Problem 174 - Chemistry: An Atoms-Focused Approach

Question

Ozone Depletion Methyl bromide \(\left(\mathrm{CH}_{3} \mathrm{Br}\right)\) is produced naturally by fungi. Methyl bromide has also been used in agriculture as a fumigant, but its use is being phased out because the compound has been linked to ozone depletion in the upper atmosphere. a. Draw the Lewis structure of \(\mathrm{CH}_{3} \mathrm{Br}\). b. Which bond in \(\mathrm{CH}_{3} \mathrm{Br}\) is more polar, carbon-hydrogen or carbon-bromine?

Step-by-Step Solution

Answer

Answer: The carbon-bromine (C-Br) bond is more polar in the methyl bromide (CH3Br) molecule.

Step 1: Draw the Lewis structure of CH3Br

To draw the Lewis structure of CH3Br, we start by counting the valence electrons in the molecule. Carbon has 4 valence electrons, hydrogen has 1 valence electron, and bromine has 7 valence electrons. The molecule has a total of 4 + 3(1) + 7 = 14 valence electrons. Next, we arrange the atoms and fill in the electrons to satisfy the octet rule for carbon and bromine, and the duet rule for hydrogen. The central atom is carbon, which is bonded to three hydrogen atoms and one bromine atom: H | H--C--Br | H Now we just need to fill in the remaining electrons. Carbon has four valence electrons, three of which are being used to bond with the hydrogen atoms, and one is used to bond with the bromine atom. Hydrogen has a duet rule, which means it only needs two electrons, and each hydrogen atom already has its required two electrons by bonding to carbon. Bromine has seven valence electrons, one of which is being used to bond with carbon, so it should have six more electrons in its valence shell. The final Lewis structure is: H | H--C--Br: : : | H

Step 2: Determine which bond is more polar

To determine the polarity of a bond, we compare the electronegativities of the involved atoms. A greater difference in electronegativities indicates a more polar bond. The electronegativity values for the involved elements are as follows: - Hydrogen (H): 2.20 - Carbon (C): 2.55 - Bromine (Br): 2.96 Now, let's compare the carbon-hydrogen (C-H) bond electronegativity difference with the carbon-bromine (C-Br) bond: C-H bond: | 2.55 - 2.20 | = 0.35 C-Br bond: | 2.96 - 2.55 | = 0.41 Since the electronegativity difference for the C-Br bond (0.41) is greater than the difference for the C-H bond (0.35), the carbon-bromine bond is more polar.

Key Concepts

Methyl Bromide

Methyl bromide (\(\mathrm{CH}_{3} \mathrm{Br}\)) is an organobromine compound produced naturally by certain fungi and plants, serving primarily as a defense against pests. It has also been manufactured for agricultural uses, acting as a fumigant to protect crops from insects, weeds, and pathogens.
However, the synthetic use of methyl bromide is being phased out as it has been linked to significant environmental issues, particularly ozone depletion. When released into the atmosphere, methyl bromide contributes to the breakdown of the stratospheric ozone layer, which is crucial for protecting living organisms from the sun’s harmful ultraviolet radiation.

• This understanding led to the regulation of methyl bromide under the Montreal Protocol, an international agreement to protect the ozone layer.
• Despite its beneficial uses, alternatives are being sought to fulfill its role in agriculture without harming the environment.

Lewis Structure

The Lewis structure is a representation of a molecule that demonstrates how atoms are bonded and how valence electrons are distributed.
For the compound \(\mathrm{CH}_{3} \mathrm{Br}\), the Lewis structure helps visualize the arrangement of atoms:

• The central carbon atom forms four bonds: three with hydrogen atoms and one with a bromine atom.
• The molecule contains 14 valence electrons in total. Carbon contributes 4, hydrogen contributes 3, and bromine contributes 7.
• The carbon atom satisfies the octet rule by sharing electrons through covalent bonds, while each hydrogen reaches the duet rule with its single bond to carbon.
• Bromine completes its octet by sharing one of its seven valence electrons, leaving six unbonded electrons.

Thus, the structure provides a clear snapshot of each atom's covalent interactions in the molecule, allowing further exploration of its chemical properties.

Polarity

Polarity in chemistry refers to the distribution of electrical charge around atoms, molecules, or chemical groups. A polar bond occurs when an atom has either a positive or negative electrical charge due to differences in electronegativity.
Considering the molecule \(\mathrm{CH}_{3} \mathrm{Br}\), polarity is analyzed by examining the two relevant types of bonds:

• Carbon-Hydrogen (C-H) bonds display slight polarity due to the difference in electronegativity values (0.35), but are often considered nonpolar because the electronegativity values are quite close.
• Carbon-Bromine (C-Br) bonds are more polar due to a larger electronegativity difference (0.41), meaning bromine pulls the shared electrons closer, creating a dipole moment.

Polar molecules like CH3Br tend to interact more strongly with polar solvents (like water) than with nonpolar ones, affecting their solubility and intermolecular interactions.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons within a chemical bond. It influences molecular properties such as bond strength and polarity. \(\mathrm{CH}_{3} \mathrm{Br}\) provides a clear example to understand variation in electronegativity:

• Carbon has an electronegativity of 2.55, compared to hydrogen at 2.20.
• Bromine is more electronegative at 2.96, indicating a stronger pull on shared electrons.

The differences in these values help determine which bonds in a molecule are polar and dictate how molecules interact with one another. For instance:

• The greater the electronegativity difference, the more polarized the bond, as observed between carbon and bromine.
• Electronegativity differences also play a key role in determining the dipole moment and thus the reactivity and physical properties of the compound.

Overall, understanding how electronegativity affects molecular interactions is crucial for predicting compound behavior in chemical reactions.