Question

Account for the fact that the \(^{29}\) Si NMR spectrum of a mixture of \(\mathrm{SiCl}_{4}\) and \(\mathrm{SiBr}_{4}\) that has been standing for \(40 \mathrm{h}\) contains five singlets which include those assigned to \(\mathrm{SiCl}_{4}\) \((\delta-19 \mathrm{ppm})\) and \(\mathrm{SiBr}_{4}(\delta-90 \mathrm{ppm})\)

Short answer

Five singlets correspond to \(\mathrm{SiCl}_4\), \(\mathrm{SiBrCl}_3\), \(\mathrm{SiBr_2Cl_2}\), \(\mathrm{SiBr}_3\text{Cl}\), and \(\mathrm{SiBr}_4\).

Step-by-step solution

Understand the Chemical Reaction

Silicon tetrachloride (\(\mathrm{SiCl}_4\)) and silicon tetrabromide (\(\mathrm{SiBr}_4\)) are both tetrahedral molecules with silicon at the center bonded to four halogens. When mixed, they can undergo a halogen exchange reaction, creating new mixed halide species such as \(\mathrm{SiBr_nCl_{4-n}}, n=0,1,2,3,4\). This can result in different chemical environments for the silicon atom.

Consider Possible Products

Since a halogen exchange can occur, the original compounds (\(\mathrm{SiCl}_4\) and \(\mathrm{SiBr}_4\)) can form mixed tetravalent silicon halides: \(\mathrm{SiCl}_4\), \(\mathrm{SiBrCl}_3\), \(\mathrm{SiBr_2Cl_2}\), \(\mathrm{SiBr_3Cl}\), and \(\mathrm{SiBr}_4\). Each of these would present a unique silicon environment, resulting in distinct NMR signals.

Assign \(\delta\) Values to Possible Compounds

In \(^29\text{Si}\) NMR, the chemical shift \(\delta\) values would vary based on the substitution pattern around the silicon atom. We already know \(\mathrm{SiCl}_4\) at \(-19\,\mathrm{ppm}\) and \(\mathrm{SiBr}_4\) at \(-90\,\mathrm{ppm}\). The three mixed halides would have shifts between these two values, and the most substituted \(\mathrm{SiCl}_n\) would likely be closest to -19, while more bromine substitution pushes it closer to -90.

Confirm Chemical Shift Count

Given five singlets in the NMR spectrum, this aligns with the presence of \(\mathrm{SiCl}_4\), \(\mathrm{SiBr}_3\text{Cl}\), \(\mathrm{SiBr_2Cl_2}\), \(\mathrm{SiBrCl}_3\), and \(\mathrm{SiBr}_4\). These five peaks imply all possible combinations of chloride and bromide ligands around silicon have formed during the 40 hours, representing five unique Si environments.

Key concepts

Halogen Exchange Reaction

The halogen exchange reaction is a fascinating process involving the swapping of halide ligands between compounds. This chemical process frequently involves silicon halides, such as silicon tetrachloride (\(\mathrm{SiCl}_4\)) and silicon tetrabromide (\(\mathrm{SiBr}_4\)). When these compounds are mixed, the halogens on the silicon atom may interchange, creating a variety of new species.
This reaction is particularly useful in synthesizing new compounds because it allows for the controlled introduction of different halogen atoms into a molecule. Halogen exchange reactions are integral in developing new materials, particularly in silicon chemistry. When silicon tetrachloride and silicon tetrabromide are left to react, they form a variety of silicon halides, such as \(\mathrm{SiBr_nCl_{4-n}}, n=0,1,2,3,4\). This creates a diverse range of silicon-centered environments, which can be analyzed and studied further through NMR spectroscopy.

Silicon Halides

Silicon halides, such as silicon tetrachloride (\(\mathrm{SiCl}_4\)) and silicon tetrabromide (\(\mathrm{SiBr}_4\)), are compounds where silicon is bonded to halogens. These are tetrahedral molecules due to the spatial arrangement of the halogen atoms around the silicon.
Silicon halides are crucial in many industrial and chemical processes. They serve as starting materials in the synthesis of silicones, which are widely used in various applications, including sealants, lubricants, and medical devices.
The reactivity of silicon halides towards halogen exchange renders them useful in creating different mixed halides, each possessing unique properties and functionalities. The presence of various halide atoms in these compounds influences not just the shape, but also the chemical behavior of the molecules. As silicon atoms interact with these halides, different molecular environments arise, making them a prime subject for molecular structure analysis.

Chemical Shifts in NMR

In NMR spectroscopy, chemical shifts are pivotal in identifying different environments around a nucleus, such as silicon in silicon halides. For \(^{29}\text{Si}\) NMR, these shifts are measured in parts per million (ppm) from a standard reference point and provide detailed insights into the molecular structure.
In the case of the silicon halides mixture, \(\mathrm{SiCl}_4\) is observed at \(-19 \text{ ppm}\) and \(\mathrm{SiBr}_4\) at \(-90 \text{ ppm}\). These shifts are influenced by the electronic environment around silicon, with chlorine creating a different interaction compared to bromine.
As the halogen exchange proceeds, the chemical shifts of the resulting species are expected to lie between these two endpoint values. Deciphering these shifts enables chemists to deduce the molecular structure and the presence of different halogens around the silicon nucleus. Hence, \(^{29}\text{Si}\) NMR serves as an indispensable tool in studying silicon-containing compounds.

Molecular Structure Analysis

Molecular structure analysis involves determining the arrangement of atoms within a molecule, a task made straightforward by techniques like \(^{29}\text{Si}\) NMR spectroscopy. Understanding the molecular structure of silicon halides requires knowing where each bromide and chloride sits around the silicon center.
The \(^{29}\text{Si}\) NMR spectra can reveal distinct lines corresponding to each unique silicon environment, indicative of different molecular structures present in the sample. In the experiment discussed, five singlets represent configurations from pure chlorides to mixed halides and pure bromides around silicon.

• \(\mathrm{SiCl}_4\)
• \(\mathrm{SiBrCl}_3\)
• \(\mathrm{SiBr_2Cl_2}\)
• \(\mathrm{SiBr_3Cl}\)
• \(\mathrm{SiBr}_4\)

This analysis provides significant insight into the nature of molecular interactions and can lead to a greater understanding of the material’s chemical behavior. Consequently, \(^{29}\text{Si}\) NMR is a critical part of the toolbox for chemists who regularly study silicon-based compounds.