Question

Each molecule in this problem can be drawn as a hybrid of five contributing structures: two Kekulé structures and three that involve creation and separation of unlike charges. Draw these five contributing structures for each molecule. (a) Chlorobenzene (b) Phenol (c) Nitrobenzene

Short answer

Question: Draw the five contributing structures (two Kekulé structures and three structures that involve creation and separation of unlike charges) for the following molecules: a) Chlorobenzene b) Phenol c) Nitrobenzene

Step-by-step solution

Understanding Kekulé structures

A Kekulé structure is a representation of a molecule that shows alternating single and double bonds in a resonance structure. Resonance structures are different ways of representing a molecule and contribute to the overall hybrid structure. In this exercise, we need to draw five contributing structures (including two Kekulé structures) for each molecule. Now let's draw the five contributing structures for each of the given molecules:

(a) Chlorobenzene: Kekulé structures

For chlorobenzene, the Kekulé structures will have alternating single and double bonds in the benzene ring, with the chlorine atom attached to one of the carbon atoms. Here are the two Kekulé structures for chlorobenzene: 1. Cl connected to a double bond: \chemfig{*6(=-=(-(-Cl)-=)-=-)} 2. Cl connected to a single bond: \chemfig{*6(=-(=(-Cl)-)-=-=)}

(a) Chlorobenzene: Structures with charge separation

We will now draw three structures with charge separation for chlorobenzene: 1. Positive charge on the carbon where Cl is attached, and negative charge on the adjacent carbon with double bond: \chemfig{*6((+)-(-Cl)-=(-(-))-=-)} 2. Positive charge on the carbon where Cl is attached, and negative charge on the other adjacent carbon with double bond: \chemfig{*6(=-=(-(-Cl)(+))-(-)=)} 3. Negative charge on the carbon where Cl is attached, and positive charge on the adjacent carbon with double bond: \chemfig{*6((+)-(-Cl(-))-=-=)}

(b) Phenol: Kekulé structures

For phenol, the Kekulé structures will have alternating single and double bonds in the benzene ring, with the hydroxyl group (OH) attached to one of the carbon atoms. Here are the two Kekulé structures for phenol: 1. OH connected to a double bond: \chemfig{*6(=-=(-(-OH)-=)-=-)} 2. OH connected to a single bond: \chemfig{*6(=-(=(-OH)-)-=-=)}

(b) Phenol: Structures with charge separation

We will now draw three structures with charge separation for phenol: 1. Positive charge on the carbon where OH is attached, and negative charge on the adjacent carbon with double bond: \chemfig{*6((+)-(-OH)-=(-(-))-=-)} 2. Positive charge on the carbon where OH is attached, and negative charge on the other adjacent carbon with double bond: \chemfig{*6(=-=(-(-OH)(+))-(-)=)} 3. Negative charge on the carbon where OH is attached, and positive charge on the adjacent carbon with double bond: \chemfig{*6((+)-(-OH(-))-=-=)}

(c) Nitrobenzene: Kekulé structures

For nitrobenzene, the Kekulé structures will have alternating single and double bonds in the benzene ring, with the nitro group (NO2) attached to one of the carbon atoms. Here are the two Kekulé structures for nitrobenzene: 1. NO2 connected to a double bond: \chemfig{*6(=-=(-(-NO_2)-=)-=-)} 2. NO2 connected to a single bond: \chemfig{*6(=-(=(-NO_2)-)-=-=)}

(c) Nitrobenzene: Structures with charge separation

We will now draw three structures with charge separation for nitrobenzene: 1. Positive charge on the carbon where NO2 is attached, and negative charge on the adjacent carbon with double bond: \chemfig{*6((+)-(-NO_2)-=(-(-))-=-)} 2. Positive charge on the carbon where NO2 is attached, and negative charge on the other adjacent carbon with double bond: \chemfig{*6(=-=(-(-NO_2)(+))-(-)=)} 3. Negative charge on the carbon where NO2 is attached, and positive charge on the adjacent carbon with double bond: \chemfig{*6((+)-(-NO_2(-))-=-=)}

