Question

Show the products from reaction of \(p\) -bromoaniline with the following reagents: (a) \(\mathrm{CH}_{3} \mathrm{I}\) (excess) (b) HCl (c) \(\mathrm{CH}_{3} \mathrm{COCl}\) (d) \(\mathrm{CH}_{3} \mathrm{MgBr}\)

Short answer

Products are p-bromo-N,N,N-trimethylanilinium iodide, p-bromoanilinium chloride, p-bromoacetanilide, and no clear product without protection in Grignard reaction.

Step-by-step solution

Reaction with excess CH3I

P-bromoaniline reacts with excess methyl iodide (\(\mathrm{CH}_{3} \mathrm{I}\)) to undergo exhaustive methylation. The -NH2 group in p-bromoaniline is methylated to form a quaternary ammonium salt. The reaction results in the formation of p-bromo-N,N,N-trimethylanilinium iodide.

Reaction with HCl

When p-bromoaniline is treated with hydrochloric acid (HCl), the amino group (\(\mathrm{-NH}_{2}\)) is protonated. This converts the amino group into an anilinium ion. The product formed is p-bromoanilinium chloride.

Reaction with CH3COCl

Methylation doesn't occur when p-bromoaniline reacts with an acid chloride such as acetyl chloride (\(\mathrm{CH}_{3} \mathrm{COCl}\)). Instead, the reaction yields an amide group. Specifically, the product is p-bromoacetanilide as the -NH2 group is protected via acetylation.

Reaction with CH3MgBr

P-bromoaniline reacts with a Grignard reagent like methylmagnesium bromide (\(\mathrm{CH}_{3} \mathrm{MgBr}\)), mainly affecting the halogen. The Grignard reagent is used to introduce the methyl group, primarily at the halogen site. However, typically p-bromoaniline doesn't react well in simple conditions as an amino group can interfere with organometallic reactions unless protected.

Key concepts

Exhaustive Methylation

P-bromoaniline can react with an excess of methyl iodide \(\mathrm{CH}_3 \mathrm{I}\) through a process called exhaustive methylation. This entails the complete substitution of hydrogen atoms in the amino group with methyl groups. The amino group \(-\mathrm{NH}_2\) in p-bromoaniline will, therefore, be transformed into a quaternary ammonium salt. In this transformation:

• The nitrogen atom becomes fully methylated.
• All three hydrogen atoms are replaced with methyl groups.
• As a result, a quaternary ammonium salt, specifically p-bromo-N,N,N-trimethylanilinium iodide, is formed, a stable compound due to its positive charge facilitated by the iodide ion.

This reaction is useful for transforming amines into strong bases or other functional groups.

Quaternary Ammonium Salt

The end product of exhaustive methylation of p-bromoaniline is a quaternary ammonium salt.

• These salts are characterized by a nitrogen atom carrying a positive charge.
• The nitrogen is bonded to four organic groups, in this case, three methyl groups and one aromatic ring from the aniline structure.

Properties and Uses:

Quaternary ammonium salts are known for their solubility in water and can act as phase transfer catalysts, enhancing the rate of reaction between substances in a biphasic system. They're commonly used in detergents and disinfectants too. The ionic nature of these compounds helps in various chemical syntheses and reactions due to their ability to stabilize negative charges.

Anilinium Ion

When p-bromoaniline encounters hydrochloric acid (HCl), the amine group is protonated to form an anilinium ion. This is because:

• The lone pair of electrons on the nitrogen atom accepts a proton.
• This conversion results in the formation of p-bromoanilinium chloride.

Protonation:

The process of protonation significantly impacts the chemical properties of amines, transforming them from a neutral to a positively charged state. The anilinium ion can significantly alter the reactivity of the compound and is more soluble in polar solvents like water. This is important in reactions where solubility and ionic interactions can play a critical role.

Grignard Reaction

The Grignard reaction involves a Grignard reagent, such as methylmagnesium bromide (\(\mathrm{CH}_3 \mathrm{MgBr}\)), which is highly reactive toward electrophilic centers. P-bromoaniline, when exposed to this reagent,

• Usually targets the halogen for substitution.
• However, due to the basic nature of the aniline amino group, direct use without protecting groups is challenging.

Challenges and Execution:

The presence of the amino group can interfere with the Grignard reaction as it reacts with the reagent, which is why they are typically problematic with p-bromoaniline. To ensure a successful reaction, protecting groups or alternative methods are needed to prevent side reactions. For optimal results, one might protect the amino group before attempting the Grignard reaction to facilitate the desired methylation at the bromine position. This reaction showcases the nuanced relationship between structure and reactivity in organic chemistry.