why is ethanoic anhydride preffered?

[Tonychet2]

why is ethanoic anhydride preffered instead of ethanoic acid to produce aspirin?

[iirene]

If ethanoic acid was used, then it would create water (ethanoic acid + salicylic acid --> acetylsalicylic acid + water). Therefore, the reaction will also drive backwards (aspirin could undergo hydrolysis and make salicylic acid + ethanoic acid). This means that the reaction will be slow and have a low yield.

Using ethanoic anhydride will increase the yield and reaction rate
ethanoic acid + ethanoic acid --> ethanoic anhydride + water
ethanoic anhydride + salicylic acid --(concentrated H2SO4)--> acetylsalicylic acid + ethanoic acid
then we can use this ethanoic acid to make more ethanoic anhydride!

[Tonychet2]

ahhhhhhh thanks alot... makes total sense