Question

Explain why is ortho nitrophenol more acidic than ortho methoxyphenol ?

Short answer

Ortho-nitrophenol is more acidic due to the electron-withdrawing nitro group stabilizing its conjugate base.

Step-by-step solution

Analyze the structures

Ortho-nitrophenol has a nitro group ( O_2) attached to the benzene ring next to the hydroxyl group, while ortho-methoxyphenol has a methoxy group (OCH_3) in the same position.

Assess the electron-withdrawing effect

The nitro group is strongly electron-withdrawing due to its resonance and inductive effect. This stabilizes the phenoxide ion formed after losing a proton, increasing the acidity of ortho-nitrophenol.

Evaluate the electron-donating effect

The methoxy group in ortho-methoxyphenol is electron-donating through resonance. This effect reduces the stability of the phenoxide ion and makes ortho-methoxyphenol less acidic than ortho-nitrophenol.

Compare the acidity

A molecule with more stable conjugate base (phenoxide ion) is more acidic. The nitro group in ortho-nitrophenol increases the stability of its conjugate base, making it more acidic compared to ortho-methoxyphenol.

Key concepts

Ortho-nitrophenol

When discussing the acidity of ortho-nitrophenol, it's essential to understand its molecular structure. The term "ortho" in organic chemistry refers to the position of a substituent on a benzene ring, specifically adjacent to another functional group. In ortho-nitrophenol, a nitro group (NO_2) is positioned next to a hydroxyl group (-OH) on the benzene ring. This proximity is significant because it affects the molecule's chemical behavior, primarily its acidity.
The nitro group acts as an electron-withdrawing group, which will be explained further in the next section. Due to this withdrawal of electrons, the hydroxyl group can release a hydrogen ion (H^+) more readily, making ortho-nitrophenol more acidic when compared to compounds with different substituents, like ortho-methoxyphenol, which contains an electron-donating methoxy group (OCH_3). This characteristic plays a crucial role in the molecule's tendency to donate a proton, thereby behaving as an acid.

Electron-withdrawing groups

Electron-withdrawing groups (EWGs) are a critical concept in understanding the acidity of certain organic compounds. These groups contain atoms that pull electron density away from other parts of the molecule through inductive or resonance effects. The nitro group (NO_2) in ortho-nitrophenol is a prime example of an electron-withdrawing group.
There are two primary ways EWGs can influence a molecule:

• Inductive Effect: This is the ability of an atom or group to attract electrons through a sigma bond. In nitro groups, the highly electronegative nitrogen and oxygen atoms pull electron density away from the benzene ring.
• Resonance Effect: EWGs can accept electron density into their structure via resonance, which involves delocalizing electrons over the molecule. This makes the molecule more stable.

In ortho-nitrophenol, the presence of the nitro group increases the acidity by stabilizing the phenoxide ion, the conjugate base, after the hydrogen ion is released.

Phenoxide ion stability

The stability of a phenoxide ion is a pivotal factor in determining the acidity of phenolic compounds. When acids like ortho-nitrophenol release a proton, they form a phenoxide ion, the conjugate base. The more stable this ion is, the more acidic the original compound.
In ortho-nitrophenol, the phenoxide ion is stabilized by the presence of the nitro group. This stability arises due to electron-withdrawing effects from the nitro group, which helps delocalize the negative charge over the entire structure of the phenoxide ion. Therefore, ortho-nitrophenol's ability to easily lose a proton is enhanced because it leads to the formation of a more stabilized conjugate base. Hence, the stability of the phenoxide ion directly correlates with increased acidity, making ortho-nitrophenol more acidic compared to compounds where the conjugate base is less stabilized.

Resonance effect

The resonance effect is a fascinating aspect of chemistry that greatly impacts the behavior and characteristics of molecules, particularly regarding acidity. In the context of ortho-nitrophenol, understanding this concept is essential. Resonance refers to the delocalization of electrons across a molecule, allowing certain structures to stabilize electron distribution.
For ortho-nitrophenol, the nitro group's resonance effect significantly contributes to the overall stability of the phenoxide ion. After the ortho-nitrophenol deprotonates to form a phenoxide ion, the resonance effect enables the negative charge to be shared across the structure and particularly into the nitro group. This delocalization reduces the energy of the ion, making it more stable.
In comparison, compounds with substituents that donate electrons through resonance, such as a methoxy group, would have less stable phenoxide ions, as electron donation increases electron density, reducing overall stability. Therefore, the resonance effect in ortho-nitrophenol enhances its acidity through better stabilization of its conjugate base.