Question

Presence of a \(-\mathrm{NO}_{2}\) group around a carbonium ion (a) reduces its stability (b) increases its stability (c) Makes no change in its stability (d) none of these

Short answer

(a) reduces its stability.

Step-by-step solution

Identify the Impact of the Nitro Group

The nitro group \(-\mathrm{NO}_{2}\) is known to be an electron-withdrawing group due to its strong electronegative nature. When attached to a carbonium ion, which is positively charged, we need to evaluate how this electron-withdrawing property impacts the ion's stability.

Examine the Effect on Positive Charge

A carbonium ion has a positive charge that benefits from electron donation (which stabilizes it) rather than electron withdrawal. Since \(-\mathrm{NO}_{2}\) pulls electrons away, it increases the electron deficiency (positive charge) of the carbonium ion.

Analyze Stability Impact

With \(-\mathrm{NO}_{2}\) increasing the electron deficiency, the carbonium ion becomes less stable because the positive charge is now less compensated by electron density. Stability in such ions generally decreases with more positive charge.

Conclusion Based on Stability Changes

Given the electron-withdrawing nature of the \(-\mathrm{NO}_{2}\) group, it reduces the electron density around the carbonium ion. Thus, the presence of the \(-\mathrm{NO}_{2}\) group reduces the stability of the carbonium ion.

Key concepts

Nitro Group Effect

The nitro group, represented as \(-\mathrm{NO}_{2}\), is well-known for its influence on the chemical stability of different compounds, particularly carbonium ions. A carbonium ion is essentially a carbon atom carrying a positive charge, often formed during organic reactions. For the nitro group, its effect largely stems from its electron-withdrawing capability. This attribute becomes significantly important when a \(-\mathrm{NO}_{2}\) group is connected to a carbon atom with a positive charge.

Because of its high electronegativity and the presence of a strongly polar nitro bond, the \(-\mathrm{NO}_{2}\) group tends to pull electrons towards itself. This makes it an electron sink, effectively drawing electrons away from the central carbon atom. In a carbonium ion, which already has a deficiency of electrons due to its positive charge, the presence of a nitro group exacerbates the deficit of electrons. Consequently, this depletes electron density further around the carbonium ion.

This behavior contrasts with groups that donate electrons, which help in stabilizing the positive charge of a carbonium ion. Therefore, rather than stabilizing the ion, the nitro group reduces its stability due to this electron-withdrawing attribute.

Electron-Withdrawing Groups

In organic chemistry, the behavior of specific functional groups can drastically change the stability of molecules and ions. These functional groups are often classified based on their ability to either donate electrons (electron-donating groups) or withdraw electrons (electron-withdrawing groups).

The nitro group \(-\mathrm{NO}_{2}\) is a classic example of an electron-withdrawing group. Electron-withdrawing groups are characterized by:

• High electronegativity, which means they attract and pull electron density toward themselves.
• The ability to stabilize negative charges due to this electron attraction, but destabilize positive charges.
• Participating in resonance, which enables them to delocalize electrons over a structure.

When attached to a carbonium ion, an electron-withdrawing group like \(-\mathrm{NO}_{2}\) pulls electrons away, intensifying the positive charge on the carbon atom. This electron withdrawal is not supportive of positive charge stability, which is why electron-withdrawing groups are crucial in predicting whether a reaction site will remain stable or will be reactive based on its environment.

Positive Charge Stability

The stability of a positively charged species such as a carbonium ion is highly dependent on the electron environment around it. Generally, a stable carbonium ion is one where the positive charge is minimized or well-compensated by surroundings that supply electron density. That is because the more the positive charge of a carbon atom is compensated by electron donation, the less reactive and more stable it becomes.

For instance, alkyl groups, which are electron-pushing or donating in nature, can increase the stability of carbonium ions by sharing their electron density with the positively charged carbon. The opposite is true with electron-withdrawing groups such as \(-\mathrm{NO}_{2}\). These groups draw away electron density, thereby intensifying the positive charge.

This has a destabilizing effect on the carbonium ion as the electron deficiency is exacerbated, leaving it far from the stable, electron-balanced state it prefers. To stabilize a carbonium ion with an electron-withdrawing group attached, either additional electron-donating groups need to be present or other compensatory stabilizing factors should be considered to help balance the charge.