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SN/E Competition - SN1/E1 Competition

The Role of the Nucleophile in the SN1 / E1 Competition

In the SN1/E1 competition, the course of the reaction subsequent to the formation of the carbenium ion depends on the chemical character of the attacking reagent. Does the reagent act as a base in an E1 elimination or as a nucleophile in a SN1 reaction? A good nucleophile with relatively low basicity favours the SN1 reaction while a poor nucleophile favours an E1 elimination. In the E1 elimination, the proton need not necessarily be abstracted by the (poor) nucleophile (base). Instead, it may also be accepted by a solvent molecule. In principle, the reaction rate of the E1 elimination increases with the increasing basicity of the nucleophile. However, if stronger bases are applied, the E1 elimination is more and more superseded by the E2 elimination. Therefore, the E1 elimination normally occurs noticeably only with poor nucleophiles that have a low basicity, or if there is no nucleophile or base present (apart from the solvent). With increasing nucleophilicity, the SN1 reaction is favoured, while the E1 elimination is preferred with an increase in basicity.

Example 1

In this case, applying sodium cyanide (NaCN) results mainly in a SN1 reaction, because the cyanide anion is a good nucleophile and a weak and soft base. In addition, the basicity of the cyanide anion is low enough so that an E2 elimination through the abstraction of the β proton by a strong base in the early course of the reaction cannot occur.

Example 2

With tetrabutylammonium chloride in acetone E1 elimination is mainly observed (about 96 %). The chloride anion, as well as the acetone, are poor nucleophiles and very weak bases. Therefore, neither a SN1 reaction through a nucleophilic attack on the carbenium ion occurs nor an E2 elimination through the abstraction of the β proton by a strong base in the early stage of the reaction can take place.

The reaction center in the trigonal bipyramidal transition state of the SN2 reaction carries five substituents, while the central carbon of the SN1 reaction's transition state possesses just three substituents. Due to this, steric interactions are much less important in the latter. Nevertheless, there are obviously steric interactions between the carbenium ion and the attacking nucleophile in the transition state of the SN1 reaction. These are stronger than in E1 eliminations. Therefore, the E1 elimination may be preferred when the carbenium ion or the nucleophile contains bulky and sterically-demanding substituents.

The nucleophilicity of the nucleophile is determined by the well-known parameters. These are the basicity of the nucleophile and the electronegativity of the attacking atom, as well as the steric demand of the nucleophile's substituents. However, due to the reasons mentioned above, the influence of steric interactions on the nucleophilicity is smaller in SN1 reactions than it is in SN2 reactions.

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