Question

Account for the fact that \(p\) -nitroaniline \(\left(p K_{\mathrm{a}}=1.0\right)\) is less basic than \(m\) -nitroaniline \(\left(p K_{\mathrm{a}}=2.5\right)\) by a factor of \(30 .\) Draw resonance structures to support your argument. (The \(\mathrm{p} K_{\mathrm{a}}\) values refer to the corresponding ammonium ions.)

Short answer

p-nitroaniline is less basic due to stronger electron-withdrawing resonance from the para nitro group compared to meta.

Step-by-step solution

Understanding pKa Values

Recognize that a lower \(pK_a\) value indicates a stronger acid, meaning its conjugate base is weaker. Therefore, \(p\)-nitroaniline with a \(pK_a\) of 1.0 is a stronger acid than \(m\)-nitroaniline with a \(pK_a\) of 2.5, indicating \(p\)-nitroaniline is the weaker base.

Understanding Basicity Factors

Since \(p\)-nitroaniline is less basic than \(m\)-nitroaniline by a factor of 30, this can be calculated as \(10^{\frac{2.5 - 1.0}{1}} = 10^{1.5} \approx 30\). This calculation shows how the difference in \(pK_a\) values translates into basicity.

Drawing the Resonance Structures for p-Nitroaniline

Draw the structure for \(p\)-nitroaniline, showing a nitro group at the para position. The nitro group, which is an electron-withdrawing group, delocalizes electrons through resonance, making it less available for protonation, hence reducing basicity.

Drawing the Resonance Structures for m-Nitroaniline

Draw the structure for \(m\)-nitroaniline with the nitro group at the meta position. Here, the resonance effect of the nitro group is not directly influencing the amine group as much as in the para position, allowing more availability of electrons for protonation, hence more basicity.

Comparing Resonance Structures

Highlight that in \(p\)-nitroaniline, the resonance helps withdraw electron density from the amine nitrogen, making it less basic compared to \(m\)-nitroaniline, where the same resonance effect is not as pronounced due to the meta positioning of the nitro group.

Key concepts

Organic Chemistry

Organic chemistry is a fascinating branch of chemistry that deals with the structure, properties, composition, reactions, and preparation of carbon-containing compounds. These compounds often include hydrogen, oxygen, nitrogen, and other elements. Understanding the principles of organic chemistry is crucial to grasping the nuances of molecular interactions and reactions.

In the case of p-nitroaniline and m-nitroaniline, organic chemistry concepts help us understand the influence of molecular structure on basicity. Basicity in organic chemistry often depends on how readily a molecule can accept protons. The position of substituents like the nitro group can drastically alter a molecule's electronic structure, impacting its ability to donate or withdraw electrons.

Thus, mastering the basics of organic chemistry allows students to predict how changes in molecular geometry can influence chemical behavior and reactivity.

Basicity

Basicity refers to the ability of a compound to accept protons (H+ ions) in chemical reactions. In organic chemistry, this concept is closely tied to the availability of lone pair electrons on heteroatoms like nitrogen. Aniline derivatives are great examples to illustrate basicity.

has a lower basicity compared to . This is due to their differing structural influences of substituent groups. A lower basicity in p-nitroaniline is indicated by its lower pKa value, making it a weaker base than m-nitroaniline. The position of the nitro group in p-nitroaniline withdraws electron density from the nitrogen atom, making it less able to stabilize additional positive charges (protons).

• Availability of Electrons: More available lone pairs mean higher basicity.
• Stabilization of Charge: The conjugate acid should be stable after protonation.

The basicity of a compound is a vital factor to consider during synthesis and chemical reactions.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) play a significant role in organic compounds, particularly in affecting acidity and basicity. These groups, like the nitro group, draw electron density away from other parts of the molecule through inductive and resonance effects.

In p-nitroaniline, the nitro group at the para position substantially decreases the basicity. It achieves this by withdrawing electron density via resonance from the amine group (NH2). Less electron density means the nitrogen atom in the amine group becomes less available to accept protons.

Comparatively, in m-nitroaniline, the nitro group at the meta position affects the amine group less through resonance effect. As a result, m-nitroaniline remains more basic.

• Inductive Effect: EWGs pull electron density through sigma bonds.
• Resonance Effect: EWGs can delocalize electrons over the pi system, affecting basicity.

Understanding how EWGs alter molecular properties is pivotal when manipulating chemical pathways in synthetic chemistry.