17.6 4 Practical Techniques In Organic Chemistry Part 2

Question 1

How to choose a solvent in solvent extraction

Answer 1

Solvent should be immiscible with the solvent containing the desired organic product

The desired organic product should be more solvent in the added solvent than in the reaction mixture

Question 2

Method for solvent extraction

Answer 2

1. Place reaction mixture in separating funnel then add chosen solvent that should form a separate layer
2. Place stopper in neck of funnel and gently agitate the contents of the funnel for a short while
3. Allow contents to settle into two layers
4. Remove stopper allow the lower layer to drain from the flask and then do the same to upper layer into a different flask

Question 3

How to agitate the contents of the funnel during solvent extraction

Answer 3

Put a finger on the stopper , invert , open tap , agitate in a circular motion , close tap and return funnel to normal position

Question 4

In solvent extraction better to add

Answer 4

Solvent in small portion as this is more efficient

Using more portions but the same total volume removes more of the desired organic product

Question 5

After solvent extraction

Answer 5

Simple distillation of fractional distillation must be used to separate desire organic product from the solvent

Question 6

Washing introduction

Answer 6

Used to remove impurities from a solid or liquid
Use of water or an organic solvent
Solvent must dissolve impurities as much as possible but as little as possible of the substance being purified

Question 7

Washing solid

Answer 7

I’m pure solid stirred in some of the solvent then the mixture filtered

Question 8

Liquid washing

Answer 8

Mixed with solvent chosen and shaken in a separating funnel
Allow to liquid layers to separate
Tap opened to allow each layer to drain into a separate container

Question 9

Drying an organic solid

Answer 9

Needs to be left in a warm place or in a desiccator with a suitable drying agent ( see image of desiccator )

Question 10

Liquid organic product

Answer 10

Often prepared using inorganic reagents which are used in aqueous solution therefore liquid organic product may be partially or completely dissolved in water - therefore water is an impurity that needs to be removed by a drying agent

Question 11

Important feature of drying agent

Answer 11

does not react with the organic liquid

Question 12

Most common drying agents

Answer 12

anhydrous metal salts usually calcium chloride ,magnesium sulfate and sodium sulfate
form hydrated salts
when come into contact with water in the organic liquid they absorb water as water of crystallisation

Question 13

Drying liquid organic product method

Answer 13

Drying agent is added to organic liquid and the mixture is swirled or shaken and then left for a period of time
before use drying agent is powdery but after absorbing water looks more crystalline . If more drying agent is added and remains powdery then indication liquid is dry
the liquid goes from cloudy to clear when water is removed
the drying agent is removed by decantation or filtration

Question 14

principle behind recrystallisation

Answer 14

A solid compound dissolve in a suitable solvent that can dissolve most of any impurities but very little of the compound being purified

Question 15

Steps or recrystallisation

Answer 15

Add impure solid into the conical flask
Add some of the chosen solvent and warm the mixture until the mixture nears the boiling point of the mixture
if there is still some undissolved solid add futher solvent and warm mixture until the mixture boils again
Continue adding solvent and heating until all of soluble solid has dissolved
if insoluble impurities are present hot filtration could be done using fluted filter paper in a heated funnel
allow liquid to cool until crystals of organic solid have formed
more crystal obtained by cooling using an ice bath
Mixture is filtered using a Buchner funnel or Hirsch funnel and dried in a desiccator or oven
see images

Question 16

for solids

Answer 16

impurities reduce the melting temperature

Question 17

How to measure melting temperature

Answer 17

Place some solid in a capillary tube attach to bulb of thermometer and then place in liquid that has boiling temperature above that melting point of solid

Question 18

For liquid boiling points

Answer 18

simple distillation apparatus can be used in most cases
have to think is toxic or flammable
may not be conclusive as thermometer may not be accurate enough - can wrongly assume is pure
can have same boiling point