Chapter 9: Problem 91
The compounds carbon tetrachloride \(\left(\mathrm{CCl}_{4}\right)\) and silicon tetrachloride \(\left(\mathrm{SiCl}_{4}\right)\) are similar in geometry and hybridization. However, \(\mathrm{CCl}_{4}\) does not react with water but \(\mathrm{SiCl}_{4}\) does. Explain the difference in their chemical reactivities. (Hint: The first step of the reaction is believed to be the addition of a water molecule to the \(\mathrm{Si}\) atom in \(\mathrm{SiCl}_{4}\).)
Short Answer
Expert verified
SiCl4 reacts with water due to available d-orbitals in silicon, enabling hydrolysis, unlike CCl4.
Step by step solution
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Tetrahedral Geometry
Tetrahedral geometry is a key concept in understanding many molecular structures. In a tetrahedral arrangement, a central atom is connected to four surrounding atoms positioned in the shape of a pyramid with triangular faces. Both carbon tetrachloride \((\mathrm{CCl}_{4})\) and silicon tetrachloride \((\mathrm{SiCl}_{4})\) have this geometry. This shape is highly symmetrical and provides a stable structure.
The angles between the bonds in this geometry are about 109.5 degrees. This is ideal for minimizing repulsion between the bonded electron pairs, leading to a stable arrangement. In tetrahedral molecules like \((\mathrm{CCl}_{4})\) and \((\mathrm{SiCl}_{4})\), the central atom (carbon or silicon) forms four covalent bonds with chlorine atoms, giving rise to this symmetrical structure.
The angles between the bonds in this geometry are about 109.5 degrees. This is ideal for minimizing repulsion between the bonded electron pairs, leading to a stable arrangement. In tetrahedral molecules like \((\mathrm{CCl}_{4})\) and \((\mathrm{SiCl}_{4})\), the central atom (carbon or silicon) forms four covalent bonds with chlorine atoms, giving rise to this symmetrical structure.
sp3 Hybridization
The concept of \(sp^3\) hybridization helps explain the bonding and geometry of molecules like \((\mathrm{CCl}_{4})\) and \((\mathrm{SiCl}_{4})\). During hybridization, one s orbital and three p orbitals mix to form four equivalent hybrid orbitals. These \(sp^3\) orbitals arrange themselves in a tetrahedral shape.
This hybridization results in strong sigma bonds with each chlorine atom, contributing to the molecule's stability. In both carbon and silicon compounds, this type of hybridization provides an understanding of how the atoms share electrons to create a solid molecular structure.
This hybridization results in strong sigma bonds with each chlorine atom, contributing to the molecule's stability. In both carbon and silicon compounds, this type of hybridization provides an understanding of how the atoms share electrons to create a solid molecular structure.
Silicon d-Orbitals
Silicon's ability to react with water is connected to its empty d-orbitals. Unlike carbon, silicon can utilize its 3d orbitals to expand its octet. This capacity makes \((\mathrm{SiCl}_{4})\) chemically reactive in ways that \((\mathrm{CCl}_{4})\) is not.
When water interacts with \((\mathrm{SiCl}_{4})\), the lone pairs on the oxygen atom can bond with silicon. The existence of empty or partially filled d-orbitals allows silicon to accommodate more electrons, enabling reactions such as hydrolysis. This feature is crucial for understanding why silicon compounds like \((\mathrm{SiCl}_{4})\) can undergo chemical changes.
When water interacts with \((\mathrm{SiCl}_{4})\), the lone pairs on the oxygen atom can bond with silicon. The existence of empty or partially filled d-orbitals allows silicon to accommodate more electrons, enabling reactions such as hydrolysis. This feature is crucial for understanding why silicon compounds like \((\mathrm{SiCl}_{4})\) can undergo chemical changes.
Carbon-Chlorine Bond
Carbon-chlorine bonds in \((\mathrm{CCl}_{4})\) are notably strong due to the electronegativity of chlorine and the stable hybridization of carbon. In general, the bond strength makes these compounds resistant to reactions like hydrolysis.
The lack of vacant orbitals in carbon means that it cannot easily accommodate extra electrons or bonds. This stability is why \((\mathrm{CCl}_{4})\) does not react with water. The strength and resistance of these bonds are central to understanding why some carbon compounds are inert in specific chemical conditions.
The lack of vacant orbitals in carbon means that it cannot easily accommodate extra electrons or bonds. This stability is why \((\mathrm{CCl}_{4})\) does not react with water. The strength and resistance of these bonds are central to understanding why some carbon compounds are inert in specific chemical conditions.
Hydrolysis Reaction
Chemical reactions like hydrolysis involve the breaking of bonds in the presence of water. In \((\mathrm{SiCl}_{4})\), hydrolysis begins with the addition of water molecules to the silicon atom. This process is facilitated by silicon's empty d-orbitals.
Once a bond forms between the lone pair on oxygen and silicon, chlorine atoms can be replaced by hydroxyl groups. The transformation of \((\mathrm{SiCl}_{4})\) into silicic acid \((\mathrm{Si(OH)}_{4})\) illustrates how silicon's electronic configuration enables it to react with water, unlike carbon compounds with more stable bonds.
Once a bond forms between the lone pair on oxygen and silicon, chlorine atoms can be replaced by hydroxyl groups. The transformation of \((\mathrm{SiCl}_{4})\) into silicic acid \((\mathrm{Si(OH)}_{4})\) illustrates how silicon's electronic configuration enables it to react with water, unlike carbon compounds with more stable bonds.
