(i) Classify the following molecules as aromatic, nonaromatic, or antiaromatic. (ii) For aromatic molecules, solve for n in Hückel’s rule. For all other molecules, explain which rule of aromaticity is being broken. (c)
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Identify the molecule: The image shows a cycloheptatriene, which is a seven-membered ring with three alternating double bonds.
Determine the number of π electrons: Cycloheptatriene has 6 π electrons from the three double bonds.
Apply Hückel's rule: For a molecule to be aromatic, it must have (4n + 2) π electrons, where n is a non-negative integer. Check if 6 fits this formula.
Check planarity and cyclic nature: The molecule must be planar and cyclic for aromaticity. Cycloheptatriene is cyclic but not planar due to the presence of a non-conjugated sp3 hybridized carbon.
Classify the molecule: Since cycloheptatriene does not meet the planarity requirement, it is nonaromatic. It does not fulfill Hückel's rule due to the sp3 hybridized carbon disrupting conjugation.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Aromaticity
Aromaticity refers to a property of cyclic compounds that exhibit enhanced stability due to the delocalization of π electrons. For a molecule to be classified as aromatic, it must be cyclic, planar, fully conjugated, and follow Hückel's rule, which states that the number of π electrons must equal 4n + 2, where n is a non-negative integer.
Hückel's rule is a criterion used to determine the aromaticity of a compound. It states that a planar, cyclic molecule is aromatic if it contains 4n + 2 π electrons in its conjugated system. This rule helps in identifying aromatic compounds by calculating the total number of π electrons and checking if they fit the formula.
Anti-aromaticity is a property of cyclic compounds that are destabilized due to the presence of 4n π electrons, which leads to increased energy and instability. Unlike aromatic compounds, anti-aromatic compounds do not satisfy Hückel's rule and often exhibit significant reactivity and strain due to their unfavorable electronic configuration.