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Related Concept Videos

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...
Relative Stabilities of Alkenes01:59

Relative Stabilities of Alkenes

The relative stability of alkenes can be determined by comparing their heats of hydrogenation. The lower heat of hydrogenation indicates the more stable alkene.  The three main factors determining the relative stability of alkenes are i) the number of substituents attached to the double-bond carbon atoms, ii) hyperconjugation, and iii) the stereochemistry of the double bond.
Stability of Substituted Cyclohexanes02:30

Stability of Substituted Cyclohexanes

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...
Benzene to 1,4-Cyclohexadiene: Birch Reduction Mechanism01:18

Benzene to 1,4-Cyclohexadiene: Birch Reduction Mechanism

Birch reduction uses solvated electrons as reducing agents. The reaction converts benzene to 1,4-cyclohexadiene. The reaction proceeds by the transfer of a single electron to the ring to form a benzene radical anion. This anion is highly basic—it abstracts a proton from the alcohol to form a cyclohexadienyl radical. Another single electron transfer gives the cyclohexadienyl anion. A proton transfer from the alcohol forms 1,4-cyclohexadiene. Since this reduction occurs via radical anion...
Stability of Conjugated Dienes01:28

Stability of Conjugated Dienes

Introduction
A comparison of the enthalpies of hydrogenation of dienes reveals that conjugated dienes release less heat on hydrogenation, rendering them more stable than their nonconjugated analogs.
Reduction of Benzene to Cyclohexane: Catalytic Hydrogenation01:28

Reduction of Benzene to Cyclohexane: Catalytic Hydrogenation

Unlike the easy catalytic hydrogenation of an alkene double bond, hydrogenation of a benzene double bond under similar reaction conditions does not take place easily. For example, in the reduction of stilbene, the benzene ring remains unaffected while the alkene bond gets reduced. Hydrogenation of an alkene double bond is exothermic and a favorable process. In contrast, to hydrogenate the first unsaturated bond of benzene, an energy input is needed; that is, the process is endothermic. This is...

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Isolating Free Carbenes, their Mixed Dimers and Organic Radicals
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Isolating Free Carbenes, their Mixed Dimers and Organic Radicals

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Carbenes with reduced heteroatom stabilization: a computational approach.

M Z Kassaee1, F A Shakib, M R Momeni

  • 1Department of Chemistry, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran. kassaeem@modares.ac.ir

The Journal of Organic Chemistry
|March 19, 2010
PubMed
Summary

This study explores how different chemical groups affect the stability and reactivity of carbenes. Amino and oxy groups significantly enhance carbene stability, influencing their electronic properties and reaction behaviors.

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Area of Science:

  • Computational chemistry
  • Quantum chemistry
  • Organic chemistry

Background:

  • Carbenes are reactive intermediates with diverse applications.
  • Understanding factors influencing carbene stability and reactivity is crucial for synthetic chemistry.
  • Previous studies have explored substituent effects, but a comprehensive analysis of novel carbenes is needed.

Purpose of the Study:

  • To investigate the impact of monoheteroatom substitution, cyclization, and unsaturation on carbene properties.
  • To compare the stability, multiplicity, and reactivity of various substituted carbenes.
  • To elucidate the electronic effects of substituents on singlet-triplet energy gaps and reactivity patterns.

Main Methods:

  • High-level Density Functional Theory (DFT) calculations.
  • Utilized isodesmic reactions to assess thermodynamic stability.
  • Analyzed singlet-triplet energy gaps (DeltaE(S-T)) and electronic properties.
  • Compared results with reference carbene molecules.

Main Results:

  • Established a stability trend: (amino ≈ oxy) > thio > phosphino > alkyl > silyl.
  • Found that amino substituents stabilize both singlet and triplet carbene states.
  • Cyclization generally increases DeltaE(S-T), while unsaturation has minor effects.
  • Amino- and phosphinoalkylcarbenes exhibit higher nucleophilicity and basicity compared to oxy- and thioalkylcarbenes.

Conclusions:

  • Monoheteroatom substitution significantly modulates carbene stability and reactivity.
  • Amino substituents provide substantial stabilization, contrary to some prior assumptions.
  • Structural modifications like cyclization and unsaturation offer further tuning of carbene properties.
  • This research provides valuable insights into the fundamental chemistry of novel carbenes.