Chapter 20: Problem 79
Arrange the following groups in order of decreasing 0 - and \(\mathrm{p}\) -directing strength. \(\mathrm{NII}_{2}\), OII, \(\mathrm{Cl}, \mathrm{R}\) (1) \(-\mathrm{Cl}>-\mathrm{R}>-\mathrm{OH}>-\mathrm{NH}_{2}\) (2) \(-\mathrm{NH}_{2}>-\mathrm{OH}>-\mathrm{R}>-\mathrm{Cl}\) (3) \(-\mathrm{OH}>-\mathrm{NH}_{2}>-\mathrm{R}>-\mathrm{Cl}\) (4) \(-\mathrm{R}>-\mathrm{CI}>-\mathrm{NH}_{2}>-\mathrm{OH}\)
Short Answer
Step by step solution
Key Concepts
These are the key concepts you need to understand to accurately answer the question.
ortho-directing groups
Groups that are typically ortho-directing have lone pairs available for donation or are strong electron donors. For example:
- Amino group (-NH2)
- Hydroxy group (-OH)
- Alkyl groups (-R)
para-directing groups
These groups can be strong electron donors, enhancing electron density both at the ortho and para positions due to their ability to either donate electrons through resonance or inductive effects. Common para-directing groups include:
- Methoxy group (-OCH3)
- Alkyl groups (-R)
electron-donating groups
- Inductive effect: Through sigma bonds as seen in alkyl groups (-R).
- Resonance effect: Through pi bonds, such as -OH and -NH2 groups.
inductive effect
Substituents like alkyl groups show a +I or positive inductive effect. They push electron density towards the benzene ring, making it more reactive.
While weaker than resonance, inductive effects are significant in determining the reactivity and directing effects of substituents. Strong +I groups like alkyls make the ortho/para positions in the benzene ring reactive to further electrophilic attack.
resonance effect
For example, groups with lone pairs like -OH and -NH2 can donate electrons into the benzene ring. This increases electron density at the ortho and para positions.
Conversely, some groups may withdraw electron density, making the benzene ring less reactive. However, in the context of ortho/para-directing groups, we focus on electron-donating substituents which stabilize intermediates and enhance reactivity at specific positions.