Problem 140 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

The basic character of ethyl amine, diethyl amine and triethyl amine in chlorobenzene is (a) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NH}_{2}<\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{NH}<\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{3} \mathrm{~N}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NH}_{2}<\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{3} \mathrm{~N}<\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{NH}\) (c) \(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{3} \mathrm{~N}<\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{NH}<\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NH}_{2}\) (d) \(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{3} \mathrm{~N}<\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NH}_{2}<\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{NH}\)

Step-by-Step Solution

Answer

The basicity order in chlorobenzene is (b).

Step 1: Understanding Basicity of Amines

The basicity of amines refers to their ability to donate a lone pair of electrons from the nitrogen atom to form a bond with a proton (H\(^+\)). The basic strength is influenced by the availability of this lone pair and the stability of the resulting cation after accepting a proton.

Step 2: Structure and Steric Hindrance

In ethylamine, diethylamine, and triethylamine, as we increase the number of ethyl groups (R groups), the steric hindrance also increases. This affects the lone pair availability for proton acceptance since bulky groups around the nitrogen make it tougher for the lone pair to be accessed by a proton.

Step 3: Role of Solvent - Chlorobenzene

Chlorobenzene is a non-polar solvent that does not solvate cations effectively. In such a solvent, the steric and electronic effects of the substituents on amines play a major role in basicity.

Step 4: Comparing Basicity of the Amines

In chlorobenzene, the basicity order is affected mainly by the steric hindrance and inductive effects of ethyl groups. Triethylamine has the highest steric hindrance, reducing its basicity the most. Diethylamine, with less steric hindrance, is more basic than triethylamine. Ethylamine, with the least steric hindrance, shows intermediate basicity as it has fewer electron-donating alkyl groups than diethylamine.

Step 5: Conclusion - Determining the Order

Considering these factors, the correct order of basicity in chlorobenzene is (b): Ethylamine (C\(_2\)H\(_5\)NH\(_2\)) < Triethylamine ((C\(_2\)H\(_5\))\(_3\)N) < Diethylamine ((C\(_2\)H\(_5\))\(_2\)NH) because triethylamine is more hindered despite having more electron-donating groups.

Key Concepts

Steric Hindrance

Steric hindrance occurs when the size of groups within a molecule affects its reactivity, particularly the availability of electrons for bonding. In the context of amines like ethylamine, diethylamine, and triethylamine, increasing the number of ethyl groups leads to significant steric hindrance. With each additional ethyl group attached to the nitrogen, the space around the nitrogen atom becomes bulkier.

This crowding around the central nitrogen atom makes it more challenging for a proton (H\(^+\)) to approach and bond with the available lone pair of electrons on the nitrogen. Thus, steric hindrance acts as a barrier to interaction, directly influencing the basicity of the amine. Generally, an amine with more steric hindrance has lower basicity because the lone pair is not easily accessible, as seen in triethylamine.

Solvent Effects

When considering the basicity of amines, the solvent plays a crucial role. In non-polar solvents like chlorobenzene, the solvent does not significantly stabilize ions formed during interactions. This means that the ability of an amine to donate its lone pair largely depends on molecular characteristics rather than stabilization factors from the solvent.

In chlorobenzene, steric and electronic effects become more pronounced influencing the basicity order of amines. Since the solvent does not assist in stabilizing the resulting ammonium ions effectively, the steric hindrance and electronic nature of substituents, such as the ethyl groups, become more crucial in determining the basic strength of ethylamine, diethylamine, and triethylamine.

Inductive Effects

Inductive effects refer to the redistribution of electron density in a molecule through sigma bonds. In amines, the alkyl groups induce electron donation towards the nitrogen atom. Ethyl groups in ethylamine, diethylamine, and triethylamine exert such an inductive effect.

The more ethyl groups attached, the greater the electron-releasing effect to the nitrogen atom. This increases the electron density around nitrogen, which can enhance the nitrogen's ability to donate its lone pair. Therefore, you'd expect the basicity to increase from ethylamine to diethylamine to triethylamine, assuming steric hindrance wasn't a factor. However, steric hindrance changes this expectation, as it diminishes the effect of lone pair donation.

Nitrogen Lone Pair

The lone pair of electrons on the nitrogen atom in amines is central to their basicity. This lone pair has the potential to accept a proton, forming an ammonium ion. The availability of this lone pair is what primarily defines an amine’s ability to act as a base.

For ethylamine, diethylamine, and triethylamine, while the nitrogen atom in each compound has a lone pair, the presence of increasing numbers of ethyl groups around the nitrogen affects the accessibility of these electrons. Ethylamine, with the least steric bulk, allows better access to the lone pair compared to diethylamine and triethylamine, where accessibility decreases due to the bulk of additional ethyl groups.

Ethyl Group Effects

Ethyl groups attached to the nitrogen atom in amines not only contribute to steric hindrance but also participate through inductive effects that influence basicity. They act as electron-releasing groups due to their electron-donating nature.

This means they can slightly enhance the negative charge around the nitrogen, making the lone pair more available for bonding with a proton. However, the steric effects introduced by additional ethyl groups can outpace their electron-donating benefit as more groups crowd the nitrogen atom. In triethylamine, for example, the steric hindrance becomes a more dominating factor, reducing its basicity despite the inductive electron donation from ethyl groups.