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Chapter 22: Problem 14

Other groups besides \(\mathrm{H}^{+}\)can act as leaving groups in electrophilic aromatic substitution. One of the best is the trimethylsilyl group, \(\mathrm{Me}_{3} \mathrm{Si}-\). For example, treatment of \(\mathrm{Me}_{3} \mathrm{SiC}_{6} \mathrm{H}_{5}\) with \(\mathrm{CF}_{3} \mathrm{COOD}\) rapidly forms \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{D}\). What are the properties of a siliconcarbon bond that allows you to predict this kind of reactivity?

Short Answer

Expert verified
Short Answer: The silicon-carbon bond in the trimethylsilyl group is reactive in electrophilic aromatic substitution reactions due to factors such as weaker bond strength resulting from longer bond lengths, the difference in electronegativity between silicon and carbon atoms, the presence of vacant d-orbitals in silicon, and the stability of the trimethylsilyl anion as a leaving group.

Step by step solution

01

Analyzing the Electrophilic Aromatic Substitution Reaction

Consider the given reaction where the trimethylsilyl group (Me₃Si-) acts as a leaving group in the electrophilic aromatic substitution. The reaction of Me₃SiC₆H₅ with CF₃COOD forms C₆H₅D. This result implies that the silicon-carbon bond in the trimethylsilyl group (Me₃Si-) can be cleaved during the reaction, indicating that it is relatively weak and allows for the electrophilic aromatic substitution to take place.
02

Silicon-Carbon Bond Properties

To understand the reactivity in this reaction, we should analyze the properties of the silicon-carbon (Si-C) bond. One important property that distinguishes silicon from carbon is its size. Silicon is larger and has longer bond lengths than that of carbon, which results in a weaker bond. Additionally, silicon is in the third period of the periodic table, which means that it has vacant d-orbitals. The vacant d-orbitals can accept electron pairs, which facilitates the cleavage of the Si-C bond during substitution reactions.
03

Electronegativity

Another factor that influences the strength and reactivity of the Si-C bond is the difference in electronegativity between silicon and carbon atoms. Silicon is less electronegative (\<\ce{Si}=1.90; \ce{C}=2.55\)) than carbon, resulting in the Si-C bond being polarized. This bond polarity makes the bond more susceptible to nucleophilic attack, which facilitates the electrophilic aromatic substitution reaction.
04

Stability of the Leaving Group

Lastly, the trimethylsilyl (Me₃Si-) anion that would form after the Si-C cleavage is stabilized by the inductive effect. The three methyl groups are electron-donating and can donate electron density to the positively charged silicon atom in the anion, making it a stable leaving group.
05

Conclusion

The reactivity of the silicon-carbon bond in electrophilic aromatic substitution can be attributed to the weaker bond strength due to the longer bond lengths, the difference in electronegativity, the available vacant d-orbitals, and the stability of the leaving group. These properties of the silicon-carbon bond allow us to predict the reactivity of the trimethylsilyl group in electrophilic aromatic substitution reactions.

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