Aromatic Hydrocarbons II: Substituent Effects, Oxidation, and Polycyclic Aromatic Hydrocarbons

Aromatic Hydrocarbons II

Introduction

This section explores advanced concepts in aromatic chemistry, focusing on the effects of substituents on benzene reactivity and orientation, the oxidation of alkyl benzenes, and the chemistry and health implications of polycyclic aromatic hydrocarbons (PAHs). Understanding these topics is essential for predicting reaction outcomes and assessing environmental and biological impacts.

Substituent Effects in Aromatic Rings

Substituted Benzenes and Electrophilic Aromatic Substitution (EAS)

Substituted benzenes undergo electrophilic aromatic substitution (EAS), where the presence of a substituent influences both the rate and position of further substitution.

• Key Question: Does a substituent affect the addition of other substituents? Yes, it can alter both reactivity and orientation.

• Possible Effects: Rate of substitution and position (ortho, meta, para) of the added substituent.

Types of Substituent Effects

• Activating Groups: Increase reactivity compared to benzene; typically direct substitution to ortho and para positions.

• Deactivating Groups: Decrease reactivity; often direct substitution to the meta position.

• Halogens: Unique; deactivate the ring but direct substitution to ortho and para positions.

Origins of Substituent Effects

Substituent effects arise from two main electronic phenomena:

• Inductive Effects: Caused by electronegativity and polarizability of the substituent, affecting electron density through sigma bonds.

• Resonance Effects: Involve delocalization of electrons via pi bonds or lone pairs, altering electron density in the ring.

Inductive Effects

• Atoms more electronegative than carbon (e.g., N, O, halogens) withdraw electrons from the ring via sigma bonds.

• Alkyl groups donate electrons by inductive effect, increasing electron density in the ring.

• Example: N and O withdraw electron density; alkyl groups donate electron density.

Resonance Effects

• Observed with substituents containing lone pairs or pi bonds directly bonded to the ring.

• Resonance can either donate or withdraw electrons, depending on the substituent.

• Electron donation: Resonance structures place a negative charge on the ring (e.g., -OH, -NH2).

• Electron withdrawal: Resonance structures place a positive charge on the ring (e.g., -NO2, -COOH).

Resonance Effects – Electron Withdrawal

• Substituents like C=O, CN, NO2 withdraw electrons from the aromatic ring by resonance.

• Pi electrons flow from the ring to the substituent.

Resonance Effects – Electron Donation

• Halogen, OH, OR, and NH2 substituents donate electrons to the ring by resonance.

• Pi electrons flow from the substituent to the ring, with the effect greatest at ortho and para positions.

Classification of Substituents by Directing Effects

• Ortho, para directors: R groups or substituents with a nonbonded electron pair directly attached to the ring (e.g., -CH3, -NH2, -OH).

• Meta directors: Substituents with a full or partial positive charge on the atom bonded to the ring (e.g., -NO2, -COOH, -SO3H).

• Halogens: Deactivate the ring but direct substitution to ortho and para positions due to resonance stabilization.

Type	General Structure	Directing Effect	Examples
Ortho, para directors & activators	Electron-donating	Ortho, para	-OH, -NH2, -CH3
Ortho, para directors & deactivators	Halogens	Ortho, para	-F, -Cl, -Br, -I
Meta directors	Electron-withdrawing	Meta	-NO2, -COOH, -SO3H, -CN

Why Substituents Activate or Deactivate a Benzene Ring

Mechanistic Explanation

Activation or deactivation is determined by the stability of the carbocation intermediate formed during the first step of EAS.

• Step 1: Addition of the electrophile (E+) forms a resonance-stabilized carbocation.

• Stabilization: Electron-donating groups stabilize the carbocation, increasing reaction rate (activation).

• Destabilization: Electron-withdrawing groups destabilize the carbocation, decreasing reaction rate (deactivation).

Equation:

• Stabilized carbocation: $\text{Substitution is faster; the ring is activated}$

• Destabilized carbocation: $\text{Substitution is slower; the ring is deactivated}$

Examples of Orientation Effects in Substituted Benzene

• CH3 Group: Ortho, para director due to electron-donating inductive effect.

