In a substitution reaction, an atom, ion, or group in one molecule is replaced b

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In a substitution reaction, an atom, ion, or group in one molecule is replaced by (substituted with) another. In an elimination reaction, atoms or groups that are bonded to adjacent atoms are eliminated, as a small molecule and the bond order between the two adjacent atoms increases. SN1 and SN2 mechanisms describe the nucleophilic substitution reactions, whereas E1 and E2 mechanisms describe the elimination reactions.

The most important reaction pathways for substitution and elimination reactions for alkyl halides can be summarized as follows:

Nature of substrate Nature of reagent Mechanism Primary alkyl halide sterically hindered strong base E2 sterically unhindered base SN2 Secondary alkyl halide strong base E2 weak base and polar aprotic solvent SN2 Tertiary alkyl halide strong base E2 weak base and polar protic solvent SN1/E1 In a substitution reaction, an atom, ion, or group in one molecule is replaced by (substituted with) another. In an elimination reaction, atoms or groups that are bonded to adjacent atoms are eliminated, as a small molecule and the bond order between the two adjacent atoms increases. SN1 and SN2 mechanisms describe the nucleophile substitution reactions, whereas E1 and E2 mechanisms describe the elimination reactions. Substitution and elimination reactions always compete with each other. Generally, a more simple structure goes through a substitution pathway, whereas a hindered structure goes through an elimination pathway. You can also control the pathway through the nucleophile reagent; changing a nucleophile that is a weaker base to a stronger base changes the mode of the reaction from substitution to elimination. It is interesting to study these reactions. The most important reaction pathways for substitution and elimination reactions for alkyl halides can be summarized as follows: Methyl halides, CH3X, are involved mainly in SN2 reactions. Primary halides are involved mainly in SN2 reactions, except with a hindered strong base such as tertiary butoxide, where an E2 reaction results. Secondary alkyl halides are involved mainly in SN2 reactions with weak bases and E2 reactions with strong bases. Tertiary alkyl halides are not involved in SN2 reactions. They undergo E2 reactions with strong bases, and they undergo the SN1 or E1 mechanism with weak bases. The polar protic solvent used decides the mechanism. If the solvent used acts as a nucleophile, the mechanism is SN1, and if the solvent used acts as a base, the mechanism is E1. These points are summarized in the table below: In each of the following reactions with alkyl halides, give the type of mechanism (SN2, SN1, E1, or E2) by which the given product is formed. Drag the appropriate labels to their respective targets.

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