The side reaction will affect the purity and percent yield that, it will decrease the purity and also decrease the present yield.
Due to side reaction, the amount of aspirin which is being produced will be less for some amount of starting material which is used. The present yield will decrease. The insoluble water which is being precipitated is formed as a result of the side-reaction which will remain with aspirin and finally contaminate the aspirin.
The purity of aspirin will also decrease.
