Starting from the 4-methylpentan-2-yl cation, $CH X_{3} - CH X^{+} - CH X_{2} - CH (CH X_{3}) - CH X_{3}$ , what is the minimum number of successive 1,2-hydride shifts required to reach a tertiary carbocation? Count only 1,2-hydride shifts (each shift counts as one step, even if the intermediate cation is no more stable than the previous one); alkyl shifts are not considered. Answer as an integer.

Numerical answer type, tolerance 0.5.

Answer: 2

The cation is at C2. Its neighbours are C1 (a methyl, so a shift would give a primary cation) and C3 (

CH X_{2}

). A hydride shift from C3 moves the charge to C3, which is still secondary (2-methylpentan-3-yl). C3 is next to C4, the tertiary carbon bearing one hydrogen. A second hydride shift from C4 gives the tertiary 2-methylpentan-2-yl cation. Minimum number of shifts

= 2

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Answer: 2

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