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Questions and Answers
What is the correct naming convention for a benzene ring with two identical substituents?
What is the correct naming convention for a benzene ring with two identical substituents?
Which of the following represents a correct way to indicate the different positions of substituents on a benzene ring?
Which of the following represents a correct way to indicate the different positions of substituents on a benzene ring?
What is a characteristic of an aromatic compound according to Hückel's rule?
What is a characteristic of an aromatic compound according to Hückel's rule?
In terms of substituent naming, how should different substituents on a benzene ring be arranged?
In terms of substituent naming, how should different substituents on a benzene ring be arranged?
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Which condition is NOT required for a compound to be considered aromatic?
Which condition is NOT required for a compound to be considered aromatic?
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What distinguishes aromatic hydrocarbons from aliphatic hydrocarbons?
What distinguishes aromatic hydrocarbons from aliphatic hydrocarbons?
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Which statement accurately describes the structure of benzene?
Which statement accurately describes the structure of benzene?
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What is the resonance energy of benzene?
What is the resonance energy of benzene?
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What is indicated by the low heat of hydrogenation of benzene?
What is indicated by the low heat of hydrogenation of benzene?
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Which reaction does benzene NOT undergo?
Which reaction does benzene NOT undergo?
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Study Notes
Pharmaceutical Organic Chemistry I (POC101) - Lecture 7: Aromaticity
- Aromatic hydrocarbons are compounds like benzene and its related compounds, which exhibit similar chemical behavior to benzene.
- Unlike aliphatic hydrocarbons which undergo addition reactions to multiple bonds (C=C, C≡C) and free radical substitution, aromatic hydrocarbons primarily undergo substitution reactions.
Aromaticity
- Benzene is a key member of a broad group of aromatic compounds.
- Aromaticity is a property of compounds that are stabilized by delocalization of π electrons.
- Resonance energy quantifies the stability gain in aromatic compounds. (e.g., benzene = 36 kcal/mol, naphthalene = 61 kcal/mol)
Benzene and Aromatic Compounds
- Benzene (C₆H₆) is the simplest aromatic hydrocarbon.
- Benzene structure is a resonance hybrid of two equivalent Lewis structures, with alternating double and single bonds.
- The π electrons are delocalized around the ring.
- Benzene has 6 π electrons.
Benzene Bond Length
- Actual C-C bond length in benzene (1.39 Å) is intermediate between single (1.53 Å) and double (1.34 Å) bonds.
- This indicates that all C-C bonds in benzene are equivalent and intermediate in length.
Stability of Benzene
- Benzene's heat of hydrogenation is lower than expected, implying unusually high stability compared to cyclohexene or cyclohexadiene.
- Benzene does not undergo addition reactions. Reaction with Br₂ results in substitution not addition.
Nomenclature of Benzene Derivatives
- Naming benzene derivatives involves identifying the substituent and adding the word "benzene."
- Common names exist for some monosubstituted benzenes (e.g., toluene, phenol, aniline).
- For disubstituted benzenes, prefixes ortho (o), meta (m), and para (p) are used to specify the relative positions of the two substituents.
- Alphabetizing substituent names is needed if more than one substituent is present.
Hückel's Rule for Aromaticity
- Cyclic: The molecule must form a closed ring.
- Planar: The molecule must be flat.
- Completely conjugated: All atoms involved in the delocalized π system must have p orbitals.
- 4n + 2 π electrons: The number of π electrons must follow this rule where 'n' is an integer (0, 1, 2…)
Examples of Aromatic and Non-Aromatic Compounds
- Cyclobutadiene (4 π electrons) is non-aromatic.
- Benzene (6 π electrons) is aromatic.
- Cycloheptatrienyl cation (6 π electrons) is aromatic cation.
- Cyclopentadienyl anion (6 π electrons) is aromatic.
- Cycloheptatriene (6 π electrons) is not fully delocalized because the CH₂ group lacks a p orbital in the π system.
Annulenes
- Completely conjugated rings larger than benzene can also be aromatic if they are planar and abide by Hückel's rule.
- Hydrocarbons with alternating double and single bonds in a single ring are called annulenes.
- The number of atoms in the ring is indicated in brackets to name an annulene.
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Description
Explore the fascinating world of aromatic hydrocarbons in this quiz on Pharmaceutical Organic Chemistry I. Learn about benzene, its resonance structures, and the unique stability conferred by aromaticity. Test your understanding of substitution reactions and the significance of resonance energy in these compounds.
