Reactions of Aromatic Compounds (overall)
Reactivity of Substituted Aromatic Compounds
The influence of substituents on the reactivity of aromatic compounds in an electrophilic aromatic substitution is discussed below.
The reaction rate of a multistep reaction is determined by the activation energy of the slowest individual step, which is called the rate-determining step. The activation energy is the energy difference between the starting products of the respective step and its transition state. The reaction rate of electrophilic aromatic substitution therefore depends on the substituent's qualities, as these influence the transition state's energy. It is important to note that a substituent does not influence the starting product's energy in the same way. The activation energy and thus the reaction rate would otherwise not vary among different substituents. In the electrophilic aromatic substitution, the rate-determining step is usually the formation of the σ complex. The σ complex resembles the transition state that is passed through when the the σ complex is formed from the starting products. This may be used as a basis for the following considerations of the substituent's influence on the reaction rate. Due to the transition state's extremely short lifetime, its structure cannot be experimentally determined. Thus, it is usually practically impossible to verify its structure. However, the fact that it can be estimated by the structure of the σ complex is quite an advantage.
According to the Polanyi-Hammond postulate, the rate-determining step's activation energy can be estimated according to the energy of the σ complex. Thus, a stable, low-energy σ complex results in a high reaction rate, while a high-energy σ complex represents a low reaction rate. The stability of the σ complex depends on the strength of the σ bond between the aromatic ring and the electrophile, as well as on the ring's and its substituent's ability to stabilize the positive charge by delocalization.
According to the considerations above, substituents may be classified into two main groups:
- Activating substituents that are capable of stabilizing the positive charge of the σ complex. Compared to benzene, such substituents result in a higher reaction rate of (a second) electrophilic aromatic substitution.
- Deactivating substituents that additionally destabilize the positive charge of the σ complex. Compared to benzene, such substituents characteristically result in a lower reaction rate of (a second) electrophilic aromatic substitution.
Principally speaking, the stabilization as well as the destabilization of the σ complex may be the outcome of two different effects.
- Inductive increase or decrease, respectively, in the aromatic system's electron density through polarization of the σ bond between the substituent and the aromatic ring. This quality of substituents is known as the electron-donating inductive effect (+I effect) or electron-withdrawing inductive effect (-I effect), respectively.
- An increase or decrease of the aromatic system's electron density through resonance (mesomerism). That is, the substituent participates in the aromatic π electron system either by donating π electron density (lone electron pair) or by accepting π electron density. These qualities of a substituent are called electron-donating mesomeric effect (+M effect) or electron-withdrawing mesomeric effect (-M effect), respectively. In Lewis formulas, the mesomeric effect of a substituent is illustrated by several possible resonance structures of the σ complex. Mesomeric effects are usually much stronger than inductive effects.