Problem 93 - A Complete Resource Book in Chemistry for JEE Main

Question

The \(\mathrm{Cl}-\mathrm{C}-\mathrm{Cl}\) angle in \(1,1,2,2\)-tetrachloroethene and tetrachloromethane will be about (a) \(120^{\circ}\) and \(109.5^{\circ}\) (b) \(90^{\circ}\) and \(109.5^{\circ}\) (c) \(109.5^{\circ}\) and \(90^{\circ}\) (d) \(109.5^{\circ}\) and \(120^{\circ}\)

Step-by-Step Solution

Answer

The angles are (a) \(120^{\circ}\) for 1,1,2,2-tetrachloroethene and \(109.5^{\circ}\) for tetrachloromethane.

Step 1: Understanding 1,1,2,2-Tetrachloroethene

1,1,2,2-tetrachloroethene is a molecule that includes a carbon-carbon double bond. The structure is similar to ethene with chlorine atoms replacing some hydrogen atoms. In ethene, the geometry around each carbon atom is trigonal planar due to the double bond, resulting in angles close to \(120^{\circ}\). Thus, the Cl-C-Cl angle in 1,1,2,2-tetrachloroethene is expected to be about \(120^{\circ}\).

Step 2: Understanding Tetrachloromethane

Tetrachloromethane (also known as carbon tetrachloride) has a single carbon atom bonded to four chlorine atoms. This forms a tetrahedral geometry around the carbon atom. Tetrahedral angles are characterized by bond angles of \(109.5^{\circ}\).

Step 3: Matching Answers with Analysis

We identified that the Cl-C-Cl angle in 1,1,2,2-tetrachloroethene is approximately \(120^{\circ}\) and in tetrachloromethane, it is \(109.5^{\circ}\). Therefore, the correct answer choice is (a) \(120^{\circ}\) and \(109.5^{\circ}\).

Key Concepts

Trigonal Planar

The trigonal planar molecular geometry is a symmetrical arrangement found in certain molecules where a central atom is surrounded by three atoms in a flat plane. This geometry is characteristic of molecules with a central atom that contains a double bond, which leads to specific bond angle properties.
For example, in 1,1,2,2-tetrachloroethene, the presence of a carbon-carbon double bond results in a trigonal planar shape around each carbon atom. This shape is favored because the sharing of double-bond electrons gives planar symmetry and minimizes electron pair repulsion through VSEPR (Valence Shell Electron Pair Repulsion) theory.
The resulting bond angle formed between any two peripheral atoms and the central atom is about \(120^{\circ}\). This unique feature makes the trigonal planar geometry distinct because it creates a stable and optimized configuration for molecules like those containing carbon-carbon double bonds.

Tetrahedral Structure

The tetrahedral structure is one of the most straightforward and common molecular shapes in chemistry. It is observed when a central atom is bonded to four peripheral atoms, creating a shape reminiscent of a three-dimensional pyramid or diamond with a triangular base. This geometry is a consequence of the VSEPR theory, which helps predict that atoms will arrange themselves as far apart from each other as possible to minimize repulsion.
In tetrachloromethane (carbon tetrachloride, \(\text{CCl}_4\)), the central carbon atom forms four single bonds with chlorine atoms, naturally adopting a tetrahedral geometry. The spatial arrangement leads to bond angles of \(109.5^{\circ}\), which is the maximal spatial configuration for minimizing the repulsion between the bonded electron pairs.
The tetrahedral structure is highly stable and prevalent in countless organic and inorganic compounds, providing a basis for understanding more complex molecular geometries.

Carbon-Carbon Double Bond

Carbon-carbon double bonds are significant in determining the shape and reactivity of organic molecules. A double bond consists of one sigma bond and one pi bond, which together result in a stronger yet more rigid connection between the two carbon atoms.
The presence of a carbon-carbon double bond confines the bonded atoms to a trigonal planar structure, as seen in molecules like 1,1,2,2-tetrachloroethene. Unlike single bonds which allow for free rotation, the pi bond in a double bond restricts such movements, leading to a fixed structural arrangement.
This rigidity gives double-bonded carbon compounds unique chemical properties and reactivity profiles, making them vital in various chemical reactions and processes involving substances like alkenes. Understanding these double bonds is crucial for predicting the behavior and geometry of a wide array of organic compounds.