Synthesis of Benzoic Acid

湛江师范学院 11化本2班 2011xxxx

1. Objectives

(1) Learn the principle and methods of preparing benzoic acid by oxidative reaction.

(2) Master the purification technique of recrystallization.

2. principle

3. Reagents

Toluene 2.7ml (0.025mol);KMnO4 8.5g; concentrated hydrochloric acid.

4. Procedure

(1) Place 100ml of water,4.5g of pure potassium permanganate and 2.7ml of toluene in a 250ml three-necked flask equipped with a sealed mechanical stirrer and reflux condenser (see Figure1-8). Stir the mixture and reflux gently until practically all the permanganate color has disappeared (about 2 hours).At this point add 4g more of potassium permanganate and reflux the mixture again until the permanganate color disappears (about 2 hours); the color of the solution can easily be seen by removing the flame and stopping the refluxing.The end of oxidation is also judged by disappearance of the permanganate of the oily toluene in the refluxing liquid.

(2) Filter the hot contents of the flask from the manganese dioxide with suction. Wash the manganese dioxide with two 12ml portions of hot water.Cool combined solution.If the color of the filtrate is purple,add a small amount of of NaHSO3 solution and re-filter.

(3) Precipitate all the benzoic acid by cautiously adding 10ml of concentrated hydrochloric acid with continul strring. When cold, filter with suction,wash the acid with cold water and dry at 80℃.Upon recrystallization from hot water,

the m.p. is raised to 120—122℃.

5. Data recording and processing

n: 0.025mol 0.025mol

m: 0.025×122.12g

The actual yield Ma=1.2g

The theoretical yield Mt=3.053g

1.2g

W= ×100％=39.31％

3.053g

6.Notes:

(1)A somewhat lower yield is obtain is obtained if all the potassium permanganate is added all at once and,furthermore,the reaction may become violent.A ddition in 2～3portions results in a more controllable reaction.

(2)If the concentrated solution is not clear, it may be clarifid by the addition of 1g of activated charcoal.

7.Questions

(1).What are the primary factors influenced the yield of benzoic acid in the process of oxidative reaction ?

Answer: Organic reaction by-products of more, the by-product will influence, and the amount of reactants will also affect the yield As well as the reaction conditions affect.

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