Relative reaction energy in partial protonation of primary versus tertiary carbon atoms

Because of the involvement of carbonium ion intermediates, rearrangement is a possibility. Reaction of t-butylethylene with hydrogen chloride in acetic acid gives both rearranged and unrearranged product. The rearranged acetate may also be 

The stereochemistry of the addition of hydrogen halides to a variety of alkenes has been investigated. The addition of hydrogen chloride to 1-methylcyclopentene is 

Addition of hydrogen bromide to cyclohexene and cis and trans-l-huient also takes place by anti addition.In other instances, additions of hydrogen halides to 

The stereochemistry of the addition depends upon the details of the addition mechanism. Two general mechanisms have been encountered for alkenes. The addition can proceed through an ion pair that involves a discrete carbonium ion  

Many alkenes, however, react via a transition state that involves the alkene, hydrogen chloride, and a third species, either solvent or halide ion. This termolecular mechanism is generally pictured as involving nucleophilic attack on an alkene-hydrogen chloride complex and bypassing a discrete carbonium ion  

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Figure 3.1. Relative reaction energy in partial protonation of primary versus tertiary carbon atoms.

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