Question

Which of the two: \(\mathrm{O}_{2} \mathrm{NCH}_{2} \mathrm{CH}_{2} \mathrm{O}^{-}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\) is expected to be more stable and why ?

Short answer

\(\mathrm{O}_{2}\mathrm{NCH}_{2}\mathrm{CH}_{2}\mathrm{O}^{-}\) is more stable due to the electron-withdrawing nitro group that provides better charge stabilization.

Step-by-step solution

Analyze the Molecular Structures

Look at the structure of each molecule. The first one is \(\mathrm{O}_{2}\mathrm{NCH}_{2}\mathrm{CH}_{2}\mathrm{O}^{-}\), which features a nitro group (\(\mathrm{NO_2}\)) attached to an ethyl alcohol group (\(\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{O}^{-}\)). The second molecule, \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{O}^{-}\), is an ethyl alcohol without additional substituents. The focus here is on how groups attached to the alkoxide ion affect stability.

Consider the Electron Withdrawing Effects

The nitro group (\(\mathrm{NO_2}\)) is a strong electron-withdrawing group due to its high electronegativity and resonance ability. When attached to the \(\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{O}^{-}\) group, it stabilizes the negative charge on the oxygen by pulling electrons toward itself. This increases the stability of \(\mathrm{O}_{2}\mathrm{NCH}_{2}\mathrm{CH}_{2}\mathrm{O}^{-}\).

Evaluate the Stability of \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{O}^{-}\)

In \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{O}^{-}\), there are no strong electron-withdrawing groups present. The stability of this alkoxide relies solely on the inherent properties of the ethyl group, which moderately stabilizes the negative charge but not as effectively as an electron-withdrawing group would.

Compare Electron Delocalization

The \(\mathrm{O}_{2}\mathrm{NCH}_{2}\mathrm{CH}_{2}\mathrm{O}^{-}\) molecule can delocalize the negative charge more effectively due to the resonance with the electron-withdrawing nitro group. Conversely, \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{O}^{-}\) lacks such resonance effects, resulting in less delocalization and thus, less stabilization of the negative charge.

Conclusion

Based on the presence of an electron-withdrawing nitro group and the increased resonance and electron delocalization this group provides, \(\mathrm{O}_{2}\mathrm{NCH}_{2}\mathrm{CH}_{2}\mathrm{O}^{-}\) is expected to be more stable than \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{O}^{-}\).

Key concepts

Electron-withdrawing groups

When we talk about electron-withdrawing groups, we refer to specific atoms or groups within a molecule that pull electron density towards themselves. These groups are highly electronegative or contain multiple bonds that can attract electrons. This electron-pulling effect stabilizes negative charges present elsewhere in the molecule. A common example of an electron-withdrawing group is the nitro group (\( \text{NO}_2 \)), which is present in the molecule \( \text{O}_2\text{NCH}_2\text{CH}_2\text{O}^- \).

The nitro group stabilizes the negative charge on the alkoxide ion (\( \text{O}^- \)) through its strong electron-withdrawing nature. This happens due to the high electronegativity of nitrogen and the resonance within the nitro group, which allows the negative charge to be shared or "spread out," reducing the overall energy of the system. This makes structures with electron-withdrawing groups often more stable compared to those without them. In contrast, the molecule \( \text{CH}_3\text{CH}_2\text{O}^- \) lacks such groups, resulting in decreased stability.

Resonance stabilization

In the context of chemical stability, resonance stabilization plays a crucial role. Resonance is a concept where electrons are delocalized across multiple atoms, allowing the charge to be spread over a larger area. This process reduces the overall energy of the molecule, which is a direct consequence of electrons being distributed rather than concentrated at one point.

In the \( \text{O}_2\text{NCH}_2\text{CH}_2\text{O}^- \) molecule, the nitro group contributes to resonance stabilization by allowing the negative charge on the oxygen to delocalize through resonance structures involving the nitro group. This efficient delocalization leads to a lower energy and more stable molecule. In contrast, the ethyl group in \( \text{CH}_3\text{CH}_2\text{O}^- \) cannot support resonance in the same way because there are no conjugated systems or electron-withdrawing groups present. Hence, the lack of resonance keeps the negative charge localized, resulting in lesser stability.

Alkoxide ions

Alkoxide ions are negatively charged species formed when an alcohol loses a proton (H⁺). The general formula is \( \text{R-O}^- \), where R is an alkyl group. These ions are strongly basic due to the presence of a negatively charged oxygen.

The stability of alkoxide ions is dependent on several factors, such as the nature of substituents attached and the ability for electron delocalization. In structures where there are electron-withdrawing groups, such as \( \text{O}_2\text{NCH}_2\text{CH}_2\text{O}^- \), the alkoxide ion is more stable because the negative charge on the oxygen is better accommodated. On the other hand, simple alkoxide ions like \( \text{CH}_3\text{CH}_2\text{O}^- \) are less stable as they lack such stabilizing elements.

As alkoxide ions are formed, the stability determines their reactivity and potential transformations in reactions. Understanding these influences is crucial for predicting the behavior of alkoxide ions in chemical reactions and designing stable compounds. The presence or absence of additional stabilizing groups like the nitro group directly impacts their chemical properties.