Question

Among the following, the molecule with the highest dipole moment is (a) \(\mathrm{CH}_{3} \mathrm{Cl}\) (b) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) (c) \(\mathrm{CHCl}_{3}\) (d) \(\mathrm{CCl}_{4}\)

Short answer

The molecule \( \mathrm{CH}_{2} \mathrm{Cl}_{2} \) has the highest dipole moment.

Step-by-step solution

Understand Dipole Moment

The dipole moment is a measure of the separation of positive and negative charges in a molecule. It is a vector quantity, meaning it has both magnitude and direction. Molecules with asymmetric charge distributions tend to have higher dipole moments.

Analyze Symmetry and Electronegativity

Chlorine is more electronegative than carbon and hydrogen, so it will attract electrons more strongly, creating a negative pole. In a symmetric molecule like \( \mathrm{CCl}_{4} \), the dipole moments cancel out due to their symmetrical arrangement.

Evaluate Each Molecule

- \( \mathrm{CH}_{3} \mathrm{Cl} \): One chlorine atom results in an asymmetrical shape, producing a significant dipole moment.- \( \mathrm{CH}_{2} \mathrm{Cl}_{2} \): The two chlorine atoms create an even greater asymmetry.- \( \mathrm{CHCl}_{3} \): The molecule is asymmetrical, but the vector sum of the dipoles reduces the overall dipole moment compared to \( \mathrm{CH}_{2} \mathrm{Cl}_{2} \).- \( \mathrm{CCl}_{4} \): The symmetry of the four chlorine atoms means dipole moments cancel out, leading to a net dipole moment of zero.

Compare and Conclude

Since \( \mathrm{CH}_{2} \mathrm{Cl}_{2} \) has two chlorine atoms and lacks symmetry, it creates a substantial dipole moment, making it the molecule with the highest dipole moment among the given options.

Key concepts

Molecular Polarity

Understanding molecular polarity helps us grasp why molecules behave the way they do in different environments. A molecule is considered polar if it has a significant separation of charge, known as a dipole. This occurs due to the uneven distribution of electrons among the atoms forming the molecule. Polar molecules have a partial positive charge on one side and a partial negative charge on the opposite side. This charge separation leads to various intermolecular forces, particularly dipole-dipole attractions, influencing properties like boiling points and solubility.
When analyzing a molecule for polarity, one crucial aspect to consider is its shape, as the geometric arrangement of atoms affects the overall polarity. For example, a simple molecule like water (H₂O) is polar because of its bent shape which causes the dipoles to not cancel each other out. In contrast, molecules with symmetrical shapes, like carbon tetrachloride (CCl₄), have dipoles that cancel out, resulting in a nonpolar molecule.
Therefore, by understanding molecular polarity, you can better predict a molecule's reactivity and interactions. This assessment is vital in fields like chemistry and pharmacology, where molecular behavior dictates effectiveness and compatibility in reactions and solutions.

Electronegativity

Electronegativity is a chemical property that describes an atom's ability to attract and bind with electrons. It's a fundamental concept when discussing molecular polarity, as differences in electronegativity between atoms dictate the distribution of electrons within a molecule.
When a molecule is composed of atoms with differing electronegativities, the more electronegative atom will attract the shared electrons more strongly. This creates a polar bond. For instance, chlorine has higher electronegativity than both carbon and hydrogen. As a result, in CH₃Cl, the electrons are drawn closer to the chlorine atom, making the molecule polar with a positive pole on the hydrogen side and a negative pole on the chlorine side.
In periodic trends, electronegativity increases across a period from left to right and decreases down a group. This trend helps us predict molecular behavior and interactions. Understanding electronegativity allows chemists to anticipate the type of bonds that will form between atoms, as well as the molecule's likely polarity.

Symmetry in Molecules

Symmetry plays a crucial role in determining the polarity of a molecule. A molecule's shape is defined by its geometry and the arrangement of its atoms in three-dimensional space. Symmetrical molecules generally have their dipole moments cancel out, leading to non-polarity.
For example, in carbon tetrachloride (CCl₄), the molecule is perfectly symmetrical with chlorine atoms evenly distributed around the central carbon atom. This symmetrical geometry means the dipole moments from each C-Cl bond counterbalance each other, resulting in a nonpolar molecule with no net dipole moment.
In contrast, take dichloromethane (CH₂Cl₂), which has two hydrogen atoms and two chlorine atoms. The lack of symmetry in the arrangement of hydrogen and chlorine leads to an uncompensated dipole moment. Therefore, molecular symmetry or lack thereof, deeply influences whether a molecule is polar or nonpolar.

• Symmetrical molecules: Dipole moments cancel out, nonpolar.
• Asymmetrical molecules: Dipole moments don't cancel, polar.

Hence, when evaluating molecular polarity, always consider both the electronegativity and the symmetry within the molecule to accurately determine its polar or nonpolar nature.