In organic chemistry, the terms "-M effect" (minus M effect) and "+M effect" (plus M effect) are used to describe the electronic effects of different groups or substituents on a molecule. These effects refer to the ability of a group to either withdraw or donate electrons through resonance or inductive effects. The order of groups based on their -M or +M effects can vary depending on the specific context and molecular structure. However, I can provide you with a general idea of the relative order of some common groups.
The -M effect refers to the electron-withdrawing ability of a group, which can stabilize negative charges and decrease electron density at a particular location in a molecule. Here are some groups in decreasing order of their -M effect:
- Nitro group (-NO2)
- Carbonyl group (-C=O)
- Cyanide group (-CN)
- Sulfonic acid group (-SO3H)
- Nitrite group (-ONO)
- Halogens (-F, -Cl, -Br, -I)
On the other hand, the +M effect refers to the electron-donating ability of a group, which can increase electron density and stabilize positive charges or polarize bonds. Here are some groups in increasing order of their +M effect:
- Alkyl groups (-R)
- Alkoxy groups (-OR)
- Amino group (-NH2)
- Hydroxyl group (-OH)
- Ether group (-O-)
- Phenyl group (-C6H5)
It's important to note that this is a general trend and there can be exceptions or variations depending on the specific molecular context. Additionally, other factors such as the proximity and conjugation effects can also influence the overall electronic effects of the substituents.