Why is anisole less reactive than phenol

Why is anisole less reactive than phenol?

Anisole and phenol have some similarities in chemical structure, but they have significant differences in reactivity. Understanding the difference in reactivity between the two compounds is important for the design of chemical reactions, industrial applications and product synthesis. This article will analyze in detail why the reactivity of anisole is lower than that of phenol, and explore the reasons.

1. Anisole and phenol structure difference

In the anisole molecule, the oxygen atom is linked to a methyl group (-CH3), while in the phenol molecule, the oxygen atom is linked to a hydroxyl group (-OH). This small structural difference plays a crucial role in the chemical reaction.

• Anisole, the presence of methyl can through the electronic effect (electron donating effect) to the benzene ring on the electron density increased, which makes the benzene ring electrophilicity decreased, thereby inhibiting the reaction.
• In contrast, phenol in the hydroxyl group has a strong electron donating effect, making the benzene ring electron density increased, thereby enhancing its electrophilic reagent affinity, resulting in phenol reactivity is higher.

This is the fundamental reason why anisole is less reactive than phenol.

2. Electronic effects

The combination of the methyl group in anisole with the benzene ring will increase the electron cloud density of the benzene ring, thereby reducing the attraction of the benzene ring to the electrophilic reagent. This effect is referred to as the "M-effect" or electron-giving effect. The increase in the electron cloud makes the affinity of the benzene ring for electrophilic reactions decrease, resulting in anisole being less reactive in many reactions.

On the other hand, the hydroxyl group in phenol is also a negative group with a lone pair of electrons, which has a stronger electron-giving effect, thus enhancing the electron cloud density of the benzene ring, making the benzene ring more likely to participate in the electrophilic reaction. Therefore, the reactivity of phenol is higher, especially in the aromatic electrophilic substitution reaction.

3. Hydrogen bond effect

The hydroxyl group in the phenol molecule can form hydrogen bonds with other molecules, which makes the phenol react more easily in solution. The formation of hydrogen bonds may change the electronic environment of phenol, making it more susceptible to reactions with other reagents, especially when interacting with strong electrophiles.

The methyl group of anisole does not form hydrogen bonds like the hydroxyl group in phenol. Therefore, anisole has a weak intermolecular interaction force and low intermolecular reactivity, which is one reason why its reactivity is lower than that of phenol.

4. Spatiosteric effect

In some cases, the difference in reactivity of anisole and phenol is also related to steric hindrance effects. The methyl group in anisole is relatively small, but its binding to the benzene ring will still affect the reaction site to a certain extent. This effect makes anisole slower in certain reactions, especially when an electrophile is required to undergo an electrophilic substitution reaction with the benzene ring.

In contrast, the hydroxyl group of phenol is usually more exposed and does not exert too much steric effect on the reaction site, so its reactivity is relatively high.

5. Summary: Anisole reactivity low reasons

From the above analysis, we can summarize the following main reasons to explain why anisole is less reactive than phenol:

1. The methyl group in anisole through the electron donating effect reduces the electrophilicity of the benzene ring and inhibits the reaction;
2. Phenol in the hydroxyl group through the electron donating effect to enhance the benzene ring electrophilicity, making it easier to participate in the electrophilic reaction;
3. Phenol can form hydrogen bonds, but anisole cannot, and hydrogen bonds promote phenol reactivity;
4. Anisole steric hindrance effect in some reactions may make its reaction rate lower.

Therefore, the reactivity of anisole is lower than that of phenol, mainly due to the comprehensive effect of its molecular structure, electronic effect and hydrogen bonding. The manifestation of these factors in chemical reactions makes anisole in many cases less reactive than phenol.