Alkyloxonium ions hydrogen halides

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... 

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... 

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. 

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. 

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 ... 

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 alkyloxonium ions of primary alcohols react with hydrogen halides via an Sj j2 mechanism in which a water molecule is displaced by the halide ion, X . 

Alkyloxonium ions of tertiary alcohols react with hydrogen halides by an Sj,jl mechanism in which a water molecule leaves in the rate-determining step. The positive charge is dispersed in the transition state between the carbon and oxygen atoms, and eventually shifts to the carbon atom. 

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