The order of reactivity is HI > HBr > HCl. Cleavage with concentrated HI or HBr at high temperatures results in the formation of tertiary halides when one of the alkyl groups is tertiary, following an SN₁ mechanism due to the stability of the carbocation formed.
Explain the order of reactivity of hydrogen halides in the cleavage of ethers, and how does the presence of a tertiary alkyl group influence the mechanism and product formed?
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The order of reactivity of hydrogen halides in the cleavage of ethers is generally HI > HBr > HCl. This order corresponds to the increasing nucleophilicity of the halide ions. Tertiary alkyl groups in ethers influence the mechanism and product by favoring an SN₁-like pathway. In the presence of a tertiary alkyl group, the carbocation intermediate is stabilized through hyperconjugation and resonance, promoting SN₁-type cleavage. The resulting products are an alkyl halide and an alcohol. The increased stability of the tertiary carbocation enhances the occurrence of this mechanism compared to SN₂-type reactions.