Tertiary alkyl halides, R3 CX, undergo spontaneous dissociation to yield a carbocation, R3 C^+,

step 1 Okay, this problem is asking us which one will react faster. And we're comparing this one and th

Tertiary alkyl halides, R3 CX, undergo spontaneous...

Tertiary alkyl halides, $\mathrm{R}_{3} \mathrm{CX}$, undergo spontaneous dissociation to yield a carbocation, $\mathrm{R}_{3} \mathrm{C}^{+},$ plus halide ion. Which do you think reacts faster, $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CBr}$ or $\mathrm{H}_{2} \mathrm{C}=\mathrm{CHC}\left(\mathrm{CH}_{3}\right)_{2} \mathrm{Br} ?$ Explain. (FIGURE CANNOT COPY)

Key Concepts

Comparison in SN1 Reactions

When comparing different substrates in SN1 reactions, the overall reaction speed depends on how well the intermediate carbocation is stabilized. A substrate that forms a resonance-stabilized carbocation, such as an allylic halide, typically reacts faster than one forming a carbocation stabilized solely by alkyl substituents, even if both are tertiary. This is because the additional resonance delocalization lowers the energy of the transition state for carbocation formation.

Carbocation Formation

In unimolecular nucleophilic substitution (SN1) reactions, the rate-determining step involves the formation of a carbocation from an alkyl halide. The ease with which this carbocation forms directly influences the overall reaction rate. Factors that stabilize the carbocation lower the activation energy barrier for its formation, resulting in a faster reaction.

Tertiary Carbocation Stability

Tertiary carbocations are generally more stable than primary or secondary ones due to greater hyperconjugation and inductive effects provided by the surrounding alkyl groups. However, the stability is also influenced by any additional stabilizing interactions present, such as resonance effects.

Resonance Stabilization

Resonance stabilization occurs when a positive charge can be delocalized over two or more atoms through overlapping p-orbitals. In the case of an allylic carbocation, the adjacent double bond allows the positive charge to be spread over the molecule, which often makes the carbocation more stable than a simple tertiary carbocation, accelerating the reaction rate.

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