Alkyloxonium ion

Step 1 Protonation of tert butyl alcohol to give an alkyloxonium ion... 

Furthermore a substance such as HCl that dissociates completely when dissolved m water also dissociates completely when dissolved m an alcohol Many important reactions of alco hols involve strong acids either as reactants or as catalysts In all these reachons the first step IS formation of an alkyloxonium ion by proton transfer from the acid to the alcohol... 

In the second step of the mechanism described m Figure 4 6 the alkyloxonium ion dissociates to a molecule of water and a carbocation, an ion that contains a positively charged carbon... 

Like tert butyloxonium ion tert butyl cation is an intermediate along the reaction pathway It is however a relatively unstable species and its formation by dissociation of the alkyloxonium ion is endothermic Step 2 is the slowest step m the mechanism and has the highest activation energy Figure 4 8 shows a potential energy diagram for this step... 

Primary alcohols do not react with hydrogen halides by way of carbo cation intermediates The nucleophilic species (Br for example) attacks the alkyloxonium ion and pushes off a water molecule from carbon m a bimolecular step This step is rate determining and the mechanism is Sn2... 

Assuming that the rate determining step in the reaction of 1 hexanol with hydrogen bro mide to give 1 bromohexane is an attack by a nucleophile on an alkyloxonium ion write an equa tion for this step Use curved arrows to show the flow of electrons... 

Because the rate determining step involves two molecules—the alkyloxonium ion and water—the overall reaction is classified as a bimolecular elimination and given the sym bol E2... 

A mechanism for the formation of these three alkenes is shown m Figure 5 9 Dissociation of the primary alkyloxonmm ion is accompanied by a shift of hydride from C 2 to C 1 This avoids the formation of a primary carbocation leading instead to a sec ondary carbocation m which the positive charge is at C 2 Deprotonation of this carbo cation yields the observed products (Some 1 butene may also arise directly from the pri mary alkyloxonium ion)... 

Step 2 The carbocation formed m step 1 reacts rapidly with a water molecule Water IS a nucleophile This step completes the nucleophilic substitution stage of the mechanism and yields an alkyloxonium ion... 

Step 3 This step is a fast acid base reaction that follows the nucleophilic substitution Water acts as a base to remove a proton from the alkyloxonium ion to give the observed product of the reaction tert butyl alcohol... 

As pointed oul m Chapter 4 Ihe firsl step m Ihe reaclion is proton Iransfer to Ihe alcohol from Ihe hydrogen halide to yield an alkyloxonium ion This is an acid-base reaclion... 

With primary alcohols Ihe nexl slage is an 8 2 reaclion m which Ihe halide ion bro mide for example displaces a molecule of water from Ihe alkyloxonium ion... 

The reactions of alcohols with hydrogen halides to give alkyl halides (Chapter 4) are nucleophilic substitution reactions of alkyloxonium ions m which water is the leaving group Primary alcohols react by an 8 2 like displacement of water from the alkyloxonium ion by halide Sec ondary and tertiary alcohols give alkyloxonium ions which form carbo cations m an S l like process Rearrangements are possible with secondary alcohols and substitution takes place with predominant but not complete inversion of configuration... 

The transition state is closer in energy to the car bocation and more closely resembles it than the alkyloxonium ion. Thus, structural features that stabilize car bocations stabilize transition states leading to them. It follows, therefore, that alkyloxonium ions derived from tertiary alcohols have a lower energy of activation for dissociation and are converted to their- corresponding carbocations faster than those derived from secondary and primar y alcohols. Simply put more stable carbocations are formed faster than less stable ones. Figure 4.17 expresses this principle via a potential energy diagran. 

The first step of this new mechanism is exactly the sane as that seen earlier for the reaction of tcrt-butyl alcohol with hydrogen chloride—fonnation of an alkyloxonium ion by proton transfer from the hydrogen halide to the alcohol. Like the earlier example, this is a rapid, reversible Brpnsted acid-base reaction. 

The alkyl halide, in this case 2-bromo-2-methylbutane, ionizes to a caibocation and a halide anion by a heterolytic cleavage of the carbon-halogen bond. Like the dissociation of an alkyloxonium ion to a caibocation, this step is rate-detennining. Because the rate-detennining step is ununolecular—it involves only the alkyl halide and not the base—it is a type of El mechanism. 

There is a strong similarity between the mechanism shown in Eigure 5.12 and the one shown for alcohol dehydration in Eigure 5.6. The main difference between the dehydration of 2-methyl-2-butanol and the dehydrohalogenation of 2-bromo-2-methylbutane is the source of the carbocation. When the alcohol is the substrate, it is the conespond-ing alkyloxonium ion that dissociates to fonn the carbocation. The alkyl halide ionizes directly to the carbocation. 

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