Problem 111 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following, the molecule with the highest dipole moment is (a) \(\mathrm{CH}_{3} \mathrm{Cl}\) (b) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) (c) \(\mathrm{CHCl}_{3}\) (d) \(\mathrm{CCl}_{4}\)

Step-by-Step Solution

Answer

CH₂Cl₂ has the highest dipole moment.

Step 1: Understanding the Concept of Dipole Moment

A dipole moment occurs when there is a separation of charge in a molecule due to differences in electronegativity between atoms. The greater the difference in electronegativity and the asymmetry of the molecule, the higher the dipole moment.

Step 2: Examine Electronegativity

Consider the electronegativity differences within each molecule. Chlorine (CCl) is more electronegative than carbon (C) or hydrogen (H), leading to an electron density shift towards chlorine atoms in these molecules.

Step 3: Analyze Molecular Geometry

The geometry of the molecule influences the net dipole moment. Asymmetrical molecules usually have a net dipole moment, while symmetrical molecules may have dipole moments that cancel out. CHCl₃ (trichloromethane) and CCl₄ (carbon tetrachloride) have tetrahedral geometry, but CCl₄ is symmetrical while CHCl₃ is not.

Step 4: Determine Net Dipole Moment

CCl₄ is nonpolar due to its symmetrical tetrahedral shape leading to cancellation of dipoles, thus it has a dipole moment of zero. CHCl₃, CH₂Cl₂, and CH₃Cl have dipole moments due to their less symmetric structures, but CH₂Cl₂ has the least symmetric position of Cl atoms around the central C atom, leading to a higher net dipole moment.

Step 5: Conclusion

Among the given molecules, CH₂Cl₂ (dichloromethane) has the highest dipole moment owing to its highest asymmetry and the positions of Cl atoms not fully balancing each other, allowing a stronger net dipole.

Key Concepts

Electronegativity

Electronegativity is a fundamental concept in chemistry, acting as a scale to measure how strongly an atom attracts bonding electrons. It plays a crucial role in determining the behavior of molecules and their interactions.
The differences in electronegativity between the atoms in a molecule create dipoles, which are regions of partial positive and negative charge. This can occur, for instance, in molecules with atoms that have significant electronegativity differences like chlorine and hydrogen.

• Chlorine is more electronegative than carbon or hydrogen, which means it strongly pulls bonding electrons toward itself.
• The greater the electronegativity difference between atoms, the more pronounced the dipole.

Understanding electronegativity helps in predicting the distribution of electrons in a molecule, which influences the molecule’s dipole moment and, ultimately, its chemical properties.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule. This structure is essential as it affects the molecule's physical and chemical properties, including its dipole moment.
The geometry can vary greatly depending on the number and type of atoms and the bonds between them. For example, both \\( \text{CHCl}_3 \\) and \\( \text{CCl}_4 \\) have tetrahedral shapes. However, their properties differ because of how atoms are arranged.

• Symmetrical geometries like in \\( \text{CCl}_4 \\) often result in dipole moments that cancel out, making the molecule nonpolar.
• Asymmetrical geometries like in \\( \text{CHCl}_3 \\) can lead to a net dipole moment as the electron distribution is uneven.

Understanding molecular geometry is key to predicting whether a molecule will have a balanced or unbalanced charge distribution, which affects its net dipole moment.

Net Dipole Moment

The net dipole moment is the vector sum of all individual bond dipoles in a molecule. It determines whether a molecule is polar or nonpolar. A key factor in understanding the dipole moment is the spatial arrangement of polar bonds within the molecule.
In molecules where these dipoles do not cancel each other out due to an asymmetric design, like \\( \text{CH}_2\text{Cl}_2 \\), a strong net dipole is present.

• \\( \text{CH}_2\text{Cl}_2 \\) has a geometry that prevents full cancellation of dipoles, leading to a higher net dipole moment than other molecules like \\( \text{CCl}_4 \\).
• A zero net dipole moment, as seen in symmetrical molecules like \\( \text{CCl}_4 \\), indicates a nonpolar molecule.

Examining these results allows chemists to infer properties like solubility and interactions with other substances, which are crucial for understanding reactivity and behavior in different environments.