Problem 145 - A Complete Resource Book in Chemistry for JEE Main

Question

Arrange the following compounds in order of increasing dipole moment 1\. Toluene, 2\. m-dichlorobenzene 3\. o-dichlorobenzene, 4\. p-dichlorobenzene (a) \(1<4<2<3\) (b) \(4<1<2<3\) (c) \(4<1<3<2\) (d) \(4<2<1<3\)

Step-by-Step Solution

Answer

The order is 4 < 1 < 2 < 3.

Step 1: Understand the concept of dipole moment

Dipole moment is a measure of the separation of positive and negative charges in a molecule. It arises from differences in electronegativity between bonded atoms and the geometric arrangement of atoms.

Step 2: Analyze the structure of toluene

Toluene consists of a benzene ring with a single methyl group attached. This group is non-polar and does not significantly contribute to the dipole moment, making toluene's dipole moment relatively low.

Step 3: Consider the structure of p-dichlorobenzene

In p-dichlorobenzene, two chlorine atoms are attached opposite each other on a benzene ring. The symmetry of the para position causes the individual dipole moments from the chlorines to cancel each other out, resulting in a very low net dipole moment.

Step 4: Examine m-dichlorobenzene

In m-dichlorobenzene, the chlorine atoms are positioned meta to each other. This arrangement leads to a partial cancellation of dipole moments, but not entirely, giving it a moderate dipole moment.

Step 5: Evaluate o-dichlorobenzene

In o-dichlorobenzene, the two chlorine atoms are adjacent to each other. This setup produces a cumulatively higher dipole moment compared to meta and para configurations as the individual dipoles do not cancel out.

Step 6: Arrange based on dipole moment

Comparing the positions of chlorine atoms, p-dichlorobenzene (lowest dipole) < toluene (low dipole) < m-dichlorobenzene (moderate dipole) < o-dichlorobenzene (highest dipole). Hence, the correct order is 4 < 1 < 2 < 3.

Key Concepts

Structure and Bonding

Molecules are structures composed of atoms bonded together. Bonds form as atoms share or transfer their outer electrons to achieve stable electron configurations. There are different types of bonding in molecules:

• Covalent Bonding: Atoms share electrons, as seen in organic compounds like toluene and dichlorobenzene. The overlap of atomic orbitals from each atom forms a molecule.
• Polar and Nonpolar Bonds: This relates to the sharing of electrons. Nonpolar bonds involve equal sharing, while polar bonds have unequal sharing due to differing electronegativities between bonded atoms.

Bonding affects physical properties like polarity and dipole moment. For example, o-dichlorobenzene has chlorines in such a position that their individual dipoles contribute to a significant overall dipole moment. Understanding the arrangement and type of bonding helps predict properties such as solubility and reactivity of a compound.

Electronegativity

Electronegativity is an atom's ability to attract and hold onto electrons. This property is crucial in determining how electron pairs are shared between atoms in a molecule.

• Elements like chlorine have high electronegativities, which means they strongly attract electrons. This can lead to the creation of polar bonds when bonded to atoms with lower electronegativity.
• The difference in electronegativity between atoms affects the dipole moment of a molecule. Larger differences result in stronger dipoles. For instance, in dichlorobenzene isomers, chlorine's high electronegativity versus carbon's leads to notable polar characteristics.

Dipole moments arise from differences in electronegativity, directing the electron density towards more electronegative atoms, influencing molecular polarity. This interaction is essential for understanding chemical behavior and interactions with other molecules.

Molecular Geometry

Molecular geometry is the three-dimensional arrangement of atoms within a molecule, greatly influencing its physical and chemical properties.

• Symmetry: Symmetrical molecules, such as p-dichlorobenzene, have their dipoles cancel out, resulting in no net dipole moment. Symmetry leads to non-polarity.
• Angle and Position: The angles and relative positions of atoms can enhance or reduce polarity. For example, o-dichlorobenzene has atoms positioned such that dipoles add together, increasing the net dipole moment compared to symmetrical arrangements.

Different molecular geometries can thus lead to varying levels of dipole moments. By visualizing the shape of a molecule, one can predict polarity and understand intermolecular forces such as hydrogen bonding, van der Waals forces, and dipole-dipole interactions in chemical reactions and molecular interactions.