The correct order of decreasing dipole moment of (I) toluene, (II) m-dichlorobenzene, (III) o-dichlorobenzene and (IV) p-dichlorobenzene.

IV < II < I < III

IV < I < II < III

I < IV < II < III

IV < I < III < II

To determine the order of decreasing dipole moment among toluene, m-dichlorobenzene, o-dichlorobenzene, and p-dichlorobenzene, we need to consider the molecular structure and the arrangement of substituents on the benzene ring. The dipole moment is a measure of the separation of positive and negative charges in a molecule, and it is influenced by both the electronegativity of the atoms involved and their spatial arrangement.

Analyzing Each Compound

Let's break down each compound to understand their dipole moments:

• Toluene (I): This compound has a methyl group (-CH3) attached to a benzene ring. The dipole moment is relatively low because the methyl group is non-polar and does not significantly contribute to the overall dipole of the molecule.
• m-Dichlorobenzene (II): In this structure, two chlorine atoms are positioned on the benzene ring at the meta positions. The dipole moments of the two Cl atoms do not cancel each other out completely, resulting in a moderate dipole moment.
• o-Dichlorobenzene (III): Here, the chlorine atoms are located at the ortho positions. The dipole moments of the two Cl atoms add up more effectively than in the meta position, leading to a higher dipole moment compared to m-dichlorobenzene.
• p-Dichlorobenzene (IV): In this case, the chlorine atoms are at the para positions. The dipole moments of the two Cl atoms cancel each other out to some extent, resulting in a lower dipole moment than o-dichlorobenzene but still higher than toluene.

Comparing Dipole Moments

Now, let's summarize the expected dipole moments based on our analysis:

• o-Dichlorobenzene (III) has the highest dipole moment due to the additive effect of the two Cl atoms in close proximity.
• m-Dichlorobenzene (II) follows, as the dipole moments of the Cl atoms do not cancel out completely.
• p-Dichlorobenzene (IV) comes next, where the dipole moments partially cancel each other out.
• Toluene (I) has the lowest dipole moment due to the non-polar nature of the methyl group.

Final Order of Decreasing Dipole Moment

Based on this analysis, the correct order of decreasing dipole moment is:

1. o-Dichlorobenzene (III)
2. m-Dichlorobenzene (II)
3. p-Dichlorobenzene (IV)
4. Toluene (I)

Thus, the final order is III > II > IV > I. Understanding the spatial arrangement of substituents and their electronegativities is crucial in predicting the dipole moments of aromatic compounds.