Question

Predict whether each of the following molecules is likely to be polar or nonpolar: (a) \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N}\) (pyridine, a molecule like benzene except that one \(-\mathrm{CH}-\) group is replaced by a nitrogen atom); (b) \(\mathrm{C}_{2} \mathrm{H}_{6}\) (ethane); (c) \(\mathrm{CHCl}_{3}\) (trichloromethane, also known as chloroform, a common organic solvent and once used as an anesthetic).

Short answer

Pyridine (\textbf C\(_5\)H\(_5\)N) is nonpolar, ethane (\textbf C\(_2\)H\(_6\)) is nonpolar, and trichloromethane (\textbf CHCl\(_3\)) is polar.

Step-by-step solution

Analyze the molecular structure of Pyridine (\textbf C\(_5\)H\(_5\)N)

Pyridine has a planar hexagonal ring structure similar to benzene, but with one nitrogen atom replacing one of the CH units. The electronegativity of nitrogen is higher than carbon, resulting in a dipole moment pointing towards nitrogen. Despite this, the overall symmetry of the molecule ensures that these dipole moments cancel each other out, making pyridine nonpolar.

Evaluate the molecular geometry of Ethane (\textbf C\(_2\)H\(_6\))

Ethane is composed of two carbon atoms single-bonded to each other, with each carbon atom also bonded to three hydrogen atoms. The molecule is symmetrical, and the electron distribution around the carbon atoms is equal, which means there is no net dipole moment. As a result, ethane is considered a nonpolar molecule.

Assess the polarity of Trichloromethane (\textbf CHCl\(_3\))

Chloroform has a tetrahedral shape with three chlorine atoms and one hydrogen atom attached to a central carbon atom. The electronegativity difference between carbon and chlorine, as well as the chlorine atoms not being symmetrically arranged, causes a significant net dipole moment in the direction of the chlorine atoms. This makes chloroform a polar molecule.

Key concepts

Electronegativity

Understanding electronegativity is crucial for predicting the polarity of a molecule. It refers to the ability of an atom to attract shared electrons in a chemical bond. Electronegativity is often represented on a scale with Fluorine being the most electronegative element.

When atoms with different electronegativities form a bond, a dipole is created, with the more electronegative atom acquiring a partial negative charge and the less electronegative atom a partial positive charge. If these dipoles do not cancel each other out due to molecular symmetry, the molecule as a whole becomes polar. For instance, in trichloromethane (CHCl₃), the high electronegativity of chlorine compared to hydrogen and carbon results in a polar molecule.

Molecular Geometry

Molecular geometry plays a pivotal role in determining whether a molecule is polar or nonpolar. The spatial arrangement of atoms around a central atom can influence whether the dipole moments cancel each other out.

Simple geometries like linear or tetrahedral might be nonpolar if the surrounding atoms are identical, as in ethane (C₂H₆). However, if the surrounding atoms are different or arranged asymmetrically, the molecule can be polar, like chloroform (CHCl₃). A molecule's geometry can often be predicted using VSEPR theory, which can provide insight into the molecular shape based on the repulsion between electron pairs.

Dipole Moment

The dipole moment is a quantitative measure of the polarity of a molecule. It is the product of the charge on the atoms and the distance between the centers of positive and negative charge. It is measured in Debye units (D).

A molecule with a symmetrical structure may have individual dipoles, but their vector addition may result in a net dipole moment of zero, classifying the molecule as nonpolar. This is the case with pyridine (C₅H₅N), where the symmetrical arrangement causes the dipole moments to cancel out. In contrast, a molecule like chloroform has a net dipole moment because it is asymmetrical, making it polar. To fully grasp this concept, one can visualize the dipole moment as an arrow pointing from the positive to the negative side of the molecule, capturing both the magnitude and direction of polarity.