Carbocations with hydrogen halides

As carbocations go CH3" is particularly unstable and its existence as an inter mediate m chemical reactions has never been demonstrated Primary carbocations although more stable than CH3" are still too unstable to be involved as intermediates m chemical reactions The threshold of stability is reached with secondary carbocations Many reactions including the reaction of secondary alcohols with hydrogen halides are believed to involve secondary carbocations The evidence m support of tertiary carbo cation intermediates is stronger yet... 

The SnI mechanism is generally accepted to be correct for the reaction of tertiary and secondary alcohols with hydrogen halides It is almost certainly not correct for methyl alcohol and primary alcohols because methyl and primary carbocations are believed to be much too unstable and the activation energies for their formation much too high for them to be reasonably involved The next section describes how methyl and primary alcohols are converted to their corresponding halides by a mechanism related to but different from S l... 

Secondary and tertiary alcohols react with hydrogen halides by a mech anism that involves formation of a carbocation intermediate m the rate determining step... 

These common features suggest that carbocations are key intermediates m alcohol dehydra tions just as they are m the reaction of alcohols with hydrogen halides Figure 5 6 portrays a three step mechanism for the acid catalyzed dehydration of tert butyl alcohol Steps 1 and 2 describe the generation of tert butyl cation by a process similar to that which led to its for matron as an intermediate m the reaction of tert butyl alcohol with hydrogen chloride... 

We have seen this situation before m the reaction of alcohols with hydrogen halides (8ection 4 11) m the acid catalyzed dehydration of alcohols (8ection 5 12) and m the conversion of alkyl halides to alkenes by the El mechanism (8ection 5 17) As m these other reactions an electronic effect specifically the stabilization of the carbocation intermediate by alkyl substituents is the decisive factor The more stable the carbo cation the faster it is formed... 

Unbranched primary alcohols and tertiary alcohols tend to react with hydrogen halides without rearrangement The alkyloxonmm ions from primary alcohols react rap idly with bromide ion for example m an Sn2 process Tertiary alcohols give tertiary alkyl halides because tertiary carbocations are stable and show little tendency to rearrange... 

When formulating a mechanism for the reaction of alkynes with hydrogen halides we could propose a process analogous to that of electrophilic addition to alkenes m which the first step is formation of a carbocation and is rate determining The second step according to such a mechanism would be nucleophilic capture of the carbocation by a halide ion... 

Reaction with hydrogen halides (Sec tion 4 7) The order of alcohol reactiv ity parallels the order of carbocation staiiility RjC" > R2CH > RCHj" > CHj" Benzylic alcohols react readily... 

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

One important experimental fact is that the rate of reaction of alcohols with hydrogen halides increases in the order methyl < primary < secondary < tertiary. This reactivity order parallels the carbocation stability order and is readily accommodated by the mechanism we have outlined. 

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. 

Tertiary alcohols undergo substitution reactions with hydrogen halides faster than do secondary alcohols because tertiary carbocations are easier to form than secondary car-bocations (Section 4.2). Thus, the reaction of a tertiary alcohol with a hydrogen halide proceeds readily at room temperature, whereas the reaction of a secondary alcohol with a hydrogen halide would have to be heated to have the reaction occur at the same rate. 

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