Question

Among the following, the strongest base is (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) (b) \(\mathrm{p}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{NH}_{2}\) (c) \(\mathrm{m}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{NH}_{2}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\)

Short answer

The strongest base is (d) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\).

Step-by-step solution

Identify Base Strength

Base strength in aromatic amines like aniline (\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\)) depends on the electron donating or withdrawing effects of substituents on the benzene ring. Electron donating groups increase basicity by increasing electron density on the nitrogen atom, whereas electron withdrawing groups decrease it.

Evaluate Electron Effects

Analyze the electron effects of substituents in each compound:- (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) has no substituents, so it is a reference.- (b) \(\mathrm{p}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{NH}_{2}\) has a para-nitro group which is electron withdrawing, reducing basicity.- (c) \(\mathrm{m}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{NH}_{2}\) has a meta-nitro group which also withdraws electrons, reducing basicity.- (d) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\) has a benzyl group, which can donate electrons to the nitrogen, increasing its basicity.

Determine Strongest Base

Based on the electron effects analysis, compound (d) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\), with an electron donating group, is the strongest base compared to those with electron withdrawing groups like the nitro groups in (b) and (c).

Key concepts

Electron Donating Groups

Electron donating groups (EDGs) play a significant role in influencing the basicity of aromatic amines. Basicity is an essential property that describes a molecule's ability to accept protons. In the case of amines, this acceptor is the nitrogen atom. EDGs enhance the basicity by increasing the electron density on the nitrogen, making it more prone to attract protons.

EDGs can include groups like alkyl chains, which, through a process known as hyperconjugation, push electron density towards the nitrogen atom. This increase in electron density makes the nitrogen atom more available and eager to bind with a proton. Consequently, an amine with an EDG is typically more basic than one without.

For example, the benzyl group in compound \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\) is an EDG. The presence of this group increases the electron density on the nitrogen, enhancing its ability to function as a base compared to other compounds in the original exercise with electron withdrawing groups.

Aromatic Amines

Aromatic amines are amines in which the nitrogen atom is directly attached to an aromatic ring, like benzene. One common example is aniline (\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\)). The aromatic ring plays a crucial role in determining the basicity of the amine.

The electron cloud of the benzene ring can either increase or decrease the availability of the nitrogen's lone pair, depending on the substituents present on the ring. Aromatic amines are generally less basic than aliphatic amines because the lone pair of electrons on the nitrogen can be partially delocalized into the aromatic ring. This reduces the nitrogen's ability to bind with an additional proton.

However, when there are substituents like an electron donating group on the benzene ring, the basicity can significantly increase as these groups can provide additional electron density to the nitrogen atom. Conversely, electron withdrawing groups can decrease the basicity further by pulling electron density away from the nitrogen.

Electron Withdrawing Groups

Electron withdrawing groups (EWGs) have the opposite effect of electron donating groups. They decrease the basicity by reducing the electron density on the nitrogen atom of aromatic amines. This leaves the nitrogen less likely to attract a proton.

EWGs include groups like nitro (\(-\mathrm{NO}_2\)) which are known for their strong electron withdrawing properties. These groups use resonance and/or inductive effects to pull electron density away from the nitrogen atom, making it less basic. This effect is more pronounced when the substituents are in positions such as the para or ortho positions on the benzene ring relative to the amine group.

In the original exercise, compounds like \(\mathrm{p}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{NH}_{2}\) and \(\mathrm{m}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{NH}_{2}\) exemplify this concept well. Both positions of the nitro groups significantly draw electrons away, thus reducing the basicity of these amines, especially compared to compounds with electron donating groups.