Question

Rank the following molecules in order of increasing polarity: \(\mathrm{CH}_{3} \mathrm{Cl}, \mathrm{CH}_{3} \mathrm{~F}\), and \(\mathrm{CH}_{3} \mathrm{Br}\).

Short answer

CH3Br < CH3Cl < CH3F

Step-by-step solution

Identify Electronegativity Values

First, identify the electronegativity values of the atoms involved in each molecule. Use the following values: C (2.55), H (2.20), F (3.98), Cl (3.16), and Br (2.96).

Determine Bond Polarity

Next, determine the polarity of each bond within the molecules. The difference in electronegativity between carbon and hydrogen in \(\text{CH}_3\) is small and does not contribute significantly to overall polarity. Focus on the carbon-halogen bonds (C-F, C-Cl, and C-Br).

Calculate Electronegativity Difference

Calculate the electronegativity difference for each bond: \( \text{C-F}: |2.55 - 3.98| = 1.43 \), \( \text{C-Cl}: |2.55 - 3.16| = 0.61 \), and \( \text{C-Br}: |2.55 - 2.96| = 0.41 \). The larger the difference, the more polar the bond.

Rank the Molecules

Rank the molecules in order of increasing polarity based on the calculated electronegativity differences: \( \text{CH}_3\text{Br} \) (0.41), \( \text{CH}_{3}\text{Cl} \) (0.61), \( \text{CH}_3\text{F} \) (1.43).

Key concepts

Electronegativity Values

Electronegativity is a measure of how strongly an atom attracts bonding electrons to itself. Different atoms have different electronegativity values. For example, fluorine (F) has the highest electronegativity value of 3.98, making it very good at attracting electrons. In contrast, carbon (C) has a lower value of 2.55, and hydrogen (H) has an even lower value of 2.20. When comparing values for chlorine (Cl) at 3.16 and bromine (Br) at 2.96, it's evident that halogens generally have higher electronegativity than carbon and hydrogen. These values help us understand how electrons are distributed in a molecule and thereby affect its overall polarity. By knowing these values, we can predict which direction the electrons are pulled in chemical bonds.

Bond Polarity

Bond polarity occurs when there is a difference in electronegativity values between two bonded atoms. If two atoms with significantly different electronegativity values form a bond, the electrons will be more attracted to the atom with the higher electronegativity. This creates a dipole, or a separation of electric charge, within the bond.
For instance, in the molecules \(\text{CH}_3\text{Cl}\), \(\text{CH}_3\text{F}\), and \(\text{CH}_3\text{Br}\), the carbon (C)-halogen (X) bonds (C-F, C-Cl, and C-Br) are polar because the halogens (F, Cl, and Br) have higher electronegativity values than carbon.

• In the C-F bond, fluorine pulls electrons much more strongly than carbon, creating a highly polar bond.
• In the C-Cl bond, chlorine also pulls electrons from carbon, but less strongly than fluorine.
• In the C-Br bond, bromine is less electronegative compared to fluorine and chlorine, resulting in a less polar bond.

It's the differing strengths of these pulls that result in varied bond polarities, affecting the overall polarity of each molecule.

Electronegativity Difference

The electronegativity difference between two bonded atoms is a key factor in determining bond polarity. This difference can be calculated by simply subtracting the smaller electronegativity value from the larger one.
Consider the calculated differences for the molecules in the exercise:

• The difference for the C-F bond in \(\text{CH}_3\text{F}\) is \(|2.55 - 3.98| = 1.43\). This is the largest difference, indicating the most polar bond.
• The difference for the C-Cl bond in \(\text{CH}_3\text{Cl}\) is \(|2.55 - 3.16| = 0.61\), which is lower than C-F but higher than C-Br.
• The difference for the C-Br bond in \(\text{CH}_3\text{Br}\) is \(|2.55 - 2.96| = 0.41\). This is the smallest difference, indicating the least polar bond.

The greater the electronegativity difference, the more polar the bond will be, leading to a more polar molecule overall. By understanding the concept of electronegativity difference, you can predict the polarity of bonds and rank molecules accordingly.