Question

 

In acid-catalyzed addition of water to alkene, alcohol is the product. The conjugate base of the acid may compete with water for the newly formed carbocation and give a different product (other than alcohol). Among the following acids, which is the best acid catalyst? Which is the worst? Depend your answers.

               HOAc     HCl      H₂SO₄       CF₃COOH

 

Answer 1

Among the given acids, acetic acid forms a quite stable conjugate base acetate anion, which is a potential alkylating agent and can compete with water give acetylated alkane instead of alcohol (OAc instead of OH). HCl is a relatively weak acid in terms with sulphuric acid as the former is a gas dissolved in water and hence will not have such high activity. Hence, this will work to give pretty good yields and also is devoid of side-reactions. Next, sulphuric acid is a strong acid that gives sulphate anion as the conjugate base. That is inert to alkanes and will not interfere with the alcohol formation. Moreover, being the strongest acid in the list, it serves to yield excellent quantities of alcohol from alkene. However, care must be taken to avoid using excess quantities of this (more than 0.4 or 0.5 equivalents depending on inertness of substrate) as too much of the acid will result in dehydration of the alcohol to give back the alkene! Finally, the nastiest choice for hydration of alkenes is trifluoroacetic acid. The conjugate base, trifluoroacetate anion is an excellent nucleophile that competes quite well with water to give trifluoroacetyl alkane apart from alcohol, in rather noticeable quantities depending on the equivalents used.

So, if the reactions conditions are tuned are bare minimum equivalents of all the acids are considered for the conversion of alkene to alcohols without any side-products, sulphuric acid is the best choice, followed by HCl and trifluoroacetic acid is the worst, succeeding acetic acid.

NOTE: Agreeable equivalents of the catalysts vary ideally from 10mol% to 30mol% and not exceeding 40mol% (0.1 equivalents to 0.3 equivalents, not more than 0.4 equivalents).