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\. \(\mathrm{m}\)-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

(b) \(4<1<2<3\)

Step 1: Understanding Dipole Moments

The dipole moment of a molecule is a measure of the separation of electric charge and depends on both the individual bond dipole moments and the molecular geometry. Keep in mind that symmetrical molecules often have a net dipole moment of zero while unsymmetrical molecules will typically have a nonzero dipole moment.

Step 2: Analyze Structure and Symmetry

1\. **Toluene:** Contains a single methyl group attached to benzene, slightly asymmetrical but low dipole due to weak polarity of the \(-\mathrm{CH_3}\) group. 2\. **m-Dichlorobenzene:** The m- (meta) position leads to partial cancellation of dipole moments of the chlorine atoms, moderate dipole moment.3\. **o-Dichlorobenzene:** The o- (ortho) position creates a non-canceling structure leading to a higher net dipole moment due to proximity of Cl atoms.4\. **p-Dichlorobenzene:** The p- (para) position results in complete dipole moment cancellation; nearly zero dipole moment.

Step 3: Order of Increasing Dipole Moment

Based on the analysis, the order of increasing dipole moment is as follows: - **p-Dichlorobenzene** has the smallest dipole moment due to perfect cancellation. - **Toluene** has a low dipole moment due to weak polarity of the methyl. - **m-Dichlorobenzene** has a moderate dipole moment due to partial cancellation of chlorine atoms. - **o-Dichlorobenzene** has the highest dipole moment because the proximity of the chlorines leads to stronger net polarity.

Step 4: Match with Given Options

Based on the analysis, the correct order is: p-Dichlorobenzene < Toluene < m-Dichlorobenzene < o-Dichlorobenzene. Seeing the options given, this matches with option (b).

Key Concepts

Molecular Geometry

Molecular geometry is the three-dimensional arrangement of atoms within a molecule. It plays a crucial role in determining the molecule's physical and chemical properties.
Understanding molecular geometry helps us predict molecule polarity, reactivity, phase of matter, color, magnetism, and biological activity.
To consider molecular geometry:

• Look at the spatial arrangement of atoms.
• Observe bond lengths and angles.
• Consider the number of electron pairs VSEPR theory can be used to predict shapes, based on electron pair repulsions.

In compounds such as dichlorobenzenes, varied substitutions lead to meta, ortho, and para geometries. These positions alter the molecular symmetry and influence the compound's overall dipole moment. For instance, in ortho-dichlorobenzene, the close proximity of chlorine atoms enhances the dipole moment due to limited symmetry.

Symmetry in Molecules

Symmetry in molecules refers to a molecule's balanced arrangement of its constituent atoms and electron clouds.
This concept is vital because it helps determine molecular properties like polarity, and chemical reactivity, and can even influence the crystal packing in solid states.
Molecular symmetry can be:

• Linear: For example, carbon dioxide (${CO}_2$) which has a linear, symmetric structure.
• Planar: Often seen in molecules like benzene, where molecules lie flat.
• Tetrahedral: Such as methane (${CH}_4$), where there is uniform distribution of atoms around the central atom.

In dichlorobenzenes, the para isomer shows complete symmetry between chlorine atoms resulting in zero net dipole moment due to perfect cancellation of dipole vectors. This symmetry causes para-dichlorobenzene to have lower polarity compared to its ortho or meta counterparts.

Polarity of Chemical Compounds

Polarity in chemical compounds arises when there is an uneven distribution of electrons between atoms, leading to areas of positive and negative charges. This is pivotal in understanding how molecules will interact with one another, with solvents, and in biological systems.
Key factors affecting polarity include:

• Differences in electronegativity: Larger differences usually result in a more polar molecule.
• Molecular geometry and symmetry: Symmetrical molecules tend to be nonpolar even if they contain polar bonds due to balanced charge distribution.
• Presence of polar bonds: Bonds between atoms with significant electronegativity differences will affect overall molecular polarity.

In the case of dichlorobenzenes, ortho positions of chlorine atoms do not allow for polarity cancellation, resulting in a higher dipole moment. Meanwhile, the para configuration enables perfect symmetry, leading to no net dipole moment.