Hydride Shift in Dehydration of 1-Butanol

Hydride Shift in Dehydration of 1-Butanol THE OVERALL REACTION  

Step 1 Protonation of the alcohol gives the corresponding alkyloxonium ion. 

Step 2 One reaction available to butyloxonium ion leads to 1-butene by an E2 pathway. 

Step 2 Alternatively, butyloxonium ion can form a secondary carbocation by loss of water accompanied by a hydride shift. 

Step 3 The secondary carbocation formed in step 2 can give 1-butene or a mixture of cw-2-butene and fran -2-butene, depending on which proton is removed. 

Hydride shifts often occur during the dehydration of primary alcohols. Thus, although 1-butene would be expected to be the only alkene formed on dehydration of 1-butanol, it is in fact accompanied by a mixture of cis- and ran5-2-butene. 

Mechanism 5.3 shows how the butyloxonium ion formed on protonation of 1-butanol can give a secondary carbocation by a hydride shift from C-2 to C-1 in the step in which a molecule of water is lost. Deprotonation of this secondary carbocation leads to 1-butene, cw-2-butene, and ra/z -2-butene. 

Using curved arrows, show how the butyloxonium ion in Mechanism 5.3 can react with water by an E2 mechanism to form 1-butene. Hint See page 204 for an example of electron flow in the E2 mechanism.) 

The ratio of isomeric alkenes obtained from a primary alcohol such as 1-butanol depends on the relative rates of dehydration by Mechanism 5.3 and the E2 process in Problem 5.21. However, the strongly acidic reaction conditions also promote alkene equilibration, which increases the proportion of 2-butenes at the expense of 1-butene. The mechanism of this equilibration is based on principles that we will consider in Chapter 6. 

This concludes discussion of our second functional group transformation involving alcohols the first was the conversion of alcohols to alkyl halides (Chapter 4), and the second the conversion of alcohols to alkenes. In the remaining sections of the chapter the conversion of alkyl halides to alkenes by dehydrohalogenation is described. 

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