Problem 89 - Chemistry An Atoms First Approach

Question

Benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\) consists of a six- membered ring of carbon atoms with one hydrogen bonded to each carbon. Write Lewis structures for benzene, including resonance structures.

Step-by-Step Solution

Answer

In benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\), a six-membered ring of carbon atoms has one hydrogen atom bonded to each carbon. To draw the Lewis structures for benzene, including its resonance structures, create a hexagonal arrangement of carbon atoms with one hydrogen atom connected to each carbon. Form double bonds between alternating carbon atoms in the ring. For the resonance structures, switch the positions of the double bonds within the ring. These two structures represent the delocalized electrons and confer extra stability to the benzene molecule.

Step 1: Review Lewis Structures

The Lewis structure is a representation of the molecule showing its atoms, the bonding between the atoms, and lone pair of electrons if any. To draw a Lewis structure for a molecule, we need to follow these steps: 1. Count the total number of valence electrons. 2. Determine the central atom (usually the one with the lowest electronegativity). 3. Connect the central atom to surrounding atoms with single bonds. 4. Distribute the remaining valence electrons as lone pairs or multiple bonds, while satisfying the octet rule.

Step 2: Draw the Basic Benzene Structure

Now let's apply these rules to draw the benzene molecule \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\): 1. Valence electrons: Carbon has 4 valence electrons (x 6 carbon atoms), and hydrogen has 1 valence electron (x 6 hydrogen atoms). This results in a total of \(4 \times 6 + 1 \times 6 = 30\) valence electrons. 2. The central atom: The structure of benzene is a six-membered ring of carbon atoms. There is no central atom, as each carbon atom is bonded to two other carbon atoms and one hydrogen atom. 3. Connect the central atom to surrounding atoms with single bonds: Since we have a ring of carbon atoms, we will bond each carbon atom to two others, making a hexagonal shape. Then, we will attach one hydrogen atom to each carbon atom.

Step 3: Distribute the Remaining Valence Electrons and Check the Octet Rule

4. Distribute the remaining valence electrons as lone pairs or multiple bonds, satisfying the octet rule: Drawing single bonds between each carbon and hydrogen and between each two adjacent carbon atoms results in every carbon atom having 3 bonds. Each carbon atom still needs one more bond to satisfy the octet rule (8 electrons per carbon). We can now form double bonds between alternating carbon atoms in the ring.

Step 4: Draw the Resonance Structure

A resonance structure is an alternative representation of a molecule that helps us to understand the distribution of electrons and bonds in a molecule. For benzene, the double bonds in the ring can be in different positions, resulting in two different resonance structures. To draw the resonance structure, alternate the positions of the double bonds within the ring (e.g., if a double bond was between C1 and C2 in the first structure, it would now be between C2 and C3). These two structures with alternating double bond positions are the resonance structures for benzene. These drawn structures indicate that the electrons are delocalized, and the alternating double bonds provide extra stability to the benzene molecule.