Name: cyclopentadienyl anion and aromaticity
Uploaded: 2023-04-30T10:31:23.000Z
Duration: 1 min 18 s
Channel: JoVE

Neutral monocyclic unsaturated hydrocarbons with an odd number of carbon atoms lack aromaticity due to the presence of an intervening sp3 carbon in the ring.
For example, cyclopentadiene is not aromatic, as it has only 4 &#960; electrons and an sp3 carbon that disrupts the continuous overlap of p orbitals.
Notably, removing a hydrogen from the CH2 group with both, one, or none of the bonding electrons converts the sp3 carbon to sp2, generating a cation, a radical, and an anion, respectively.
Compared to the cation and radical, only the anion has the required number of (4n + 2) &#960; electrons.
Moreover, the availability of a 2p orbital at the sp2 carbon facilitates a continuous overlap of p orbitals and delocalization of the negative charge throughout the ring, making the cyclopentadienyl anion aromatic.
Additionally, the &#960; electrons occupying the bonding molecular orbitals and the five resonance structures further corroborate the unusual stability of the aromatic anion.

aromatic hydrocarbon anions: structural overview

Aromatic Hydrocarbon Anions: Structural Overview

Neutral hydrocarbons like cyclopentadiene with an odd number of carbon atoms and one intervening CH2 group in the ring are not aromatic. Cyclopentadiene with 4 &#960; electrons does not satisfy the 4n + 2 &#960; electron rule. Additionally, the intervening CH2 group is sp3 hybridized and lacks a vacant p orbital, thereby interrupting the overlap of p orbitals in a continuous manner and preventing the delocalization of &#960; electrons throughout the ring.
 Due to the absence of continuous overlap of p orbitals, cyclopentadiene does not comply with aromaticity criteria. However, the removal of one hydrogen with both or one or no electrons from the CH2 group drives the conversion of sp3 carbon to sp2, thereby converting cyclopentadiene into cyclopentadienyl cation, radical, and anion, respectively, having a vacant p orbital. Among all the three species, cyclopentadienyl cation and radical with four and five &#960; electrons, respectively, do not meet the criteria of 4n + 2 &#960; electrons and are not aromatic. On the other hand, cyclopentadienyl anion with 6 &#960; electrons, a continuous overlapping ring of p orbitals, and delocalized &#960; electron density becomes aromatic. The electrostatic potential map of the anion corroborates the delocalization of &#960; electrons throughout the ring. Moreover, the five resonance structures of cyclopentadienyl anion and the occupancy of bonding molecular orbitals by all 6 &#960; electrons in the frost diagram validate the unusual stability of the anion.

Neutral hydrocarbons like cyclopentadiene with an odd number of carbon atoms and one intervening CH2 group in the ring are not aromatic. Cyclopentadiene ...

aromatic-hydrocarbon-anions-structural-overview

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Organic Chemistry

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Aromatic Compounds

KEY TERMS AND CONCEPTS

cyclopentadienyl anion and aromaticity

Cyclopentadiene is a neutral hydrocarbon that is not aromatic. It contains an odd number of carbon atoms and one CH2 group in the ring. With only 4 pi electrons, it does not follow the 4n + 2 pi electron rule. The CH2 carbon is sp3 hybridized, so it has no vacant p orbital to keep p orbitals overlapped around the ring.

That break in overlap stops pi electrons from spreading across the entire ring. As a result, cyclopentadiene does not meet the usual aromaticity criteria. If one hydrogen is removed from the CH2 group, the carbon changes from sp3 to sp2 hybridization. This creates a vacant p orbital and gives three related species: the cyclopentadienyl cation, radical, and anion.

These three species do not behave the same way. The cyclopentadienyl cation has 4 pi electrons, and the radical has 5 pi electrons. Neither one satisfies the 4n + 2 rule, so neither is aromatic. The cyclopentadienyl anion has 6 pi electrons, a continuous ring of overlapping p orbitals, and delocalized pi electron density. That makes it aromatic and unusually stable.

The electrostatic potential map of the anion supports this electron delocalization across the ring. Its five resonance structures also show how the electrons can be spread out. The frost diagram gives the same result, since all 6 pi electrons fill bonding molecular orbitals. Together, these features explain why the cyclopentadienyl anion is more stable than the neutral ring, the cation, or the radical.

Cyclopentadienyl Anion and Aromaticity

aromatic compounds: overview

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H&uuml;ckel Rule and Aromatic Ring Criteria

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h&uuml;ckel's rule diagram of &pi; mos: frost circle

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The Frost circle is a graphical method for showing the relative energies of pi molecular orbitals in planar, fully conjugated, monocyclic compounds. It is also called the inscribed polygon method. A. A. Frost and Boris Musulin first described this method in 1953. To build a Frost circle, draw a polygon with the same number of edges as the number of carbons in the cyclic system. Place one vertex pointing downward. Then draw a circle around the polygon so that every vertex touches the circle.

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Using Frost Circles to Test Aromaticity

Frost circles help test whether a cyclic compound is aromatic by using the inscribed polygon method and Hückel’s 4n + 2 rule. A compound is stable and aromatic when every bonding molecular orbital (MO) is completely filled with a pair of electrons. If electrons occupy non-bonding or antibonding orbitals, the compound is unstable and not aromatic. Frost circle diagrams for cycloalkenes with 4 to 8 carbons show a clear pattern. The number of bonding MOs is always odd. The electrons needed to...

Cyclopentadiene is a neutral hydrocarbon that is not aromatic. It contains an odd number of carbon atoms and one CH2 group in the ring. With only 4 pi electrons, it does not follow the 4n + 2 pi electron rule. The CH2 carbon is sp3 hybridized, so it has no vacant p orbital to keep p orbitals overlapped around the ring. That break in overlap stops pi electrons from spreading across the entire ring. As a result, cyclopentadiene does not meet the usual aromaticity criteria. If one hydrogen is...

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nmr spectroscopy of aromatic compounds

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key terms and concepts

Learn why cyclopentadiene is not aromatic and how the cyclopentadienyl anion becomes stable through 6 pi electrons, resonance, and orbital overlap.

Cyclopentadienyl Anion and Aromaticity | JoVE K12

organic chemistry

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Cyclopentadienyl Anion and Aromaticity

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