EAS:Synergistic and Competitive Groups: Videos & Practice Problems

Steric effects in polysubstituted benzene during electrophilic aromatic substitution (EAS) reactions refer to the spatial hindrance caused by bulky groups already attached to the benzene ring. These bulky groups can block certain positions on the ring, making them less reactive or completely unreactive. For example, if two tert-butyl groups are present on the benzene ring, the position between them will be highly sterically hindered and thus, the least likely to react. Understanding steric effects is crucial for predicting which positions on the benzene ring will be available for further substitution.

Synergistic directing groups in polysubstituted benzene enhance the reactivity of a specific position on the benzene ring during EAS reactions. When multiple substituents direct incoming electrophiles to the same position, they are considered synergistic. For instance, if both a nitro group and an aldehyde group are present on the benzene ring and both direct to the same meta position, the electrophile will preferentially add to that position. This synergy results in a higher yield of the desired product because all directing groups are cooperating, making the targeted position more reactive.

When competitive directing groups are present in a polysubstituted benzene, they direct incoming electrophiles to different positions on the benzene ring, leading to a mixture of products. For example, if a nitro group and an aldehyde group are present and they direct to different positions, the electrophile will add to both positions, resulting in multiple products. The major product is determined by the strongest activator among the groups. The strongest activator will direct the electrophile to its preferred position more effectively, resulting in a higher yield of that product compared to the others.

To determine the major product in EAS reactions involving competitive directing groups, you need to identify the strongest activator among the substituents. The strongest activator is the group that most effectively increases the reactivity of the benzene ring towards electrophilic attack. For example, if a nitro group (a strong deactivator) and an aldehyde group (a moderate deactivator) are present, the aldehyde group will be the stronger activator. Therefore, the major product will be the one where the electrophile adds to the position directed by the aldehyde group. The relative positions of the groups on the activity chart help in making this determination.

The activity chart is a crucial tool for predicting the outcomes of EAS reactions in polysubstituted benzene. It ranks substituents based on their activating or deactivating effects on the benzene ring. Activating groups increase the ring's reactivity towards electrophiles, while deactivating groups decrease it. When multiple substituents are present, the activity chart helps determine which group will dominate the directing effects. The substituent higher on the chart will generally dictate the major product by directing the electrophile to its preferred position. This understanding helps in predicting the major and minor products in complex EAS reactions.