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Related Concept Videos

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...
E1 Reaction: Stereochemistry and Regiochemistry02:43

E1 Reaction: Stereochemistry and Regiochemistry

One of the critical aspects of the E1 reaction mechanism, as also observed in E2, is the regiochemistry, with multiple regioisomers obtained as products. In the example discussed, the presence of water as a weak base favors elimination over substitution to generate two alkenes. Given that alkenes’ stability increases with the number of alkyl groups across the double bond, typically, E1 reactions lead to the Zaitsev product, for this is more substituted and stable than the Hofmann product.
Directing and Steric Effects in Disubstituted Benzene Derivatives01:18

Directing and Steric Effects in Disubstituted Benzene Derivatives

When disubstituted benzenes undergo electrophilic substitution, the product distribution depends on the directing effect of both substituents. When the directing effects of both substituents reinforce each other, a single product is obtained. For example, bromination of p-nitrotoluene occurs ortho to the methyl group and meta to the nitro group, which is the same position, resulting in a single product. However, if the directing effects of the two groups oppose each other, the more strongly...
Inductive Effects on Chemical Shift: Overview01:27

Inductive Effects on Chemical Shift: Overview

The protons in unsubstituted alkanes are strongly shielded with chemical shifts below 1.8 ppm. Methine, methylene, and methyl protons appear at approximately 1.7, 1.2 and 0.7 ppm, while the proton signal from methane appears at 0.23 ppm. An electronegative substituent, such as chlorine, withdraws the electron density from the protons, increasing their chemical shift. Progressive substitution of the hydrogens in methane by chlorine shifts the proton signals increasingly downfield, to 3.05 ppm in...
Directing Effect of Substituents: meta-Directing Groups01:09

Directing Effect of Substituents: meta-Directing Groups

Substituents on the benzene ring that direct an incoming electrophile to undergo substitution at the meta position are called meta directors. All meta directors either have a positive charge on the atom directly bonded to the ring or a partial positive charge. These groups function by withdrawing electrons from the ring through inductive and resonance effects. Consider the carbocation intermediates formed upon the addition of an electrophile on nitrobenzene at the ortho, meta, and para...
Directing Effect of Substituents: ortho–para-Directing Groups01:14

Directing Effect of Substituents: ortho–para-Directing Groups

Ortho–para directors are substituent groups attached to the benzene ring and direct the addition of an electrophile to the positions ortho or para to the substituent. All electron-donating groups are considered ortho–para directors. They donate electrons to the ring and make the ring more electron-rich. The ring is therefore susceptible to the addition of electrophiles. Substituents such as amino, hydroxy, or alkoxy, containing lone pairs on the atom adjacent to the ring, donate electrons...

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Related Experiment Video

Updated: Jul 10, 2026

Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex
10:52

Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex

Published on: July 27, 2022

Substituent effects on dynamics at conical intersections: alpha,beta-enones.

A M D Lee1, J D Coe, S Ullrich

  • 1Steacie Institute for Molecular Sciences, National Research Council, Ottawa, ON, Canada.

The Journal of Physical Chemistry. A
|November 8, 2007
PubMed
Summary

Methyl substitution significantly impacts alpha,beta-enone relaxation dynamics. While S2 state decay is similar, S1 state relaxation and product branching ratios vary based on methyl group position, suggesting dynamical factors influence conical intersection passage.

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Facile Preparation of (2Z,4E)-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate
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Facile Preparation of (2Z,4E)-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate

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A Two-Step Protocol for Umpolung Functionalization of Ketones Via Enolonium Species
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A Two-Step Protocol for Umpolung Functionalization of Ketones Via Enolonium Species

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Last Updated: Jul 10, 2026

Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex
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Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex

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Facile Preparation of (2Z,4E)-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate
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Facile Preparation of (2Z,4E)-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate

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A Two-Step Protocol for Umpolung Functionalization of Ketones Via Enolonium Species

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Area of Science:

  • Photochemistry
  • Molecular Dynamics
  • Quantum Chemistry

Background:

  • Alpha,beta-enones are crucial in photochemistry.
  • Understanding their excited-state dynamics is key to controlling photochemical reactions.

Purpose of the Study:

  • Investigate methyl substitution effects on the electronic dynamics of alpha,beta-enones.
  • Elucidate the relaxation pathways following excitation to the S2(pipi*) state.

Main Methods:

  • Femtosecond time-resolved photoelectron spectroscopy.
  • High-level ab initio theoretical calculations.

Main Results:

  • Excited enones rapidly access conical intersections, leading to S1(npi*) and then ground state relaxation.
  • S2 decay times show minor variations, but S1 relaxation rates and product branching ratios are sensitive to methyl group position.
  • Ab initio calculations did not fully explain observed relaxation variations.

Conclusions:

  • Methyl substitution position critically influences S1 state relaxation rates and product distributions in alpha,beta-enones.
  • Dynamical factors, not just static geometry, likely govern the efficiency of passage through S1/S0 conical intersections.