Covers arenes and their defining features: the cyclic, sp² hybridised benzene ring, planarity, cyclic conjugation and Hückel's 4n+2 rule for aromaticity. Walks through the stepwise mechanism of electrophilic aromatic substitution, using bromination of benzene with FeBr3 as the example, then activating and deactivating ring substituents, polycyclic aromatic hydrocarbons and procarcinogens such as those in tobacco. Several sections are illustrated with pictures.
Topic 20 of 42 in the MU Sofia syllabus for Chemistry (Medicine, Year 1). The free AceMed note covers it in 7 sections, about 582 words (4 min of reading), with 18 flashcards.
In this note
Aromatic hydrocarbons (Arenes).
Aromaticity–Hückel’s rule.
Reactions of electrophilic substitution .
Orientation in benzene ring.
Activating and deactivating groups.
Polycyclic aromatic hydrocarbons.
Procarcinogens.
Aromatic hydrocarbons (Arenes).
Aromatic hydrocarbons, also known as arenes, are a class of organic compounds characterized by specific structural features and properties.
1. Structure:
Cyclic: Carbon atoms are joined in a ring structure.
Sp² hybridization: Each carbon atom in the ring is sp² hybridized, meaning they form three sigma (σ) bonds and one pi (π) bond.
Benzene ring: The most common aromatic structure is the benzene ring, which consists of six carbon atoms arranged in a hexagonal shape with alternating single and double bonds. Hydrogen atoms can be attached to the carbons around the ring.
Aromaticity: This term refers to a specific set of properties, not just the presence of a benzene ring. Aromatic compounds exhibit:
Planar structure: The ring atoms lie in the same plane due to the delocalization of pi electrons.
Cyclic conjugation: Pi electrons are delocalized across the entire ring system, creating a stable configuration.
Hückel's rule: The number of delocalized pi electrons must be 4n + 2, where n is an integer (0, 1, 2, etc.). Benzene, with 6 pi electrons, follows this rule (4 x 1 + 2 = 6).
3. Examples:
Benzene (C6H6): The simplest and most common aromatic hydrocarbon. (First picture)
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