Problem 154 - A Complete Resource Book in Chemistry for JEE Main

Question

Arrange the following compounds in order of increasing dipole moment. \(\mathrm{Cl}\) Clc1cccc(Cl)c1Cl Clc1ccc(Cl)cc1 Clc1ccc(Cl)c(Cl)c1 (I) (II) (III) ClC=C=C=CCl Clc1cc(Cl)cc(Cl)c1 (IV) (V) (a) \(\mathrm{II}=\mathrm{IV}<\mathrm{V}<\mathrm{I}<\mathrm{III}\) (b) \(\mathrm{IV}<\mathrm{III}<\mathrm{I}\) (c) \(\mathrm{II}<\mathrm{IV}<\mathrm{III}<\mathrm{I}<\mathrm{V}\) (d) \(\mathrm{V}=\mathrm{II}=\mathrm{IV}\)

Step-by-Step Solution

Answer

The correct order is (a) II = IV < V < I < III.

Step 1: Understand Dipole Moment

The dipole moment is a measure of the separation of charge in a molecule, which occurs due to differences in electronegativity between atoms. Polar bonds contribute to a molecule's dipole moment, and the net dipole moment depends on both individual bond moments and molecular geometry.

Step 2: Analyze Individual Compounds and Their Structures

We need to consider how the arrangement of chlorine atoms affects electronegativity and geometry to determine the dipole moment: 1. Compound (I): Contains three chlorine atoms on a benzene ring. 2. Compound (II): Contains two chlorine atoms on a benzene ring at positions 1,4. 3. Compound (III): Contains three chlorines on a benzene ring at positions 1,2,4. 4. Compound (IV): Linear structure compound with chlorines at each end. 5. Compound (V): Contains three chlorines on a benzene ring at positions 1,3,5.

Step 3: Compare Symmetry and Distribution of Atoms

Symmetry in a molecule influences the net dipole moment. - Molecules with higher symmetry generally have lower dipole moments because their bond dipoles cancel each other. - Compound (II) and (IV) are highly symmetrical, leading to lower dipole moments. - Compounds with asymmetric distribution or more polar bonds will generally have higher dipole moments.

Step 4: Evaluate Each Compound

Let's evaluate: - Compound (II): Symmetrical (1,4-dichlorobenzene), expected low dipole moment. - Compound (IV): Symmetrical linear structure, expected low dipole moment. - Compound (V): 1,3,5-trichlorobenzene, symmetrical, dipole moments cancel. - Compound (I): Three chlorines, less symmetry than (II), higher than (II). - Compound (III): Asymmetric, with chlorines at 1,2,4 positions, expected highest dipole moment among these.

Step 5: Arrange the Compounds in Order of Increasing Dipole Moment

Based on our analysis, we arrange the compounds from lowest to highest dipole moment: - II = IV < V < I < III.

Key Concepts

Molecular Geometry

Molecular geometry is like a 3D blueprint of a molecule. It tells us how atoms are arranged in space relative to one another. This arrangement is crucial because it affects physical and chemical properties. For instance, the geometry can influence the type and strength of intermolecular forces as well as the molecule's reactivity.
In simple terms, understanding geometry helps us figure out how molecules interact with each other and with other substances. Different geometric shapes arise from the number of atoms bonded and the presence of lone pairs of electrons. For example:

• A molecule with two atoms is linear.
• Three atoms can form an angular or a planar shape.
• Tetrahedral is for more complex shapes, like when you have four atoms bonded.

The shape can also impact the dipole moment, as we saw in the benzene derivatives with chlorine atoms. The symmetry in compounds like dichlorobenzene plays a significant role in determining the molecular geometry, which in turn influences their chemical behavior.

Electronegativity

Electronegativity refers to an atom's ability to attract and hold onto electrons within a bond. Think of it as a tug-of-war where stronger atoms have a better grasp on the shared electrons.
The higher the electronegativity of an atom, the more it tends to pull electrons toward itself in a bond. Chlorine is a good example of a highly electronegative element, which is why it's often involved in polar bonds. In our compounds, chlorine's electronegativity is key to creating differences in electron density and thus causing dipole moments. To better understand electronegativity, remember these points:

• Fluorine is the most electronegative element.
• Electronegativity generally increases across a period and decreases down a group in the periodic table.
• The difference in electronegativity between bonded atoms determines if a bond is nonpolar covalent, polar covalent, or ionic.

Knowing these basics enriches comprehension of why certain molecules behave the way they do.

Symmetry in Molecules

Symmetry is nature's way of packing molecules efficiently. In chemical terms, symmetry can mask or reveal interesting properties of a molecule. When it comes to dipole moments, symmetry—or lack thereof—can either nullify or enhance them.
Highly symmetrical molecules often have no net dipole moment since the polar bonds may counterbalance each other. Let’s break it down further:

• If a molecule is symmetric, like a perfect circle, its dipoles cancel out, resulting in no net dipole moment.
• Less symmetry or asymmetrical molecules have uneven electron distribution, yielding a higher dipole moment.

For instance, compound (II) dichlorobenzene, with its symmetric 1,4 positions, shows low dipole moment as its bond dipoles cancel. Meanwhile, in asymmetrical arrangements like compound (III), net dipole moments arise due to different spatial placements of atoms, leading to significant polarity.

Benzene Derivatives

Benzene is a cornerstone in organic chemistry. Derivatives of benzene play a major role in understanding chemical reactivity and properties. When other atoms or groups replace hydrogen atoms on a benzene ring, benzene derivatives are formed.
Benzene itself is aromatic, showcasing a unique stability due to its structure, involving six carbon atoms forming a planar hexagonal ring. Substituents on benzene, like chlorine in our compounds, can influence the molecule's properties significantly.

• They can either be electron-donating, which increases electron density on the benzene ring, or electron-withdrawing, decreasing it.
• This affects the reactivity of the benzene and its derivatives.

The arrangement of substituents also impacts molecular geometry, which can alter physical properties like dipole moment and boiling point. Understanding derivatives of benzene helps in predicting behaviors in chemical reactions and applications in industrial processes.

Polarity in Chemistry

Polarity in chemistry speaks to the distribution of electric charges within a molecule. This determines how molecules interact with each other and their environments. It's like a dance between positives and negatives.
Polar molecules have an uneven distribution of electron density, resulting in a dipole moment, where one side is slightly positive and the other is slightly negative. Here's how polarity emerges:

• Dipole moments are vector quantities—meaning they have both magnitude and direction.
• The spatial arrangement and bonding relationships in the molecule influence polarity.
• The bigger the difference in electronegativity between atoms, typically, the more polar the bond.

Understanding polarity is important as it affects everything from solubility to boiling points, reactivity, and intermolecular forces. In our exercise, the molecular geometry and the electronegativity differences of atoms generated varying dipole moments, demonstrating the impact of polarity.