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Published on: January 17, 2020
Pressure-dependent OH yields in alkene + HO2 reactions: a theoretical study
Judit Zádor1, Stephen J Klippenstein, James A Miller
1Combustion Research Facility, Sandia National Laboratories, Livermore, California 94551-0969, USA. jzador@sandia.gov
The Journal of Physical Chemistry. A
|August 9, 2011
Summary
Reactions between alkenes and hydroperoxyl radicals (HO(2)) are crucial for combustion. This study calculates temperature- and pressure-dependent rates for these reactions, highlighting the importance of pressure-dependence for accurate combustion modeling.
Area of Science:
- Chemical Kinetics
- Combustion Chemistry
- Atmospheric Chemistry
Background:
- Alkyl + O(2) reactions primarily form alkenes and hydroperoxyl radicals (HO(2)).
- Alkene + HO(2) reactions yield alkylperoxy radicals, hydroxyl radicals, cyclic ethers, and hydroperoxyalkyl species.
- Allylic hydrogen abstraction competes with addition, complicating reaction outcomes.
Purpose of the Study:
- To investigate the kinetics of alkene + HO(2) reactions.
- To calculate temperature- and pressure-dependent rate coefficients and branching fractions.
- To assess the significance of these reactions in unsaturated compound combustion.
Main Methods:
- Utilized previously established potential energy surfaces.
- Employed multiwell master equation calculations.
- Calculated temperature (300-1200 K) and pressure-dependent rate coefficients and branching fractions.
Main Results:
- Studied reactions involving ethene, propene, 1-butene, trans-2-butene, isobutene, cyclohexene, and vinyl alcohol with HO(2).
- Determined rate coefficients and branching fractions for six alkene + HO(2) reactions and one unsaturated oxygenate + HO(2) reaction.
- Demonstrated the critical role of pressure-dependence in accurate rate coefficient calculations.
Conclusions:
- Alkene + HO(2) reactions significantly impact combustion chemistry of unsaturated compounds.
- Accurate combustion modeling requires the inclusion of pressure-dependent rate coefficients.
- This research provides crucial kinetic data for understanding combustion processes.
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02:47Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation
Introduction
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.
02:36Regioselectivity and Stereochemistry of Hydroboration
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.
02:45Acid-Catalyzed Hydration of Alkenes
Alkenes react with water in the presence of an acid to form an alcohol. In the absence of acid, hydration of alkenes does not occur at a significant rate, and the acid is not consumed in the reaction. Therefore, alkene hydration is an acid-catalyzed reaction.
03:08Hydroboration-Oxidation of Alkenes
In addition to the oxymercuration–demercuration method, which converts the alkenes to alcohols with Markovnikov orientation, a complementary hydroboration-oxidation method yields the anti-Markovnikov product. The hydroboration reaction, discovered in 1959 by H.C. Brown, involves the addition of a B–H bond of borane to an alkene giving an organoborane intermediate. The oxidation of this intermediate with basic hydrogen peroxide forms an alcohol.
02:34Regioselectivity and Stereochemistry of Acid-Catalyzed Hydration
The rate of acid-catalyzed hydration of alkenes depends on the alkene's structure, as the presence of alkyl substituents at the double bond can significantly influence the rate.
02:13Reduction of Alkenes: Catalytic Hydrogenation
Alkenes undergo reduction by the addition of molecular hydrogen to give alkanes. Because the process generally occurs in the presence of a transition-metal catalyst, the reaction is called catalytic hydrogenation.
Metals like palladium, platinum, and nickel are commonly used in their solid forms — fine powder on an inert surface. As these catalysts remain insoluble in the reaction mixture, they are referred to as heterogeneous catalysts.
The hydrogenation process takes place on the surface of...
Metals like palladium, platinum, and nickel are commonly used in their solid forms — fine powder on an inert surface. As these catalysts remain insoluble in the reaction mixture, they are referred to as heterogeneous catalysts.
The hydrogenation process takes place on the surface of...