Question

Consider the following compounds: (1) \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{NH}_{2}\) (2) \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{NH}-\mathrm{C}_{6} \mathrm{H}_{5}\) (3) \(\left(\mathrm{C}_{6} \mathrm{H}_{5}\right)_{3} \mathrm{~N}\) (4) \(\mathrm{CH}_{1}-\mathrm{CH}_{2}-\mathrm{NH}_{2}\) 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

(d) 4 > 1 > 2 > 3

Step-by-step solution

Analyze Basicity Concept

Basicity of a compound is primarily determined by the availability of a lone pair of electrons on the nitrogen atom. The greater the availability, the more basic the compound is, as it can donate the lone pair more readily. Additionally, electron-withdrawing groups decrease basicity because they reduce electron density on nitrogen, making the lone pair less available.

Analyze Compound (1) - Aniline

Compound (1) is aniline \(\mathrm{C}_6\mathrm{H}_5-\mathrm{NH}_2\). The lone pair on the nitrogen in aniline is less available because it is delocalized into the aromatic ring through resonance, reducing its basicity.

Analyze Compound (2) - Diphenylamine

Compound (2) is diphenylamine \(\mathrm{C}_6\mathrm{H}_5-\mathrm{NH}-\mathrm{C}_6\mathrm{H}_5\). The lone pair on nitrogen is less available due to delocalization into both phenyl rings. This further decreases its basicity compared to aniline.

Analyze Compound (3) - Triphenylamine

Compound (3) is triphenylamine \((\mathrm{C}_6\mathrm{H}_5)_3\mathrm{~N})\). The lone pair on nitrogen is delocalized into three phenyl rings, which significantly decreases basicity. This compound is the least basic because the electron density on nitrogen is greatly reduced.

Analyze Compound (4) - Ethylamine

Compound (4) is ethylamine \(\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{NH}_2\). There is no delocalization or electron-withdrawing group affecting the lone pair on nitrogen, making it freely available. Consequently, ethylamine is the most basic compound of the group.

Arrange Compounds by Basicity

Based on the analyses, arrange the compounds from most to least basic: Compound (4) (ethylamine) is more basic, followed by Compound (1) (aniline), then Compound (2) (diphenylamine), and finally Compound (3) (triphenylamine). Thus, the order is 4 > 1 > 2 > 3.

Key concepts

Electron Delocalization

Electron delocalization is a key concept in understanding the basicity of amine compounds. In chemistry, electrons aren't always fixed in location; they can spread out or "delocalize" over several atoms. Imagine the electrons as a cloud that can extend over multiple parts of a molecule. This spreading out means that the electrons can interact with different areas, and this interaction is essential for determining how a molecule behaves, especially regarding its reactivity and basicity.
In amine compounds, the nitrogen atom typically has a pair of electrons that it can donate in chemical reactions, known as a lone pair. Whether these electrons are concentrated on the nitrogen or spread over the entire molecule affects the compound's ability to donate them. When delocalized, these electrons are less available to participate in chemical reactions, such as when a base donates an electron pair, reducing the compound's basicity. Understanding this concept is crucial when predicting the behavior of different amine compounds in chemical reactions.

Resonance Effect

The resonance effect can significantly influence the basicity of compounds, especially in cyclic structures like benzene rings attached to amines. Resonance refers to the ability of a molecule's electrons to be shared across different atomic frameworks, effectively creating various "resonance structures." The real structure of the molecule is then a hybrid of these resonance forms.
In amine compounds like aniline, resonance can impact the lone pair on the nitrogen atom. The lone pair can overlap with the adjacent aromatic ring, spreading out over the ring structure as part of its electron cloud. While this sharing stabilizes the molecule energetically, it also decreases the electron availability on the nitrogen for reaction, thus decreasing its basicity. The more resonance structures a molecule has, the more its electron density is spread out, leading to decreased basicity. This effect is a critical factor in comparing basicity among different amines.

Amine Compounds

Amine compounds are nitrogen-containing organic molecules classified by the number of organic groups attached to the nitrogen. Considered in primary, secondary, and tertiary forms, these compounds are essential in a wide range of biological and chemical processes. A primary amine has one alkyl or aryl group attached, a secondary amine has two, and a tertiary amine has three.
The basicity of these compounds is dependent on several structural factors, including the number of attached groups and their ability to undergo resonance. Each added alkyl group generally increases the electron-donating capability, while aromatic groups tend to decrease it through electron delocalization. This variance in structure affects the reactivity of the nitrogen's lone pair, crucial for the compound's ability to act as a base. An understanding of how different amines behave based on their structure is essential for predicting their reactions in various chemical environments.

Nitrogen Lone Pair Availability

The availability of the nitrogen lone pair is a cornerstone concept for assessing basicity in amine compounds. A lone pair refers to the two electrons on the nitrogen atom that do not participate in bonding but are available for donation when the compound acts as a base.
Factors affecting this availability include the presence of electron-withdrawing or donating groups and the potential for resonance. In a free state, like in ethylamine, the lone pair is readily available, enhancing basicity. However, when attached to aromatic groups or involved in resonance, the electrons are less available, reducing basicity.
Modifiers such as electron-withdrawing groups can pull electron density away from the nitrogen, decreasing its lone pair availability. Conversely, electron-donating groups enhance the electron presence at the nitrogen, making the electron pair more accessible for bonding, thus increasing basicity. Understanding these intricate details provides insight into the chemical behavior of amines in various reactions.