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

Short answer

(b) 4<1<2<3

Step-by-step solution

Understand Dipole Moments

Dipole moment is a vector quantity that represents the polarity of a molecule. It depends on the difference in electronegativity between atoms and the geometry of the molecule. Molecules with asymmetric electron distributions typically have higher dipole moments.

Analyze Toluene

Toluene is an aromatic compound with a methyl group attached to the benzene ring. The dipole moment here is mainly due to the slight electron-donating effect of the methyl group, making toluene less polar compared to dichlorobenzene isomers.

Analyze p-dichlorobenzene

In p-dichlorobenzene, the two chlorine atoms are opposite each other. This symmetrical arrangement leads to the dipole moments canceling each other out, resulting in a relatively low dipole moment.

Analyze m-dichlorobenzene

In m-dichlorobenzene, the chlorine atoms are situated on the 1st and 3rd positions of the benzene ring, creating a slightly polar structure with moderate dipole moment due to an asymmetric distribution of electronegative atoms.

Analyze o-dichlorobenzene

For o-dichlorobenzene, the chlorine atoms are adjacent, resulting in the most polar structure of the three isomers. The dipole moments augment each other, leading to the highest dipole moment among the isomers.

Arrange In Order

Now, based on the analyses: - Toluene has a low dipole moment due to minimal polarity. - p-dichlorobenzene has low dipole moment due to cancellation. - m-dichlorobenzene has a higher dipole moment because of its geometry. - o-dichlorobenzene has the highest dipole moment due to maximum asymmetry. Thus, the increasing order is: p-dichlorobenzene < toluene < m-dichlorobenzene < o-dichlorobenzene.

Key concepts

Toluene

Toluene, a simple aromatic hydrocarbon, plays an essential role in understanding molecular polarity as it only possesses slight polar characteristics. It consists of a benzene ring with a single methyl group attached. The methyl group can slightly donate electrons to the benzene ring due to its structure, but because the benzene ring itself is nonpolar, toluene as a whole has a minimal dipole moment.
Toluene's low polarity translates into a relatively low dipole moment when compared to other compounds with more electronegative elements or different arrangements, like dichlorobenzene isomers. This makes toluene less polar.
In exercises involving order of dipole moments, toluene typically falls behind compounds with electronegative substitutions which are key factors in polarity and dipole moment strength. Understanding toluene's position in terms of molecular polarity gives us insight into how subtle changes in molecular structure can impact physical properties.

Dichlorobenzene Isomers

Dichlorobenzene isomers refer to the three different arrangements of chlorine atoms attached to a benzene ring: ortho (o-), meta (m-), and para (p-). These arrangements can drastically alter the polarity and, consequently, the dipole moment of the compound.

• **o-Dichlorobenzene**: Features chlorine atoms adjacent to each other on the benzene ring. This proximity results in their dipole moments reinforcing one another, leading to the highest dipole moment among the isomers.
• **m-Dichlorobenzene**: The chlorine atoms are positioned with one carbon atom between them. This asymmetrical placement creates a less aligned dipole compared to the ortho configuration but still retains more polarization than the para isomer.
• **p-Dichlorobenzene**: The chlorine atoms are opposite each other. This setup is symmetric, causing the dipole moments from the chlorine atoms to effectively cancel out each other, resulting in the lowest dipole moment.

The differences in these isomers highlight how crucial both the number and arrangement of electronegative atoms are in determining molecular polarity.

Molecular Polarity

Molecular polarity arises from the uneven distribution of electrons within a molecule. It is a crucial factor that dictates many of a molecule's properties, like solubility, boiling point, and interactions with other substances.
The polarity of a molecule depends on two main factors: the electronegativity of the atoms within the molecule and the geometry of the molecule itself:

• **Electronegativity**: The tendency of an atom to attract electrons towards itself. When atoms with different electronegativities form a bond, electrons are unequally shared, creating dipoles.
• **Molecular Geometry**: The 3D arrangement of atoms influences how bond dipoles interact. In symmetric molecules, these dipoles might cancel out, making the molecule nonpolar, even if it contains polar bonds.

For example, while the individual C-Cl bonds in dichlorobenzenes are polar due to chlorine's high electronegativity, overall polarity varies with the isomeric structure. Understanding how these factors interact helps in predicting the behavior and properties of molecules.

Electronegativity Difference

Electronegativity difference between atoms is one of the key reasons for the existence of a dipole moment in a molecule. Electronegativity is a measure of how strongly an atom can attract electrons. When two atoms with different electronegativities form a bond, the electron pair is not equally shared, resulting in a partial negative charge on the more electronegative atom and a partial positive charge on the other.
This charge separation creates what is known as a dipole.

• If the electronegativity difference is large, the resulting dipole is strong, leading to significant molecular polarity.
• In contrast, if the difference is small or zero, the bond is likely nonpolar or weakly polar.

For example, in dichlorobenzenes, the chlorine atoms are highly electronegative compared to the carbon atoms they bond with, creating distinct dipoles. These contribute significantly to the overall dipole moment, especially in asymmetric isomers, thereby affecting the entire molecule's polarity. Understanding these differences is essential for predicting and explaining molecular behaviors.