2-Methylpropene hydration

We can extend the general principles of electrophilic addition to acid catalyzed hydration In the first step of the mechanism shown m Figure 6 9 proton transfer to 2 methylpropene forms tert butyl cation This is followed m step 2 by reaction of the car bocation with a molecule of water acting as a nucleophile The aUcyloxomum ion formed m this step is simply the conjugate acid of tert butyl alcohol Deprotonation of the alkyl oxonium ion m step 3 yields the alcohol and regenerates the acid catalyst... 

FIGURE 6 9 Mechanism of acid catalyzed hydration of 2 methylpropene... 

IS reversible with respect to reactants and products so each tiny increment of progress along the reaction coordinate is reversible Once we know the mechanism for the for ward phase of a particular reaction we also know what the intermediates and transition states must be for the reverse In particular the three step mechanism for the acid catalyzed hydration of 2 methylpropene m Figure 6 9 is the reverse of that for the acid catalyzed dehydration of tert butyl alcohol m Figure 5 6... 

Although 2 methylpropene undergoes acid catalyzed hydration m dilute sulfuric acid to form tert butyl alcohol (Section 6 10) a different reaction occurs m more concentrated solutions of sulfuric acid Rather than form the expected alkyl hydrogen sulfate (see Sec tion 6 9) 2 methylpropene is converted to a mixture of two isomeric C Hig alkenes... 

Arrange the following compounds in order of increasing rate of acid-catalyzed hydration ethylene, 2-cyclopropylpropene, 2-methylpropene, propene, 1-cyclopro-pyl-l-methoxyefliene. Explain the basis of your prediction. 

Reaction of 2-methylpropene with CH3OH in the presence of H2SO4 catalyst yields methyl tert-butyl ether, CP OQCHT, by a mechanism analogous to that of acid-catalyzed alkene hydration. Write the mechanism, using curved arrows for each step. 

The catalytic hydration of olefins can also be performed in a three-phase system solid catalyst, liquid water (with the alcohol formed dissolved in it) and gaseous olefin [258,279,280]. The olefin conversion is raised, in comparison with the vapour phase processes, by the increase in solubility of the product alcohol in the excess of water [258]. For these systems with liquid and vapour phases simultaneously present, the equilibrium composition of both phases can be estimated together with vapour-liquid equilibrium data [281]. For the three-phase systems, ion exchangers, especially, have proved to be very efficient catalysts [260,280]. With higher olefins (2-methylpropene), the reaction was also performed in a two-phase liquid system with an ion exchanger as catalyst [282]. It is evident that the kinetic characteristics differ according to the arrangement (phase conditions), i.e. whether the vapour system, liquid vapour system or two-phase liquid system is used. However, most kinetic and mechanistic studies of olefin hydration were carried out in vapour phase systems. 

Only in one case (the hydration of 2-methylpropene over a H2S04—Si02 catalyst [278]) was the so-called Rideal mechanism proposed as a preferable model and expressed by the rate equation for single-site adsorption with retardation by the product alcohol, viz. 

Exercise 10-15 Arrange ethene, propene, and 2-methylpropene in order of expected ease of hydration with aqueous acid. Show your reasoning. 

In conventional hydration of alkenes by water in the presence of a strong acid, there is little direct stereochemical consequence, but we mention it as a useful contrast to the content of the next section. A proton initially adds to the alkene, and in the case of 2-methylpropene (44) gives the intermediate carbocation 45 (Scheme 4.9), which is then captured by the nucleophile, water, and yields the product 2-methylpropan-2-ol (46). 

The usage of metal sulfates as catalysts is not new. In 1901, aluminum sulfate was used as the dehydration catalyst for the formation of 2-methylpropene from 2-methyl-2-propanol (29) and, in 1923, as the hydration catalyst for the formation of ethanol from ethylene 30). 

Acid-Catalyzed Hydration of 2-Methylpropene 241 6.9 Epoxidation of an Alkene 259... 

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