Problem 94 - Chemistry The Central Science In Si Units

Question

1,2-dihydroxybenzene is obtained when two of the adjacent hydrogen atoms in benzene are replaced with an OH group. A skeleton of the molecule is shown here. (a) Complete a Lewis structure for the molecule using bonds and electron pairs as needed. (b) Are there any resonance structures for the molecule? If so, sketch them. (c) Are the resonance structures in (a) and (b) equivalent to one another as they are in benzene?

Step-by-Step Solution

Answer

(a) Complete the structure with OH groups and lone pairs. (b) Yes, there are resonance structures. (c) They are not equivalent due to OH groups.

Step 1: Analyze the Molecular Structure

1,2-Dihydroxybenzene, also known as catechol, has two hydroxyl (-OH) groups attached to adjacent carbons on a benzene ring. Begin by drawing a benzene ring with alternating single and double bonds, then select any two adjacent carbon atoms to attach the OH groups.

Step 2: Complete the Lewis Structure

Add the OH groups to the two neighboring carbon atoms identified in Step 1. Ensure each carbon has four total bonds, and each oxygen atom in the OH group has two lone pairs to complete their octet.

Step 3: Determine Possible Resonance Structures

For dihydroxybenzene, consider the movement of double bonds in the benzene ring: the pi electrons can move around the ring. Draw alternative structures by changing the positions of the double bonds while keeping the OH groups fixed.

Step 4: Evaluate the Resonance Structures

Compare the resonance structures to see if they all contribute equally to the description of the molecule. Note that while benzene itself has equivalent resonance structures, the presence of OH groups disrupts this symmetry.

Step 5: Conclude on Resonance Structure Equivalence

The resonance structures of 1,2-dihydroxybenzene are not equivalent like those of pure benzene. The position of the OH groups causes electron distribution effects, making some resonance forms more stable than others.

Key Concepts

Lewis structure

Lewis structures are diagrams that represent the bonding of atoms within a molecule along with any lone pairs of electrons that may exist. To create a Lewis structure for 1,2-dihydroxybenzene, you start by drawing its basic skeleton:

• A six-membered carbon ring representing benzene, typically shown with alternating single and double bonds.
• Two hydroxyl groups (OH) added to two adjacent carbons.

Here's how to proceed:

• Add OH groups to adjacent carbon atoms on the benzene ring.
• Each carbon atom must have four bonds in total, considering the existing carbon-carbon bonds from the ring and the newly attached OH groups.
• Every oxygen atom should exhibit two lone pairs to satisfy the octet rule.

This method ensures that each atom in catechol achieves a full valency, reflecting how molecules share electrons to form bonds.

resonance structures

Resonance structures illustrate different ways to represent the electron arrangement within a molecule, highlighting the possible configurations of pi electrons. In the context of 1,2-dihydroxybenzene, we can explore the resonance by moving around the double bonds located within the benzene ring. To identify resonance structures:

• Keep the OH groups fixed; they're bonded to the same carbon atoms throughout each structure.
• Focus on shifting the pi bonds around the ring.
• This involves interchanging single and double bonds between the carbon atoms of the benzene ring.

The resonance forms illustrate that while the benzene core is conjugated with moving pi electrons, the presence of the hydroxyl groups affects the stabilization and appearance of these forms. Not all generated forms will contribute equally to the real structure; this is where analyzing stability and equivalence becomes crucial.

benzene ring

A benzene ring is a highly stable, cyclic hydrocarbon structure consisting of six carbon atoms connected in a hexagonal arrangement, with alternating single and double bonds. This creates what is called a conjugated system. In benzene:

• The electrons in the pi bonds are delocalized, meaning they are shared across the entire ring.
• This delocalization leads to a resonance-stabilized structure, giving benzene its well-known stability and symmetry.

The introduction of substituents, like hydroxyl groups in 1,2-dihydroxybenzene, modifies this perfect symmetry. The hydroxyl groups exert an electron-donating effect through resonance, affecting the electron cloud's uniformity and subsequently the resonance structures. Thus, while the base benzene ring exhibits interchangeable resonance forms, adding functional groups like OH disrupts this equivalence by drawing the electron density preferentially toward themselves, impacting the overall electron distribution of the molecule.