required practical 5
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what is the aim of this practical?
distollation of a product from a reaction
which reaction is used in this practical?
the preparation of cyclohexene from cyclohexanol (dehydration)
what is the method for producing the liquid containing cyclohexene from the cyclohexanol?
• use a spatula to add a few anti-bumping granules to a round-bottomed flask • use a measuring cylinder to measure 15cm3 of cyclohexanol and put in the round-bottomed flask • slowly add 6cm3 of concentrated phosphoric acid to the flask and swirl to ensure the contents are mixed • set up the distillation apparatus - round-bottomed flask with thermometer in, Liebig condenser, clamp, collection flask • use a heating mantle to gently heat the flask, distill and collect the distillate
what is the method for purifying the liquid containing cyclohexene?
• pour the distillate into a separating funnel and add an equal volume of sodium carbonate solution • place the bung on the separating funnel and invert to mix the contents • shake, then open the tap to release pressure • two layers will form - the upper, less dense layer contains the alkene and should be put in a test tube while the lower, more dense aqueous layer contains impurities and should be discarded • add a few pieces of anhydrous calcium chloride to the crude alkene, swirl and decant the liquid into a round-bottomed flask • redistill the crude alkene • place a small amount of the alkene in a test tube and test using bromine water
how is the yield of product maximised during distillation?
by collecting the distillate at the approximate boiling point of the alkene and not higher
why is concentrated acid added to the cyclohexanol?
it acts as an acid catalyst and is a dehydrating agent so removes the water molecule from the alcohol
why are anti-bumping granules added to the round-bottomed flask?
they ensure only small bubbles form not large ones to prevent vigorous boiling - this means the liquid doesn’t boil into the condenser so impurities don’t contaminate the product
why is sodium carbonate added to the distillate?
to react with any phosphoric acid which is still present
why is anhydrous calcium chloride added to the alkene?
to remove any remaining water
why is a heating mantle used for distillation?
• it allows you to control the temperature so you can collect the distillate in the correct range • it is less risky than a Bunsen burner because alcohols are flammable
where should the bulb of the thermometer by positioned in distillation?
at the T junction connecting the condenser to the flask so it can measure the correct temperature
where does water enter the condenser during distillation?
at the bottom so it goes against gravity - this ensures it fills the condenser to prevent backflow and maximises the heat transfer for condensation so cooling is more efficient