Problem 183 - A Complete Resource Book in Chemistry for JEE Main

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 12003 (a) \(\mathrm{CH}_{3} \mathrm{NH}_{2}<\mathrm{NH}_{3}<\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}<\mathrm{NH}_{1}<\mathrm{CH}_{3} \mathrm{NH}_{2}\) (c) \(\mathrm{NH}_{3}<\mathrm{CH}_{3} \mathrm{NH}_{2}<\left(\mathrm{CH}_{2}\right)_{2} \mathrm{NH}\) (d) \(\mathrm{CH}_{3} \mathrm{NH}_{2}<\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}<\mathrm{NH}_{3}\)

Step-by-Step Solution

Answer

The correct order of increasing basic nature is (c) NH_3 < CH_3NH_2 < (CH_3)_2NH.

Step 1: Analyzing the basicity of NH_3

The basic nature of a compound is determined by its ability to donate a lone pair of electrons. In the case of ammonia (NH_3), the nitrogen atom has one lone pair of electrons, making it a basic compound.

Step 2: Introduction of the Methyl Group

When a methyl group (CH_3) is introduced, as in methylamine (CH_3NH_2), the electron-donating effect of the methyl group increases the electron density on nitrogen, thus enhancing the basicity compared to ammonia.

Step 3: Comparing Two Methyl Groups

In dimethylamine ((CH_3)_2NH), there are two methyl groups attached to nitrogen. Both groups donate electron density to the nitrogen simultaneously, further increasing the nitrogen's ability to share its lone pair of electrons, making it more basic than both ammonia and methylamine.

Step 4: Conclusion: Establish Relative Basicities

Based on the inductive effect, the more the number of alkyl groups attached to the nitrogen, the more basic the compound. Therefore, the increasing order of basic strength is: NH_3 < CH_3NH_2 < (CH_3)_2NH.

Key Concepts

Inductive Effect

The inductive effect is a fundamental concept in organic chemistry that helps determine the basicity of amines. It refers to the ability of an atom or group of atoms to polarize a covalent bond by pulling or pushing electron density through the sigma bonds of a molecule. This effect can either enhance or reduce a molecule's basicity, depending on the group involved.

In the case of amines, attaching alkyl groups like the methyl group \(\text{CH}_3\) can increase basicity due to their +I effect, which means electron-donating inductive effect. Alkyl groups push electron density towards the nitrogen atom, enhancing its ability to donate its lone pair of electrons. This makes the nitrogen more basic.

For example:

• Ammonia \(\text{NH}_3\) has no alkyl group, and hence its basicity is solely due to its lone pair of electrons.
• Methylamine \(\text{CH}_3\text{NH}_2\), with one methyl group, has increased electron density on the nitrogen, making it more basic than ammonia.
• Dimethylamine \( (\text{CH}_3)_2\text{NH} \), with two methyl groups, receives even more electron density, further enhancing its basicity.

So, the presence and number of alkyl groups significantly influence the basicity of amines through the inductive effect.

Electron Donation

Electron donation is a core mechanism behind the basicity of amines. The more effectively an amine can donate electrons, the stronger its basic properties. This process primarily involves the nitrogen atom's lone pair of electrons that can participate in forming bonds with protons \(\text{H}^{+}\).

Amines are derivatives of ammonia \(\text{NH}_3\), where hydrogen atoms are often replaced by alkyl or aryl groups. Substituting with alkyl groups enhances electron donation through the inductive effect. These groups push electron density towards the nitrogen atom, reinforcing its lone pair's availability for bonding. As a result, these nitrogen compounds become stronger bases.

Taking a closer look:

• Ammonia \(\text{NH}_3\) has only hydrogen atoms and a lone pair available for donations.
• Substituting one hydrogen atom with a methyl group, as in methylamine \(\text{CH}_3\text{NH}_2\), increases electron availability, thus boosting the basicity.
• Adding another methyl group, as seen in dimethylamine \( (\text{CH}_3)_2\text{NH} \), further enhances this donation potential.

Therefore, the electron donation capacity significantly affects the overall basic strength of an amine.

Lone Pair

The lone pair refers to a pair of valence electrons not involved in bonding, and for amines, they are located on the nitrogen atom. This lone pair's availability is central to the basic nature of amines, as it determines the compound's ability to accept protons or form bonds with them.

Without any substituents, ammonia \(\text{NH}_3\) has a basic nature solely due to its lone pair. Once you introduce alkyl groups like methyl, they influence the lone pair's effectiveness.

The introduction of substituents affects the orbital where the lone pair resides and the electron density. When more electron-rich substituents, like a methyl group, are attached, they increase electron density, making the lone pair readily available for bonding.

• In ammonia, the lone pair is free to bond, giving it basic characteristics.
• In methylamine \(\text{CH}_3\text{NH}_2\), the methyl group makes the lone pair even more ready to donate electrons, hence more basic.
• In dimethylamine \( (\text{CH}_3)_2\text{NH} \), two methyl groups make this capacity even stronger.

Thus, the lone pair's role is amplified or diminished based on the substitution pattern, demonstrating its crucial role in amine basicity.