• NH2 Group: Ortho, para director with additional resonance stabilization.

• NO2 Group: Meta director; meta attack avoids destabilized carbocation intermediate.

• Halogens: Ortho, para-directing deactivators; inductive withdrawal outweighs resonance donation.

Halogenation of Activated Benzenes

Polyhalogenation and Monosubstitution

• Polyhalogenation: Occurs with strong electron-donating groups; multiple halogen atoms can be introduced.

• Monosubstitution: Can occur without added catalyst in activated rings.

Disubstituted Benzene: Rules for Substitution

• Rule 1: If two groups reinforce, the new substituent is placed as directed by both.

• Rule 2: If two groups oppose, the more powerful activator determines the position.

• Rule 3: No substitution occurs between two meta substituents due to steric crowding.

Oxidation of Alkyl Benzene

Benzylic Oxidation

Alkyl benzenes with at least one benzylic C-H bond are oxidized to benzoic acid under strong oxidizing conditions.

• Reagents: $\text{KMnO}_4$, $\text{Na}_2\text{Cr}_2\text{O}_7$/H2SO4, heat

• General Reaction: Alkylbenzene $\xrightarrow{[O]}$ Benzoic acid

• Equation: $\text{C}_6\text{H}_5\text{CH}_3 \xrightarrow{[O]} \text{C}_6\text{H}_5\text{COOH}$

• Substrates with multiple alkyl groups: Oxidized to dicarboxylic acids (e.g., xylene to phthalic acid).

• 3° Alkyl groups: Not oxidized due to lack of benzylic C-H bond.

Polycyclic Aromatic Hydrocarbons (PAH)

Structure and Properties

• Definition: PAHs are aromatic compounds composed of multiple fused benzene rings.

• Physical Properties: Can be colorless, white, or pale yellow/green solids.

• Occurrence: Found naturally and as a result of human activities.

Carcinogenicity of PAHs

• Some PAHs are carcinogenic due to their ability to damage DNA.

• PAHs are flat molecules that can intercalate between DNA base pairs.

• Once intercalated, PAHs can react with DNA bases or the phosphate backbone, leading to strand breaks or mutations during transcription.

• Example: Oxidation of PAH by liver enzymes produces DNA-binding products.

Sources of PAHs

• Formed during incomplete burning of coal, oil, gas, garbage, and tobacco.

• Released from volcanoes, forest fires, exhaust.

• Found in coal tar, crude oil, dyes, plastics, pesticides, BBQ/smoked meat and fish.

Routes of Human Exposure

• Inhalation of air releases

• Contact with contaminated soil

• Ingestion of contaminated water, cow's milk, foods

• Consumption of charred/smoked meat and fish, cereals, vegetables, fruits

• Exposure indoors, especially via second-hand smoke

Protection Strategies

• Ensure proper enclosure, ventilation, and protective equipment

• Wash immediately after exposure and before going home

• Change clothes at work, launder separately

• Avoid second-hand smoke

• Use properly installed stoves

• Avoid smoked foods; remove charred parts if barbecuing

• Do not smoke

Summary Table

Topic	Key Points
Substituted benzene	Electron donor: activator, -o, -p director Electron withdrawal: deactivator, -m director Halogens: electron withdrawal, deactivator, -o, -p director
Oxidation of alkyl benzene	Oxidized to benzoic acid YES: 1° or 2° alkyl (benzylic C-H) NO: 3° alkyl
Polycyclic aromatic hydrocarbons	Multiple rings Carcinogenic

References

• Carey, F.A. (2008) Organic Chemistry, 7th ed. McGraw Hill

• McMurry, J. (2008) Organic Chemistry, 7th ed. Thomson Brooks Cole

• Bruice, P.Y. (2017) Organic Chemistry, 8th ed. Prentice Hall International

• Bruice, P.Y. (2016) Essential Organic Chemistry: Study Guide and Solution Manual, 3rd ed. Pearson

• Brown, W.H., Poon, T. (2016) Introduction to Organic Chemistry, 6th ed. Wiley