Alkylation of Xylocaine with benzyl chloride to form the quaternary ammonium salt will give you Denatonium chloride, which is more known as its benzoate salt (Denatonium Benzoate), as it is the world’s bitterest known substance. 30 ppm of it added to any liquid (such as ethanol) renders it too bitter for most human test subjects to tolerate it.