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01:11ortho–para-Directing Activators: –CH3, –OH, –NH2, –OCH3
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All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...
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01:26Photochemical Electrocyclic Reactions: Stereochemistry
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The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
Selection Rules: Photochemical Activation
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01:11Radical Reactivity: Overview
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Radicals, the highly reactive species, gain stability by undergoing three different reactions. The first reaction involves a radical-radical coupling, in which a radical combines with another radical, forming a spin‐paired molecule. The second reaction is between a radical and a spin‐paired molecule, generating a new radical and a new spin‐paired molecule. The third reaction is radical decomposition in a unimolecular reaction, forming a new radical and a spin‐paired...
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01:34Woodward–Hoffmann Selection Rules and Microscopic Reversibility
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Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...
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02:36Regioselectivity and Stereochemistry of Hydroboration
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A significant aspect of hydroboration–oxidation is the regio- and stereochemical outcome of the reaction.
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
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01:12Cycloaddition Reactions: MO Requirements for Photochemical Activation
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Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.
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Vibrational Quantum-State-Controlled Reactivity in the O2+ + C3H4 Reaction.
C Zagorec-Marks1,2, G S Kocheril1,2, T Kieft1,2
1Department of Physics, University of Colorado, Boulder, Colorado 80309, United States.
The Journal of Physical Chemistry Letters
|April 6, 2026
Summary
Researchers achieved quantum-state control in chemistry by demonstrating vibrational-state-dependent reactivity in an ion-molecule reaction. This selective control opens new pathways for targeted chemical synthesis.
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Area of Science:
- Physical Chemistry
- Chemical Physics
- Quantum Chemistry
Background:
- Quantum-state control is a key goal in physical chemistry.
- Understanding ion-molecule reactions is crucial for chemical synthesis.
Purpose of the Study:
- To investigate the vibrational-state-dependent reactivity of O2+ with C3H4 isomers.
- To demonstrate quantum-state control in ion-molecule reactions.
Main Methods:
- Studied the reaction between vibrationally distinct O2+ ions and allene/propyne.
- Analyzed product branching ratios and formation pathways.
Main Results:
- Most products formed irrespective of O2+ vibrational state.
- Vibrational excitation influenced product branching ratios.
- A new product, C2O+, formed exclusively in excited-state reactions.
Conclusions:
- Vibrational excitation selectively activates specific reaction pathways.
- Provides direct evidence of quantum-state control in reactivity.
- Represents a significant step toward quantum-controlled chemistry.