Question

Draw a three-dimensional representation for each molecule. Indicate which ones have a dipole moment and in what direction it is pointing. (a) \(\mathrm{CH}_{3} \mathrm{~F}\) (b) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) (c) \(\mathrm{CH}_{2} \mathrm{ClBr}\) (d) \(\mathrm{CFCl}_{3}\) (e) \(\mathrm{CCl}_{4}\) (f) \(\mathrm{CH}_{2}=\mathrm{CCl}_{2}\) (g) \(\mathrm{CH}_{2}=\mathrm{CHCl}\) (h) \(\mathrm{HC} \equiv \mathrm{C}-\mathrm{C} \equiv \mathrm{CH}\) (i) \(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{N}\) (j) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{O}\) (k) \(\mathrm{BrCH}=\mathrm{CHBr}\) (two answers)

Short answer

Question: Determine the dipole moments and their directions for the following molecules: (a) CH3F and (b) CH2Cl2. Answer: (a) CH3F has a dipole moment pointing towards the F atom. (b) CH2Cl2 has a net dipole moment pointing towards the Cl atoms.

Step-by-step solution

Draw the Lewis structure

Draw the Lewis structure of the molecule by connecting the atoms and adding non-bonding electron pairs.

Determine molecular geometry

Determine the molecular geometry by analyzing the distribution of electron pairs around the carbon atom. CH3F has a tetrahedral molecular geometry.

Analyze electron density and identify dipole moment, if any

The electron density around the F atom is higher than on the H atoms, so there is a dipole moment pointing towards the F atom. **For molecule (b) CH2Cl2**

Draw the Lewis structure

Draw the Lewis structure of the molecule by connecting the atoms and adding non-bonding electron pairs.

Determine molecular geometry

Determine the molecular geometry by analyzing the distribution of electron pairs around the carbon atom. CH2Cl2 has a tetrahedral molecular geometry.

Analyze electron density and identify dipole moment, if any

The electron density around the Cl atoms is higher than on the H atoms, so there is a net dipole moment pointing towards the Cl atoms. We can follow the same steps for the remaining molecules (c)-(k). When creating the 3-dimensional representation always draw the bonds and atoms according to the determined molecular geometry and use arrows to indicate the presence and direction of dipole moments. Remember that a molecule will only have a dipole moment if its electron distribution is asymmetric. If a molecule has a symmetrical distribution of electron density or if the atoms in the molecule have the same electronegativity, it will not have a dipole moment.

Key concepts

Lewis Structure

Understanding the Lewis structure is fundamental to predicting the behavior of molecules in reactions and their physical properties. In simple terms, a Lewis structure is a diagram that represents the valence electrons of atoms within a molecule. These electrons are illustrated as dots that are arranged around the atoms, showing how the atoms in the molecule are bonded together.

When drawing the Lewis structure, start by identifying the total number of valence electrons available for bonding. Then, connect the atoms with single lines to represent shared electron pairs, or bonds. The remaining valence electrons are placed around the atoms as non-bonding electron pairs, or lone pairs. This visual representation helps us determine the bonding patterns and can reveal whether a molecule may have a dipole moment due to asymmetric electron distribution.

Electron Density

Electron density refers to the probability of finding electrons in a certain area around the atomic nucleus. In chemistry, it's vital for predicting how electrons are distributed in a molecule and how they affect molecular shape and polarity. When atoms with different electronegativities form a bond, the shared electrons are pulled more towards the more electronegative atom, resulting in an uneven electron distribution.

For instance, in a molecule of water, oxygen is more electronegative than hydrogen, which draws the electrons closer to itself, leading to a higher electron density around the oxygen atom. This concept is crucial in determining whether a molecule has a dipole moment which is essentially created by an asymmetric electron distribution leading to partial negative and positive charges.

Tetrahedral Molecular Geometry

Tetrahedral molecular geometry arises when a central atom is surrounded by four other atoms or groups, with all the atoms at the corners of a tetrahedron and the central atom at its center. This configuration is one of the most common molecular shapes and is pivotal for understanding many organic molecules' structure and reactivity.

In a tetrahedral geometry, the bond angles are approximately 109.5 degrees. Such a shape promotes even distribution of electron density when all the surrounding atoms or groups are the same. However, if substituents differ in their electronegativity, the molecule may become polar, and the electron distribution within the molecule will no longer be symmetric.

Asymmetric Electron Distribution

For a molecule to have a dipole moment, the electron distribution must be asymmetric. This means the molecule's shape or the electronegativities of the atoms cause the electron density to be unevenly distributed. Asymmetry in electron distribution creates partial charges within the molecule, with denser electron regions becoming partially negative, and regions from where electrons are drawn becoming partially positive.

Many factors contribute to this asymmetry, including molecular geometry and the presence of different atoms with varying electronegativities. Molecules like CH3F have an asymmetric distribution because fluorine has a higher electronegativity compared to hydrogen, inducing a dipole moment pointing towards the fluorine atom. Recognizing these non-uniform distributions is key in predicting molecular polarity and behavior in electric fields.