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Related Concept Videos

Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

4.2K
Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
Removing one hydrogen from the intervening CH2 group...
4.2K
Aromatic Hydrocarbon Anions: Structural Overview01:18

Aromatic Hydrocarbon Anions: Structural Overview

4.2K
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 π electrons does not satisfy the 4n + 2 π 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 π electrons throughout the ring.
Due to the absence of continuous...
4.2K
Frost Circles for Different Conjugated Systems01:18

Frost Circles for Different Conjugated Systems

4.1K
The inscribed polygon method is consistent with Hückel’s 4n + 2 rule and helps to learn whether the given cyclic compound is aromatic or not. The compound is stable and aromatic if every bonding molecular orbital (MO) is completely filled with a pair of electrons. However, if the non-bonding or antibonding orbitals are filled with electrons, the compound is unstable and not aromatic. Consider the Frost circle diagrams for cycloalkenes containing 4 to 8 carbons.
4.1K
Conformations of Cyclohexane02:11

Conformations of Cyclohexane

16.8K
Cyclohexane does not exist in a planar form due to the high angle and torsional strain it would experience in the planar structure. Instead, it adopts non-planar chair and boat conformations.
The chair form is the most stable and derives its name from its resemblance to the “easy chair.” In the chair conformation, two carbon atoms are arranged out-of-plane — one above and one below, minimizing the torsional strain. In the chair form, the bond angle is very close to the ideal...
16.8K
Stability of Substituted Cyclohexanes02:30

Stability of Substituted Cyclohexanes

16.7K
This lesson discusses the stability of substituted cyclohexanes with a focus on energies of various conformers and the effect of 1,3-diaxial interactions.
The two chair conformations of cyclohexanes undergo rapid interconversion at room temperature. Both forms have identical energies and stabilities, each comprising equal amounts of the equilibrium mixture. Replacing a hydrogen atom with a functional group makes the two conformations energetically non-equivalent.
For example, in...
16.7K
Carbocations02:10

Carbocations

14.3K
Carbocations are one of the reaction intermediates formed during several nucleophilic substitutions or elimination reactions. A carbocation is an electron-deficient species with the central carbon atom having six electrons and three bonded atoms. The central carbon in a carbocation is sp2 hybridized with trigonal planar geometry. It has an empty p orbital perpendicular to the plane of the structure that can accept electrons. Thus, carbocations act as strong electrophiles and may react with any...
14.3K

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Updated: Mar 16, 2026

Accessing Valuable Ligand Supports for Transition Metals: A Modified, Intermediate Scale Preparation of 1,2,3,4,5-Pentamethylcyclopentadiene
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Accessing Valuable Ligand Supports for Transition Metals: A Modified, Intermediate Scale Preparation of 1,2,3,4,5-Pentamethylcyclopentadiene

Published on: March 20, 2017

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Characterizing a nonclassical carbene with coupled cluster methods: cyclobutylidene.

Xiao Wang1, Jay Agarwal, Henry F Schaefer Iii

  • 1Center for Computational Quantum Chemistry, University of Georgia, Athens, GA 30602, USA. ccq@uga.edu.

Physical Chemistry Chemical Physics : PCCP
|August 20, 2016
PubMed
Summary

Cyclobutylidene, a unique carbene, rearranges via 1,2-C migration, not 1,2-H migration, resolving theoretical and experimental disparities. This study provides accurate rate constants and characterization for this reactive compound.

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Isolating Free Carbenes, their Mixed Dimers and Organic Radicals
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Preparation of a Corannulene-functionalized Hexahelicene by CopperI-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units

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Area of Science:

  • Quantum Chemistry
  • Organic Chemistry
  • Computational Chemistry

Background:

  • Carbenes are reactive compounds with a lone electron pair on a carbon atom.
  • Cyclobutylidene exhibits unique nonclassical behavior due to transannular interactions in its four-membered ring.
  • Understanding carbene reactivity is crucial in organic synthesis and reaction mechanisms.

Purpose of the Study:

  • To investigate the rearrangement pathways of cyclobutylidene using high-level computational methods.
  • To resolve discrepancies between theoretical predictions and experimental observations of cyclobutylidene's reactivity.
  • To accurately determine the preferred reaction path and kinetics for cyclobutylidene.

Main Methods:

  • High-level coupled-cluster (CC) methods, including CCSDT(Q), were employed.
  • Extrapolation techniques to the complete basis set (CBS) limit were used for high accuracy.
  • Canonical variational transition state theory (CVT) was applied to calculate reaction rate constants.

Main Results:

  • Cyclobutylidene is predicted to preferentially undergo 1,2-C migration over 1,2-H migration.
  • Calculated rate constants show good agreement with experimental data.
  • The singlet-triplet gap was determined to be -9.3 kcal mol(-1) at the CCSDT(Q)/CBS level.
  • Anharmonic vibrational frequencies were computed using VPT2.

Conclusions:

  • The study successfully resolves the long-standing theoretical-experimental disparity for cyclobutylidene reactivity.
  • Computational methods provide accurate predictions for carbene reaction mechanisms.
  • Detailed characterization of cyclobutylidene, including its energetic and vibrational properties, is reported.