Problem 141 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following halogen containing compounds (i) \(\mathrm{CHCl}_{3}\) (ii) \(\mathrm{CCl}_{4}\) (iii) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) (iv) \(\mathrm{CH}_{3}^{+} \mathrm{Cl}\) (v) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}_{2}\) The compounds with a net zero dipole moment are (a) (ii) and (v) only (b) (iii) only (c) (iii) and (iv) only (d) (iv) only

Step-by-Step Solution

Answer

The compounds with a net zero dipole moment are (ii).

Step 1: Understand Dipole Moments

The dipole moment of a molecule depends on the difference in electronegativity between atoms and the geometry of the molecule. A molecule will have a net zero dipole moment if it is symmetrical and the polarities within the molecule cancel each other out.

Step 2: Analyze Each Compound

For each compound, we must examine its molecular geometry and the arrangement of its atoms to determine if the polarities cancel out:- **(i) \( \text{CHCl}_3 \)**: Tetrahedral geometry but asymmetric due to different atoms; it has a net dipole moment.- **(ii) \( \text{CCl}_4 \)**: Tetrahedral geometry with symmetrical arrangement of chloride atoms; it has no net dipole moment.- **(iii) \( \text{CH}_2\text{Cl}_2 \)**: Tetrahedral geometry but asymmetrical; it has a net dipole moment.- **(iv) \( \text{CH}_3^+\text{Cl} \)**: Linear geometry due to open valency in the carbocation; it has a net dipole moment.- **(v) \( \text{C}_6\text{H}_5\text{Cl}_2 \)**: Assuming ortho, meta, or para substitution, these usually result in dipole moment due to asymmetric substitution in the benzene ring.

Step 3: Identify Symmetrical Compounds

From the analysis:- **(ii) \( \text{CCl}_4 \)** is symmetrical with a zero net dipole moment as the polarities cancel.- **(v) \( \text{C}_6\text{H}_5\text{Cl}_2 \)** can sometimes be symmetrical (para), thus having a zero dipole moment. Despite this, without a specific conformation, it is generally assumed not to be fully symmetrical.

Step 4: Choose the Correct Option

Among the options provided, most accurately based on the typical consideration of symmetry and substitution:The correct option is (a) which lists \( \text{CCl}_4 \) as a net zero dipole moment compound.

Key Concepts

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It is a crucial factor in determining the properties of a molecule, including its dipole moment and reactivity. The molecular geometry is primarily defined by the number of electron pairs surrounding a central atom, which includes both bonding and non-bonding pairs.

• Tetrahedral Geometry: In a tetrahedral geometry, which is common for carbon compounds like \( ext{CH}_2 ext{Cl}_2\) and \( ext{CCl}_4\), the central atom is bonded to four atoms arranged symmetrically in 3D. This gives a bond angle of approximately 109.5 degrees.
• Linear Geometry: As seen in \( ext{CH}_3^+ ext{Cl}\), the presence of a carbocation leads to fewer electrons around the central atom, often resulting in a linear geometry with 180-degree angles.

Understanding the geometry helps in predicting whether a molecule is symmetrical or not, and thus whether its dipole moments will cancel out.

Electronegativity

Electronegativity is the tendency of an atom to attract electrons in a chemical bond. In the context of dipole moments, it is a significant factor because the difference in electronegativity between atoms in a bond can introduce polarity.

• Polar Bonds and Dipoles: When atoms in a molecule like \( ext{CH}_2 ext{Cl}_2\) have differing electronegativities, the electrons are not shared equally, resulting in a polar bond. The molecule develops a dipole moment, which is a measure of the separation of positive and negative charges.
• Electronegativity Values: Halogens, such as chlorine in \( ext{CCl}_4\), are highly electronegative and tend to attract electrons from less electronegative atoms like carbon, potentially creating strong dipoles if not symmetrically arranged.

Understanding electronegativity helps explain why certain molecules have a dipole moment and others do not.

Symmetrical Molecules

Symmetrical molecules are those in which the distribution of atoms around the central atom is balanced, leading to cancellation of any dipole moments. Symmetry is a key factor in determining if a molecule will have a net dipole moment.

• Cancellation of Dipoles: In symmetrical molecules like \( ext{CCl}_4\), the dipoles from the four \( ext{C-Cl}\) bonds cancel out because they point in opposite directions. This results in a zero net dipole moment.
• Impact of Substitution: For molecules like \( ext{C}_6 ext{H}_5 ext{Cl}_2\), the symmetry can be disrupted by substitution patterns, such as ortho or meta, which prevents the cancellation of dipoles unless in specific configurations like the para position.

Understanding symmetry in molecular structures is essential for predicting the presence or absence of a dipole moment.

Halogen Compounds

Halogen compounds are organic molecules that contain one or more halogen atoms (such as chlorine, bromine, or iodine). These compounds often exhibit interesting chemical properties due to the high electronegativity of halogens.

• Reactivity and Properties: Halogens are highly reactive, which makes halogen compounds, such as \(\text{CHCl}_3\) or \(\text{CCl}_4\), versatile in a range of chemical reactions. They can participate in nucleophilic substitution reactions due to their tendency to attract electrons.
• Polar Nature: The halogen’s high electronegativity introduces polarity into the molecule. This polarity affects the physical properties, like boiling and melting points, and the solubility of the compound in various solvents.

Understanding the role of halogens in molecules helps to predict how these compounds behave chemically and physically.