Radical-Pair Formation in Hydrocarbon (Aut)Oxidation.
Lakshmanan Sandhiya1, Hendrik Zipse1
1Department Chemie, Ludwig-Maximilians-Universität München, 81377, München, Germany.
Chemistry (Weinheim an Der Bergstrasse, Germany)
|May 7, 2019
Summary
Benzyl hydroperoxide decomposition pathways were investigated. Molecule-induced radical formation (MIRF) offers a lower energy route for hydrocarbon oxidation initiation compared to unimolecular dissociation.
Area of Science:
- Chemical Kinetics
- Reaction Mechanisms
- Oxidation Chemistry
Background:
- Hydrocarbon autoxidation involves complex radical initiation steps.
- Benzyl hydroperoxide is a key intermediate in oxidation processes.
- Understanding decomposition pathways is crucial for controlling reactions.
Purpose of the Study:
- To elucidate the reaction profiles of uni- and bimolecular decomposition of benzyl hydroperoxide.
- To compare the energetic barriers of unimolecular versus bimolecular decomposition pathways.
- To investigate the role of benzyl hydroperoxide in hydrocarbon oxidation initiation.
Main Methods:
- Computational studies of reaction profiles.
- Analysis of unimolecular dissociation pathways.
- Investigation of bimolecular reactions including self-reaction and reaction with toluene.
Main Results:
- Unimolecular dissociation involves a hydrogen-bonded radical-pair minimum.
- Molecule-induced radical formation (MIRF) in reactions with toluene and self-reaction has a lower enthalpic barrier than unimolecular O-O bond cleavage.
- MIRF reactions form radical pairs stabilized by hydrogen-bonding to a water molecule.
Conclusions:
- Bimolecular reactions, specifically MIRF, are more kinetically favorable initiation pathways for hydrocarbon oxidation than unimolecular decomposition.
- The coupled O-O bond homolysis and C-H bond abstraction in MIRF significantly lowers the activation energy.
- Hydrogen-bonding interactions play a role in stabilizing radical products in MIRF reactions.
Related Concept Videos
01:03Radical Formation: Overview
2.6K
A bond can be broken either by heterolytic bond cleavage to form ions or homolytic bond cleavage to yield radicals. A fishhook arrow is used to represent the motion of a single electron in homolytic bond cleavage. There are two main sources from which radicals can be formed:
Radicals from spin-paired molecules:
Radicals can be obtained from spin-paired molecules either by homolysis or electron transfer. While two radicals are formed in the former, an electron is added in the...
Radicals from spin-paired molecules:
Radicals can be obtained from spin-paired molecules either by homolysis or electron transfer. While two radicals are formed in the former, an electron is added in the...
2.6K
00:54Radical Formation: Homolysis
4.3K
A bond is formed between two atoms by sharing two electrons. When this bond is broken by supplying sufficient energy, either two electrons can be taken up by one atom forming ions by the cleavage called heterolysis, or the two electrons are shared by two atoms, with one each creating radicals by the cleavage called homolysis.
4.3K
00:47Radical Formation: Addition
2.2K
Radicals can be formed by adding a radical to a spin-paired molecule. This is typically observed with unsaturated species, where the addition of a radical across the π bond leads to the production of a new radical by dissolving the π bond. For example, the addition of a Br radical to an alkene yields a carbon-centered radical.
Similar to charge conservation in chemical reactions, spin conservation is implicit for radical reactions. Accordingly, the product formed must possess an...
Similar to charge conservation in chemical reactions, spin conservation is implicit for radical reactions. Accordingly, the product formed must possess an...
2.2K
00:47Radical Formation: Abstraction
4.4K
The electron of an atom can be abstracted from a compound by a relatively unstable radical to generate a new radical of relatively greater stability. For example, an initiator which forms radicals by homolysis can abstract a suitable species like a hydrogen atom or a halogen atom from a compound to generate a new radical. This ability of radicals to propagate by abstraction is a crucial feature of radical chain reactions.
Even though homolysis produces radicals, it is different from radical...
Even though homolysis produces radicals, it is different from radical...
4.4K
00:51Radical Formation: Elimination
2.3K
Another method of radical formation is the elimination process. It is the opposite of the addition route and is driven by the instability of the radical. For example, as depicted in Figure 1, dibenzoyl peroxide yields a pair of unstable radicals upon homolysis. Given its instability, this radical spontaneously undergoes elimination via a C–C bond cleavage to form a relatively more stable phenyl radical. The mechanism involves cleavage of the bond between the α and β positions with respect...
2.3K
01:21Radical Oxidation of Allylic and Benzylic Alcohols
2.9K
Activated manganese(IV) oxide can selectively oxidize allylic and benzylic alcohols via a radical intermediate mechanism. Primary allylic alcohols are oxidized to aldehydes, while secondary allylic alcohols yield ketones. The redox reaction of potassium permanganate with an Mn(II) salt such as manganese sulfate (under either alkaline or acidic conditions), followed by thorough drying, yields the oxidizing agent: activated MnO2. While MnO2 is insoluble in the solvents used for the reaction, the...
2.9K