RP 5 - organic

RP 5

preparation of an organic liquid

general steps for the synthesis of an organic liquid

1. preparation - react suitable quantities of reactants to make crude product, often heated under reflux

2. separation of the crude (impure) product - liquids usually by simple/fractional distillation

3. purification - removes impurities like unreacted reagents and by products and carried out by solvent extraction

4. dry - swirling with a drying agent

5. final distillation - identity and purity check

preparation of cyclohexene by dehydration and distillation of cyclohexanol

1. pour cyclohexanol into pear shaped flask thats been weighed, reweigh and record mass

2. use plastic graduated dropping pipette to add H2SO4 to flask

3. add anti bumping granules

4. assemble distillation apparatus and heat gently distilling any liquid which boils below 100 degrees

5. pour distillate into separating funnel and add saturated NaCl solution. shake and allow layers to separate

6. run lower level into beaker and transfer upper layer (contains crude cyclohexene) into small conical flask

7. add anhydrous CaCl2 to crude cyclohexene to remove water

8. stopper flask, shake and allow to stand until liquid becomes clear

9. decant into sample container thats been weighed

10. reweigh container, calculate mass produced and determine % yield

testing distillate

use bromine water to confirm alkene - goes colourless

why can the cyclohexene be separated in this way (step 3)

has a lower boiling point

what does saturated NaCl act as

drying agent

what does step 5 do

removes water, a little cyclohexanol and H3PO4

assumption made when calculating mass

all dry distillate is cyclohexene

common mark scheme method

1. acidify potassiu managanate soilution combining volume of it with equal volume of dilute H2SO4

2. pour mixture into pear shaped flask with a still head containing a thermometer

3. attach to condenser with ice cooled collecting vessel

4. add few anti bumping granules

5. heat flask gently

6. collect sample at bp of desired product

7. cooled collection vessel is essential to reduce evaporatation of product

diagram

why are secondary alcohols refluxed

reactions can be slow as contains strong covalent bonds?

whys a water bath used

if there are flammable substances present

what do anti bumping granules do

• prevent large bubbles forming

• ensure liquid does not boil too vigorously - mixture would boil over into condenser and undesired impurities would contaminate product

condenser angle

tilted slightly down so any liquid can run into collection flask

where should thermometer bulb be

at the T junction connecting to the condenser to measure the correct bp

condenser water flow

water enters lowest point and leaves highest to go against gravity to

• ensure water fills the condenser (prevents backflow of water)

• maximise heat transfer for condensation (more efficient cooling)

collection flask

must not be sealed to condenser

• system should not be air tight as it is heated the air inside expands

• if air tight the air cannot escape and may cause apparatus to crack

preparation of ethanal by oxidation and distillation of ethanol (separation?)

1. make oxidising agent by dissolving potassium dichromate in dilute H2SO4 - conc should be 1g in every 10cm3

2. use measuring cylinder to measure it and pour into boiling tube

3. cool in beaker of cold water

4. use teat pipette to add ethanol drop wise shaking to mix contents

5. add anti bumping granules

6. attach to a bung fitted with a right angled glass delivery tube

7. clamp boiling tube in water so delivery tube goes to a test tube immersed in cold water

8. gently heat to slowly distill some liquid distillate

9. use tollens to test

whys cold water used

keep test tube cool to avoid loss of volatile ethanal

tollens silver mirror test

1. add NaOH to AgNO3 in test tube

2. add just enough dilute ammonia solution to dissolve the brown ppt completely

3. using beaker of hot water (50-60 degrees) gently warm in test tube

4. add distillate

5. wait a few mins and record observations - silver mirror should form

safety precautions

• make sure tollens reagent is disposed of thoroughly by rinsing it away with plenty of water

• rinse glassware that has contained reagent with little dilute H2SO4 when finished

drawing thermometer on diagrams

bulb is opposite to the exit to the condenser so the temp of vapour exiting, which condenses to form product, can be recorded

how can the purification be done

using solvent extraction

• involves shaking the organic liquid with an aqueous solution

• impurities are more soluble and move into it - extracted

• organic liquid and aqueous solution are immiscible and can be separated

solvent extraction steps

1. place organic liquid in seperating funnel and add portion of aqueous solution (often NaHCO3)

2. stopper and shake, releasing the pressure by inverting and opening the tap

3. allow separating funnel to stand until the layers settle and separate

4. remove stopper and open tap to run off bottom layer into beaker

5. run off second later into separate beaker

6. place organic later back in separating funnel and repeat

what do you do step 4 when bottom layer is nearly all drained

close the tap partially to slow down the flow and avoid any of the top layer leaving the funnel

why do you do it twice

more effective to carry out solvent extraction twice with smaller portions of solvent rather than once with a large portion

deciding which layer is the organic liquid

1. more dense liquid is bottom layer

2. or add some water and observe which layer increases in size (aqueous layer)

drying

purified organic liquid may contain water

1. add spatula of drying agent to organic liquid in beaker/conical flask

2. swirl

3. add more drying agent until liquid changes from cloudy to clear

4. filter or decant off the liquid into a clean dry flask

final distillation

boiling point measuring

what if flask contains mixture of liquid in fractional distillation

the boiling liquid produces a vapour that is richer in the most volatile of the liquids present - one with lowest bp

whys % yeild less than 100%

theoretical reasons

1. side reactions occur so by products may be produced rather than expected

2. reagents may be impure

3. incomplete reaction

practical reasons

1. some product lost in purification steps

2. some product lost in distillation

why must conc H2SO4 be added slowly and with cooling

on dilution it gives out a lot of heat - the slow addition dissipates this heat and avoids splashing (would occur if the mixture became hot)

why is conc H2SO4 used

its a catalyst, also a dehydrating agent and removes water which promotes the forward reaction

whats the nature of antibumping granuals

small pieces of silica or broken, unglazed pottery

whys the distillate cooled ie when its ethanal

• ethanal has a low bp which is close to room temp

• the ice cools the flask and ensures its collected as a liquid ie does not evaporate

if 2cm3 of distillate containing ethanal, ethanol and water is added to a beaker of boiling water, explain what would happen

• ethanal and ethanol can hydrogen bond with water - they are miscible with water

• the liquids would boil as both have bp less than 100 degrees

precaution when using solid K2CR2O7

must wear gloves and avoid inhaling any dust

• highly toxic

• category 2 carinogen

• irritant

whens fractional distillation used

to separate liquids with similar boiling points

explain 3 improvements

• place thermometer at side arm of condenser to get accurate boiling temp

• add reflux condensor to top of flask to have better separation

• use multiple distillation adapter to prevent contamination

why is a fractioning column needed

• gives longer path for reaction mixture to separate

• allows ethanol to boil off while water condenses back into the round bottomed flask

changes made to the apparatus set up to change from reflux to distillation

• add a still head

• add a thermometer

• condenser moved from upright to sideways

• receiver on the end of the condenser

suggest why NaCO3 is used to wash the distillate rather than NaOH when use haloalkanes

• NaCO3 is a weaker alkali

• NaOH would hydrolyse the haloalkane