Differentiating between substitution and elimination reactions may well be one of the most difficult topics that you will cover in your organic chemistry course. Part of the reason has to do with the way these reactions are taught. You learn the SN1 reaction, then SN2 then E1 and finally E2. Or perhaps you learn these out of order. And you gain a false confidence of knowing what to do for each one when presented by itself. Suddenly you are faced with a reaction that asks you to provide the products without specifying the reaction type, and this is where you get stuck.
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 helps you determine between a unimolecular and bimolecular attack based on its ability to form a carbocation, or be attacked by a strong base or nucleophile. Unimolecular SN1 and E1 reactions proceed via a carbocation intermediate. And so you have to ensure that this can form. Carbocations are very stable on a tertiary carbon, also stable on a secondary carbon, but cannot form if the atom in question is primary or methyl. The more stable the C+ intermediate, the faster the leaving group departs from the chain
The '2' type reactions, meaning SN2 or E2 differ slightly. An SN2 reaction occurs via backside attack and so you're looking for a molecule that has an easily accessible leaving group. There are 2 things to consider here. 1- The nucleophile prefers to attack a methyl or primary carbon. Secondary is still accessible but tertiary carbons cannot be attacked by a nucleophile 2- The nearby carbons should be minimized so as not to block the approach of the nucleophile to the carbon that will be attacked
An E2 reaction is slightly different. Since the base attacks the nearby beta-hydrogen atom rather than the carbon holding the leaving group, substitution of this carbon is irrelevant. Instead we're looking for a beta-hydrogen that is easy to access, while at the same time will provide with the most substituted and thus stable pi bond. This means an E2 reaction can take place for tertiary, secondary and primary carbons, but it cannot take place on a methyl given that there are no beta carbons present
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 helps you determine between a unimolecular and bimolecular attack based on its ability to form a carbocation, or be attacked by a strong base or nucleophile. Unimolecular SN1 and E1 reactions proceed via a carbocation intermediate. And so you have to ensure that this can form. Carbocations are very stable on a tertiary carbon, also stable on a secondary carbon, but cannot form if the atom in question is primary or methyl. The more stable the C+ intermediate, the faster the leaving group departs from the chain
The '2' type reactions, meaning SN2 or E2 differ slightly. An SN2 reaction occurs via backside attack and so you're looking for a molecule that has an easily accessible leaving group. There are 2 things to consider here. 1- The nucleophile prefers to attack a methyl or primary carbon. Secondary is still accessible but tertiary carbons cannot be attacked by a nucleophile 2- The nearby carbons should be minimized so as not to block the approach of the nucleophile to the carbon that will be attacked
An E2 reaction is slightly different. Since the base attacks the nearby beta-hydrogen atom rather than the carbon holding the leaving group, substitution of this carbon is irrelevant. Instead we're looking for a beta-hydrogen that is easy to access, while at the same time will provide with the most substituted and thus stable pi bond. This means an E2 reaction can take place for tertiary, secondary and primary carbons, but it cannot take place on a methyl given that there are no beta carbons present
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