Question

1. Briefly explain why  

(a) you should not heat organic solvents over a Bunsen burner flame.  

(b) you should add activated carbon to a cool solution and then heat the mixture to boiling rather than add the carbon to a boiling solution.  

2. Indicate a procedure to solve the following recrystallization problems, 

(a) oiling out  

(b) lack of crystal formation  

(c) presence of colored impurities  

(d) premature recrystallization in the funnel stem during gravity filtration

Answer 1

1. a. Organic solvents should not be heated on Bunsen burner because organic solvents may catch fire, they are potentially flammable.

Organic solvents have very low vaporization points, which equals to a very low combustion temperature because of which they get ignited soon as come in contact of fire flames. That is why the use of Bunsen burner is usually restricted in the organic chemistry lab. Instead for heating of the organic solvents it is is advisable to use water bath or hot plate.

b. Adding of activated charcoal to a boiling solution will cause the solution to boil over, which may burn skin. Hence activated charcoal is added to the cool solution.

2. a. Oiling out:

'oil' refers to non-polar hydrocarbon and related substances, but in this context, oil just refers to the consistency of the precipitate formed.

It occurs if the compound contains undesirable impurities, which is enough reason to prevent recrystallization . Or it is caused if recrystallizing solution has lower melting point than the boiling point of the solvent solution used for recryatallisation avoid recrystalisation.

Solution:

Use a recrystallizing solvent with a boiling point lower than that of the melting point of the recrystallized compound will avoid the oiling out and recryatllisation will complete.

b. Lack of crystal formation:

This problem can be resolved by adding Seed crystals (a small amount of the pure solid) to jump start the recrystalization process, or scratching the side of the glass.

c. Presence of colored impurities:

The problem of colored impurities can be resolved by using activated carbon "decolorizing carbon" which has a high affinity to absorb the colored impurities. Then filter mixture using gravity filtration method.

d. premature recrystallization in the funnel stem during gravity filtration.

Pre-heating the funnel shortly before filtration will avoid the premature recrystallization in the funnel stem during gravity filtration.