Reaction Mechanism: Ammonia acts as a nucleophile, using its lone pair to attack the electron-deficient carbon in a halogenoalkane. This process initially forms an intermediate salt, which then reacts with a second molecule of ammonia to release the free primary amine.
The Challenge of Multiple Substitution: Because the resulting primary amine still possesses a lone pair, it can act as a nucleophile itself, attacking further halogenoalkane molecules. This leads to a mixture of primary, secondary, and tertiary amines, as well as quaternary ammonium salts.
Control of Product: To maximize the yield of a primary amine, a large excess of ammonia must be used to ensure that ammonia is the most likely nucleophile to encounter the halogenoalkane. Conversely, using an excess of the halogenoalkane favors the production of quaternary ammonium salts.
Methodology: Nitriles (compounds containing the group) can be reduced to primary amines using strong reducing agents. This method is often preferred for synthesizing pure primary amines because it avoids the multiple substitution issues seen with halogenoalkanes.
Reagents and Conditions: The reduction can be achieved using Lithium Aluminum Hydride () in dry ether, followed by dilute acid. Alternatively, industrial applications often use hydrogen gas () with a nickel catalyst at elevated temperatures and pressures.
Carbon Chain Extension: This synthetic route is particularly useful because the formation of the nitrile precursor (from a halogenoalkane and ) adds one carbon atom to the original chain, allowing for the synthesis of amines with longer carbon skeletons.
Step 1: Nitration: The process begins with the nitration of benzene using a mixture of concentrated nitric acid () and concentrated sulfuric acid () at temperatures between and degrees Celsius to produce nitrobenzene.
Step 2: Reduction: Nitrobenzene is reduced to phenylamine using tin (Sn) and concentrated hydrochloric acid (HCl) under reflux. This initially produces an acidic mixture containing the phenylammonium ion ().
Step 3: Liberation and Purification: Sodium hydroxide () is added to the acidic mixture to remove the proton from the ammonium ion, liberating the free phenylamine. The final product is typically separated from the reaction mixture via steam distillation.
| Feature | Haloalkane + Ammonia | Nitrile Reduction |
|---|---|---|
| Product Purity | Mixture of amines | High purity amine |
| Chain Length | Remains the same | Increases by one carbon |
| Mechanism | Nucleophilic Substitution | Reduction |
Reagent Specificity: Always specify 'ethanolic' ammonia when discussing the reaction with halogenoalkanes. If water is used as a solvent instead of ethanol, the hydroxide ions may lead to the formation of alcohols via competing substitution reactions.
The 'Excess' Rule: In exam questions, if the goal is to produce a primary amine from a halogenoalkane, you must explicitly state that excess ammonia is used. If the goal is a quaternary salt, specify excess halogenoalkane.
Naming Conventions: For secondary and tertiary amines, use the N-substituted notation (e.g., N-methylethanamine) to indicate that the smaller alkyl groups are attached to the nitrogen atom rather than the main carbon chain.
