Organic synthesis
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Hi, I’m currently doing an experiment on "Preparation of Aspirin". May I know the reasons for my low percentage yield of the aspirin?
Igloo writes ...
It all depends what you mean by “low”! Are you referring you the yield of crude or purified aspirin? A typical yield of non-purified aspirin would be in the region of 70-90%, whereas if you purify this solid using, shall we say, two recrystallisations, the yield might well drop to between 40 and 60%.
Crude Aspirin
A low yield could be explained by any of the following:
1 Not allowing the reaction mixture to stand long enough, or a failure to reflux the mixture gently for long enough to “complete the reaction”.
2 Impure ethanoic anhydride. If the bottle of ethanoic anhydride was “old” then hydrolysis might have taken place, forming ethanoic acid. This would mean that not all the salicylic acid would have reacted.
3 A failure to cool the reaction mixture sufficiently in an ice-bath at the end, thus losing some of the product remaining in solution.
Purified Aspirin
A low yield here could be explained by:
1 Use of too much hot water for recrystallisation, so that a signiifcant proportion of the product failed to recrystallise on cooling, and therefore was lost in the filtrate.
2 A failure to allow the solution to cool sufficiently in an ice-bath, thus once again losing some of the product remaining in solution.
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 10 June 2007
