Sunday, 31 August 2014

Oxidation of toluene by potassium permanganate


Benzoic acid is a versatile reagent that can be implemented in a plethora of syntheses (e.g. benzene [Ref. 1], methyl benzoate [Ref. 2], etc). Today, benzoic acid is most commonly encountered as benzoate due to food  its preserving properties[Ref 3.]. Benzoic acid can be prepared in the home lab through the oxidation of toluene using Potassium Permanganate. Interestingly, Potassium permanganate can oxidise any alkyl benzene so long as it contains a benzylic hydrogen. The mechanism not completely understood, however it is thought that potassium permanganate abstracts a hydrogen forming a radical, although the rest is not known[Ref. 4][Ref. 5] The scheme is given below:

  Reaction equation: 

C6H5CH3 (aq) + 2 KMnO4(aq) C6H5COOK(aq)+ 2 MnO2(S) H2O(aq)+ KOH(aq)
In this experiment we will make Benzoic acid via oxidation by potassium permanganate 
To synthesise Benzoic acid on a laboratory scale.
Safety and Disposal: 
Before attempting any experiment be sure to read all relevant safety information on the chemicals you will be handling/producing. In this experiment it is highly recommended to wear gloves, wear eye protection, wear a lab coat and work in a well ventilated area (preferably out side or in fume hood). Concentrated Hydrochloric acid is highly corrosive along with its fumes. Potassium permanganate is highly oxidising and stains everything. Toluene posses an inhalation hazard, and is toxic.Benzoic acid is not particularly toxic, however exposure should be limited. Be sure to read all relevant disposal regulations, as it different depending on where you live the and author can't give such information. The author of this blog will not be held responsible for any damage caused by following directly or indirectly instructions/ gaining information from this blog. 

Note: this experiment was based on the procedure found here:

Weigh out 8.00 (50.622 mmol)grams of potassium permanganate and add to the round bottom flask

To the Round bottom flask add 90 ml of distilled water and 4.664 (50.622 mmol) grams of toluene (~10 ml) (note*) add a magnetic stir bar, fit the reflux condenser and begin water flow (thermometer not necessary)
Reflux the solution for 2-2.5 hours with vigorous magnetic stirring
as the reaction progresses the intense purple permanganate ion will gradually disappear as it is reduced from manganese (VII) to manganese (IV) which precipitates as brown manganese dioxide 

After 2.5 hours the reflux was stopped and the manganese dioxide was vacuum filtered with a Buchner funnel (do not wash the precipitate) 

The filtrate was put into a small separatory funnel and the aqueous phase was drained into an appropriately sized beaker.
The aqueous layer was then acidified with excess (until no more precipitate forms) conc. hydrochloric acid. 
The benzoic acid precipitate was vacuum filtered and washed with ~ 2x 60 ml distilled water. (note** ) The precipitate was pressed with a glass stopper until no more mother liquor passed through the filter frit. Air was then drawn though the product until dry.

note*:the required amount of toluene is 25.31 mmol, however the reaction works best with toluene in excess, as this allows for a greater phase boundary surface area, as toluene is not very soluble in water (0.52 g/L (20 °C)). It also eliminates a step in the original procedure to reduce any remaining permanganate ion. 
note**: this proved to a possible cause for low yield. 2x 30ml with ice cold distilled water seems to be more appropriate and would result in less product loss. However, it could be suggested that if the mother liquor was boiled down to less than half its volume and placed in an ice bath, some of the product could be recovered.
The aim of the experiment was achieved; benzoic acid was prepared on a small scale. Although the product has not been characterised due to lack of resources, a qualitative test can be carried out using a soluble copper (ii) salt to form the benzoate which precipitates out of solution. Yield was ~ 2 grams or a ~32% yield. This is most likely due to the over oxidation of toluene to carbon dioxide, small scale and mechanical losses. However, as discussed in note**, some of the product may be recovered from the washes. On a larger scale a better yield should be obtained.
[Ref.1] James F Norris, JFN (1924), Experimental Organic Chemistry, Second Edition,  MCGRAW-HILL BOOK COMPANY, INC, USA 
[Ref. 2] Arthur I. Vogel, AIV (1948), Practical Organic Chemistry, Third Edition,  LOWE AND BRYDONE (PRINTERS) LTD, GBR
[Ref 3.] 2004. Compound Summary for: CID 243 Benzoic Acid. [ONLINE] Available at: [Accessed 31 August 14]. 
[Ref.4] Master Organic Chemistry. 2012. Oxidation of aromatic alkanes with KMnO4 to give carboxylic acids. [ONLINE] Available at: [Accessed 31 August 14].
[Ref. 5]Sukalyan Dash , Sabita Patel , Bijay K. Mishra ,(2008), Oxidation by permanganate: synthetic and                   mechanistic aspects, Tetrahedron, 65(4), 724

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