Question

Which of the following will not have dipolemoment as zero? (1) p-dichlorobenzene (2) p-xylene (3) p-quinol (4) p-dinitrobenzene

Short answer

p-Dinitrobenzene

Step-by-step solution

Understand Dipole Moment

Dipole moment is a measure of the separation of positive and negative charges in a molecule. Molecules with a symmetrical arrangement of identical substituents usually have a dipole moment of zero.

Analyze p-Dichlorobenzene

p-Dichlorobenzene has two chlorine atoms positioned opposite each other. This symmetrical arrangement means their individual dipole moments cancel out, resulting in a net dipole moment of zero.

Analyze p-Xylene

p-Xylene has two methyl groups positioned opposite each other on a benzene ring. The symmetrical arrangement of the methyl groups leads to a cancellation of their dipole moments, resulting in a net dipole moment of zero.

Analyze p-Quinol

p-Quinol has two hydroxyl groups positioned opposite each other on a benzene ring. This symmetrical arrangement cancels out their individual dipole moments, resulting in a net dipole moment of zero.

Analyze p-Dinitrobenzene

p-Dinitrobenzene has two nitro groups positioned opposite each other. Despite their opposite positioning, the electrons in nitro groups create a stronger dipole moment that does not cancel out completely. Thus, p-dinitrobenzene will not have a dipole moment of zero.

Conclusion

After analyzing the molecules, it is evident that p-dinitrobenzene is the molecule with a non-zero dipole moment.

Key concepts

Symmetry in Molecular Structure

Symmetry plays a vital role in determining whether a molecule has a net dipole moment. When a molecule is symmetrical, it means that the distribution of atoms and their dipole moments are balanced. This balance usually results in the dipole moments canceling each other out. For example, in p-dichlorobenzene, the two chlorine atoms are placed opposite each other on a benzene ring, creating a symmetrical structure. Because of this symmetry, the individual dipole moments of the chlorine atoms cancel out, leading to a net dipole moment of zero. Similarly, p-xylene and p-quinol both have symmetric arrangements that cause their dipole moments to cancel out.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons. It plays a significant role in the formation of dipole moments within a molecule. When two bonded atoms have different electronegativities, the more electronegative atom will attract the bonding electrons more strongly. This unequal distribution of electrons creates a dipole moment. For instance, chlorine is more electronegative than carbon, resulting in a dipole moment in bonds like C-Cl. In the cases of p-dichlorobenzene, p-xylene, and p-quinol, although there are differences in electronegativity, their symmetric arrangements ensure that these dipole moments cancel out. However, in p-dinitrobenzene, the strong electron-withdrawing nature of the nitro groups causes a more substantial dipole moment that does not entirely cancel out due to lack of perfect symmetry.

Vector Addition of Dipole Moments

The concept of vector addition of dipole moments is crucial to determining the net dipole moment in a molecule. Each bond dipole moment can be represented as a vector, having both magnitude and direction. When adding these vectors, if they point in opposite directions and have equal magnitudes, they will cancel out. For instance, in p-dichlorobenzene and p-xylene, the dipole moments due to substituents (chlorine and methyl groups, respectively) are equal in magnitude but opposite in direction, leading to a net dipole moment of zero. In contrast, p-dinitrobenzene's dipole moments, although also positioned oppositely, do not completely cancel out due to the large difference caused by the electron-withdrawing nitro groups.

Molecular Polarity

Molecular polarity is determined by the symmetry and electronegativity differences within a molecule. A polar molecule has a net dipole moment because of asymmetric electron distribution. Polarity affects various properties, such as solubility and boiling point. In our examples, p-dichlorobenzene, p-xylene, and p-quinol are non-polar due to their symmetrical structures, which allow for the cancellation of individual dipoles. On the other hand, p-dinitrobenzene is polar, as the dipole moments created by nitro groups do not entirely cancel each other out, resulting in a net dipole moment. This makes p-dinitrobenzene distinct from the other molecules, which have net dipole moments of zero.