Acid-Catalyzed Hydration of 2-Methylpropene

Acid-Catalyzed Hydration of 2-Methylpropene THE OVERALL REACTION  

Step 1 Protonation of the carbon-carbon double bond in the direction that leads to more stable carbocation  

Step 2 Water acts as a nucleophile to capture rf-butyl cation  

Step 3 Deprotonation of f r -butyloxonium ion. Water acts as a Brpnsted base  

The rates of hydration of the two alkenes shown differ by a factor of over 7000 at 25°C. Which isomer is the more reactive Why  

Protonation of ethylene, the least reactive alkene of the three, would give a primary carbocation protonation of 2-methylpropene, the most reactive, gives a tertiary carbocation. The more stable the carbocation, the faster its rate of formation and the faster the overall reaction rate. 

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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... 

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

Acid-Catalyzed Hydration of 2-Methylpropene 227 Hydroboration of 1-Methylcyclopentene 233 Oxidation of an Organoborane 235 Bromine Addition to Cyclopentene 237 Epoxidation of Bicyclo[2.2.1]-2-heptene 240 Free-Radical Addition of Hydrogen Bromide to 1-Butene 243... 

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. 

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... 

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... 

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. 

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