Why doesn't OCH3 show -M effect? (O-R groups) Is presence of a lone pair mandatory for M effect?

Answer 1

Let's elaborate further on the topic of -M (negative mesomeric effect) and +M (positive mesomeric effect) effects and whether the presence of a lone pair is mandatory.

1. OCH3 and -M Effect:

- The methoxy group (OCH3) is electron-donating due to the presence of a lone pair on oxygen. This lone pair can engage in resonance with adjacent pi systems. However, this effect is not classified as a -M effect; rather, it is considered a +M effect.

2. Positive Mesomeric Effect (+M Effect):

- In the context of the methoxy group, the lone pair on oxygen can resonate with the adjacent pi system, such as in an aromatic ring. This resonance leads to the delocalization of electrons, stabilizing the system.

- Example: Consider an aromatic ring with a methoxy substituent (Ar-OMe). The lone pair on the oxygen can delocalize into the aromatic ring, resulting in increased electron density and stabilizing the overall structure.

3. Lone Pair vs. Pi Bonds in Mesomeric Effects:

- While lone pairs are commonly associated with mesomeric effects, pi bonds can also participate. The critical factor is the movement of electrons, leading to resonance stabilization.

- Example: In the case of ethene (C2H4), the pi electrons in the double bond can participate in resonance, contributing to a mesomeric effect. This is known as the +M effect.

4. Mandatory Lone Pair for Mesomeric Effects:

- Lone pairs are not mandatory for mesomeric effects. As mentioned, pi bonds can also contribute to resonance, leading to mesomeric effects.

- Example: In the nitro group (NO2), the pi electrons in the double bond and the oxygen lone pairs can engage in resonance, creating a -M effect.

In summary, the methoxy group (OCH3) exhibits a +M effect, and while lone pairs are commonly involved in mesomeric effects, pi bonds can also contribute. The key is the movement of electrons and the resulting resonance that leads to stabilization.

Answer 2

It would show a +M effect (positive mesomeric effect) I believe. I don't think a lone pair is mandatory since I believe you can have M effect from interaction of 2 pi bonds, OR interaction of a bond and a lone pair of electrons. Hopefully someone more well versed in O-Chem. will weigh in on this topic.