Aim:

To look at the solubility of impure acetanilide in the
three solvents. Then to select an appropriate recrystallizing solvent. After, separate
and purify acetanilide from a mixture of a recrystallization. The final aim is
to weight the purified solid product of acetanilide and calculate the
percentage recovery.

Introduction

Impure substances have some impurities that usually
contaminate organic compounds that have been synthesized in the laboratory or
isolated from natural sources. Recrystallization is a purification process
which Is used to remove impurities from organic compounds that are solid at
room temperature. This procedure is based on the evidence that the solubility
of a compound in a solvent increase with temperature. On the other hand, the
solubility of the compound decreases as the solution cools down and the
crystals is formed.

Selecting an appropriate recrystallizing solvent to use is the
main step of purification and recrystallisation. When choosing a
recrystallizing solvent, the primary consideration is the degree to which the
compound and impurities are soluble in the solvent at high and low
temperatures. The compound that is recrystallized has to be very soluble in the
chosen solvent at high temperatures, but just about insoluble in the cold
solvent.  Impurities should be soluble in
the chosen solvent at all temperatures so that impurities stay in solution but
it should be insoluble at all temperatures so they can be filtered from the hot
solutions.

  

 

 

The purification of solids by recrystallisation depends on
the differences in their solubility in a given solvent or mixture of solvents.
The method is to dissolve the solid in some suitable solvent at or near the
boiling point, then to filter the hot solution to remove insoluble impurities
(when necessary). This will allow the hot solution to cool which causes the
dissolved substances to crystallise out. Then crystals can be separated from
the supernatants solution. Finally, crystals can be dried out.

A suitable solvent is one which will dissolve the solid
readily when hot, but only to a relatively small amount when cold. It should yield
with well-formed crystals of the purified compound and it must be able to
easily from the crystals of the purified compound.

The solution has to be filtered by gravity filtration
through a funnel containing a fluted filter paper to remove any insoluble
compound. Using a fluted filter paper, it increases the surface area inside the
funnel and speeds the filtering process. Vacuum filtration is the best
technique for separating the crystals from the mother liquor, or a solvent. In
vacuum filtration, a receiver flask with a sidearm, called a filter flask, is
connected by heavy-walled vacuum tubing to a vacuum source. A Buchner funnel is
fitted to the filter flask with a rubber stopper or filter adapter. A typical
gravity filtration apparatus is shown in Figure 2.

 figure 2

Percent
recovery is calculated by: dividing the mass of the recrystallized

compound by the mass of the crude compound before
recrystallization.

 

Assessing
Purity of a recrystallized compound is assessed by observing its colour:  If the compound has an off-white colour, the compound
should again be recrystallized using activated carbon. If the compound is pure
white it is purified.

 

Apparatus:

Ø  Conical
flask

Ø  Watch-glass

Ø  Suction
filtration apparatus

Method

1. Selection of a solvent

·        
The solubility of acetanilide in methanol, water, and light
petroleum (80-100°C boiling fraction) should be examined by carrying out the
test tube experiments (The results of these experiments are reported in the
table). By examining the result that’s obtained and by reading the experimental
procedure, select an appropriate solvent for the recrystallisation of
acetanilide.

·        
Place acetanilide (approximately 0.1g) in three test tubes and
each of the possible solvents should be added drop by drop, with continuous
shaking, to one of these test-tubes.

·        
After, 1 cm3 of each solvent had been added, the solid-liquid
mixtures are then heated to boiling using a water bath for the organic
solvents.

·        
If the acetanilide dissolved
in the hot solvents, the tubes has to be cooled to determine whether
crystallisation occurred. The results obtained are shown below for the three
solvents.

 

 

Recrystallisation
of acetanilide

·        
First, weigh out commercial acetanilide (4g) into a 250 cm3
conical flask. Add enough amount of solvent (water) to wet the crystals. Boil
the mixture for a few minutes for acetanilide to dissolve. If the acetanilide
does not dissolve, slowly add a few more cm3 of solvent and boil again for a
few minutes. Repeat this procedure until the solid has dissolved.

·        
Allow the solution to cool slowly by covering the conical flask
containing the hot filtrate with a watch glass. When the solution has cooled substantially
place the flask in an ice-bath to aid the crystallisation process.

·        
When this is complete, separate the solid from the mother liquor
by vacuum (suction) filtration. Wash the crystals twice with portions (5 cm3)
of cold solvent. Maintain a vacuum on the system until the solid is dry.

·        
 For solid to dry rapidly,
place the crystals on a watch glass in a vacuum oven, providing the temperature
of the latter is below the melting point of the solid.

·        
Finally, Weigh the product and calculate the percentage recovery
and place it in a properly labelled sample bag and hand in to the laboratory
supervisor.

The melting point is not required.

 

 

 

 

 

 

 

 

 

 

 

 

 

Evaluation

In this experiment the aim was to use a technique called
purification by recrystallization to carry out purification of acetanilide.
Then the percentage recovery of acetanilide was to be calculated. Our result
was 88% of recovered acetanilide as shown in the report form.  In this experiment we have successfully obtained
the amount of dried crystal which was further used to calculate the percentage
recovery of acetanilide. The experiment showed that the suitable solvent has to
be used to dissolve the solute so recrystallisation can be carry out.

 Acetanilide is
soluble in methanol and less soluble in light petroleum. We observe the
temperature has influence in its solubility because acetanilide is more soluble
in hot water. Even after a solid has been recrystallized, it was still not pure
as the result we achieved was 88%. This could have been influenced due to some
human errors and because of acetanilide containing some insoluble impurities.

