Sn1 vs. Sn2 Mechanisms. Strong or Weak Nucleophiles?

[premed]

One can predict by which mechanism a reaction will occur, Sn2 or Sn1, by the substrate's order( i.e. primary, secondary, or tertiary carbon) and by the strength of the nucleophile. How can I tell if a nucleophile is strong or weak? For instance, one problem showed methanol combining with cyclohexyl bromide. I thought the methoxide would be a strong nucleophile but it was in fact weak and went through the Sn1 mechanism instead of Sn2. How could I tell if it is strong or weak. Thanks.

 

[chemisttree]

It is a bit more complicated than "strong nucleophile" vs "weak nucleophile". Are you sure it didn't go through an E2 process in the presence of methoxide? You have two or more competing processes at work. One process occurs most readily with increasing carbon substitution and in the presence of strong bases. Methoxide is a strong base and will readily deprotonate things...

Methoxide will not make methoxycyclohexane from cyclohexyl bromide by any mechanism!
You don't get substitution in that case.

 

[Blueshift5]

it is important to understand the differences between the Sn1 and Sn2 mechanisms and how they are influenced by the strength of the nucleophile. The Sn1 mechanism involves a two-step process where the leaving group leaves first, followed by the nucleophilic attack. This mechanism is favored for tertiary substrates due to the stability of the carbocation intermediate. On the other hand, the Sn2 mechanism involves a one-step process where the nucleophile attacks the substrate at the same time as the leaving group leaves. This mechanism is favored for primary substrates due to the steric hindrance in tertiary substrates.

In terms of nucleophile strength, it is important to consider both the basicity and the nucleophilicity of the compound. A strong nucleophile is one that is both highly basic and highly nucleophilic. Basicity refers to the ability of a compound to donate a pair of electrons, while nucleophilicity refers to the ability of a compound to attack an electron-deficient atom. For example, methoxide (CH3O-) is a strong nucleophile because it is both highly basic (due to the lone pair of electrons on oxygen) and highly nucleophilic (due to the negative charge on oxygen). This makes it a good candidate for the Sn2 reaction.

In the case of methanol combining with cyclohexyl bromide, the methoxide may have been a strong nucleophile, but it is also a weak base. This means that it is less likely to deprotonate the cyclohexyl bromide and more likely to participate in the Sn1 mechanism. Other factors that can affect nucleophile strength include solvent effects, steric hindrance, and the leaving group ability of the substrate.

In summary, to determine if a nucleophile is strong or weak, one must consider both its basicity and nucleophilicity, as well as other factors that may influence its reactivity. It is important to carefully analyze the substrate and the reaction conditions to predict which mechanism is most likely to occur. I hope this information helps you in your research.