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REQUIRED PRACTICAL : MAKING AN ORGANIC LIQUID (ESTER)
The exact method to prepare an organic liquid depends on the actual
liquid being prepared but, in general, an organic liquid can be synthesised
and purified using the stages shown below.
PREPARATION React suitable quantities of the reactants to
produce a crude product. The reactants are often
heated under reflux
SEPARATION OF
THE CRUDE
(IMPURE)
PRODUCT
Liquids are usually separated by simple (or
fractional) distillation
PURIFICATION OF
THE PRODUCT
This removes impurities such as unreacted
reactants and by-products and is carried out by
SOLVENT EXTRACTION
DRY By swirling with a drying reagent
FINAL
DISTILLATION
This is an identity and purity check – if the liquid
is pure the boiling point will be sharp
PREPARATION
When preparing an organic liquid the reactions are often slow as the
organic reagents contain strong covalent bonds. It is often
necessary to REFLUX f or some time.
M
Reflux is the continual boiling and condensing of a reaction mixture,
with the reactants falling back into the reaction vessel, to ensure
that the reaction takes place, without the flask boiling dry.
Most organic compounds are flammable and so a water bath is used
for heating. This is only suitable if the temperature needed is less
than 100oC – otherwise an electric heater needs to be used.
Some organic compounds are prepared by distillation. For example,
an aldehyde prepared by the oxidation of a primary alcohol by
acidified potassium dichromate.
SEPARATION OF THE CRUDE PRODUCT
Rearrange the apparatus for distillation
IF A QUESTION ASKS ABOUT SOLVENT EXTRACTION, ALWAYS
CHECK IF THE DENSITIES ARE GIVEN, AND DETERMINE
WHICH IS THE ORGANIC LAYER.
DRYING
The purified organic liquid may still contain water and must be
dried.
Add a spatula of a drying agent, for example anhydrous calcium
chloride or anhydrous magnesium sulfate.
Swirl.
Add more of the drying agent until the liquid changes from cloudy
to clear.
FILTER or DECANT off the liquid into a clean, dry flask.
FINAL DISTILLATION
The product can be redistilled. The boiling point measured during the distillation indicates the identity and the purity of the substance.
If the boiling point is sharp then the product is pure. If the boiling point recorded during distillation is the same as given in data books, the the correct product has been obtained. If the boiling points of the reactants and products are very similar, then fractional distillation is used. WHY IS THE PERCENTAGE YIELD LESS THAN 100%? Theoretical reasons Side reactions occur, so by-products maybe made instead of the expected products. The reagents used maybe impure. The reaction is incomplete. Practical reasons Some product is lost in the purification steps, for example in washing, separating in the separating funnel and in transfer between apparatus. Some product is lost in distillation. WHY IS THE PERCENTAGE YIELD MORE THAN 100%? Theoretical reasons None Practical reasons The product was not dried fully and so mass of water is being measured too. QUESTIONS Q1. Cyclohexene (boiling point = 83 °C) can be prepared by the dehydration of cyclohexanol (boiling point = 161 °C) using concentrated phosphoric acid. A student prepared cyclohexene by placing 10 cm^3 of cyclohexanol (density = 0.96 g cm–3) into a round-bottomed flask. 3 cm^3 of concentrated phosphoric acid were then carefully added to the flask. The student added a few anti-bumping granules and set up the apparatus shown in the diagram.
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(2) (c) Suggest why sodium carbonate solution was used to wash the distillate.
(1) (d) Explain why it is important to open the tap of the separating funnel periodically.
(1) (e) Give a property of anhydrous calcium chloride, other than its ability to absorb water, that makes it suitable as a drying agent in this preparation.
(1) (f) Describe the apparatus used to remove the drying agent by filtration under reduced pressure. Your description of the apparatus can be either a labelled diagram or a description in words. (2) (g) A sample of cyclohexene has been contaminated with cyclohexanol. The cyclohexene can be separated from the cyclohexanol by column chromatography. Silica gel is used as the stationary phase and hexane as the mobile phase. Explain why cyclohexene has a shorter retention time than cyclohexanol.
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(2) (h) Explain how an infrared spectrum would confirm that the cyclohexene obtained from the chromatography column did not contain any cyclohexanol.
(1) (Total 13 marks) Q2. Ethanol can be oxidised by acidified potassium dichromate(VI) to ethanoic acid in a two-step process. ethanol ethanal ethanoic acid (a) In order to ensure that the oxidation to ethanoic acid is complete, the reaction is carried out under reflux. Describe what happens when a reaction mixture is refluxed and why it is necessary, in this case, for complete oxidation to ethanoic acid.
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(2) (e) A student obtained a sample of a liquid using the apparatus in part (c). Describe how the student could use chemical tests to confirm that the liquid contained ethanal and did not contain ethanoic acid.
(5) (Total 16 marks) Q3. Ethanoic acid, propyl ethanoate and propan-1-ol are all colourless liquids. Esters do not give a positive result with any of the usual tests for functional groups. State how you could use chemical tests to show the presence of ethanoic acid and propan-1-ol in a mixture of the acid, the alcohol and the ester.
(Total 4 marks)
