Alkyloxonium ions halides

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... 

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 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. 

As pointed out in Chapter 4, the first step in the reaction is proton transfer to the alcohol from the hydrogen halide to yield an alkyloxonium ion. This is an acid-base reaction. 

With primary alcohols, the next stage is an Sn2 reaction in which the halide ion, bromide, for exfflnple, displaces a molecule of water from the alkyloxonium ion. 

Unbranched primary alcohols and tertiary alcohols tend to react with hydrogen halides without reanangement. The alkyloxonium ions from primary alcohols react rapidly with bromide ion, for exfflnple, in an Sn2 process. Tertiaiy alcohols give tertiaiy alkyl halides because tertiaiy caibocations are stable and show little tendency to reanange. 

Halide formation proceeds at a useful rate only in the presence of strong acid, which can be furnished by excess hydrogen bromide or, usually and more economically, by sulfuric acid. The alcohol accepts a proton from the acid to give an alkyloxonium ion, which is more reactive in subsequent displacement with bromide ion than the alcohol (by either SN2 or SN1 mechanisms) because H20 is a better leaving group than eOH ... 

The transition state for carbocation formation begins to resemble the carbocation. If we assume that structural features that stabilize carbocations also stabilize transition states that have carbocation character, it follows that alkyloxoninm 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 primary alcohols. Figure 4.14 depicts the effect of alkyloxonium ion stmctnre on the activation energy for, and thus the rate of, carbocation formation. Once the carbocation is formed, it is rapidly captured by halide ion, so that the rate of alkyl hahde formation is governed by the rate of carbocation formation. 

Unlike tertiary and secondary carbocations, primary carbocations are too high in energy to be intermediates in chemical reactions. Since primary alcohols are converted, albeit rather slowly, to aUcyl halides on treatment with hydrogen halides, they must follow some other mechanism that avoids carbocation intermediates. This alternative mechanism is believed to be one in which the carbon-halogen bond begins to form before the carbon-oxygen bond of the alkyloxonium ion is completely broken. 

The hahde nucleophile helps to push off a water molecule from the alkyloxonium ion. According to this mechanism, both the halide ion and the alkyloxonium ion are involved in the same bimolecular elementary step. In Ingold s terminology, introduced in Section 4.11 and to be described in detail in Chapter 8, nucleophilic substimtions characterized by a bimolecular rate-determining step are given the mechanistic symbol Sn2. 

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