Nucleophile: A nucleophile is an electron-rich species, such as an ion or molecule, that possesses a lone pair of electrons available for donation to an electron-deficient center. Common examples include the hydroxide ion (), cyanide ion (), and ammonia ().
Electrophilic Center: In halogenoalkanes, the carbon atom bonded to the halogen is electron-deficient () due to the higher electronegativity of the halogen atom (). This polar bond makes the carbon susceptible to attack by nucleophiles.
Substitution Reaction: This is a chemical process where one functional group in a compound is replaced by another. In this context, the halogen atom is displaced as a halide ion (), which acts as the leaving group.
Formation of Alcohols: Halogenoalkanes react with aqueous sodium or potassium hydroxide ( or ) under reflux. The hydroxide ion acts as the nucleophile to produce an alcohol.
Formation of Nitriles: Reaction with potassium cyanide () in ethanol under reflux introduces a cyano group (). This is a critical synthetic step as it extends the carbon chain by one atom.
Formation of Primary Amines: Reaction with excess concentrated ammonia () in ethanol under pressure produces a primary amine. Excess ammonia is required to minimize further substitution of the amine product.
| Feature | Mechanism | Mechanism |
|---|---|---|
| Steps | Two-step process | One-step (concerted) process |
| Intermediate | Carbocation formed | Transition state only |
| Kinetics | First order: Rate = | Second order: Rate = |
| Substrate | Favored by tertiary halogenoalkanes | Favored by primary halogenoalkanes |
| Stereochemistry | Results in racemization | Results in inversion of configuration |
(Substitution Nucleophilic Unimolecular): The leaving group departs first to form a stable carbocation, which is then attacked by the nucleophile. Tertiary substrates favor this because alkyl groups stabilize the carbocation through inductive effects.
(Substitution Nucleophilic Bimolecular): The nucleophile attacks the carbon from the opposite side of the leaving group simultaneously as the bond to the halogen breaks. Primary substrates favor this because there is less steric hindrance for the backside attack.
Identify the Reagent State: Always check if the hydroxide is aqueous or ethanolic. Aqueous leads to substitution (alcohol), whereas ethanolic with heat typically leads to elimination (alkene).
Mechanism Selection: When asked to draw a mechanism, look at the classification of the halogenoalkane. Use for primary (one step, transition state) and for tertiary (two steps, carbocation).
Curly Arrow Precision: Ensure curly arrows start exactly from a lone pair or a bond and point directly to the atom forming the new bond. A common mistake is starting the arrow from the atom symbol rather than the electron pair.
Rate Predictions: If comparing reactivity, always cite bond enthalpy as the reason why iodoalkanes react faster than chloroalkanes, even though is more polar.
