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Related Concept Videos

Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

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 with both...
Five-Membered Heterocyclic Aromatic Compounds: Overview01:13

Five-Membered Heterocyclic Aromatic Compounds: Overview

Heterocyclic aromatic compounds are cyclic compounds that are aromatic and have one or more heteroatoms—atoms other than carbon, in the ring. Depending upon the number of atoms present in the ring, they can be either five or six-membered. Examples of five-membered heterocyclic aromatic compounds include pyrrole, furan, thiophene, and imidazole. Pyrrole consists of one nitrogen atom having one lone pair of electrons. Furan and thiophene have one oxygen and one sulfur heteroatom, respectively.
Aromatic Hydrocarbon Anions: Structural Overview01:18

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 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 overlap of p...
Criteria for Aromaticity and the Hückel 4n + 2 Rule01:20

Criteria for Aromaticity and the Hückel 4n + 2 Rule

Like benzene, cyclobutadiene and cyclooctatetraene are cyclic compounds with alternate single and double bonds. However, their chemical behavior differs from benzene, as they are unstable and not aromatic. So, what are the structural characteristics of unsaturated compounds categorized as aromatic?
For the first time, Eric Hückel, a German chemical physicist, derived a set of structural features for a compound to be classified as aromatic. This is now known as Hückel’s rule or the 4n + 2 rule.
Carbocations02:10

Carbocations

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...
Frost Circles for Different Conjugated Systems01:18

Frost Circles for Different Conjugated Systems

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.

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Related Experiment Video

Updated: Jun 29, 2026

Palladium N-Heterocyclic Carbene Complexes: Synthesis from Benzimidazolium Salts and Catalytic Activity in Carbon-carbon Bond-forming Reactions
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Palladium N-Heterocyclic Carbene Complexes: Synthesis from Benzimidazolium Salts and Catalytic Activity in Carbon-carbon Bond-forming Reactions

Published on: July 30, 2017

A persistent P,N-heterocyclic carbene.

Guido D Frey1, Maoying Song, Jean-Baptiste Bourg

  • 1UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957), Department of Chemistry, University of California, Riverside, CA 92521-0403, USA.

Chemical Communications (Cambridge, England)
|October 3, 2008
PubMed
Summary
This summary is machine-generated.

Researchers prepared conjugate acids of cyclic (amino)(phosphino)carbenes (P-NHCs). Deprotonation revealed diverse reaction pathways, yielding metastable P-NHCs, azomethine ylides, and bicyclic phosphiranes.

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Palladium N-Heterocyclic Carbene Complexes: Synthesis from Benzimidazolium Salts and Catalytic Activity in Carbon-carbon Bond-forming Reactions
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Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization

Published on: November 29, 2018

Area of Science:

  • Organophosphorus chemistry
  • Carbene chemistry
  • Organic synthesis

Background:

  • Cyclic (amino)(phosphino)carbenes (P-NHCs) are a unique class of carbene ligands.
  • Their conjugate acids are key intermediates in various transformations.
  • Understanding their deprotonation behavior is crucial for synthetic applications.

Purpose of the Study:

  • To synthesize and characterize the conjugate acids of cyclic (amino)(phosphino)carbenes (P-NHCs).
  • To investigate the reaction pathways occurring during the deprotonation of these conjugate acids.
  • To identify the products formed under different deprotonation conditions.

Main Methods:

  • Synthesis of P-NHC conjugate acids via protonation of parent P-NHCs.
  • Deprotonation studies using various bases and reaction conditions.
  • Characterization of reaction products using NMR spectroscopy and mass spectrometry.

Main Results:

  • Successful preparation of several cyclic (amino)(phosphino)carbene conjugate acids.
  • Observation of multiple deprotonation pathways, including the formation of a metastable P-NHC.
  • Identification of azomethine ylides and bicyclic phosphiranes as significant deprotonation products.

Conclusions:

  • The deprotonation of P-NHC conjugate acids is a complex process with diverse outcomes.
  • Metastable P-NHCs, azomethine ylides, and bicyclic phosphiranes represent key species accessible through these reactions.
  • This study expands the known reactivity of P-NHCs and offers new synthetic routes.