Problem 56 - A Complete Resource Book in Chemistry for JEE Main

Question

The molecule having largest dipole moment among the following is (a) \(\mathrm{CHI}_{3}\) (b) \(\mathrm{CH}_{4}\) (c) \(\mathrm{CHCl}_{3}\) (d) \(\mathrm{CCl}_{4}\)

Step-by-Step Solution

Answer

The molecule with the largest dipole moment is  extrm{CHCl}_3.

Step 1: Understand Dipole Moment

The dipole moment of a molecule is a measure of the separation of positive and negative charges within the molecule. It depends on both the magnitudes of individual bond dipoles (due to differences in electronegativity) and the spatial arrangement of these bond dipoles. Molecules with high electronegativity differences and asymmetrical shapes typically have larger dipole moments.

Step 2: Analyze Each Molecule's Geometry

Let's consider each molecule's geometry. -  extrm{CHI}_3  and  extrm{CHCl}_3  have tetrahedral geometries where one atom differs from the others, creating an asymmetrical molecule. -  extrm{CH}_4  and  extrm{CCl}_4  also have tetrahedral geometry but are symmetrical, as all surrounding atoms are the same.

Step 3: Evaluate Symmetry and Polarity

-  extrm{CH}_4 and  extrm{CCl}_4 are nonpolar due to symmetrical arrangements, so their dipole moments are zero. - In  extrm{CHI}_3 , differences in electronegativity between carbon, iodine, and hydrogen induce minor polarity. - In  extrm{CHCl}_3 , the high electronegativity of chlorine and the asymmetry induced by the presence of the hydrogen atom leads to a substantial nonzero dipole moment, since the dipole vectors do not cancel out.

Step 4: Compare Individual Dipole Moments

Finally, to find the molecule with the largest dipole moment, compare the individual results: -  extrm{CHI}_3 and  extrm{CHCl}_3 are polar. - Among these,  extrm{CHCl}_3  is expected to have a higher dipole moment due to the greater electronegativity difference between hydrogen and chlorine as compared to iodine and hydrogen or iodine and carbon.

Key Concepts

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. This shape influences many physical and chemical properties, including the dipole moment of the molecule. Understanding the geometry can help explain why certain molecules are polar or nonpolar.

Tetrahedral geometry is one common arrangement, where a central atom is bonded to four surrounding atoms positioned at the corners of a tetrahedron. This structure is found in molecules like \(\text{CH}_4\), \(\text{CCl}_4\), \(\text{CHCl}_3\), and \(\text{CHI}_3\).

In \(\text{CH}_4\) and \(\text{CCl}_4\), all surrounding atoms are identical, leading to a perfectly symmetrical distribution of charge and thereby making these molecules nonpolar. However, in \(\text{CHCl}_3\) and \(\text{CHI}_3\), the presence of different atoms like chlorine and iodine disrupts this symmetry, resulting in polar molecules due to an uneven distribution of charge.

Electronegativity

Electronegativity is a measure of an atom's capacity to attract and hold onto electrons within a bond. It is a critical factor in determining how electrons are distributed in a molecule and whether the resulting bonds will be polar or nonpolar.

Atoms with different electronegativities form polar bonds. In polar molecules like \(\text{CHCl}_3\), carbon, chlorine, and hydrogen have varying electronegativities. Chlorine, being highly electronegative, pulls shared electrons closer, creating a negative charge on one side of the \(\text{CHCl}_3\) molecule.

Conversely, atoms with similar electronegativities, as seen in \(\text{CH}_4\) and \(\text{CCl}_4\), result in nonpolar bonds since electrons are equally shared. In these cases, the electronegativity difference is nearly zero, leading to a neutral distribution of charge and, consequently, no dipole moment.

Molecular Polarity

Molecular polarity arises from the distribution of electric charge around a molecule. For a molecule to be polar, it needs to have polar bonds and an asymmetrical shape that allows the dipole moments to not cancel each other out.

In \(\text{CHCl}_3\), the molecule is polar because the three chlorine atoms and one hydrogen atom create an asymmetrical charge distribution. Although each \(\text{C-Cl}\) bond is polar due to chlorine's high electronegativity, the structure is such that the individual bond dipoles add up, rather than cancel out, resulting in a net dipole moment.

By contrast, molecules like \(\text{CCl}_4\) and \(\text{CH}_4\) are nonpolar. Although they contain polar bonds (in the case of \(\text{CCl}_4\)), the symmetrical tetrahedral geometry ensures that the bond dipoles cancel out, leaving no net dipole moment. Thus, understanding both the electronegativity and molecular geometry is crucial for determining molecular polarity.