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Alkylhalides: Substitution reactions 3 (Sn2)

Alkylhalides: Substitution

(Ir)reversibility

Basic rule with substitution reactions: a Sn2 reaction always goes in the direction where the stronger base drives the weaker base away. The stronger base will the place of the weaker base, thus it is a irreversible reaction. For example have a look on following reaction: 

CH3-Cl  +   OH-     >>       CH3-OH   +   Cl-

Chloride is a weaker base than the added hydroxide ion, this means that the reaction is NOT reversible, thus irreversible.

Solvent

The problem with second row (O, N, ...) nucleophiles is that they are very small. Because they are so small, polar solvents are not fit for these nucleophiles. These nucleophiles get solvated very fast by these polar solvents, this blocks the nucleophile attacking the alkylhalide on the backside, and thus no reaction will take place. Conclusion: water and alcohol are NOT fit as solvent for 2nd row nucleophiles used with Sn2 reactions.

What solvents must be used then? The answer lies in Polar aprotic solvents. The most used of these solvents are DMF, DMSO and Acetonitril (CH3CN). These solvents can solvate a cation better than an anion, because their partial negative charge is on the surface of the molecule, the partial positive charge is covered in the molecule, thus more hidden for the nucleophile to react with. The nucleophile will be less likely solvated so this means it will be more reactive in these kind of solvents. 

Sn2 advantages

A lot of molecules with different functional groups (a very wide range of diversity) can be made with Sn2 reactions, a small overview can be found below:




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