Textbook Question
Ciprofloxacin is a member of the fluoroquinolone class of antibiotics.
a. Which of its rings are aromatic?
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18. Aromaticity
Basicity of Aromatic Heterocycles
4:03 minutesProblem 35f
Textbook Question
Some of the following compounds show aromatic properties, and others do not.
1. Predict which ones are likely to be aromatic, and explain why they are aromatic.
2. Predict which nitrogen atoms are more basic than water and which are less basic.
(f) 
Verified step by step guidance1
Step 1: Analyze the structure of the compound provided. The image shows a six-membered ring with alternating double bonds (a conjugated system) and a positively charged nitrogen atom with a hydrogen attached. This structure resembles pyridinium.
Step 2: Determine aromaticity using Huckel's rule. Huckel's rule states that a compound is aromatic if it is cyclic, planar, fully conjugated, and contains (4n + 2) π electrons, where n is an integer. Count the π electrons in the conjugated system of the ring.
Step 3: Evaluate the π electron count. The ring has three double bonds, contributing 6 π electrons. Since 6 π electrons satisfy Huckel's rule (4n + 2, where n = 1), the compound is aromatic.
Step 4: Assess the basicity of the nitrogen atom. The nitrogen in pyridinium is positively charged, which significantly reduces its ability to donate electrons. Therefore, this nitrogen is less basic than water.
Step 5: Summarize the findings. The compound is aromatic due to its cyclic, planar, conjugated structure and 6 π electrons. The nitrogen atom is less basic than water because it is positively charged and unable to effectively donate electrons.
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Video duration:
4mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Aromaticity
Aromaticity refers to the special stability and unique properties of cyclic compounds that follow Hückel's rule, which states that a molecule must have (4n + 2) π electrons, where n is a non-negative integer. Aromatic compounds are typically planar, fully conjugated, and exhibit resonance, leading to lower reactivity compared to non-aromatic compounds. The presence of a nitrogen atom in a ring can influence aromaticity depending on its hybridization and the electron density it contributes.
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Basicity of Nitrogen
The basicity of nitrogen atoms in organic compounds is determined by their ability to donate a lone pair of electrons. Factors such as hybridization, electronegativity, and the presence of electron-withdrawing or electron-donating groups affect basicity. In general, sp3-hybridized nitrogen is more basic than sp2-hybridized nitrogen due to the greater availability of the lone pair for protonation, while aromatic nitrogen can be less basic if it participates in the aromatic system.
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Hückel's Rule
Hückel's rule is a criterion for determining the aromaticity of cyclic compounds. It states that a planar, cyclic molecule must have a total of (4n + 2) π electrons in its conjugated system to be considered aromatic. This rule helps predict the stability and reactivity of compounds, as aromatic systems are generally more stable than their non-aromatic counterparts. Understanding this rule is essential for evaluating the aromatic nature of the compounds in the question.
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