Practical techniques in organic chemistry - Part 2

Question 1

5 other methods of separation not involving heating and distillation are:

Answer 1

• solvent extraction
• drying
• washing
• filtration
• recrystallisation

Question 2

solvent extraction is:

Answer 2

-using a solvent to remove the desired organic product from other substances

Question 3

the choice of solvent used in solvent extraction depends on

Answer 3

• the solvent added should be immiscible (does not form a mixture) with the solvent containing the desired product
• the desired organic product should be much more soluble in the added solvent than in the reaction mixture

Question 4

the method for solvent extraction is:

Answer 4

-place the reaction mixture in a separating funnel and then add the chosen solvent-it should form a separate layer
(this should be added in small portions rather than one go)
-place the stopper in the neck of the funnel and gently agitate the contents or the funnel for a short while
(place finger on stopper, invert, ope the tap, agitate in a circular motion, close the tap and return funnel to its normal position
-allow contents to settle into two layers
-remove the stopper and open the tap to allow the bottom layer to drain into a flask, then do the same for the upper layer in a separate flask
-the solution containing the desired organic product must now be distilled off using simple or fractional distillation

Question 5

washing is used to ……………..

Answer 5

remove impurities from both solids but also liquids using a solvent (can be water or an organic solvent)

Question 6

the method for washing is :

Answer 6

• an impure solid is stirred in some of the solvent chosen so that it iwill siddolve little, if any of the solid but as much of the impurity as possible.
• the mixture is then filtered
• a liquid would be mixed with a solvent chosen so that it will dissolve little, if any of liquid to be purified.
• the mixture is then shaken in a separating funnel and the layers are drain off one at a time

Question 7

an important feature of a drying agent is

Answer 7

that it does no react with the organic product

Question 8

the two possible methods for drying an organic solid are:

Answer 8

• leave the mixture in a warm place

- use a desiccator with a suitable drying agent

Question 9

the method for drying an organic liquid is:

Answer 9

-select a drying agent that does not react with the organic liquid
(usually an anhydrous metal salt, such as calcium chloride, magnesium sulfate and sodium sulfate. these will absorb water as water of crystallisation to form hydrated salts.)
-add he drying agent to the mixture and swirl and shake it, then leave to settle
-add some more drying agent until it remains powdery, indicating that the liquid is dry (the liquid also goes form cloudy to clear when water is removed)
-the drying agent is removed either by decantation (pouring the organic liquid off the solid drying agent) or by filtration

Question 10

recrystallisation is used to

Answer 10

remove impurities by dissolving a solid compound in a solvent where all or most of any impurities but very little of the desired compound is dissolved.

Question 11

the method for purification by recrystallisation is:

Answer 11

• add the impure sold to a conical flask
• add some of the chosen solvent and warm until the mixture nears to the boiling temperature of the mixture
• if there is still some undissolved solid, add further solvent and warm until the mixture boils again
• continue adding further solvent and heating until all of the soluble solid has dissolved
• if insoluble impurities are present, then hot filtration could be done using fluted filter paper in a heated funnel
• allow the liquid to cool until crystals of the organic have formed
• more crystals can be obtained by cooling the solution below room temperature in an ice bath
• the mixture is then filtered to remove soluble impurities using a Buchner funnel or a Hirsch funnel, and then dried in a desiccator or oven

Question 12

both Buchner and Hirsch funnels use

Answer 12

vacuum pumps which allow for filtration under reduced pressure.

Question 13

the set up for a Buchner funnel is:

Answer 13

-a conical-shaped flask with a link to a vacuum pump , with a holder attached to the neck of the flask where a layer of filter paper is underneath the organic solid

Question 14

the set up for a Hirsch funnel is:

Answer 14

a cone linked to a test tube which is itself linked to a vacuum pump

Question 15

for solids, impurities …………the melting temperature

Answer 15

reduce

Question 16

the traditional way of measuring melting temperature of a solid is:

Answer 16

• place some of the solid in a small capillary tube attached to the bulb of a thermometer and then place this assembly into a liquid that has a boiling temperature of above that of the melting temp. of the solid
• apply heat and measure the temperature to record the solid’s melting temp.

Question 17

for liquids, impurities……………the boiling temperature

Answer 17

increase

Question 18

for measuring the boiling temperature of a liquid, the appraturs used is usually………………….but this depends on

Answer 18

• usually the apparatus for simple distillation

- depends on the volume of liquid available, and whether it is toxic or flammable

Question 19

measuring the boiling temperature of a liquid as a test for purity may not be conclusive because:

Answer 19

-may not be possible to measure the temperature accurately enough (thermometer might read too high or too low)
-different organic compounds can, by coincidence, have the same boiling temperature
1-chloropentane and 2-methylpropan-1-ol have the boiling temp. of 108 degrees Celsius