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01:26Photochemical Electrocyclic Reactions: Stereochemistry
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
01:12Cycloaddition Reactions: MO Requirements for Photochemical Activation
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.
01:11Radical Reactivity: Overview
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 molecule. These three...
03:06Reaction Mechanisms
Chemical reactions often occur in a stepwise fashion, involving two or more distinct reactions taking place in a sequence. A balanced equation indicates the reacting species and the product species, but it reveals no details about how the reaction occurs at the molecular level. The reaction mechanism (or reaction path) provides details regarding the precise, step-by-step process by which a reaction occurs.
For instance, the decomposition of ozone appears to follow a mechanism with two steps:
For instance, the decomposition of ozone appears to follow a mechanism with two steps:
01:17Thermal Electrocyclic Reactions: Stereochemistry
The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
01:18Carboxylic Acids to Acid Chlorides
Carboxylic acids react with SOCl2 or PCl5 to form acid chlorides. Amongst the carboxylic acid derivatives, acid chlorides are the most reactive and synthetically important derivatives. They are useful reagents for Friedel–Crafts acylation of some aromatic compounds.
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Photochemical reaction dynamics in SO(2)-acetylene complexes.
Vladimir I Makarov1, Sergei A Kochubei, Igor Khmelinskii
1Department of Physics, University of Puerto Rico, Rio Piedras, P.O. Box 23343, San Juan 00931-3343, Puerto Rico.
The Journal of Chemical Physics
|June 17, 2010
Summary
Photoinduced reactions in sulfur dioxide-acetylene van der Waals complexes were studied. The primary channel involves SO2 excitation leading to HOSO and C2H radical formation.
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Area of Science:
- Chemical Dynamics
- Photochemistry
- Molecular Spectroscopy
Background:
- Van der Waals (vdW) complexes offer insights into intermolecular interactions and reaction dynamics.
- Understanding photoinduced reactions is crucial for chemical synthesis and atmospheric chemistry.
Purpose of the Study:
- To investigate the dynamics of photoinduced reactions within SO2-acetylene vdW complexes.
- To identify the primary photodecomposition channel and characterize the resulting photofragments.
Main Methods:
- Excitation of SO2 using tunable dye laser radiation.
- Resonance-enhanced multiphoton photoionization (REMPI) to detect C2H radicals.
- Infrared (IR) emission spectroscopy to detect HOSO radicals.
Main Results:
- The primary photodecomposition channel was identified as SO2(*) + C2H2 --> HOSO + C2H.
- Experimental evidence confirmed that photofragments originate from the SO2...C2H2 vdW complex.
- Analysis of action spectra and IR emission revealed energy distribution among photofragments, with C2H formed vibrationless but rotationally/translationally excited, and HOSO vibrationally/rotationally/translationally excited.
Conclusions:
- The reaction mechanism involves H atom transfer from acetylene to SO2, followed by non-statistical dissociation of the vdW complex.
- Theoretical analysis supported a C(s) symmetry for the SO2-acetylene vdW complex.
- Kinetic studies yielded a characteristic radical build-up time of 0.64 microseconds.