Question

Outline a synthesis of tert-butyl isopropyl ether using either alcohol dehydration or alkene addition, as appropriate.

Answer 1

Acids other than hydrogen halides also add to the carbon–carbon bond of alkenes. Concentrated sulfuric acid, for example, reacts with certain alkenes to form alkyl hydrogen sulfates.

Addition of Sulfuric Acid to Propene

Notice in the following example that a proton adds to the carbon that has the greater number of hydrogens, and the hydrogen sulfate anion (OSO₂OH) adds to the carbon that has the fewer hydrogens.

Markovnikov’s rule is obeyed because the mechanism of sulfuric acid addition to alkenes, illustrated for the case of propene in Mechanism 6.4, is analogous to that described earlier for the electrophilic addition of hydrogen halides.
Alkyl hydrogen sulfates can be converted to alcohols by heating them with water. This is called hydrolysis, because a bond is cleaved by reaction with water. It is the oxygen–sulfur bond that is broken when an alkyl hydrogen sulfate undergoes hydrolysis.

The combination of sulfuric acid addition to propene, followed by hydrolysis of the resulting isopropyl hydrogen sulfate, is the major method by which over 109 lb of isopropyl alcohol is prepared each year in the United States.

We say that propene has undergone hydration. Overall, H and OH have added across the carbon–carbon double bond. In the same manner, cyclohexanol has been prepared by hydration of cyclohexene:

Other Method for the hydration of alkenes is by reaction with water under conditions of acid catalysis.

Unlike the addition of concentrated sulfuric acid to form alkyl hydrogen sulfates, this reaction is carried out in a dilute acid medium. A 50% water/sulfuric acid solution is often used, yielding the alcohol directly without the necessity of a separate hydrolysis step. Markovnikov’s rule is followed:

Mechanism 6.5 extends the general principles of electrophilic addition to acidcatalyzed hydration. In the first step of the mechanism, proton transfer to 2-methylpropene forms tert-butyl cation. This is followed in step 2 by reaction of the carbocation with a molecule of water acting as a nucleophile. The alkyloxonium ion formed in this step is simply the conjugate acid of tert-butyl alcohol. Deprotonation of the alkyloxonium ion in step 3 yields the alcohol and regenerates the acid catalyst.

Acid-Catalyzed Hydration of 2-Methylpropene