Show how you would use direct alkylation to synthesize the following compounds.
(a) benzyltrimethylammonium iodide 
(b) pentan-1-amine 
(c) benzylamine

Verified step by step guidance

1. Step 1: Understand the concept of direct alkylation. Direct alkylation involves the reaction of an amine (or ammonia) with an alkyl halide to form a substituted amine. This reaction proceeds via an SN2 mechanism, where the nucleophilic amine attacks the electrophilic carbon of the alkyl halide, displacing the halide ion.
2. Step 2: For (a) benzyltrimethylammonium iodide, start with trimethylamine as the nucleophile. React trimethylamine with benzyl iodide (C6H5CH2I). The nucleophilic nitrogen in trimethylamine will attack the benzyl carbon, displacing the iodide ion and forming benzyltrimethylammonium iodide.
3. Step 3: For (b) pentan-1-amine, begin with ammonia (NH3) as the nucleophile. React ammonia with 1-bromopentane (CH3(CH2)4Br). The nucleophilic nitrogen in ammonia will attack the primary carbon of 1-bromopentane, displacing the bromide ion and forming pentan-1-amine.
4. Step 4: For (c) benzylamine, use ammonia (NH3) as the nucleophile again. React ammonia with benzyl bromide (C6H5CH2Br). The nucleophilic nitrogen in ammonia will attack the benzyl carbon, displacing the bromide ion and forming benzylamine.
5. Step 5: Ensure that the reaction conditions are controlled to avoid over-alkylation. For primary amines like pentan-1-amine and benzylamine, use an excess of ammonia to minimize the formation of secondary and tertiary amines. For quaternary ammonium salts like benzyltrimethylammonium iodide, use a stoichiometric amount of the alkyl halide to achieve the desired product.

Key Concepts

Direct Alkylation

Direct alkylation is a method used in organic synthesis to introduce an alkyl group into a molecule. This process typically involves the reaction of an alkyl halide with a nucleophile, such as an amine or an alkoxide. The choice of alkyl halide and the reaction conditions can significantly influence the yield and selectivity of the desired product.

Quaternary Ammonium Salts

Benzyltrimethylammonium iodide is a quaternary ammonium salt formed by the alkylation of a tertiary amine. These salts are characterized by a positively charged nitrogen atom bonded to four carbon-containing groups. They are often used as phase transfer catalysts in organic reactions due to their ability to solubilize ionic compounds in organic solvents.

Amine Synthesis

The synthesis of amines, such as pentan-1-amine and benzylamine, can be achieved through direct alkylation of ammonia or primary amines. This process involves the nucleophilic attack of the amine on an alkyl halide, leading to the formation of the corresponding amine. The choice of alkyl halide and the reaction conditions are crucial for achieving the desired amine product.