Recrystallising acetanilide

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The aim of this experiment is to establish the most suitable solvent required for recrystallising acetanilide, and to exhibit the methods of vacuum filtration and recrystallisation.

Experimental Method (for selection of a suitable solvent):

  • Four solvents: dichloromethane, methylated sprits, petroleum ether and water were tested to see how appropriate they were for recrystallising acetanilide.
  • Impure acetanilide (0.1g) was inserted into a small test tube and whilst continually shaking the test the solvent was added drop by drop.
  • After 1cm3 of the solvent was added!!!!? the mixture was then heated till it was boiling using the water for the organic solvents bath in the fume cupboard once approximately 1mL of solvent had been added.
  • As the acetanilide was dissolved the test tube was cooled to see whether recrystallisation occurred.
  • The same method was repeated out for all four solvents.

Experimental Method (for recrystallisation of acetanilide):

  1. Impure Acetanilide (4g) was weighed out using a rough balance into a conical flask (250mL).
  2. A suitable amount of the solvent from the previous method [water (1mL)] was used to wet the crystals.
  3. The mixture was then boiled gently, and more solvent was added until all the sold had dissolved.
  4. Using the filter paper obtained from the pre-heated funnel the boiling solution was filtered. This was done into a pre-heated container so that rapid cooling as avoided.
  5. The flask containing the hot solvent was covered and allowed to cool slowly. When a noticeable amount of solid had formed and the solution had cooled considerably, the solution was placed in an ice-bath to speed up the process.
  6. The filter paper was wet with the solvent then using vacuum filtration the crystals were separated from the rest of the solution.
  7. Using cold solvent (5mL approximately) the crystals were washed.
  8. The crystals were kept on the Büchner funnel until they were almost dry. The damp material was transferred to a labelled watch glass and then this was positioned into a vacuum oven of a set temperature lower than the melting of the pure solid.
  9. The dry purified product was weighed and packaged into a labelled plastic bag.

Why I chose water for the larger scale recrystallisation:

This is because when cooling, water out of the four solvents dissolved completely and the most crystals were formed. Also water is the most suitable solvent because it has the best polarity.

The covalent bonding between an oxygen atom and two atoms of hydrogen form a water molecule. Atoms share electrons between these covalent bonds. The sharing of these electrons is not equal in water as the hydrogen atoms have a weaker attraction to the electrons than the oxygen atoms. Hence the distribution of charge in water is asymmetrical. Molecules are known as polar when they have ends with positive and partially negative charges. The reason for why water is able to dissolve a number is substances, is due to this polarity that enables water to separate polar solute molecules.

Waters polarity makes gives it the property of being a good solvent. Water molecules surround polar or ionic compounds that enter water. One molecule of solute can be surrounded by many water molecules as the size of water is relatively small. The positive dipoles are attracted to negatively charged components of the solute and vice versa for the negative dipoles. The hydrogen bonds in water are always being formed and broken hence liquid water has a partially ordered structure. The high cohesiveness of water is due to the strong hydrogen bonds and this also results in high surface tension. This is why water remains together as drops when on a non-soluble plane. Additional reasons for why water was the most suitable solvent: i. It was cheap to use water as the solvent ii. When the water was boiled at a temperature below the acetanilide melting point. iii. At room temperature the acetanilide appeared to have low solubility when it was in water. iv. The water was also not toxic or flammable. v. When the acetanilide was heated to boiling in water it seemed to have high solubility.

The main points in the procedure where the sample is lost:

There are a number of ways in which the sample can be lost throughout the experiment:

One of the reasons is whilst carrying out the practical some of our sample was spilt accidently by one of our neighbouring colleagues so our yield was less that what would have been expected.

               The sample may have been lost during the transfer stages i.e. when the sample was transferred to the conical flask some of the sample may have remained in the weighing boat as it is unlikely you can get a 100% transfer. Further sample could have been lost when it was transferred from the conical flask to the Büchner funnel when washing the conical flask using distilled water. Furthermore, sample could have been lost by the transfer of sample from the Büchner funnel to the watch glass. Also when the crystals were transported into the glass plate as part of the sample may have got trapped to the pot and remained in the vacuum. When we used the filter paper to filter the acetanilide crystals some of the residue maybe have remained on the filter paper resulting in less sample.

  • · Why low boiling point solvent such as diethyl ether (BP: 35⁰C) or dichloromethane (BP: 40⁰C) are general les suitable for recrystallisation than higher boiling point solvents, such as water or ethanol, irrespective of their polarity:
  • One of the technicians had said in the practical that Dichloromethane has an occupation explosive limit (OEL)...........!!!. Both diethyl ether and dichloromethane are the best solvents to consider as they are both flammable which is a safety hazard. Dichloromethane (DCM) is not as suitable because it evaporates more readily at room temperature (due to its low boiling point) which does not allow time for the crystals to dissolve. Also the impure compound would not have enough time to dissolve as well as the DCM would evaporate before hand.

  • · Why the solubility of paracetamol in water is several times higher than that of acetanilide by looking at the structures:
  • Because the paracetamol contains an OH group hence it makes it more polar than acetanilide so it dissolves more readily in water which is also polar.

Conclusion:

In conclusion, water was found to be the most suitable solvent to dissolve acetanilide out of the four solvents in investigation. The yield of crystals formed is greater with a larger scale of production.

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