Problem 55 - Chemistry: The Central Science

Question

(a) What are trihalomethanes (THMs)? (b) Draw the Lewis structures of two example THMs.

Step-by-Step Solution

Answer

(a) Trihalomethanes (THMs) are organic compounds composed of a central carbon atom bonded to a hydrogen atom and three halogen atoms (fluorine, chlorine, bromine, or iodine). They are byproducts of drinking water chlorination and are associated with potential health risks. (b) Lewis structures for two example THMs: 1. Chloroform (CHCl3): \(H - C - Cl\) \(-Cl\) \(-Cl\) 2. Bromodichloromethane (CHBrCl2): \(H - C - Br\) \(-Cl\) \(-Cl\)

Step 1: (a) Defining Trihalomethanes (THMs)

Trihalomethanes (THMs) are a group of organic compounds that consist of a central carbon atom bonded to a hydrogen atom and three halogen atoms. Halogens include fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). THMs are commonly formed as byproducts during the chlorination of drinking water when chlorine reacts with naturally occurring organic matter. They have been associated with potential health risks, such as increased risk for cancer and reproductive issues.

Step 2: (b) Drawing Lewis Structures of Two Example THMs

To draw the Lewis structures of two example THMs, we will follow these steps: 1. Identify the central atom: The central atom is the carbon atom, as it is bonded to a hydrogen atom and three halogen atoms. 2. Determine the total number of valence electrons: Add the valence electrons of all the atoms in the molecule. Carbon has 4 valence electrons, hydrogen has 1, and halogens typically have 7. 3. Distribute the valence electrons to create bonds and fulfill the octet rule: Place electrons between atoms to create single bonds, and then distribute the remaining electrons to complete the octet around each atom (except hydrogen, which only needs 2 electrons to complete its valence shell). Example 1: Chloroform (CHCl3) 1. Carbon is the central atom. 2. Total valence electrons: 4 (Carbon) + 1 (Hydrogen) + 7*3 (Chlorine) = 26 electrons 3. Distribute electrons: - Place 2 electrons between C-H, and 2 electrons between C-Cl (for each of the three chlorine atoms) to create single bonds. - Carbon has 8 electrons (fulfilled). - Complete the octet for each chlorine atom by adding 6 more electrons, making a complete set of 8. Lewis structure of Chloroform: \(H - C - Cl\) \(-Cl\) \(-Cl\) Example 2: Bromodichloromethane (CHBrCl2) 1. Carbon is the central atom. 2. Total valence electrons: 4 (Carbon) + 1 (Hydrogen) + 7 (Bromine) + 7*2 (Chlorine) = 26 electrons 3. Distribute electrons: - Place 2 electrons between C-H, 2 electrons between C-Br, and 2 electrons between C-Cl (for both chlorine atoms) to create single bonds. - Carbon has 8 electrons (fulfilled). - Complete the octet for bromine and each chlorine atom by adding 6 more electrons, making a complete set of 8. Lewis structure of Bromodichloromethane: \(H - C - Br\) \(-Cl\) \(-Cl\)