Explain nucleophilic substitution of alkyl halides?

Question

Christian Anderson · Accepted Answer

Nucleophilic substitution of alkyl halides is the substitution of the halogen by another group called a nucleophile.

In most of the alkyl halides, the C-X bond is polar.

That gives the α carbon a partial positive charge

The positive charge makes that carbon susceptible to attack by a nucleophile.

In one scenario, called the #"S"_"N"2# mechanism, the halide leaves at the same time as the nucleophile attacks.

In a different scenario, the #"S"_"N"1# mechanism, the halide ion leaves first, forming a carbocation.

The nucleophile attacks the carbocation in a second step, forming the product.

Here's a video on the nucleophilic substitution of alkyl halides.

Isaac Aronson · Answer

undefined Nucleophilic substitution of alkyl halides involves the replacement of a halogen atom (such as chlorine, bromine, or iodine) in an alkyl halide molecule by a nucleophile. The process typically occurs in three main steps: 
1. Nucleophilic attack: The nucleophile, which is an electron-rich species, attacks the electrophilic carbon atom bearing the halogen. This results in the formation of a new bond between the nucleophile and the carbon atom, and the breaking of the bond between the carbon and the halogen.
2. Leaving group departure: The halogen atom, which is a good leaving group due to its ability to stabilize negative charge, leaves the molecule, taking with it the pair of electrons that formed the bond with the carbon atom. This step generates a carbocation intermediate.
3. Carbocation stabilization: The carbocation intermediate is stabilized by nearby electron-donating groups or resonance structures.
4. Nucleophilic substitution: Finally, a nucleophile attacks the carbocation, forming a new bond with the carbon atom and displacing the leaving group. This results in the formation of the desired product, where the nucleophile has replaced the halogen atom.
This process is influenced by factors such as the nature of the alkyl halide, the strength and nucleophilicity of the nucleophile, and the reaction conditions.

Addison Armstrong · Answer

undefined Nucleophilic substitution of alkyl halides is a chemical reaction in which a nucleophile replaces a halogen atom bonded to a carbon atom in an alkyl halide molecule. The general reaction mechanism involves two main steps: nucleophilic attack and leaving group departure.

1. Nucleophilic attack: The nucleophile, which is typically a negatively charged or electron-rich species, such as a halide ion or an oxygen or nitrogen atom from a molecule like water or an alcohol, attacks the carbon atom bonded to the halogen in the alkyl halide. This results in the formation of a new bond between the carbon and the nucleophile, leading to the displacement of the halogen.

2. Leaving group departure: As the nucleophile attacks, the bond between the carbon and the halogen weakens, and the halogen atom becomes a leaving group. The leaving group departs, taking with it the pair of electrons that formed the carbon-halogen bond. This step is typically facilitated by the presence of a polar solvent, which stabilizes the leaving group.

Overall, nucleophilic substitution reactions of alkyl halides result in the replacement of the halogen atom with a nucleophile, leading to the formation of a new organic compound. The outcome of the reaction depends on factors such as the nature of the alkyl halide, the strength and nucleophilicity of the nucleophile, and the reaction conditions, including the solvent and temperature. Common nucleophilic substitution reactions include SN1 (unimolecular) and SN2 (bimolecular) mechanisms, each characterized by distinct reaction kinetics and stereochemical outcomes.