Question

Consider the following compounds: (1) \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{NH}_{2}\) (2) \(\mathrm{C}_{6} \mathrm{H}_{3}-\mathrm{NH}-\mathrm{C}_{6} \mathrm{H}_{5}\) (3) \(\left(\mathrm{C}_{6} \mathrm{H}_{5}\right)_{3} \mathrm{~N}\) (4) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{NH}_{2}\) Arrange these compounds in decreasing order of their basicity. (a) \(1>2>3>4\) (b) \(2>3>1>4\) (c) \(3>2>1>4\) (d) \(4>1>2>3\)

Short answer

The correct order of basicity is option (d): 4>1>2>3.

Step-by-step solution

Understanding Basicity

Basicity is the ability of a compound to accept protons. For amines, basicity typically depends on the availability of the lone pair of electrons on the nitrogen atom for protonation. Electron-donating groups increase basicity, while electron-withdrawing groups decrease it.

Analyzing Compound (1)

Compound (1), \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{NH}_{2}\), is aniline. The phenyl group (\(\mathrm{C}_{6} \mathrm{H}_{5}\)) is an electron-withdrawing group due to resonance, which reduces the availability of the lone pair on nitrogen, thus decreasing its basicity compared to aliphatic amines.

Analyzing Compound (2)

Compound (2), \(\mathrm{C}_{6} \mathrm{H}_{3}-\mathrm{NH}-\mathrm{C}_{6} \mathrm{H}_{5}\), is a diarylamine. Both phenyl groups exert an electron-withdrawing effect, further reducing the nitrogen's lone pair availability more than compound (1), resulting in lower basicity.

Analyzing Compound (3)

Compound (3), \((\mathrm{C}_{6} \mathrm{H}_{5})_{3} \mathrm{~N}\), is triphenylamine. With three phenyl groups, the lone pair on nitrogen is even less available due to extensive electron-withdrawing effects, making it less basic than compounds (1) and (2).

Analyzing Compound (4)

Compound (4), \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{NH}_{2}\), is ethylamine, an aliphatic amine with no electron-withdrawing groups. The ethyl group is electron-donating, increasing the availability of the lone pair on nitrogen, making this compound the most basic of the four.

Comparing Basicity and Concluding

Based on the effects of substituents on the nitrogen atom, the order of decreasing basicity is: Compound (4) > Compound (1) > Compound (2) > Compound (3). Thus, option (d) is correct: \(4>1>2>3\).

Key concepts

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are elements or functional groups attached to molecules that pull electron density away from other parts of the molecule. In the context of amines, these groups are crucial because they impact the availability of the lone pair of electrons on the nitrogen atom. When they pull electron density away, less electron density is available on the nitrogen, making the amine less basic.

In the compounds considered in the exercise, the phenyl group (\(\text{C}_6 \text{H}_5\)) is an example of an electron-withdrawing group. This effect is particularly significant in compounds like aniline (compound 1) and diarylamine (compound 2) because these phenyl rings involve the lone pair in resonating structures. When the lone pair is involved in resonance, it is less available for bonding with protons, thus reducing basicity. Consequently, the presence of EWGs in a molecule substantially reduces the basicity of amines by making the nitrogen less nucleophilic.

Electron-Donating Groups

In contrast to electron-withdrawing groups, electron-donating groups (EDGs) have the opposite effect. These groups push electron density towards other parts of the molecule, enhancing the availability of the lone pair of electrons on the nitrogen atom. For amines, this increases the basicity because the nitrogen has a greater ability to donate its lone pair and bond with protons.

Take ethylamine (compound 4) as an example, which contains an ethyl group \(\text{CH}_3\text{-CH}_2\). This alkyl group is an excellent electron-donating group. It donates electron density towards the nitrogen, boosting the lone pair's availability for protonation. This makes ethylamine more basic than compounds where nitrogen is bound to electron-withdrawing groups. Understanding how these groups adjust electron density can help predict and reason the behavior of different amines in comparison.

Protonation of Nitrogen

Protonation is the process where a proton (a hydrogen ion, \(\text{H}^+\)) is added to a molecule. For amines, protonation primarily involves the nitrogen atom, which has a lone pair available to bond with the proton. This interaction forms a positively charged ammonium ion. The ease with which an amine is protonated profoundly impacts its basicity.

In a situation where the nitrogen has a high electron density (like when assisted by electron-donating groups), it can easily accommodate a proton. When electron-withdrawing groups are present, however, the nitrogen becomes less nucleophilic, and its ability to bind a proton decreases. For instance, in aniline and its derivatives, the nitrogen's lone pair is less effective in bonding with protons due to the electron density being shared or pulled away. Protonation is, therefore, an essential aspect of getting to grips with why one amine may be more basic than another.

Aromatic vs Aliphatic Amines

Aromatic amines, like aniline, contain a nitrogen atom attached to one or more aromatic rings. Due to the effect of resonance within the aromatic structure, these nitrogen atoms often have their lone pairs less available for bonding. This happens because the lone pair can delocalize and participate in the electron cloud of the aromatic system.

On the other hand, aliphatic amines, such as ethylamine, do not involve such resonance structures. In these compounds, the nitrogen's lone pair is focused and available, leading to higher basicity. Aliphatic amines tend to be more basic because the saturated carbon chains do not withdraw or delocalize electron density, unlike aromatic systems.

Understanding the distinction between these types of amines is crucial in predicting and explaining their reactivity, particularly when assessing their basicity in chemical reactions. The differences between aromatic and aliphatic amines highlight the significant impact molecular structure has on the chemical properties of compounds.