Question

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 properties of a silicon-carbon bond allow you to predict this kind of reactivity?

Short answer

Answer: The properties that make a silicon-carbon bond a good leaving group in electrophilic aromatic substitution reactions are the difference in electronegativity between silicon and carbon, the weaker bond strength of the Si-C bond compared to C-C bonds, and the presence of a bulky trimethylsilyl group with electron-donating methyl groups that increase the electrophilic nature of the silicon center.

Step-by-step solution

Understanding Electrophilic Aromatic Substitution

Electrophilic aromatic substitution is a reaction in which an electrophile, such as CF3COOD, reacts with an aromatic ring and replaces one of the substituents on the ring. In our exercise, the trimethylsilyl group (Me3Si) is acting as the leaving group, which is being replaced by the deuterium atom (D) from CF3COOD.

Properties of Silicon-Carbon bond

One of the first things to consider is the difference in electronegativity between silicon and carbon. Silicon has an electronegativity of 1.9, while carbon has an electronegativity of 2.55. This difference indicates that the bond between silicon and carbon is somewhat polar, with silicon being slightly electron-deficient. Another important property of the silicon-carbon bond is its bond strength, which is weaker compared to carbon-carbon bonds. The bond dissociation energy of a Si-C bond is approximately 75 kcal/mol, whereas a C-C bond is about 85 kcal/mol. The weaker bond strength makes it easier for the bond to break during the reaction, acting as a leaving group.

Understand the impact of the trimethylsilyl group

The trimethylsilyl group (Me3Si) consists of three methyl groups bonded to a silicon atom (Si(CH3)3). The presence of these three methyl groups contributes to the electron-deficient nature of the silicon, making the silicon more susceptible to electron donation from the aromatic ring during the electrophilic aromatic substitution reactions. Additionally, the steric bulk of the trimethylsilyl group may also have an effect, contributing to the ease of the leaving group departing from the aromatic ring.

Conclusion

The properties of a silicon-carbon bond that allow it to act as a good leaving group in electrophilic aromatic substitution reactions are: 1) the difference in electronegativity between silicon and carbon, which results in a polar bond with electron-deficient silicon, 2) the weaker bond strength of the Si-C bond compared to C-C bonds, and 3) the bulky trimethylsilyl group with electron-donating methyl groups that increase the electrophilic nature of the silicon center. These factors contribute to the silicon-carbon bond being more susceptible to being replaced by the incoming electrophile, CF3COOD, during the reaction.