Problem 104 - Chemistry

Question

Some mothballs used when storing clothes are made of naphthalene \(\left(\mathrm{C}_{10} \mathrm{H}_{8}\right)\), which has the following incomplete structure. (a) Add double bonds where needed to draw a complete electron-dot structure. (b) Starting from this structure, use curved arrows to indicate how a new resonance structure can be drawn.

Step-by-Step Solution

Answer

Add alternating double bonds in rings & use arrows to show electron movement.

Step 1: Understand Naphthalene Structure

Naphthalene is a polycyclic aromatic hydrocarbon with the chemical formula \(C_{10}H_8\). It consists of two six-membered benzene rings fused together, sharing two carbon atoms. Each carbon atom forms three bonds: either double or single to maintain aromaticity.

Step 2: Complete the Electron-Dot Structure

Begin by drawing two hexagons that represent the fused benzene rings. Add alternating double bonds in each ring to follow the pattern of an aromatic compound. Each carbon atom should either have a double bond with one neighbor or a single bond with two neighbors. Add hydrogen atoms to each carbon to complete four bonds per carbon.

Step 3: Identify Resonance in Naphthalene

In aromatic compounds like naphthalene, electron delocalization occurs, meaning electrons can move around in the structure. Within the naphthalene, these double bonds can shift positions. Identify locations in the structure where double bonds can be relocated while respecting the aromatic nature of naphthalene.

Step 4: Use Curved Arrows for Resonance

Use curved arrows to indicate the movement of electrons that generate new resonance structures. Typically, you will move a pair of electrons from a double bond to a adjacent bond position, ensuring that overall aromaticity and electron count per atom are maintained.

Step 5: Draw the Resonance Structures

From the original structure, apply the electron movements indicated by the curved arrows and draw the new resonance structure. Ensure the number of bonds around each carbon and overall charge conservation are maintained.

Key Concepts

Aromatic Hydrocarbons

Aromatic hydrocarbons are fascinating group of organic compounds known for their stable ring-like structures. What makes them special and often chemically stable is the arrangement of their carbon atoms in closed rings, where atoms form alternating single and double bonds. This structure contributes to their high resonance energy, meaning they are less reactive than other hydrocarbons.
When referring to aromatic hydrocarbons like naphthalene, it's crucial to understand the concept of aromaticity. This describes the electron-delocalization within the conjugated bond system, enhancing the molecule's stability. Aromatic compounds often provide pleasant smells, and are used in many products like mothballs, dyes, and even perfumes. In naphthalene, the aromatic rings are what allow it to resist reactions, making it effective as a moth-repellent.
Some facts about these hydrocarbons include:

• They usually have a distinct smell.
• They exist widely in natural and synthetic forms.
• They include benzene as a key member.

Electron-Dot Structure

The electron-dot structure, commonly known as the Lewis structure, is a helpful tool to visualize how atoms in a molecule are bonded together and how the electrons are distributed around them.
In the case of naphthalene, it's vital to first understand that these structures represent the connections between atoms using dots for electrons. For naphthalene, to draw the electron-dot structure:

• Start by drawing the two hexagonal rings sharing two carbons, resembling fused benzene rings.
• Place alternating double bonds on the rings. This pattern is vital for maintaining aromaticity.
• Each carbon should form four bonds in total, ensuring its octet is complete, which may involve attaching hydrogen atoms.

An accurate electron-dot structure is essential since it lays the groundwork for understanding how electrons may shift in resonance structures.

Resonance Structures

The concept of resonance in chemistry is crucial, especially for molecules like naphthalene. It involves the distribution of electrons across more than one structure, which collectively depict the molecule's electron arrangement.
In naphthalene and similar compounds, no single Lewis structure can fully represent the molecule. Instead, multiple resonance structures show possible configurations. This is crucial because resonance hybrid (the average of these structures) better describes the true nature of the molecule, depicting delocalized electrons that are spread over several atoms.
To draw resonance structures for naphthalene:

• Indicate electron movement with curved arrows.
• Shift double bonds while maintaining the molecule's aromatic characteristics.
• Ensure each new structure adheres to proper bonding and electron arrangements.

This process highlights the electrostatic stability conferred through electron delocalization.

Chemical Bonding

Understanding chemical bonding is vital to comprehend how atoms connect to form complex structures like naphthalene. At a basic level, chemical bonds involve the sharing or transferring of electrons between atoms to achieve stability.
In aromatic hydrocarbons such as naphthalene, bonding intricacies are fascinating. With naphthalene, each carbon atom forms part of a pi-bond system. Double bonds join select carbons while single bonds connect others to adjacent atoms or hydrogens to satisfy every carbon's tetravalence.
Chemical bonding types in naphthalene include:

• Sigma Bonds: These are single covalent bonds that occur when two orbitals overlap along the internuclear axis, creating robust connections between carbon and hydrogen atoms.
• Pi Bonds: Found in double bonds, these arise in regions above and below the plane of atoms, adding layers of electronic interaction essential for aromaticity.

Overall, chemical bonding underpins the molecule’s formation, driving its structural and functional behavior.

Organic Chemistry

Organic chemistry is the branch of chemistry that deals with the study of carbon-based compounds. These compounds make up the things around and inside humans. It includes endless structures made of carbon and hydrogen atoms, known as hydrocarbons, and more complicated molecules that contribute to the chemistry of life.
Naphthalene is one of these organic compounds and serves as a prime example of the diversity and complexity within organic chemistry. With its two benzene rings, naphthalene shows how simple carbon-based structures can become quite complex and possess unique properties like aromaticity and resonance.
Essential elements to bear in mind about organic chemistry:

• Carbon Atoms: They can bond in numerous ways, forming chains, rings, and frameworks critical to all organic compounds.
• Aromatic Compounds: Beyond just hydrocarbons, they include diverse molecules pivotal in pharmaceuticals, agriculture, and materials science.
• Versatility: Organic compounds range from simple to extremely complex structures, demonstrating adaptability and intricacy.

Organic chemistry’s application goes far beyond simple substances, significantly impacting industrial and everyday life.