Organic synthesis

Question 1

What can distillation be used for

Answer 1

• to separate miscible compounds by their boiling point
• chemicals with the lowest boiling point will distil first

Question 2

in which type of flask is the distillation and reflux of alcohols carried out in

Answer 2

pear-shaped or round bottomed flask

Question 3

why are anti bumping granules used in distillation

Answer 3

to promote smooth boiling as they prevent the formation of large gas bubbles that cause violent boiling

Question 4

why do joints in quickfit often have a layer of silicon grease smeared over them

Answer 4

to give a better seal as well as to make it easier to disassemble the equipment

Question 5

how does a steady and constant stream of water pass through the condenser

Answer 5

enters at the bottom of the condenser and the drainage pipe removes the water from the top of the condenser

Question 6

how is the reaction mixture heated and why

Answer 6

using a heating mantle because the temperature can be controlled, and because you are using chemicals which are flammable

Question 7

during the formation of aldehyde from a primary alcohol, which is distilled first

Answer 7

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 8

what’s the difference between distillation and reflux

Answer 8

• In distillation, you are trying to separate a (miscible) chemical or product from a mixture
• When heating under reflux, you aim to keep all the chemicals inside the reaction vessel

Question 9

in reflux, reaction mixture can be heated using…

Answer 9

heating mantle or water bath

Question 10

what’s the difference in the setup of quickfit apparatus between distillation and reflux

Answer 10

• in reflux, condenser is vertical not horizontal
• in reflux there’s an open top with no thermometer (prevents pressure build up)

Question 11

what are the different methods of purifying organic liquids

Answer 11

• Use of a separating funnel
• Use of drying agents
• Redistillation

Question 12

what can separating funnels be used to separate

Answer 12

• separates pure liquids from impurities
• separates two immiscible liquids that have different boiling points

Question 13

what can be done to remove acid impurities in organic reaction mixture

Answer 13

• Na2CO3 added to neutralise it

Question 14

what’s redistillation

Answer 14

• Sometimes when an organic liquid is obtained via distillation there can be traces of other liquids
• may occur if any liquids in the reaction vessel or other by-products have a similar boiling point to the desired organic liquid
• 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

Question 15

define miscible

Answer 15

liquids that are soluble and form homogenous solution

Question 16

how can you tell which is the aqueous layer in a separating funnel

Answer 16

add water and the aq layer will increase in volume

Question 17

what are drying agents

Answer 17

anhydrous inorganic salt that readily takes up water to become hydrated

Question 18

what are drying agents used for

Answer 18

removing traces of water in organic liquid

Question 19

examples of drying agents

Answer 19

CaCl2
CaSO4
MgSO4

Question 20

how do you know when no water is left after adding drying agents

Answer 20

solid no longer stuck in a lump and some drying agent is now dispersed in solution as fine powder

Question 21

equation for the dehydration of hexane using CaCl2

Answer 21

• CaCl2 (s) + C6H14 (aq) —-> CaCl2.H2O (s) + C6H14 (l)
• hexane goes from (aq) to (l) as it started of ‘wet’