![]() Often alkyl iodides are reactive enough to be difficult to store, so the the common choices for reactions are alkyl chlorides and alkyl bromides. Practically, alkyl fluorides are not used for S N2 reactions because the C-F bond is too strong. This leads to the following reactivity order for alkyl halides Since the bond between the carbon and the leaving group is being broken in the transition state, the weaker this bond is the lower the activation energy and the faster the reaction. It turns out that the two factors lead to the same prediction for halogen leaving group ability: There are two main factors: The strength of the C-X bond, and the stability of the X group after it has left. As you may imagine, however, the nature of the leaving group is an important consideration: if the C-X bond does not break, the new bond between the nucleophile and electrophilic carbon cannot form, regardless of whether the substitution is S N1 or S N2. In our general discussion of nucleophilic substitution reactions, we have until now been designating the leaving group simply as “X”. The electrophile (a) Structure of the alkyl group So in general we want a strong nucleophile. If we have a strong nucleophile, the S N2 reaction will happen faster a weak nucleophile will react more slowly and may not even react. ![]() This is because the nucleophile is almost “naked” in aprotic solvents, whereas in polar protic solvents it is surrounded by a cage of solvent molecules. Regarding the solvent, polar aprotic solvents such as DMSO, DMF, acetone or acetonitrile are popular choices for S N2 reactions, because rates are generally faster than with polar protic solvents (water, alcohols, etc.).
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