4.6 -Organic Synthesis - Techniques

Question 1

What does distillation allow you to do

Answer 1

Separate compounds by their boiling point

Question 2

How do you produce a aldehyde from a primary alcohol

Answer 2

1. place the primary alcohol and acidified potassium dichromate solution into a round bottomed flask
2. use anti bumping granules to promote smooth boiling
3. Quickfit apparatus is set up including a still head and a condenser
4. A quickfit thermometer is used - sitting where the vapours will pass into the condenser
5. a steady and constant stream of water passees through the condeser
6. heated until it boils
7. the Distillate which forms into the condenser drips directly into a receiving vessel

Question 3

what do anti bumping granules do

Answer 3

promote smooth boiling

Question 4

What are the rules when collecting distillate (3)

Answer 4

The distillate which should be collected, is that which is given off at +/-2 °Cof the
boiling point of the desired product

Some distillate may be given off below this temperature - this needs to be discarded
and a clean vessel used to collect the desired product

Stop collecting the distillate if the temperature rises above +/-2°C of the boiling point
of the desired product

Question 5

Why can we use distillation of a primary alcohol to gain a aldehyde

Answer 5

The aldehyde product has a lower boiling point than the alcohol (since it has lost the H-
bonding) so it can be distilled off as soon as it forms

Question 6

why do we heat under reflux

Answer 6

organic reactions are often slow at RT
By heating under reflux you can produce an organic liquid
allowing the mixture to react as fully as possible without losing reactants products or solvent
you aim to keep all the chemicals inside the reaction vessel

Question 7

What are 2 examples where you heat under reflux

Answer 7

The production of a carboxylic acid from a primary alcohol using acidified potassium
dichromate
The production of an ester from an alcohol and acid in the presence of an acid catalyst

Question 8

what are the steps of heating under reflux

Answer 8

• reaction mixture is placed into a round bottomed flask
• add bumping granules
• the flask is placed on a heating mantle or in a water bath
• Quickfit apparatus is set up with the condenser clamped vertically in place with open top
• water flows through condenser
• It is heated and the reaction mixture is allowed to boil
• the heat is stopped and the mixture comes back to RT

Question 9

what are the steps of heating under reflux

Answer 9

• reaction mixture is placed into a round bottomed flask
• add bumping granules
• the flask is placed on a heating mantle or in a water bath
• Quickfit apparatus is set up with the condenser clamped vertically in place with open top
• water flows through condenser
• It is heated and the reaction mixture is allowed to boil
• the heat is stopped and the mixture comes back to RT

Question 10

What are 3 ways to purify organic liquids

Answer 10

Using a separating funnel
use of drying agents
redistillation

Question 11

how do you use a separating funnel

Answer 11

When organic products are being formed water can often be obtained along with the organic product
it will usually form a aqueous layer with the product in the organic layer
to identify which layer this is you can add water and see which layer grows
the contents of the reaction vessel are transferred to a separating funnel and a stopper is added
invert this and release the stopcock to release pressure (repeat 15-20 times)
allow the 2 layers to separate
open the stop cock to allow the aqueous layer to drain away

Less dense layer will be on top

Question 12

How do you use drying agents

Answer 12

they can remove water from an organic product
they are usually anhydrous inorganic salts

add a spatula of drying agent to the organic product and swirl
if the drying agent clumps together there is still water in the organic liquid
add more until some remains as fine powder
the dry organic liquid can then be decanted or filtered

Question 13

what drying agent is used to dry hydrocarbons

Answer 13

calcium chloride

Question 14

How do you redistil

Answer 14

Sometimes when an organic liquid is obtained via distillation there can be traces of other
liquids

This is particularly the case if any liquids in the reaction vessel or other by-products
have a similar boiling point to the desired organic liquid

In this situation, the distillation apparatus is cleaned, dried and set up for a second
distillation

The aim of the second distillation is to obtain just the desired organic liquid

This can be done by aiming for a narrower boiling point range in the distillate

The narrower the boiling point range, the more pure the organic liquid is