A reaction mechanism is a series of smaller reactions that form an overall reaction. Substitutions and elimination reactions are very important in the study of organic chemistry mechanisms.[2]
[change]Unimolecular substitution (SN1)
A unimolecular substitution mechanism occurs in steps. First, an atom called the leaving group breaks away from a molecule, leaving the molecule with a positive electric charge. This intermediate is called a carbocation. Second, a nucleophile forms a chemical bond with the carbocation. If the nucleophile has a neutral charge, a third step is required. The third step uses a reagent to remove the positive charge. The shorthand symbol for unimolecular substitution is SN1.
[change]Bimolecular substitution (SN2)
In a bimolecular substitution, a nucleophile, which is a reagent that forms a chemical bond, replaces another atom attached to a molecule. The original atom is called the leaving group because it leaves the molecule. The bonding of the nucleophile and the departure of the leaving group are concerted and happen at the same time instead of one at a time. The shorthand symbol for a bimolecular substitution is SN2.
[change]Unimolecular elimination (E1)
The first step of a unimolecular elimination reaction is an atom, the leaving group, breaks away from a molecule. The loss of this atom forms an intermediate carbocation. Then a basic reagent attacks a hydrogen atom and forces a double bond. The reagent in an elimination reaction is a base. These are the same steps as in a bimolecular elimination, but they are not concerted and occur one at a time. The shorthand symbol for unimolecular elimination is E1.
[change]Bimolecular elimination (E2)
The reagent in an elimination reaction is a base. The base takes a proton from the leaving group, forcing electrons to form a double chemical bond and break the bond to the leaving group. Like bimolecular substitution, this is a concerted step. The shorthand symbol for bimolecular elimination is E2.
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