Question

Dichlorobenzene exists in three isomers-ortho, meta and para isomers. Out of these three isomers, one with highest dipole moment will be (a) ortho-isomer (b) meta-isomer (c) para-isomer (d) all will have the same dipole moment.

Short answer

The isomer with the highest dipole moment is the ortho-isomer.

Step-by-step solution

Understand the molecular structures of isomers

The three isomers of dichlorobenzene have different structures. The ortho-isomer has two chlorine atoms on the adjacent carbons, the meta-isomer has the chlorine atoms separated by one carbon atom, and the para-isomer has the chlorine atoms on opposite sides of the benzene ring.

Analyze the dipole moments

Each C-Cl bond in dichlorobenzene has a polarity due to a difference in electronegativity between Chlorine and Carbon, giving rise to individual dipole moments. The total dipole moment of a molecule is resultant of the vector sum of these individual dipole moments. For ortho and meta-isomers, two dipole moments don't cancel each other out, due to their position and angle. However, in the para-isomer, the two chlorine atoms are symmetrically placed and two dipole moments are of same magnitude but opposite in direction, hence they cancel each other out.

Identify the isomer with the highest dipole moment

Based on the orientations of dipole moments, the ortho-isomer and meta-isomer have discernable dipole moments while the para-isomer has a net dipole moment of zero. Between ortho and meta, because ortho isomer's dipoles are closer hence giving rise to a higher overall dipole moment. Thus, the ortho-isomer should have the highest dipole moment.

Key concepts

Dipole Moment

The concept of the dipole moment is paramount in understanding how molecules interact with electric fields and nearby particles. In simple terms, the dipole moment of a molecule reflects the extent of charge separation within the molecule. This separation occurs because atoms in a molecule can have differing electronegativities, or tendencies to attract electrons. When two atoms form a bond, the more electronegative atom will draw electrons closer to itself, creating a partial negative charge (denoted as \( \delta- \) ), while leaving the less electronegative atom with a partial positive charge (denoted as \( \delta+ \) ).

The resulting polarity from this charge imbalance is what we describe as a dipole moment, and it is measured in Debye units. For molecules with more than two atoms, like dichlorobenzene, each C-Cl bond has its own dipole moment, arising from chlorine's higher electronegativity compared to carbon. The overall dipole moment of the molecule is then determined by how these individual dipoles align and interact within the molecule's three-dimensional structure.

Molecular Structures of Isomers

Isomers are molecules with the same molecular formula but different molecular structures. These structural differences are crucial as they can lead to varying physical and chemical properties. With dichlorobenzene, the three isomers are distinguished by the positions of the chlorine atoms on the benzene ring.

• The ortho-isomer has the chlorine atoms on adjacent carbon atoms.
• The meta-isomer has the chlorine atoms separated by one carbon atom.
• The para-isomer has the chlorine atoms opposite each other on the benzene ring.

This positioning affects the molecule's symmetry, which, in turn, influences the dipole moment. While all three isomers have the same formula, the differing geometrical arrangements lead to the distinct possibility of dipoles either reinforcing each other or canceling out.

Vector Sum of Dipoles

To determine the total dipole moment in a molecule like dichlorobenzene, we consider the vector sum of dipoles. Dipole moments are vector quantities, meaning they have both magnitude and direction. When multiple bonds with dipole moments are present in a molecule, their individual vectors must be summed to get the overall dipole moment.

In the case of dichlorobenzene, the ortho and meta-isomers have vectors that don't align symmetrically. Hence, their dipole moments don't fully cancel each other out. For the ortho-isomer, the closeness of the dipoles leads to a strengthening of the resultant dipole. On the other hand, the para-isomer's symmetrically opposed chloride atoms lead their dipoles to cancel out, resulting in a net dipole moment of zero. Therefore, the analysis of dipole moments in terms of their vector properties is key to predicting the molecule's behavior in an electromagnetic field.