This reaction can be used for making bromo-derivative of acetanilide in
identification of organic compounds in UG as well as in PG level (2).
The aim of this experiment is to brominates acetaldehyde to form different product which to find out the melting point of which product is formed as there is three possibility para-. Ortho- and meta-.
Literature value:
para-bromoacetanilide- 168oc
McGraw-Hill dictionary of chemistry 2nd Edition pg 52
ortho-bromoacetanilide- 99 oc
Introduction to organic laboratory techniques: small scale approach by Donald L.Pavia pg 349
Hazard and risk analysis
When handling chemicals, there are many hazards and risk associated with the. This experiment uses acetanilide, Bromine, glacial acetic acid, aqueous sodium hydrogen sulphite and ethanol. Bromine is toxic, corrosive and a severe irritant to lung and eye tissue, therefore, it must be done in a fume hood and it is not allowed to be taken in an open laboratory. Protective gloves are advisable while dispensing the bromine solution. To destroy access of bromine use sodium hydrogen sulphite. Ethanoic (acetic) acid is flammable, corrosive and has an irritating vapour, handle in the fume hood, avoid breathing vapour or spillage on the skin. Sodium hydrogen sulphite (sodium metabisulphite) solution can irritate the skin after prolonged contact.
Before going to the lab, little research been done to fill the COSHH assessment and was bring along in the experiment.
Method
The experiment started in the fume hood, 4.5 g of acetanilide was placed in a 100 cm3 of conical flask before it was dissolved in glacial acetic acid(20 cm3) which was measuring cylinder was used. The burette was provided and it was dispensed 7.0 cm3 of bromine in acetic acid solution which was directly into a conical flask in one portion and the flask was swirled to mix the contents. Then the conical flask was covered with an up-turned small glass beaker and it was left in the fume hood for at least 30 min and occasionally the flask was swirled.
The excess of bromine was destroyed by adding aqueous sodium hydrogen sulphite solution until the brown color of bromine was discharged. Approximately 100 cm3 of water was put into a 250 cm3 glass beaker and was poured the reaction mixture into it, whilst it was stirred with a glass rod. The reaction flask was rinsed with water using the wash bottle which was transferred any residue and also was to break up the solid in the beaker which was dispersed the solid particle and then the reaction mixture was transferred to the open bench. The solid was filtered by suction filtration using a Buchner funnel and was washed with distilled water (2x25 cm3 portions). Before it was transferred into 100 cm3 conical flask, the water was removed from the solid as possible by suction filtration. The product was recrystallise and it was used a mixture of ethanol: water (2:1) and it was a minimum mixture at its boiling point and portions was added using the steam bath (50 cm3 was more than enough), until all the crystal was cleared and it was set aside to cool to a near room temperature and then the flask was cooled thoroughly in an ice/water bath mixture.
Last but not least, before the crystals was collected by suction filtration using a Hirsch funnel, the crystals was washed with approximately 1 cm3 of ice/cold ethanol/water mixture and then finally the crystal was dried by suction filtration as thoroughly possible and the mass and the melting point of the purified product was recorded. The purified product was transferred in the test tube and was covered with clock and it was labeled with name on it before submitting.
Results
Observed melting point range= 164 – 170 oc
Mass of acetanilide = 4.5 g
Mass of purified product = 5.38 g
Therefore, Percentage yield= actual mass x 100
Theoretical mass
= 4.50 x 100 = 83.64 %
5.38
Comparing the literature value of para, and ortho- bromoactanilide with the theoretical value this experiment results shows the product is p-bromoacetanilide, as the abserved melting point is 164 – 170 oc which is close to the literature value which 168 oc.
The molecular weight of p-bromoacetanilide= 214.06 g/mol
Molar mass = mass = 5.38 = 0.02513 mol
Molecular weight 214.06
The molecular weight of acetanilide= 135.00 g/mol
Molar mass = mass = 4.50 = 0.03333 mol
Molecular weight 135.0
% yield = 0.02513 x 100 = 64.60 %
0.03333
Discussion
Comparing the literature value of para, and ortho- bromoactanilide with the theoretical value this experiment results shows the product is p-bromoacetanilide, as the abserved melting point is 164 – 170 oc which is close to the literature value which 168 oc. Furthermore, the percentage yield of mass is high which is shown in the result 83.64 %, this indicate the better purity of p-bromoacetanilide.
The kind of mechanism reaction which is carried out in this experiment is electrophilic aromatic substitution (EAS), this mechanism helps in understanding how substituted acetanilide undergo EAS. However, many aromatic compounds are readily brominated when treated with bromine in the presence of catalyst of a Lewis acid which is Iron (III) bromide as the mechanism is shown in the introduction. This experiment is runned in mild conditions which glacial acetic is used. This catalyst increase the stability of aromatic compound (p-bromoacetanilide) to make the reaction more vigorous conditions to react, this is why it is preferred positively charged electrophile for this reaction as the mechanism shown in introduction. Iron (III) bromides cause a multiple risk brominations, so weaker agent is used glacial acetic acid. Furthermore, sodium hydrogen sulphite is used to remove the excess of bromine and also to neutralize the acid.
The p-bromoacetanilide was formed with fairly good yield at approximately 64.60%. There were a few steps that might have resulted in loss of product There is some error in this experiment which result inaccurate yielding. There was an error in measuring the melting point was inaccurate. However, it is believed that the mercury thermometer was fairly functional and accurate but in the melting apparatus made it difficult to control the pace of heating of the crude product. Another type error. If more ethanol is added in recrystallization then the melting point would lower and become more soluble which make the product ortho- instead of para-.
Finally, there is another way to identify p-bromoacetanilide using NMR spectra, a copy of a typical NMR shift table has been included as Table below shown. It can also be used to distinguish the para-isomer from its meta- and ortho-substituted isomers in which there four different types of aromatic hydrogens, each with integration one.
Conclusion
To sum it up, the product is p-bromoacetanilide as the observed melting point and the literature value are close as it shown in the result. The highest the percentage yields the better the purity of p-bromoacetanilide. Furthermore, melting point is a good method of determining the purity of p-bromoacetanilide. Another way of identifying the product is by using NMR spectra.
Reference
- College Practical Chemistry by V K Ahluwalia, Sunita Dhingra Adarsh Gulati Page 238
- Lecture notes from Professor Mark Wainwright- Aromatic Chemistry
-
Organic chemistry; structure and function, 6th edition, Peter Vollhardt, Neil Schore-published 2009 page 674
- Organic synthesis, Michael B.Smith, McGraw Hill international editions page 186-191