Key concepts

Kekulé Structures

Kekulé structures are a specific type of resonance structure used to represent aromatic compounds, like benzene rings, in organic chemistry. These structures depict alternating single and double bonds within the ring, conveying the possibility of electron delocalization. In reality, the electrons are more fluidly shared across the bonds than depicted, providing stability to the molecule through resonance.
For example, benzene is often drawn with alternating single and double bonds in either of the two Kekulé forms, but neither fully represents the true structure, which is a hybrid of both.

• Kekulé structures help in visualizing potential stability granted by resonance.
• They demonstrate the concept of delocalization indirectly.
• In representation, the positions of double bonds shift yet maintain symmetry.

Kekulé structures are fundamental in understanding aromatic compounds' reactivity, as they are approachable via resonance and hybridization theories.

Chlorobenzene Structures

Chlorobenzene is an aromatic compound where a chlorine atom is attached to a benzene ring. Representation often involves considering both Kekulé structures and those maintaining charge separation due to electronegativity differences.

• In Kekulé representations, chlorine alternates between being attached to a single or a double bond; both demonstrate the possibility of resonance.
• Kekulé forms showcase benzene's delocalized electrons, offering insights into chlorobenzene's aromatic stability.

Charge-separated structures illustrate polarization caused by chlorine, an electronegative element, influencing electron density.

• Positive charge visually rests on the carbon with chlorine, while negative charges appear on adjacent carbons, indicating possible electron shifts.
• Such structures explain potential reactivity patterns and bond polarities of chlorobenzene.

Phenol Structures

Phenol involves a hydroxyl group (-OH) attached to a benzene ring. Like chlorobenzene, its resonance is depicted through Kekulé structures as well as charge-separated forms.
Phenol's Kekulé arrangements involve alternating single and double bonds with the hydroxyl group directly influencing electron distribution.

• The hydroxyl group adds hydrogen bonding capabilities, affecting phenol's physical properties.
• Two main resonance contributors show the -OH connected via different bonds, emphasizing resonance stability.

Charge-separated structures illustrate potential resonance forms involving the hydroxyl group:

• Positive charges may manifest on the oxygen due to electronegativity, with negative charges on adjacent carbons.
• These forms stress the electron-donating nature of -OH, contributing to phenol's reactive nature.

Nitrobenzene Structures

Nitrobenzene, featuring a nitro group (-NO₂) attached to a benzene ring, is understood through Kekulé structures and possible charge-separated forms.

• Its Kekulé models demonstrate the aromatic ring's consistent electron flow despite NO₂'s presence.
• The nitro group, being an electron-withdrawing entity, influences the reactivity of nitrobenzene in chemical reactions.

In charge-separated images, the nitro group significantly alters electron distribution:

• Positive charges commonly appear on the carbon bonded to N, translating to increased electron pull.
• The unequal electron density impacts nitrobenzene’s chemistry, particularly in electrophilic aromatic substitution reactions.
• Such structures provide a visual understanding of electron trial across the molecule.

Charge Separation in Organic Molecules

Charge separation in organic molecules refers to arrangements where electrons are distributed unequally across atoms, resulting often in a molecule polarizing temporarily.
This mechanism is crucial in appreciating how certain groups within molecules, like halogens or nitro groups, influence overall molecular behavior:

• More electronegative atoms induce a dipole by pulling electron density towards themselves.
• Charge separation often leads to increased reactivity, providing sites for various chemical processes.

This characteristic is frequently represented in resonance structures, which help visualize different states a molecule might adopt.

• These visual frameworks allow chemists to predict a molecule's interactions and preferred reaction pathways.
• Understanding charge separation facilitates the design of reactions in synthesis and analytical chemistry.

Charge separation is a pervasive theme across organic chemistry, underpinning many concepts on molecular structure and reactivity.