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Chapter 21: Problem 17

Construct MO energy diagrams for the cyclopropenyl cation, radical, and anion. Which of these species is aromatic according to the Hückel criteria?

Short Answer

Expert verified
Answer: The cyclopropenyl cation is aromatic according to the Hückel criteria.

Step by step solution

01

Determine the number of π-electrons

Cyclopropenyl cation has two π-electrons, the radical has three π-electrons, and the anion has four π-electrons.
02

Construct the MO energy diagram for the cyclopropenyl cation

Draw an energy diagram with two energy levels (1 and -1). Place two electrons in the lower energy level (-1). The MO energy diagram for cyclopropenyl cation is: E =------------- | ↑↓ --------- (1) | ↑↓ E =------------- -------- (-1)
03

Construct the MO energy diagram for the cyclopropenyl radical

Draw an energy diagram with three energy levels (1,0, and -1). Place three electrons in the energy levels: two in the lower level (-1) and one in the highest level (1). The MO energy diagram for the cyclopropenyl radical is: E =------------- | ↑ --------- (1) | E =------------- | ↑↓ -------- (-1)
04

Construct the MO energy diagram for the cyclopropenyl anion

Draw an energy diagram with three energy levels (1,0, and -1). Place four electrons in the energy levels: two in the lower level (-1) and two in the highest level (1). The MO energy diagram for the cyclopropenyl anion is: E =------------- | ↑↓ --------- (1) | E =------------- | ↑↓ -------- (-1)
05

Apply the Hückel criteria and determine aromaticity

- Cyclopropenyl cation: The Hückel criteria require (4n+2) π-electrons. With two π-electrons, this corresponds to n=0, and thus the cation is aromatic. - Cyclopropenyl radical: With three π-electrons, the Hückel criteria are not met, and thus the radical is not aromatic. - Cyclopropenyl anion: With four π-electrons, the Hückel criteria are not met, as it doesn't follow the (4n+2) rule. Therefore the anion is not aromatic. In conclusion, only the cyclopropenyl cation is aromatic, according to the Hückel criteria.

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