Delocalized Electrons in Organic Chemistry

Delocalized electrons are critical in organic chemistry, affecting concepts addressed in later chapters.
This chapter focuses on representation of delocalized electrons and their implications on pKa values, carbocation stability, and product formation from electrophilic addition reactions.
Delocalized electrons are shared by three or more atoms, as opposed to localized electrons that belong to a single atom or are shared between two atoms.
- Example of localized electrons:
  - Structure: CH3 NH2
- Example of delocalized electrons:
  - Structure: COO− where two p electrons are shared across three atoms (two oxygens and one carbon).
The chapter includes recognition of compounds with delocalized electrons and the understanding of their special characteristics and wide-ranging effects on organic reactions.

Early chemists, lacking knowledge of delocalized electrons, struggled to explain the structure of benzene (C6H6).
Benzene's stability and the uncharacteristic reactions (not undergoing typical electrophilic addition like alkenes) contributed to confusion.
Only one product results from mono-substitution of benzene, while three distinct products arise from disubstitution.
Benzene has eight fewer hydrogens than the acyclic alkane formula CnH2n+2, indicating a degree of unsaturation equal to four, implying total number of rings and double bonds is four.
Structures compared:
- Two acyclic configurations yield only two disubstituted products, conflicting with observed results.
Cyclohexene with not having alternating single and double bonds:
- Bond lengths: 1.39 Å (benzene) < 1.54 Å (single bond) < 1.33 Å (double bond).
The six carbon-carbon bonds are identical in length, indicative of delocalization, which provides stability through resonance.

Friedrich Kekulé proposed that benzene is a resonance hybrid of two structures rapidly interconverting, explaining the observed products from disubstitution.
The confirmation of benzene's structure came with the work of Paul Sabatier in 1901 through catalytic hydrogenation producing cyclohexane under extreme conditions.
Raman and X-ray diffraction techniques confirmed a planar structure in the 1930s, enhancing scientific understanding of resonant structure stability.

Each carbon in benzene is sp2 hybridized, creating 120° bond angles, forming a planar molecule.
Overlapping p orbitals among six carbons result in a doughnut-shaped cloud of electrons above and below the benzene ring, denoting delocalization of six p electrons.
Benzene is often simplified in structures, represented either with a hexagon or circles to indicate bond lengths and behaviors without showing traditional double bonds.

While resonance contributors illustrate the delocalized electrons, they do not accurately show how many p electrons are involved.
The actual molecular structure is a resonance hybrid, averaging properties from contributing resonance structures.
The configurations using double-headed arrows denote electron delocalization across multiple resonance contributors, operating under the principle of visualizing average electron distribution.
The resonance hybrid of nitroethane, for instance, shows equal distribution across its contributing resonance structures, demonstrating characteristics of shared charge among electrophilic structures.

General rules for drawing resonance structures include:
- Atoms remain stationary, only moving the p electrons within allowed configurations.
- The net charge remains constant across structures, ensuring all resonance contributors adhere to overall charge balance.
- Only lone pairs and pi electrons can shift in the resonance forms, avoiding p orbitals of sp3 hybridized atoms.
Example illustrations highlight how to move electrons effectively to visualize the resonance contributors for different molecular structures.

Delocalization energy stabilizes compounds containing delocalized electrons, enhancing resonance energy above localized configurations.
Alkene and conjugated diene experiments illustrate diminished heat of hydrogenation correlating to increased stability of compounds due to electron delocalization, highlighting how energy differences govern molecular stability.

Isolated Dienes: Distinct double bonds separated by more than one single bond (lower stability).
Conjugated Dienes: Double bonds separated by a single bond allowing for electron delocalization (higher stability).
Reactivity and relative stabilities of substituted systems are also influenced by conjugation in diene structures.

Carboxylic acids exhibit stronger acidity than alcohols partly due to the resonance stabilization of their conjugate base (carboxylate ion) through inductive withdrawal by adjacent electronegative atoms.
The increased delocalization energy from carboxylate formation is contrasted with localized electrons in the conjugate base of alcohols, which leads to less stability.
Similar analysis applies to differences in acidity observed between phenols and cyclohexanol, further delineated through comparative pKa evaluations.

Aromatic compounds like benzene react through electrophilic aromatic substitution reactions: Reactions where electrophiles replace hydrogen atoms in benzene under specific conditions.
Electrophilic reactions during substitution preserve aromaticity, in contrast to alkene addition reactions, explaining their unique reactivity and resistance to typical addition modalities.

An electrophile (Y+) adds to the benzene ring, creating a carbocation intermediate.
This intermediate undergoes deprotonation where a base (B:) removes the hydrogen to restore aromaticity. This results in the aromatic substitution product, completed through single-step concerted mechanisms.

The discuss of delocalized electrons emphasizes connections between structural reactivity and stability, fusion to electrophilic addition reactions across various organic families.
The interaction between reactants and structural features significantly influences organic reactions, emphasizing how knowledge of electron delocalization guides the predictability of substitution and addition products in synthetic applications.

Understanding delocalized electrons and their implications in terms of pKa values, stability of intermediates and products, and the overall reactivity of aromatic compounds in coordination with their electronic structures is essential in further advancements in organic chemistry and synthesis.
The chapters consolidate how resonance, electronic effects, and the behavior of aromatic compounds form the foundation of organic reactions encountered in higher-level chemistry studies.