Question

Identify whether each of the following reactions occurs through an Sn1 , Sn2, E1, or E2 mechanism.

Identify whether each of the following reactions occurs through an SN1, SN2, E1, or E2 mechanism.

Answer 1

Concepts and reason

Three reactions are given with their product, the mechanism followed by the formation of given product needs to be written. Generally, the rate of mechanism of reaction depends on the various parameters for instance, substrate, type of nucleophile, and the leaving group.

Firstly, identify whether the product formed is substitution product or the elimination product and the write the suitable mechanism.

Fundamentals

Substitution reaction can be categorized as, ${{\rm{S}}_{\rm{N}}}1$ and ${{\rm{S}}_{\rm{N}}}2$ depending on the type of mechanism followed.

• ${{\rm{S}}_{\rm{N}}}1$ reaction is unimolecular nucleophilic substitution reaction which takes place between a haloalkane and a nucleophile. The reaction takes place in two step through a carbocation intermediate and the reaction product are formed with both retention as well as inversion of configuration. Consider the following scheme of a ${{\rm{S}}_{\rm{N}}}1$ reaction:

In haloalkane, the order of reactivity for the ${{\rm{S}}_{\rm{N}}}1$ substitution reaction is, $3^\circ > 2^\circ > 1^\circ > {\rm{C}}{{\rm{H}}_3}$ like the stability of carbocation.

• ${{\rm{S}}_{\rm{N}}}2$ reaction is bimolecular nucleophilic substitution reaction which takes place between a haloalkane and a nucleophile. The reaction takes place in one step through a transition state. Consider the following scheme of a ${{\rm{S}}_{\rm{N}}}2$ reaction:

In haloalkane, the order of reactivity for the ${{\rm{S}}_{\rm{N}}}2$ substitution reaction is, $3^\circ < 2^\circ < 1^\circ < {\rm{C}}{{\rm{H}}_3}$ .

Similarly, elimination reaction can be categorized as, ${\rm{E}}1$ and ${\rm{E}}2$ depending on the type of mechanism followed.

• Elimination reaction can be classified as ${\rm{E - }}1$ and ${\rm{E - 2}}$ . In ${\rm{E - }}1$ elimination reaction the reaction completes in two step and involves the formation of carbocation intermediate. In haloalkane, the order of reactivity for the ${\rm{E - }}1$ elimination is, $3^\circ > 2^\circ > 1^\circ > {\rm{C}}{{\rm{H}}_3}$ like the stability of carbocation.

• In E-2 elimination reaction the reaction takes place in one step through the formation of a transition state. In haloalkane, the order of reactivity for the ${\rm{E - 2}}$ elimination is, $3^\circ < 2^\circ < 1^\circ < {\rm{C}}{{\rm{H}}_3}$ .

Part (a)

Consider the given reactants as shown below:

As the product form is alkene from alkane therefore, it must follow elimination reaction.

Now, identify the type of elimination mechanism in the given reaction by observing the structure of haloalkane and the strength of base as shown below:

As the given reactants are secondary haloalkane and a strong base, therefore, the reaction will undergo ${\rm{E}}2$ elimination reaction mechanism.

Part (b)

Consider the given reactants as shown below:

As the product formed is an ether from haloalkane by the substitution of Br therefore, it must follow substitution reaction.

Now, identify the type of substitution mechanism in the given reaction by observing the structure of haloalkane and the strength of base as shown below:

As the given reactants are secondary haloalkane and a strong base, therefore, the reaction will undergo ${{\rm{S}}_{\rm{N}}}2$ substitution reaction mechanism.

Part (c)

Consider the given reactants as shown below:

As the product formed is an ether from alcohol by the substitution of methoxy group therefore, it must follow substitution reaction.

Now, identify the type of substitution mechanism in the given reaction by observing the structure of alcohol and the solvent as shown below:

As the given reactants are secondary alcohol in the presence of polar solvent, therefore, the reaction will undergo ${{\rm{S}}_{\rm{N}}}1$ substitution reaction mechanism.

Ans: Part (a)