Question

Draw the resonance structures for the following compounds. Show the electron shift using curved-arrow notation. (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NO}_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCHO}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CHO}\) (e) \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}_{2}\) (f) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH} \mathrm{C} \mathrm{H}_{2}\)

Short answer

Draw each molecule's structure, use curved arrows to show electron shifts, and generate resonance forms.

Step-by-step solution

Draw the Structure for C6H5OH

To find resonance structures, start by drawing the Lewis structure for phenol (\(\mathrm{C}_6\mathrm{H}_5\mathrm{OH}\)). The benzene ring consists of alternating double bonds, and an OH group is attached to one carbon atom. Ensure all atoms have the correct number of bonds and no formal charges.

Identify Location of Lone Pairs and π Bonds in C6H5OH

Identify the π bonds of the benzene ring and the lone pairs on the oxygen atom of the OH group. This step is crucial as these electrons participate in resonance.

Draw Curved Arrows for C6H5OH

Use curved arrows to show potential electron movement from a lone pair on the oxygen to form a double bond with the benzene ring, while moving another π bond within the ring. Ensure that each atom fulfills the octet rule.

Generate Resonance Structures for C6H5OH

Create two or more resonance structures by moving electrons either around the ring or towards the oxygen atom, keeping the aromatic ring intact. Each structure should depict a possible distribution of electrons.

Repeat Steps for C6H5NO2

Begin with the nitrobenzene structure, identifying the lone pairs on oxygen in \(\mathrm{NO}_2\) and the π bonds in the benzene ring. Use curved arrows to show electron movement from one oxygen to another via resonance, creating multiple structures.

Repeat Steps for CH3CH=CHCHO

Start with the linear Lewis structure of cinnamaldehyde, then use curved arrows to show possible electron shifts, resulting in resonance structures involving the \(\mathrm{C}=\mathrm{O}\) bond and the \(\mathrm{C}=\mathrm{C}\) bond.

Repeat Steps for C6H5-CHO

Draw benzaldehyde with the benzene ring and the CHO group. Identify π bonds and lone pairs, then use curved arrows to indicate possible resonance, focusing on the movement of electrons in the ring and between the ring and the carbonyl group.

Repeat Steps for C6H5-CH2

Note the benzyl group structure with an empty π bond ready for resonance with the benzene ring. Use arrows to show possible delocalization of π electrons within the ring, generating different resonance forms.

Repeat Steps for CH3CH=CHCH2

Draw the conjugated diene structure, highlighting the lone pairs and the π bonds. Show via curved arrows how electrons shift from the \(\mathrm{C}=\mathrm{C}\) double bond, resulting in resonance forms including cations or anions if necessary.

Key concepts

Curved-Arrow Notation

Curved-arrow notation is a helpful tool in organic chemistry for illustrating how electrons move in a molecule. This technique is particularly useful when depicting resonance structures and reaction mechanisms.
In curved-arrow notation, arrows start where the electrons are currently located and point towards where the electrons are moving. If you have lone pairs, the arrow originates from them. If you're showing a double bond moving, the arrow begins in the middle of the bond.

• This helps to visualize the flow of electrons, which is essential in understanding resonance structures.
• The direction of arrows can indicate shifts in bonds, creation of new bonds, or the movement of lone pairs.

When dealing with resonance structures like those in aromatic compounds, curved-arrow notation shows how electrons delocalize to create different electron distributions across the molecule. This method ensures that every step obeys the rules of chemistry, such as the octet rule.

Electron Shift

Electron shift refers to the movement of electrons within a molecule and is a fundamental aspect in understanding resonance and aromaticity. In organic molecules, electrons can shift as lone pairs or through bonds, facilitating the formation of different resonance structures.
Resonance structures are alternative ways that electrons can be arranged in a molecule while maintaining the basic structure. These electron shifts help stabilize the molecule by distributing charge over a larger area, leading to more stability.

• During resonance, electrons move to create new areas of electron density.
• Ensure the overall charge and atom positions remain constant between resonance forms.
• The concept of electron shift is essential when dealing with functional groups like carbonyl (C=O) and nitro groups (NO₂) where electrons often shift to stabilize the molecule.

Understanding electron shifts is a powerful way to predict how molecules behave in reactions and their properties.

Aromatic Compounds

Aromatic compounds are a class of molecules defined by their stability and unique electronic arrangement. The quintessential example is benzene, which has a hexagonal ring structure with alternating single and double bonds.
The electrons in aromatic compounds are delocalized across the ring, allowing for extra stability which is sometimes referred to as "aromatic stabilization."

• This delocalization is due to resonance and is represented using multiple resonance structures.
• To be classified as aromatic, a molecule must obey Huckel's rule: it must be cyclic, planar, and have \(4n + 2\) π electrons.
• Aromatic compounds exhibit a high degree of stability, impacting their reactivity and presence in many biological molecules.

Aromatic compounds are foundational in studying chemistry, especially when learning about electron shifts and resonance, as these concepts explain their unique stability and reactivity.

Lewis Structures

Lewis structures provide a simplified way to represent the bonds and lone pairs in a molecule. They offer a visual approach to determining how atoms in a molecule interact by showing where electrons are likely to be found.
Lewis structures can assist in identifying possible resonance structures by highlighting areas with lone pairs or π bonds, which are potential sites for electron shift.

• Begin by counting the total number of valence electrons available for bonding.
• Arrange atoms to satisfy the octet rule (or duet rule for hydrogen), showing bonds as either lines (bond pairs) or dots (lone pairs).
• Lewis structures help to predict the distribution of electrons, formal charges, and the overall geometry of molecules.

Using Lewis structures effectively allows for predictions about molecular shape and reactivity, and they serve as a foundational tool in more complex concepts like resonance and aromaticity.