Question

Which is most acidic here? (a) COc1ccccc1[O-] (b) COc1cccc([O-])c1 (c) COc1ccc([O-])cc1 (d) all are equally acidic

Short answer

(c) is the most acidic because the methoxy group is at the meta position, minimizing electron donation.

Step-by-step solution

Understand Acidity

Acidity refers to the ability of a molecule to donate a proton (H⁺). In organic chemistry, the acidity of phenol derivatives can be influenced by electron-withdrawing or electron-donating groups attached to the aromatic ring. A group that stabilizes the negative charge on the oxygen after losing a proton can increase acidity.

Analyze Structures

Let's analyze each structure: 1. Structure (a): "COc1ccccc1[O-]" is a methoxyphenoxide ion, with the methoxy group on the benzene ring. 2. Structure (b): "COc1cccc([O-])c1" has the oxygen at the para position to the methoxy group. 3. Structure (c): "COc1ccc([O-])cc1" has the oxygen at the meta position to the methoxy group. Each of these structures contains a methoxy group (electron-donating) and an oxygen anion.

Effects of Substituents on Acidity

The methoxy group is electron-donating and can reduce the stability of the phenoxide ion, thereby affecting acidity. The position of the methoxy group relative to the oxygen anion is crucial: - Para (position in (b)) allows for resonance stabilization between the methoxy group and the phenoxide ion, enhancing acidity less. - Meta (position in (c)) has little to no resonance impact, thus providing less stabilization. - Ortho (position in (a)) does not provide resonance stabilization, but steric effects can sometimes enhance acidity by preventing other interactions that destabilize the ion.

Compare Acidities

Position (c) has the methoxy group in the meta position, where resonance does not stabilize the negative charge on the oxygen. This makes it the most acidic among the given options. In (a), the ortho position still allows for some interaction, but not as strong as the para. In (b), para reduces acidity due to resonance with the methoxy group.

Conclusion

Among the options, the most acidic compound is the one where the electron donating effect is minimized. Therefore, the phenoxide with the methoxy group in the meta position (structure (c)) is the most acidic.

Key concepts

Electron-withdrawing groups

Electron-withdrawing groups (EWGs) play a significant role in determining the acidity of organic compounds. These groups pull electron density away from other parts of the molecule, often stabilizing negative charges that form when a molecule loses a proton. This increased stabilization leads to higher acidity as it makes the compound more willing to donate its proton.

In the case of phenol derivatives, the presence and position of electron-withdrawing groups on the aromatic ring can drastically alter acidity. When a strong electron-withdrawing group is directly attached to the aromatic ring, it can delocalize the negative charge formed after deprotonation, thereby increasing the stability of the resultant anion. This process directly enhances the acidity of the compound.

Some common electron-withdrawing groups include:

• Halogens (F, Cl, Br)
• Nitro group (-NO₂)
• Cyano group (-CN)
• Carbonyl groups (C=O)

All these groups can increase the acidity of phenol derivatives depending on their positions relative to the hydroxyl group.

Resonance stabilization

Resonance stabilization is another critical factor affecting the acidity of compounds in organic chemistry. It refers to the delocalization of electrons across certain atoms, which can spread out and thus stabilize any resulting charges.

Take phenol derivatives, for instance. When a phenol loses a proton, it forms a phenoxide ion. The negative charge on the oxygen is more stable if it can resonate with the aromatic ring, leading to resonance stabilization. This process is enhanced when specific substituent groups are capable of participating in the resonance, such as electron-donating groups at para and ortho positions.

Key points about resonance in acidity include:

• The para position often allows for optimal resonance stabilization, creating a significant effect on acidity due to the smooth flow of electrons.
• Meta positions, on the other hand, do not typically feature effective resonance paths, leading to lesser stabilization of the charge.

The interplay of the electron flow, the positions of substituents, and their nature (donating or withdrawing) can guide us in predicting acidity trends in aromatic compounds.

Phenol derivatives

Phenol derivatives are compounds where a phenolic (an aromatic ring-bound hydroxide, \(\text{OH}\)) group is present. The acidity of these compounds is interesting to study due to how substituents attached to the aromatic ring influence electronic structures.

The phenol molecule itself is acidic mainly due to its ability to stabilize the resulting phenoxide ion through possible resonance with the aromatic ring. However, the specific nature of substituents attached can drastically affect the acidity:

• Substituents like methoxy (electron-donating groups) generally decrease acidity by destabilizing the anion, whereas electron-withdrawing groups can increase acidity by enhancing stability.
• The position relative to the phenol group—ortho, para, or meta—also changes how these effects are felt:
• Ortho and para positions may allow for more significant resonance and inductive effects.
• Meta positions typically do not facilitate effective resonance.

Ultimately, understanding phenol derivatives means engaging with both the structural and electronic nuances affected by different substituents to predict and explain changes in their acidity.