Which of the following are arranged in the decreasing order of dipole moment? (a) \(\mathrm{CH}_{3} \mathrm{Cl}, \mathrm{CH}_{3} \mathrm{Br}, \mathrm{CH}_{3} \mathrm{~F}\) (b) \(\mathrm{CH}_{3} \mathrm{Cl}, \mathrm{CH}_{3} \mathrm{~F}, \mathrm{CH}_{3} \mathrm{Br}\) (c) \(\mathrm{CH}_{3} \mathrm{Br}, \mathrm{CH}_{3} \mathrm{Cl}, \mathrm{CH}_{3} \mathrm{~F}\) (d) \(\mathrm{CH}_{3} \mathrm{Br}, \mathrm{CH}_{3} \mathrm{~F}, \mathrm{CH}_{3} \mathrm{Cl}\)

Grasping the concept of electronegativity is crucial when exploring the dipole moment in organic molecules. Electronegativity refers to the ability of an atom to attract shared electrons in a chemical bond. Picture it as a tug-of-war competition between different atoms; the one with higher electronegativity pulls harder on the electron rope.

A dipole moment arises when there is a difference in the electronegativity between two atoms in a bond, leading to an uneven distribution of electron density. This creates a dipole: a pair of equal and oppositely charged or magnetized poles separated by a distance. The dipole moment (μ) of a molecule can be expressed mathematically as the product of the charge difference (Q) and the distance (r) between the charges: μ = Q × r.

The polarity of a bond — whether it tends to be more ionic or covalent — is directly linked to the dipole moment. The greater the difference in electronegativity, the more polar the bond and the higher the dipole moment. In organic molecules, this concept helps us predict and explain the physical properties, such as boiling points, solubility, and reactivity.

Haloalkanes, which are alkanes with one or more halogen atoms substituted for hydrogen, showcase interesting variations in dipole moment. The presence of a halogen greatly influences the molecule's overall dipole moment because halogens bring with them a substantial electronegativity.

For instance, within a series of haloalkanes such as chloromethane (CH3Cl), bromomethane (CH3Br), and fluoromethane (CH3F), the differences in dipole moment are determined largely by the electronegativity of the halogen connected to the carbon atom. Since fluorine has the highest electronegativity among the halogens, fluoromethane will have the strongest dipole moment. Here, it's not just about which atom is pulling harder, but also how far the 'tug-of-war' battle is stretching, meaning the bond length plays a role too.

However, in the context of haloalkanes, where the carbon-halogen bond lengths do not differ significantly, the primary determining factor of the dipole moment is the halogen's electronegativity. Therefore, understanding the order of electronegativity (F > Cl > Br) is essential to predict the order of dipole moments in haloalkanes.

The concept of molecular polarity extends from the dipole moments of individual bonds to the entire molecule. A molecule is deemed polar if it has an uneven distribution of electron density, leading to a net dipole moment. For a molecule to be polar, it must have polar bonds, and those bonds must be arranged asymmetrically.

Take water (H2O) for example. The molecule is bent, and because of the difference in electronegativity between hydrogen and oxygen, there are two polar bonds. These bonds do not cancel each other out because of the bent geometry, resulting in a polar molecule with a net dipole moment.

In contrast, carbon dioxide (CO2) has two polar bonds as well, but the molecule is linear, and the bond dipoles cancel each other out, resulting in a nonpolar molecule. When assessing molecular polarity, the shape of the molecule, the dipole moment of its bonds, and the symmetry of the molecule all play pivotal roles. For students grappling with these concepts, visualizing molecular geometry and understanding symmetry can greatly aid in determining whether a molecule is polar or nonpolar.