R.P 5 (Distillation of a product from a reaction) Flashcards
(28 cards)
Outline the 5 steps in the general method of separating an organic liquid produced from distillation
(1) Put the distillate of impure product into a separating funnel
(2) Wash product
(3) Allow the layers to separate in the funnel, and then run and discard the aqueous layer
(4) Run the organic layer into a clean, dry conical flask and add three spatula loads of drying agent to dry the organic liquid. When dry the organic liquid should appear clear
(5) Carefully decant the liquid into the distillation flask
(6) Distil to collect pure product
What are the 2 methods of washing the product when separating an organic liquid, stating the role of both
- Add sodium hydrogencarbonate solution , shaking and releasing the pressure from CO2 produced
- Sodium hydrogencarbonate will neutralise any remaining reactant acid
-Add Saturated sodium chloride solution
-Sodium chloride will help separate the
organic layer from the aqueous layer
Give 2 examples of drying agents and state what characteristics a drying agent must have
e.g. anhydrous sodium sulphate, calcium chloride/sulphate
The drying agent should
*be insoluble in the organic liquid
* not react with the organic liquid
Which layer is usually the organic layer when using a separating funnel
The layer with lower density will be the upper layer. This is usually the organic layer
Outline the 5 stages in preparing cyclohexene from cyclohexanol
a) Pour about 20 cm3 of cyclohexanol into a weighed 50 cm3 pear-shaped flask. Reweigh the flask and record the mass of cyclohexanol.
b) Using a plastic graduated dropping pipette, carefully and with frequent shaking, add to the flask approximately 8 cm3 of concentrated phosphoric acid.
c ) Add a few anti-bumping granules to the flask and assemble the distillation apparatus, so that the contents of the flask may be distilled. Heat the flask gently, distilling over any liquid which boils below 100 °C.
d) THEN PURIFY using general method of separation
e) Weigh sample container which used to decant liquid into, then reweigh container to find mass of dry cyclohexene produced
What is the purpose of anti bumping granules
Added to the prevent vigorous, uneven boiling by making small bubbles form instead of large bubbles
What is the catalyst in the conversion of cyclohexanol to cyclohexene
Conc H2SO4
What is the reagent and conditions for the partial oxidation of a primary alcohol to form an aldehyde
Reagent: potassium dichromate (VI) solution and dilute sulphuric acid.
Conditions: (use a limited amount of dichromate) warm gently and distil out the aldehyde as it forms:
Write an equation showing the oxidation of propanal using [O] as oxidising agent
Write the full equation for the oxidation of ethanal (using dichromate) into ethanoic acid
3CH3CHO + Cr2O72- + 8H+ —> 3 CH3COOH + 4H2O + 2Cr3+
Outline the 3 stages in the partial oxidation of propan-1-ol to form propanal
1) Place about 10 cm3 of dilute sulphuric acid in a flask and add about 3g of potassium dichromate(VI) and 2 or 3 anti-bumping granules. Shake the contents of the flask until solution is complete (do not warm)
2) Add 1.5 cm3 of propan-1-ol in drops from a dropping pipette, shaking the flask so as to mix the contents, and then assemble distillation apparatus
3) Gently heat and slowly distil 2 cm3 of liquid into a test tube
Why do we not use an excess of dichromate and not under reflex when undergoing partial oxidation of a primary alcohol when an aldehyde is the desired product
Because these conditions will form a carboxylic acid
Sketch a distillation set up
Where should the bulb of the thermometer be when setting up a distillation
The bulb of the thermometer should be at the T junction connecting to the condenser to measure the correct boiling point
Which way should water come in from when setting up a distillation
The water goes in the bottom of the
condenser to go against gravity. This allows more efficient cooling and prevents back flow of water.
On this diagram water our should actually be at the top
Why do we often use an electric heater rather than a Bunsen burner when distilling organic compounds
Because organic chemicals are normally highly flammable and could set on fire
with a naked flame.
What is the reagent and conditions for the full oxidation of primary alcohols into carboxylic acids
Reagent: potassium/sodium dichromate(VI) solution and sulphuric acid
Conditions: use an excess of dichromate, and heat under reflux: (distil off product after the reaction has
finished)
Write an equation for the full oxidation of propan-1-ol to form propanoic acid
CH3CH2CH2OH + 2[O] —-> CH3CH2COOH + H2O
What is observed when propan-1-ol is oxidised into propanoic acid
Orange dichromate ion turns to green Cr3+ ion
What is observed when propan-1-ol is oxidised into propanal
Orange dichromate ion turns to green Cr3+ ion
Outline the 5 stages in the full oxidation of propan-1-ol into propanoic acid
(1) Measure 5 cm3 of water into a boiling tube. Add 6 g of sodium dichromate(VI), shake and set aside to dissolve.
(2) Put about 1.5 cm3 propan-1-ol into a 50 cm3 round bottomed flask and add about 5 cm3 of water and two or
three anti-bumping granules. Put a condenser on the flask for reflux, as shown in figure below.
(3) Add 2 cm3 of concentrated sulphuric acid down the condenser in drops from a dropping pipette. While the
mixture is still warm, start to add your sodium dichromate(VI) solution down the condenser in drops from a
dropping pipette. The energy released from the reaction should make the mixture boil. Add the solution a drop
at a time so that the mixture continues to boil without any external heating.
(4) When all the sodium dichromate(VI) solution has been added, use a low Bunsen burner flame to keep the
mixture boiling for 10 minutes, not allowing any vapour to escape.
(5) At the end of that time remove the Bunsen burner and arrange the apparatus for distillation. Gently distil 2-3
cm3 of liquid into a test tube
Sketch the setup for reflux
When do we use reflux
When heating organic reaction mixtures for long periods
Why do we never seal the end of the condenser when refluxing
The build up of gas pressure could cause the apparatus to explode
