Sunday, 17 August 2014

Aspirin Hydrolysis.

Acetylsalicylic acid hydrolysis

In this experiment we will make salicylic acid

Salicylic acid's formula is C6H4(OH)COOH and its IUPAC name is 2-hydroxybenzoic acid. It finds use today ( among others ) as the active metabolite of acetylsalicylic acid (ASA), an acne treatment (a chemical exfoliant) and as a chemical reagent.In this experiment we will hydrolyse acetylsalicylic acid to salicylic acid (SA) with hydrochloric acid and reflux. Here is a look at the reaction scheme
One mole of ASA is hydrolysed to form one mole of SA and acetic acid (vinegar). Esters are generally formed through a condensation reaction called "Fischer-Speier esterification" where a carboxylic acid  is refluxed with an alcohol (usually in excess) to form an ester and a water molecule. this reaction requires an acid catalyst, which is usually concentrated sulphuric acid as it is a strong acid with dehydrating properties which drives the reaction to the right ( small amounts of the ester will be hydrolysed in water as the reaction is reversible) Although ASA is formed in a different way it  is technically an ester. This reaction is in essence the reverse where  water and dilute acid yield an acid and an alcohol ( but this reaction forms two acids ) 

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. Strong fumes of acetic acid may be present in the flask after reflux. Salicylic acid can cause chemical burns. All of the acids used can burn and blind you and thus you must be prepared for such a situation if it would occur. 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.

1. As always, time for some maths. first is calculation of theoretical yield: 
As seen from the reaction scheme the reaction is a 1:1 molar ratio, that is to say 1 mole of ASA hydrolyses to form 1 mole of SA. that makes calculations easy!
M= molar mass 
m= mass of sample
n= number of moles
From the formula n= m/M we can determine how much our theoretical yield will be from our sample
M(ASA)= 180.157 g/mol
the author's sample weighed 2.230 grams
From the formula n=m/M
n= 0.0123 mol
if we rearrange the formula n=m/M we can determine the theoretical yield;
 m= nM 
M(SA)=138.121 g/mol
m(SA)= 0.0123x138.121
m(SA)= 1.706 g 
So there we have it, with a few simple calculations we can determine how much SA we get from this reaction.

     2. place 2.230 g of ASA into a 100 ml round bottom flask 
                   3. measure out in 50 ml graduated cylinder 1 ml of  >28 % w/w HCl and add 35 ml of distilled or deionised water. add this solution to the round bottom flask. add a few boiling chips to promote smooth boiling.

4. Clamp the flask just above a hotplate ( ensuring that the bottom of the flask is not directly touching the hotplate)
5. Attach a condenser and set up for reflux ( ensuring that the cold water is flowing from the bottom of the condenser up) reflux for 30-45 minutes 

6. Once refluxed the flask is stoppered and allowed to cool down to room temperature (make sure that the stopper is heavily greased) The SA is quite soluble at higher temperatures 
after about 15 minutes the SA precipitates due to its low solubility at room temperature (2g/L 20°C)

7. The product was vacuum filtered on a glass frit and was washed with 3x 30 ml ice cold distilled water ( probably over kill). The product was dried by drawing air over the product until it no more liquid was coming off and it was a free flowing powder.
              the final product was weighed and found to weigh 1.635 g representing a 95 % yield!

 No melting point was taken ( as the author does not have the apparatus)  but it is highly recommended . The product may be re-crystalised  from acetone. Make sure your apparatus has no iron contaminants as it will produce a pink complex and will ruin your product.


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