Show how you would convert aniline to the following compounds.
(a) fluorobenzene
(b) chlorobenzene

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Step 1: Begin with aniline (C6H5NH2), which is a benzene ring with an amino group (-NH2) attached. The amino group is highly activating and ortho/para-directing in electrophilic aromatic substitution reactions.

Step 2: Convert the amino group (-NH2) into a diazonium salt (-N2+). This is achieved by treating aniline with sodium nitrite (NaNO2) and hydrochloric acid (HCl) at low temperatures (0–5°C). The reaction forms benzene diazonium chloride (C6H5N2+Cl−).

Step 3: For part (a), to convert the diazonium salt into fluorobenzene (C6H5F), perform a Sandmeyer reaction using fluoroboric acid (HBF4). The diazonium salt reacts with HBF4 to produce fluorobenzene and nitrogen gas as a byproduct.

Step 4: For part (b), to convert the diazonium salt into chlorobenzene (C6H5Cl), perform a Sandmeyer reaction using cuprous chloride (CuCl). The diazonium salt reacts with CuCl to produce chlorobenzene and nitrogen gas as a byproduct.

Step 5: After the reactions, purify the products (fluorobenzene and chlorobenzene) using techniques such as distillation or recrystallization, depending on the physical properties of the compounds.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. In the case of converting aniline to fluorobenzene and chlorobenzene, EAS is crucial as it allows the introduction of halogen substituents onto the benzene ring. The presence of the amino group (-NH2) in aniline activates the ring towards electrophilic attack, making it more reactive.

Halogenation of Aromatic Compounds

Halogenation involves the introduction of halogen atoms (like fluorine or chlorine) into an organic compound. For aniline, this process typically requires the use of a halogenating agent, such as bromine or chlorine, often in the presence of a catalyst like iron or aluminum chloride. The choice of halogenating agent and conditions can influence the selectivity and yield of the desired halogenated product.

Deactivation of the Amino Group

While the amino group in aniline is a strong activating group for electrophilic substitution, it can also lead to complications in halogenation reactions. The amino group can be deactivated under certain conditions, such as when converting aniline to chlorobenzene, where the amino group may need to be temporarily protected or converted to a less reactive form to prevent multiple substitutions or side reactions.

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