 

The
choice of solvent was the most important step in the process.  The right solvent had to be selected to form
a product of high purity and good recovery. Water was chosen as the best
solvent because when cooling, out of the three solvents water dissolved
completely and the most crystals were formed. Furthermore, water is the most
suitable solvent due to it having the best polarity hence giving it the
property of best solvent. Polar or ionic compounds are surrounded by water
molecule. The positive dipoles of hydrogen in water are attracted to negatively
charged components of the solute and vice versa for the negative dipoles. The
hydrogen bonds in water are formed and then broken therefore liquid water contains
partly ordered structure. Furthermore, water was chosen because at room
temperature the acetanilide appeared to have low solubility when it was in
water, and when the acetanilide was heated to boiling in water it seemed to
have high solubility. It is also nontoxic and not flammable.  

 

Accuracy
and precision of the measurements taken in the experiment can influence the
data obtained. There are a number of ways in which the sample can be lost
throughout the experiment. The sample could have been lost in a transfer stage.
For example, while transferring the sample to the conical flask from the weighing
boat. Some of the sample may have remained in the weighing boat as it is
unlikely you can get a 100% transfer. Furthermore, the sample was not 100% as
some of it could have been lost when it was transferred from the conical flask
to the Buchner funnel when washing the conical flask using distilled water. As well
as this, a part of the sample may have been trapped to the pot and the vacuum when
the crystals were transported into the glass plate. We also used the filter
paper to filter the acetanilide crystals. This means some of the residue maybe
have remained on the filter paper resulting in less sample. Experimental error
in the results may have been due to the use of inaccurate glassware,
calibration of measuring equipment which all a have % error. There is a
systematic error for each of the measuring devices such as the scale had a
systematic error of ±0.0002g. it contained a Percentage error (scale) =
(0.0002/3.52) x 100 = 0.0057 %. This will influence the percentage of recovery.
In addition, the data could be influenced due to human errors such as
misjudging the colour of the crystal.

 

In
future if the whole experiment was to be repeated, the errors that can be
connected with volumetric glass accuracy can be adjusted for by careful
calibration of the glassware. we can minimalize errors using A class volumetric
glass. We can also minimalize errors by weighing the sample at the end more
than three times and working out the average. This will make sure to
give an accurate percentage recovery.

Conclusion

In
conclusion, water was found to be the most suitable solvent to dissolve
acetanilide out of the three solvents in investigation. The yield of crystals
formed was 3.52 grams. The percentage recovery was not 100% but 88%. This
suggests that in acetanilide there are there insoluble
impurities and due to some human errors and we might have lost 12% from the
original substance.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Report
form

 

EXPERIMENT 2

 

Purification
of Acetanilide by Recrystallisation

 

1. Solvent selected

 

Solvent selected for recrystallisation of
acetanilide was:  Distilled water

 

2. Details of Recovered Acetanilide

 

Amount obtained: 3.52 grams             % Recovery:  88 %

 

Calculation:

 

 

Product appearance:  white
crystals

 

 

 

 

 

 

 

Questions

 

1. Why should the use of diethyl
ether as a solvent for recrystallisation be avoided wherever possible?

Ø  The use of diethyl ether as a solvent for
recrystallisation should be avoided because the boiling point of diethyl ether
is very low (34.5 0 ) hence it evaporates quickly. The cooling that
takes place during evaporation causes water from the atmosphere to condense on its
surface which would lead to crystal being wet. Furthermore, when filtering the
purified organic crystals “ice crystals” would be formed on the
filter paper when the diethyl ether evaporates. 

 

2. Would you expect a polar substance to be more soluble in
polar solvents or non-polar solvents?

Ø  The
polar substance is expected to be more soluble on a polar solvent because molecules
mix readily as both types of molecules engage in hydrogen bonding. Since the
intermolecular attractions are roughly equal, the molecules can break away from
each other and form new solute.

3. When would you use mixed solvents for recrystallisation
of a compound?

Ø  We
would use mixed solvent for recrystallisation If no solvent can be found in
which the solute is relatively insoluble in the solvent at room temperature but
is lot more soluble in the solvent at higher temperature. Mixture of solvent
can be used If the compound that is being purified is freely soluble in one
solvent and practically insoluble in another solvent. If two solvents in which
the solute has different solubility characteristics are mixed, an appropriate
solvent can be found. For instant, if you dissolve a compound in warm solvent A,
in which it dissolves very easily. Then you begin to add warm solvent B to the
solution, a little at a time, until the mixture starts to cloud. This is the
point where the compound is at the limit of its solubility in the mixture. If the
solution is now set aside and allowed to be cooled, pure crystals of your
compound will form and can be filtered off. The idea is that the impurities
will remain in solution. Using a single solvent may need excessive solvent to
dissolve a sample and if there are impurities, they can be difficult to
distinguish from an insoluble product.

 

4. Name one other way of purifying compounds. Give a brief
explanation of the principles behind it.

Ø  Distillation
is another way of purifying compound. It is used to separate two or more
liquids that are miscible. Distillation is s method which is designed to
separate a mixture containing two or more liquids which has different boiling
points.  The process includes heating the
mixture in a flask. One compound (compound A) will have different boiling point
to another compound (compound B). The compound with the lower boiling point
evaporates first, e.g. compound A. This vapour is then cooled and condensed
inside the condenser to form a pure liquid. The thermometer shows the boiling
point of the pure liquid of compound A.  When all the compound has evaporated from the
solution, the temperature rises and the compound B evaporates. Apparatus and
the steps are shown below: