RP5 : Distillation of a product from a reaction

Question 1

what’s the reaction and conditions needed for partial oxidation of primary alcohols

Answer 1

reaction : primary alcohol => aldehyde
reagent : potassium dichromate solution and dilute sulphuric acid
conditions : warm gently and distil out the aldehyde as it forms

Question 2

what are some safety with partial oxidation of aldehydes

Answer 2

• must wear safety gloves when handling solid potassium dichromate since it’s highly toxic and it’s also an irritant
  -avoid inhaling any dust
• Conc sulphuric acid is corrosive

Question 3

what’s the observation with partial oxidation of alcohols

Answer 3

orange dichromate ion ( Cr2O7 2-) reduces to the green Cr3+ ion

Question 4

what’s the method of partial oxidation of propan-1-ol

Answer 4

• place about 10cm3 of dilute sulphuric acid in a flask and add about 3g of potassium dichromate and 2 or 3 anti bumping granules . Shake contents of the flask until solution is complete
• Add 1.5cm3 of propan-1-ol in drops from a dropping pipette , shaking the flask as to mix the contents and then assemble distillation apparatus
• Gently heat and slowly distill 2cm3 of liquid into a test tube , taking care that none of the reaction mixture splashes over

Question 5

what should the distillation apparatus look like

Answer 5

• The bulb of the thermometer should be at the T junction connecting to the condenser to measure the correct boiling point
• water goes in the bottom of the condenser to go against gravity . This allows more efficient cooling and prevents back flow of water
• electric heaters are used to heat organic chemicals . This is because they’re normally highly flammable and could set on fire with a naked flame

Question 6

what’s the reaction and reagents for full oxidation of primary alcohols

Answer 6

reaction : primary alcohol => carboxylic acid
Reagent : potassium dichromate and sulphuric acid
conditions : excess of potassium dichromate and heat under reflux

Question 7

what’s the detailed method for full oxidation of alcohols

Answer 7

• measure 5cm3 of water into boiling tube .
• add 6g of sodium dichromate , shake and set aside to dissolve
• put about 1.5cm3 of propan-1-ol into a 50cm3 round bottomed flask and add about 5cm3 of water and two or 3 antibumping granules . Put a condenser on the flask for reflux
• add 2cm3 of conc sulphuric acid down condenser in the drops from a dropping pipette .
  While the mixture is still warm , start to add your sodium dichromate solution down the condenser in drops from a dropping pipette . 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
• when all the sodium dichromate solution has been added , use a low bunsen burner flame to keep the mixture boiling for 10 mins , not allowing any vapour to escape .
• At the end of that time remove the bunsen burner and arrange apparatus for distillation and gently distill 2/3 cm3 of liquid into a test tube

Question 8

what shouldn’t you do in a reflux reaction

Answer 8

• never seal the end of the condenser as the build up of gas pressure could cause the apparatus to explode .
• antibumping granules are added to the flask in both distillation and reflux to prevent vigorous , uneven boiling by making small bubbles form instead of large bubbles

Question 9

how should a reflux apparatus look

Answer 9

• don’t draw lines between flask and condenser
• don’t have top of condenser sealed
• condenser must have outer tube for water that is sealed at top and bottom
• condenser must have two opening for water in and out that are open

Question 10

what’s the fractional distillation method in the lab

Answer 10

• heat flask with a bunsen burner or electric mantle
• this causes vapors of all the components in the mixture to be produced
• vapor’s pass up the fractionating column
• the vapour of the substance with the lower boiling point reaches top of fractionating
  column first
• thermometer should be at least below the boiling point of the most volatile substance
• the vapours with higher boiling point condense back into the flask
• only the volatile vapor passes into the condenser
• the condenser cools the vapor and condenses to a liquid
  and is collected

Question 11

EQ : In order to ensure that the oxidation to ethanoic acid is complete , the reaction is carried out in reflux . Describe what happens when a reaction mixture is refluxed and why it’s necessary in this case for complete oxidation to ethanoic acid (3)

Answer 11

• a mixture of liquids is heated to a boiling point for a prolonged time
• vapour is formed which escapes from liquid mixture and is changed back into liquid and returned to the liquid mixture
• any ethanal and ethanol that initially evaporates can then be oxidised

Question 12

EQ : Describe how you would obtain a sample of ethanal from a mixture of ethanol , ethanal and ethanoic acid . Include in your answer a description of the apparatus you would use and how you would minimise the loss of ethanal . (5)

Answer 12

• mixture heated in a suitable flask
• with still head containing a thermometer
• water cooled condenser connected to the still head and suitable cooled collecting vessel
• collect sample at the boiling point of ethanal
• cooled collection vessel necessary to reduce evaporation of ethanal