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 trimethylsilyl group is an effective leaving group in electrophilic aromatic substitution reactions due to several factors, including the bond polarity resulting from silicon's greater electropositivity compared to carbon, the longer and weaker Si-C bond due to silicon's larger atomic size, and the stability of the trimethylsilyl leaving group provided by the three methyl groups. These properties collectively contribute to the reactivity of the silicon-carbon bond and make the trimethylsilyl group an excellent leaving group in EAS reactions.

Step-by-step solution

Identify the electrophilic aromatic substitution reaction

The given reaction is an electrophilic aromatic substitution (EAS) where the trimethylsilyl group in \(\mathrm{Me}_{3} \mathrm{SiC}_{6} \mathrm{H}_{5}\) acts as a leaving group. The EAS reaction involves the replacement of a leaving group (in our case, the trimethylsilyl group) with another electrophile (in our case, the deuterium from \(\mathrm{CF}_{3} \mathrm{COOD}\)) on the aromatic ring.

Analyze the properties of silicon

Silicon is located in the same group as carbon in the periodic table (Group 14). However, it is larger in size and possesses some unique properties compared to carbon. These properties include its higher electropositivity, larger atomic size, and the ability to form weaker bonds with elements like oxygen.

Discuss the silicon-carbon bond polarity

Silicon is more electropositive than carbon, resulting in a polar Si-C bond where the electron density is shifted towards silicon. This effect, in turn, makes the leaving group electron-deficient, which is a desirable property for leaving groups in electrophilic aromatic substitution reactions.

Analyze the bond strength

The larger atomic size of silicon compared to carbon results in a longer Si-C bond. Longer bonds are generally weaker, meaning the Si-C bond is weaker compared to a C-C bond, making it easier for the bond to break during the reaction. Moreover, Silicon can accommodate the negative charge on the leaving group more effectively due to its larger atomic size, further stabilizing the transition state during the reaction.

Examine the stability of the leaving group

The trimethylsilyl group, \(\mathrm{Me}_{3} \mathrm{Si}- \), can form a stable species upon leaving the aromatic ring, as the three methyl groups, when combined with the partial negative charge on silicon, provide stability in the form of anions or silyl ethers.

Conclusion

The reactivity of the silicon-carbon bond in electrophilic aromatic substitution reactions can be attributed to several factors including the bond polarity due to silicon's greater electropositivity, the relatively weaken Si-C bond as compared to C-C bond due to larger atomic size, and the stability of the trimethylsilyl leaving group. These properties make the trimethylsilyl group an excellent leaving group in EAS reactions.