel or sharp knife, cut a small piece of sodium no larger than 2 x 2 x 2 mm. Place the small piece in a separate bottle of oil, or use immediately. Return the larger piece to its bottle.
Procedure
HEALTH & SAFETY: Wear goggles and disposable nitrile gloves throughout.
a Place a few lumps of ice and some water in a 250 cm3 beaker, to make an ice bath.
b Measure out 5 cm3 of hex-1-ene using a measuring cylinder and pour it into a boiling tube.
c Cool the hex-1-ene in the ice bath for about three minutes.
d Measure out 5 cm3 of 75% sulfuric acid into another measuring cylinder.
e Slowly add the acid to the hex-1-ene, keeping the tube in the ice bath and stirring the mixture with a glass rod.
f When all the acid has been added, continue to stir the mixture until it becomes homogeneous (just one layer is visible). This will take about 5 minutes.
g Allow the mixture to stand in the ice bath for another 5 minutes, then carefully add an equal volume of cold water, using another boiling tube. The mixture should separate into two layers. The top layer is impure hexan-2-ol, the lower layer contains mainly sulfuric acid.
Distillation of the product
SAFETY: Wear eye protection throughout
a Ask students to tip the contents of their boiling tubes into a beaker. Then pour the contents into a separating funnel, stopper, shake and allow the contents to settle.
b Remove the stopper and run off the lower layer (containing sulfuric acid) into the beaker and discard it by washing it carefully down the sink with plenty of water.
c Add 10 cm3 distilled water to the separating funnel, stopper and shake. This washes the
hexan-2-ol. Remove the stopper and run off the bottom aqueous layer and discard it.
d Place 25 g of anhydrous sodium carbonate in a 250 cm3 conical flask. Run the pentan-2-ol out of the separating funnel into the flask. Swirl the flask frequently for twenty minutes to dry the hexan-2-ol.
e Set up a distillation apparatus (see diagram). Decant as much as possible of the hexan-2-ol from the conical flask into the distilling flask and then filter the rest into the distilling flask.
f Distil, collecting the fraction that boils between 130o and 160oC. This is hexan-2-ol. Note that hex-1-ene boils at a much lower temperature, 63°C.
g Using a few drops of hex-1-ene and hexan-2-ol for each test, compare them by:
• Shaking (in a corked test-tube) with about 1 cm3 bromine water.
• Shaking (in a corked test-tube) with about 1 cm3 acidified potassium manganate(VII) solution.
• In a clean dry test tube, adding a small piece of sodium metal.
• Placing a few drops on a crucible lid and igniting with a lit splint.
Teaching notes
The overall preparation reaction is:
C6H12 + H2O → C6H13OH
The mechanism is shown by:
The secondary carbocation is formed rather than the primary one where the positive charge is on the end carbon. This is because of stabilisation by inductive effects from the two carbon atoms and is an example of Markownikoff’s rule.
The alkyl hydrogensulfate reacts with water to give the alcohol and regenerates sulfuric acid, but of course it is the secondary alcohol hexan-2-ol, CH3CH2CH2CH2CH(OH)CH3, that is formed, rather than the primary hexan-1-ol.
Hexan-2-ol has hydrogen bonding between its molecules giving it a much higher boiling point than hex-1-ene, which just has van der Waals' forces.
The test reactions are:
Bromine water
C6H12 + Br2 → C6H12Br2
1,2-dibromohexane is formed. The brown colour of the bromine is decolorised; there is no reaction with hexan-2-ol
Potassium manganate(VII)
Potassium manganate(VII) saturates the double bond in pentene by oxidation – the purple colour is decolorised; there is no reaction with hexan-2-ol in the cold.
Sodium
2C6H13OH + 2Na → 2C6H13ONa + H2
Fizzing should be seen; there is no reaction with hexene.
Combustion
The hexene should burn with a more smoky flame than the alcohol because it is unsaturated.
Health & Safety checked, October 2007
Web Links
www.chem.uic.edu/web1/OCOL-II/WIN/ALKENE/60/ALKENE3.HTM has details of lots of addition reactions (including this one) and their mechanisms.
www.chemguide.co.uk/organicprops/alkenes/h2so4.html has detail of this reaction and its mechanism.
(Websites accessed June 2008)
Updated 29 Oct 2008
3 out of 5
25 g of anhydrous sodium carbonate completely absorbed all the liquid, immediately terminating the experiment.
2.5 g had a similar effect, absorbing all the liquid!
Please advise.
Submitted by: Nicky Le Good on 8 May 2009