Benzoic acid
Premise
As we know any food product on the market is subject to deterioration because of various chemical natural processes caused by micro organisms. Since the remotest times, mankined has left, for several reasons, the need to stop or, at least to slow down such natural chemical processes in order to keep food as long as possible. The simple essay an benzoic acid had archived the aim of showing how it is employed by the modern food industry and how it cam be obtained by means of different chemical methods. The essay also present the chemical formulae and the data need to obtain benzoic acid on an industrial scale.
Benzoic acid
The simplest carboxylic acid can be found in many plants, particularly in resins and balsams, both in a free and in an asterified condition; the oldest source of this substance used to be the ippuric acid in the wrine of horses, obtained eithen by acid or alkaline hydrolysis, or by bacterial decomposition. Benzoic acid inhibits the fermentation and rotting of foodstuff and it has been used as sodic salt to preserve food. Besides, it is volatile in a vapour stream and it can be easily sublimed at about 100°C. The presence of micro organisms in food can produce toxins, and it can alter the organoleptic characteristics. Benzoic acid is employed in fruit juices, margarines, jams and marmalades, just to avoid this actions. Benzoic acid can be harmful if ingested, inhaled or touched without any adequate precautions. When ingested, it can cause irritation to the digestive tract, with ensuring heartburn, nausea, vomit and diarrhoea. When inhaled, it can cause irritation to the respiratory system, dyspnoea, cough and possible oedemas of different gravity depending on the concentration and the exposure time, while a mere contact can cause slight local irritation, without any special systemic affects. A derivate of benzoic acid, the orthohydroxybenzoic acid, also called salicylic acid, is the forerunner of Aspirin, a well-know pain-killer and anti-inflammatory drug. The following draft illustrates its synthesis.
Methods of preparation of benzoic acid
Benzoic acid can be obtained in several ways, e.g. by hydrolysis of benzamide, by oxidation acetophenone and by oxidation of toluene.
Hydrolysis of benzamide
We weigh a certain amount of benzamide and we pour it into an Erlenmeyer flask, adding some sulphuric acid at 10% to catalyze the reaction. We put the flask in a bain-marie at about 80°C and we wait for the complete dispersion of the benzamide and then for the formation of a thin film on the surface of the solution, when the hydrolytic reaction has occurred. After waiting for ten more minutes since the solution has become limpid, we put the flask to cool down at room temperature and then in ice-bath so as to obtain a white precipitate of benzoic acid. After the precipitation we proceed with the filtration, that will be carrie out with a Buckner funnel and device for vacuum filtration, and with a stove desiccation at about 80-90°C. After the desiccation we weigh the funnel and then we proced with the recristallization stage. Here we explain the hydrolysis mechanism through the reaction:
Oxidation of acetophenone to benzoic acid
We weigh some acetophenone in a pre-weighed bar and we add some bleach or clorex. Then we stir for a few minutes and we add some Na2S03 to destroy the exceeding oxidant. After slowly adding the sodium sulphite we should extract the mixture twice with 2 mls of ethyl ether and we should keep the liquid phase. Following this operation, we should acidify the watery solution with HC] 3M, checking the pH with a universal indicator. We wait for about fifteen minutes for the pH to stabilize and then filter the precipitate formed after the addition of the acid with a Hirsch fummel and a device for vacuum filtration. Before proceeding with the stove desiccation, the crystals should be washed in a little ice-cold water, so as to reduce the amount of benzoic acid that will be dissolved. After the desiccation the crystals are ready to be weighed again, so as to work out the yields and carry out the IR spectrum. The process of the oxidating reaction is explained below:
Oxidation of toluene to benzoic acid
Toluene can be trasformed into benzoic acid by oxiding its side chain, i.e. the CH3 group, by means of permanganates or dichromates. The reaction illustred below shows how a mole of toluene, when exposed to oxidising agents such as the permanganates, will turn into a mole of benzoic acid. This reaction can be carried out with all arenes which, like toluene, get oxidized and finally produce benzoic acid.
In the following pages we present the description of the workings of an industrial plant, meant for the oxdation of toluene to benzoic acid.
