CHM 211 Lecture 1

CHM 211: Organic Chemistry I

• Instructor: Dr. Bilkisu A.A

Content Overview

• Chemistry of aromatic compounds

• Structures of simple sugars, starch, and cellulose

• Peptides and proteins

• Mechanisms of substitution, elimination, addition, and rearrangement reactions

Chemistry of Aromatic Compounds

Aromatic compounds, derived from the Greek word aroma meaning pleasant smell, refer to a specific class of hydrocarbons known as ‘arenes.’ These organic compounds primarily consist of carbon and hydrogen, characterized by sigma bonds and delocalized pi electrons existing between carbon atoms in a cyclic arrangement. Aromatic compounds can be broadly categorized into benzenoids, which contain at least one benzene ring, and nonbenzenoids, which do not, such as furan. Notably, the benzene ring, a highly unsaturated structure, retains its unsaturation during reactions, unlike typical alkenes. Consequently, aromatic compounds exhibit a distinct reactivity pattern; they do not participate in standard electrophilic addition reactions as this could compromise the aromaticity of the ring. Instead, they undergo reactions facilitated through an aromatic electrophilic substitution mechanism.

Structure of Benzene

Benzene was first isolated by Michael Faraday in 1825, with the molecular formula C6H6 indicating a high degree of unsaturation. Its stable structure was established through experimental evidence such as the formation of triozonide, implicating three double bonds. The existence of a single monosubstituted derivative inferred that all carbons and hydrogens in benzene are equivalent. August Kekulé proposed a structural model for benzene in 1865 featuring a cyclic arrangement of six carbon atoms, interspersed with alternating single and double bonds. However, the formation of only one ortho disubstituted product contradicted this model.

Kekulé introduced the oscillation of double bonds to address this issue. Although this adjustment highlighted benzene’s resonance characteristic, it failed to fully elucidate its unusual stability and preference for substitution over addition reactions. Presently, Valence Bond Theory articulates this resonance phenomenon more clearly, suggesting a hybrid representation of benzene with a circle indicating delocalized electrons shared across the six carbon atoms. All carbon atoms in benzene are sp² hybridized, forming six sigma bonds with adjacent carbon atoms and hydrogen, while retaining unhybridized p orbitals that facilitate the formation of overlapping π bonds.

Delocalization and Stability of Benzene

The lateral overlap of the unhybridized p orbitals generates a delocalized π bond, allowing the electrons to oscillate freely among the six carbon nuclei. Evidence from X-ray diffraction indicates that benzene is planar and possesses equal bond lengths among its C–C bonds. This resonance stabilizes benzene beyond that of a hypothetical cyclohexatriene, informing our understanding of its reactivity.

Aromaticity Criteria

Aromaticity describes the unique chemical behavior of cyclic, planar molecules exhibiting resonance bonds, conferring greater stability than analogous structures without these characteristics. To be classified as aromatic, a molecule must satisfy four structural criteria:

1. Cyclic Structure: Must be arranged in a ring.

2. Planarity: All p orbitals must align for effective delocalization.

   • Example: Cyclooctatetraene is nonplanar and reacts like typical alkenes.

3. Conjugation: Every atom in the ring must have a p orbital, allowing for overlap.

4. Hückel’s Rule: Must contain a specific number of π electrons defined by the formula 4n+2, where n is a non-negative integer.

   • This means the numbers of π electrons must fit the sequence: 2, 6, 10, 14, and so forth.

Classifying π Electron Counts

According to Hückel's theory, benzene's aromatic stability stems from its 6 π electrons harmonizing with the 4n+2 rule. Compounds categorized as antiaromatic adhere to the condition of being cyclic, planar, and conjugated but possess 4n π electrons (e.g., N=4 gives 4, 8, 12). Nonaromatic compounds lack continuity in p orbital overlap or planarity, preventing them from being aromatic or antiaromatic.

Stability Comparisons

Molecular Orbital Theory in Aromaticity

In benzene, the six sp² carbon atoms are configured such that each forms two bonds with adjacent carbons while using the third bond for hydrogen. The p orbitals, which remain unhybridized, are perpendicular to the plane, collectively creating molecular orbitals categorized into bonding and anti-bonding. Frost’s circle aids in visualizing aromaticity; prisms can reveal energy levels and orbital occupancy. Aromatic compounds feature fully occupied bonding molecular orbitals, while antiaromatic compounds may comprise unfilled orbitals, rendering them less favorable in terms of stability.

Examples of Aromatic Compounds

Here are a few examples:

• Benzene (6 π electrons): Parent aromatic compound.

• Naphthalene (10 π electrons): Core of many synthetic dyes.

• Phenanthrene and Anthracene (14 π electrons): Prominent polycyclic aromatic hydrocarbons.

Aromatic Heterocycles

Heterocycles like pyridine and pyrrole illustrate aromatic behavior with heteroatoms that can share electrons in the π system. Pyridine acts as a base, while pyrrole has a structure capable of resonance due to the nitrogen atom's lone pair. Azulene, a non-benzenoid aromatic compound, lacks a benzene structure yet meets the aromatic criteria.