Question

Rank the following compounds in order of increasing basicity: (a) \(p\) -Nitroaniline, \(p\) -aminobenzaldehyde, \(p\) -bromoaniline (b) \(p\) -Chloroaniline, \(p\) -aminoacetophenone, \(p\) -methylaniline (c) \(p\) -(Trifluoromethyl)aniline, \(p\) -methylaniline, \(p\) -(fluoromethyl)aniline

Short answer

(a) p-Nitroaniline < p-Aminobenzaldehyde < p-Bromoaniline; (b) p-Aminoacetophenone < p-Chloroaniline < p-Methylaniline; (c) p-(Trifluoromethyl)aniline < p-(Fluoromethyl)aniline < p-Methylaniline.

Step-by-step solution

Understand Basicity

Basicity is a measure of a compound's ability to accept protons. In anilines, this depends largely on the electron-donating or electron-withdrawing nature of substituents on the benzene ring. Substituents that release electrons into the ring (electron-donating) increase basicity, while those that withdraw electrons (electron-withdrawing) decrease basicity.

Analyze Compounds in Group (a)

(a) For the compounds: - **p-Nitroaniline:** Strong electron-withdrawing nitro group, decreasing basicity. - **p-Aminobenzaldehyde:** Aldehyde group is electron-withdrawing. - **p-Bromoaniline:** Bromine is weakly electron-withdrawing. Order of increasing basicity: p-Nitroaniline < p-Aminobenzaldehyde < p-Bromoaniline.

Analyze Compounds in Group (b)

(b) For the compounds: - **p-Chloroaniline:** Chlorine is weakly electron-withdrawing. - **p-Aminoacetophenone:** Ketone group (acetyl) is electron-withdrawing. - **p-Methylaniline:** Methyl group is electron-donating. Order of increasing basicity: p-Aminoacetophenone < p-Chloroaniline < p-Methylaniline.

Analyze Compounds in Group (c)

(c) For the compounds: - **p-(Trifluoromethyl)aniline:** Trifluoromethyl is a strong electron-withdrawing group. - **p-(Fluoromethyl)aniline:** Fluorine in this group is more weakly electron-withdrawing than in trifluoromethyl. - **p-Methylaniline:** Methyl group is electron-donating. Order of increasing basicity: p-(Trifluoromethyl)aniline < p-(Fluoromethyl)aniline < p-Methylaniline.

Key concepts

Electron-Donating Groups

In organic chemistry, electron-donating groups (EDGs) play a key role in increasing the basicity of compounds, such as aniline derivatives. These groups are capable of releasing electrons into the aromatic ring, enhancing the electron density on the nitrogen atom of the aniline. This increased electron density makes it easier for the nitrogen to accept a proton, thus increasing basicity.

One common example of an EDG is the methyl group, as seen in compounds like \( p \)-methylaniline. The presence of a methyl group pushes electrons toward the aromatic ring through the inductive effect, which is basically the transfer of electron density through a sigma bond. This addition of electron density onto the nitrogen atom increases its ability to bind protons, making \( p \)-methylaniline more basic compared to anilines with electron-withdrawing groups.

To easily identify EDGs, look for alkyl groups or substituents containing lone pairs, such as amino groups, which exhibit similar electron-releasing effects.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) reduce the basicity of aniline derivatives by pulling electron density away from the aromatic ring. This removal of electrons occurs through either the inductive effect or resonance. When electron density on the nitrogen atom is decreased, its ability to accept protons diminishes, thereby reducing basicity.

Some of the strongest EWGs include nitro groups (\(-\text{NO}_2\)) and trifluoromethyl groups, which are highly electronegative and pull electrons away effectively. As seen in \( p \)-nitroaniline and \( p \)-(trifluoromethyl)aniline, these groups dramatically lower basicity, as the nitrogen atom in these compounds is deprived of electron density needed to attract and hold onto protons.

Even halogens like chlorine and bromine can act as EWGs, albeit more weakly, as they also exhibit electronegativity and pull electrons from the aromatic ring onto themselves. This effect is visible in compounds like \( p \)-chloroaniline and \( p \)-bromoaniline, where the presence of these groups slightly decreases basicity.

Aniline Derivatives

Aniline derivatives are a group of organic compounds that consist of an aniline (phenylamine) structure with various substituents attached to the benzene ring. These substituents significantly influence the compound's properties, including basicity. In analyzing aniline derivatives, attention must be paid to whether these substituents are electron-donating or electron-withdrawing groups, as these will impact the overall electron density around the nitrogen atom.

Substituents such as methyl groups increase basicity by elevating the electron density at the nitrogen, while groups like nitro or acetyl will do the opposite. The influence of these substituents is evident when arranging compounds in order of increasing basicity, as electron-donating groups typically lead to a more basic compound.

Aniline derivatives are not only fascinating for their chemistry but also for their broad application in various fields, including dyes, pesticides, and pharmaceuticals. Understanding the influence of different substituents on their basicity is crucial in tailoring these compounds for specific functions and efficiencies.