2.3: Acid-catalyzed SN1 reaction

The rate determining step of an S_N1 reaction requires the leaving group to leave. If the electrophile is covalently bonded to a poor leaving group, such as HO^- or RO^-, this step can be so slow that the reaction does not proceed at all.

In these cases, adding an acid catalyst can protonate the leaving group before it leaves, turning it into a much better leaving group such as H₂O or ROH. This significantly increases the rate of the reaction.  For example, consider the substitution reaction between 2-methylpropanol and methanol, shown below. The leaving group, HO^- is poor, so an acid catalyst, such as CH₃OH₂⁺, is necessary to enable the reaction.

In the mechanism of this reaction, the first step is an acid-base reaction where the acid catalyst protonates the alcohol to make it a better leaving group. Steps 2 and 3 are the standard steps of an S_N1 mechanism: the leaving group leaves, and then the nucleophile attacks the carbocation intermediate. The result is the protonated substituted product, similar to what was shown in Section 2.2 when neutral nucleophiles were used.  The fourth and final step is another acid/base reaction to deprotonate this molecule. In many S_N1 reactions, the nucleophile is used as the solvent. This makes the nucleophile/solvent a good choice to use as a base for the final deprotonation, since (1) it has two electrons available to act as a base (recall that nucleophiles have lone pair electrons) and (2) they are available in excess (since it is the solvent). Note that this final step reproduces the acid, making it catalytic.

Guided:
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Interactive: