SN1 SN2 E1 E2 Reactions are typically taught one at a time so that you recognize the starting molecule, reaction sequence and products. The difficult arises when you are asked to differentiate between the unimolecular and bimolecular substitution and elimination reactions given a random set of starting molecules, chemical reagents and solvents, where the specific reaction pathway is not specified
Just like any other organic chemistry reaction, the answer lies in a logical and systematic approach. There are 4 things you want to consider, each of which will help you validate or eliminate a specific reaction sequence. The 4 things to consider are the alkyl chain holding the leaving group, the leaving group itself, the attacking nucleophile or base, and finally the solvent where the reaction takes place. Temperature may also play a role
The alkyl chain is analyzed for SN1 SN2 E1 E2 as follows. First determine if a stable carbocation can form allowing the unimolecular SN1 or E1 reaction to take place. The trend for carbocation stability is as follows: Tertiary carbons form very stable carbocations, secondary carbons form ok carbocations, primary and methyl carbons will not form a stable carbocation and therefore cannot undergo an SN1 or E1 reaction
The nucleophilic substitution and beta elimination bimolecular reactions cannot be lumped together the way we did with the unimolecular reactions. This is because the mode of attack between them differs greatly. The SN2 reaction occurs when the nucleophile attacks the carbon chain directly kicking out the leaving group in a single step. Thus the nucleophile must be able to reach the carbon holding the leaving group and therefor prefers a methyl or primary carbon. The nucleophile can attack a secondary carbon, but anything more substituted is too hindered for this reaction to take place
The bimolecular beta elimination reaction does not involve the carbon-leaving group directly and so its substitution is irrelevant. Instead the base will find a nearby hydrogen atom on a beta carbon and pull that away from the molecule instead. The resulting pi bond will be stabilized by the degree of substitution on the carbons that are double bound. Therefor the base is looking for the most substituted beta hydrogen atom to attack. Even more so, your goal is to identify which beta hydrogen, when removed, will result in the most substituted pi bond for the most stable elimination product
Just like any other organic chemistry reaction, the answer lies in a logical and systematic approach. There are 4 things you want to consider, each of which will help you validate or eliminate a specific reaction sequence. The 4 things to consider are the alkyl chain holding the leaving group, the leaving group itself, the attacking nucleophile or base, and finally the solvent where the reaction takes place. Temperature may also play a role
The alkyl chain is analyzed for SN1 SN2 E1 E2 as follows. First determine if a stable carbocation can form allowing the unimolecular SN1 or E1 reaction to take place. The trend for carbocation stability is as follows: Tertiary carbons form very stable carbocations, secondary carbons form ok carbocations, primary and methyl carbons will not form a stable carbocation and therefore cannot undergo an SN1 or E1 reaction
The nucleophilic substitution and beta elimination bimolecular reactions cannot be lumped together the way we did with the unimolecular reactions. This is because the mode of attack between them differs greatly. The SN2 reaction occurs when the nucleophile attacks the carbon chain directly kicking out the leaving group in a single step. Thus the nucleophile must be able to reach the carbon holding the leaving group and therefor prefers a methyl or primary carbon. The nucleophile can attack a secondary carbon, but anything more substituted is too hindered for this reaction to take place
The bimolecular beta elimination reaction does not involve the carbon-leaving group directly and so its substitution is irrelevant. Instead the base will find a nearby hydrogen atom on a beta carbon and pull that away from the molecule instead. The resulting pi bond will be stabilized by the degree of substitution on the carbons that are double bound. Therefor the base is looking for the most substituted beta hydrogen atom to attack. Even more so, your goal is to identify which beta hydrogen, when removed, will result in the most substituted pi bond for the most stable elimination product
About the Author:
To learn even more about choosing between Nucleophilic Substitution and Beta Elimination reactions, be sure to watch my FREE YouTube videos series where I break down every aspect of the SN1 SN2 E1 E2 process