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
There are 4 aspects that you want to consider when differentiating between substitution and elimination reactions, as well as between unimolecular and bimolecular reactions. These are as follows: Alkyl halide or carbon chain holding the leaving group; ability of the leaving group to break away from the molecule and remain stable in solution, strength of the attacking nuleophile or base, and finally the solvent where the reaction takes place. In this article I will help you understand the nature of the carbon holding the leaving group
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 SN2 and E2 reactions cannot be lumped together the way we did with the unimolecular reactions. This is because the mode of attack is very different for substitution and elimination. An SN2 reaction occurs via a backside attack, meaning the carbon holding the leaving group must be accessible to such an attack. Thus an SN2 reaction will require an easy to access leaving group such as that bound to a methyl or primary carbon. Secondary can also take place but tertiary is too hindered
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
There are 4 aspects that you want to consider when differentiating between substitution and elimination reactions, as well as between unimolecular and bimolecular reactions. These are as follows: Alkyl halide or carbon chain holding the leaving group; ability of the leaving group to break away from the molecule and remain stable in solution, strength of the attacking nuleophile or base, and finally the solvent where the reaction takes place. In this article I will help you understand the nature of the carbon holding the leaving group
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 SN2 and E2 reactions cannot be lumped together the way we did with the unimolecular reactions. This is because the mode of attack is very different for substitution and elimination. An SN2 reaction occurs via a backside attack, meaning the carbon holding the leaving group must be accessible to such an attack. Thus an SN2 reaction will require an easy to access leaving group such as that bound to a methyl or primary carbon. Secondary can also take place but tertiary is too hindered
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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Want to find out more about Substitution and Elimination Reactions, Then be sure to watch my YouTube video series to learn about every aspect of SN1 SN2 E1 E2 reactions
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