Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Non è quello che usi tu?Cambia libro di testo

Bruice 8th Edition

Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene

Problema 50a

Capitolo 8, Problema 50a

Which compound in each set is aromatic? Explain your choice.
a.

Guida verificata passo dopo passo

Step 1: Recall the criteria for aromaticity. A compound is aromatic if it satisfies the following conditions: (1) It is cyclic, (2) It is planar, (3) It has a conjugated π-electron system, and (4) It follows Huckel's rule, which states that the molecule must have (4n + 2) π-electrons, where n is a non-negative integer.

Step 2: Analyze cyclopropene. Cyclopropene is cyclic and planar, but it has only two π-electrons in its double bond. Since 2 does not satisfy Huckel's rule (4n + 2 = 2 is not possible for any integer n), cyclopropene is not aromatic.

Step 3: Analyze the cyclopropenyl cation. The cation is cyclic and planar, and it has a conjugated π-electron system. The cation has two π-electrons from the double bond and an additional empty p-orbital on the positively charged carbon. This results in a total of 2 π-electrons, which does not satisfy Huckel's rule. Therefore, the cyclopropenyl cation is not aromatic.

Step 4: Analyze the cyclopropenyl anion. The anion is cyclic and planar, and it has a conjugated π-electron system. The anion has two π-electrons from the double bond and two additional π-electrons from the lone pair on the negatively charged carbon. This results in a total of 4 π-electrons, which satisfies Huckel's rule (4n + 2 = 6, where n = 1). Therefore, the cyclopropenyl anion is aromatic.

Step 5: Conclude that among the given compounds, the cyclopropenyl anion is aromatic because it satisfies all the criteria for aromaticity, including Huckel's rule.

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Aromaticity

Aromaticity refers to a property of cyclic compounds that exhibit enhanced stability due to delocalized π electrons. For a compound to be aromatic, it must be cyclic, planar, fully conjugated, and follow Hückel's rule, which states that it should have 4n + 2 π electrons, where n is a non-negative integer.

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Intro to Aromaticity

Cyclopropene

Cyclopropene is a three-membered cyclic alkene with one double bond. It does not meet the criteria for aromaticity because it has only two π electrons and is not fully conjugated. Its structure leads to significant angle strain, making it less stable than larger cyclic compounds.

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01:49

General properties of cyclopropanation.

Cyclopropenyl Cation and Anion

The cyclopropenyl cation and anion are derived from cyclopropene by adding or removing a proton. The cyclopropenyl cation has 2 π electrons and is not aromatic, while the cyclopropenyl anion has 4 π electrons, which also disqualifies it from being aromatic. However, the anion is more stable due to the presence of a negative charge that can be delocalized.

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