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SN1 - First-order Nucleophilic Substitution

Importance of the Leaving Group in SN1 Reactions

In the rate-determining step of SN1 reactions, the substrate is cleaved into a positively charged carbocation and a negatively charged leaving group. However, the reaction rate is not only influenced by the stability of the carbocation, but also by the stability of the leaving group. The more stable the leaving group is, the more easily the C-X bond is also cleaved and, thus, the more effectively the C-X bond is cleaved, the higher the reaction rate also is. The leaving group may be characterised as a base, due to its lone electron pair. According to the HSAB principle, the C-X bond is more efficiently cleaved, the softer the base is.

Fig.1
Comparison of typical leaving groups.

If alcohols are applied as substrates in SN1 reactions, the leaving group will usually be water, as, under the usual acidic reaction conditions, the hydroxy group is protonated.

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