Question

The correct order of increasing basic nature for the bases \(\mathrm{NH}_{3}, \mathrm{CH}_{3} \mathrm{NH}_{2}\), and \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\) is (a) \(\mathrm{CH}_{3} \mathrm{NH}_{2}<\mathrm{NH}_{3}<\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}<\mathrm{NH}_{3}<\mathrm{CH}_{3} \mathrm{NH}_{2}\) (c) \(\mathrm{NH}_{3}<\mathrm{CH}_{3} \mathrm{NH}_{2}<\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\) (d) \(\mathrm{CH}_{3} \mathrm{NH}_{2}<\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}<\mathrm{NH}_{3}\)

Short answer

(c) NH₃ < CH₃NH₂ < (CH₃)₂NH

Step-by-step solution

Understand Basicity

Basicity is the ability of a compound to accept a proton (H+). For amines, basicity is largely affected by the electron-releasing capability of the substituents on the nitrogen atom. Amines with alkyl groups, which release electrons to the nitrogen, are generally more basic than ammonia (NH\(_3\)).

Analyze Ammonia (NH₃)

Ammonia is the simplest amine with no alkyl groups to push electrons toward nitrogen. Therefore, it has basicity but is usually less basic than its alkyl-substituted counterparts.

Analyze Methylamine (CH₃NH₂)

Methylamine has one methyl (CH\(_3\)) group attached to nitrogen. This alkyl group releases electrons into the nitrogen atom, enhancing its ability to accept a proton and thus increasing its basicity compared to ammonia.

Analyze Dimethylamine ((CH₃)₂NH)

Dimethylamine has two methyl groups attached to the nitrogen. Both methyl groups push more electron density towards the nitrogen compared to methylamine, which enhances its basicity further.

Arrange the Compounds by Increasing Basicity

With the knowledge that dimethylamine has the most electron-pushing alkyl groups and is thus the most basic, we arrange the compounds in order of increasing basicity: NH\(_3\) < CH\(_3\)NH\(_2\) < (CH\(_3\))\(_2\)NH.

Key concepts

Basicity

Basicity is a fundamental property of chemical compounds that indicates their ability to accept protons. In simpler terms, it can be thought of as how "hungry" a compound is for hydrogen ions (H\(^+\)). Amines, which are compounds derived from ammonia (NH\(_3\)), have nitrogen atoms that contribute electron pairs, making them potentially good at grabbing onto protons.
A molecule's basicity can be influenced by the atoms or groups attached to the nitrogen. Generally, if these groups can donate electrons to the nitrogen, the compound becomes more basic. This is because it can hold onto protons more tightly, as it now has extra electron density. Therefore, understanding the basicity of amines revolves around two main factors: the structure of the compound and the electron influences of the surrounding groups.

Proton Acceptance

Proton acceptance is a key trait in understanding a substance's basic nature. Essentially, it describes how well a molecule can attract and hold a hydrogen ion. When amines function as bases, they do so by accepting protons using the lone pair of electrons situated on their nitrogen atom.
The capacity of an amine to accept a proton strongly relates to the electron availability at the nitrogen atom. If this availability increases, the amine will generally become more basic because it is more effective in holding onto protons. Proton acceptance is crucial to analyzing the basicity of the three compounds: ammonia, methylamine, and dimethylamine, and determining why they have different levels of basicity.

Alkyl Groups

Alkyl groups play an essential role in affecting the basicity of amines. They are essentially hydrocarbon chains that can attach to the nitrogen atom in an amine molecule. Due to their structure, alkyl groups are known for having a tendency to push electrons towards the nitrogen atom—a trait derived from their electron-donating properties.
In the context of increasing the basicity of amines, the more alkyl groups attached, generally the greater the electron density around the nitrogen. For example, ammonia, with no alkyl groups, is less basic compared to methylamine, which has one alkyl group. Even more, dimethylamine, with two alkyl groups, exhibits an even higher basicity than methylamine due to increased electron donation.

Electron-Releasing Effects

The electron-releasing effects come into play significantly when examining the basic nature of amines. As alkyl groups are known to release electrons, they donate these electrons to the nitrogen atom in an amine, enhancing its electron density. This donation increases the compound's proton acceptance ability, as a more electron-rich nitrogen can hold onto protons more securely.
Methyl groups (\(CH_3 \)), a type of alkyl group, are particularly effective at releasing electrons. Therefore, when comparing ammonia, methylamine, and dimethylamine, dimethylamine with two methyl groups shows the most robust electron-releasing effect, making it the most basic. As a result, the order of increasing basicity is straightforwardly given by the ability of these groups to enrich the nitrogen atom with more electrons.