Introduction
Friedel-Crafts reaction is a reaction to attach a substituents to an aromatic ring. It involves an electrophilic substitution. Friedel-Crafts acylation is the acylation of aromatic rings with an acyl chloride using a strong Lewis acid catalyst. Friedel-Crafts acylation is also possible with acid anhydrides. Reaction conditions are similar to the Friedel-Crafts alkylation mentioned above.(1)
Mechanism :
Benzophenone is the organic compound with the formula (C6H5)2CO, generally abbreviated Ph2CO. Benzophenone is a widely used building block in organic chemistry, being the parent diarylketone.(2) 4-Bromobenzophenone with the formula of (C13H9BrO) is obtained by the reaction between bromobenzene and benzoyl chloride under Friedel-Crafts reaction. The introduction of an acyl group into an aromatic ring is accomplished by an electrophilic substitution by the acylium ion generated by the reaction between an acyl halide and aluminium chloride . In bromobenzene, the benzene is a deactivating and ortho, para-directing substituent. However, this reaction gives mainly the para-isomer, presumably ortho-substitution which might also be expected is sterically less favoured.
Apparatus
Steam bath
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Aromatic hydrocarbons are relatively not reactive. They have a special stability due to resonance (their pi electrons are completely delocalized). Aromatic compounds do not undergo addition reactions, because they would lose this special stability. Alkenes, however, are much more reactive. They have electron-rich double bonds that allow them to undergo addition reactions. What is the overall ...
Rotary Evaporator
IR
Materials
Bromobenzene
Benzoyl Chloride
Anhydrous aluminium chloride
Sodium Hydroxide
Diethyl ether
Light Petroleum
pH indicator paper
Procedure
Bromobenzene and benzoyl chloride are placed in a 50 mL Erlenmeyer flask. Aluminium chloride is added in three portions, shaking and stirring in between additions. The flask is then heated on a boiling water bath for 20 minutes. After 20 minutes, it is left cooled. The dark red liquid is poured onto an ice and the remaining contents are washed out by adding 10% NaOH solution to dissolve any benzoic acid present and also aluminum salt. The alkaline solution is heated. The solution is removed and the brown oily precipitate is collected. The brown solid is dissolved with the addition of NaOH and ether. The mixture is extracted with ether three times. The organic extract is dried over MgSO4 and the ether is removed on the rotary evaporator. 4-Bromobenzophenone remained and may be recrystallized from light petroleum to give a colourless solid. The yield is recorded and the melting point of the product is determined. The purified materials is analyzed using IR spectroscopy. Calculation
No. of mol bromobenzene :
= massmolarmass= 3.0 g 157.01 g/mol= 0.0191 mol
No. of mol benzoyl cloride :
= massmolarmass= 4.0 g 126.58 g/mol= 0.30316 mol
-Therefore, bromobenzene is the limiting reactant.
Theoretical yield of 4-Bromobezophenone :
= no. of mol × molar mass
= 0.0191 mol × 261.11 g/mol
= 4.9872 g
Melting point : 79- 84 oC.(3)
Result and Observation
In this experiment, Friedel-Crafts reaction is carried out by the reaction between bromobenzene and acyl chloride in order to obtained 4-bromobenzophenone. However, the product is not obtained at the end of this experiment. This is maybe due to some errors occur during handling this experiment. For the first 20 minutes, aluminum chloride is used as the catalyst. Aluminium chloride is divided into three portions before adding. While waiting for shaking and stirring process done before adding it one by one, the aluminium chloride may be exposed too long to the heat and moisture. It caused it to be quite unstable to react afterwards. After 20 minutes, the dark red liquid is left to be cooled. However it may not be left too cool enough to precipitate. It influences the amount of product produced. During extraction, ether is used. It is because the extraction involves liquid-liquid extraction and ether is immiscible with water.
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During the separation, product may be extracted together with the unwanted ones. Light petroleum is used to recrytallized. It is because it has high volatility and insoluble in water. After all, product is still not obtained maybe due to these errors. The theoretical yield should be 4.9872 g and the melting point range is between 79-84oC. From the 1HNMR spectrum attached, a peak is formed between 6.0-9.0 ppm. It shows an aromatic compound. While from the IR spectrum, three peaks is produced. First peak which is between 600-800 cm-1 is benzene peak. Second peak formed between 1600-1800 cm-1 is carbonyl C=O peak. Lastly, peak of C-H saturated is produced between 2800-3000 cm-1. Overall, it could give a possible compound of 4-Bromobenzophenone.
