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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.
01:04Hydrogen Bonds
A hydrogen bond is formed when a weakly positive hydrogen atom already bonded to one electronegative atom (for example, the oxygen in the water molecule) is attracted to another electronegative atom from another polar molecule, such as water (H2O), hydrogen fluoride (HF), or ammonia (NH3). The huge electronegativity difference between the H atom (2.1) and the atom to which it is bonded (4.0 for an F atom, 3.5 for an O atom, or 3.0 for an N atom), combined with the very small size of an H atom...
00:26Hydrogen Bonds
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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.
01:32Aldehydes and Ketones with HCN: Cyanohydrin Formation Overview
Cyanohydrins are compounds that contain –CN and –OH groups on the same carbon atom. They are formed by the nucleophilic addition of the cyanide ions to the carbonyl group. Cyanide ions are highly basic and nucleophilic and can be generated from HCN under aqueous conditions. However, since HCN is a weak acid, the number of cyanide ions generated is very small. Hence, a small amount of base or KCN/NaCN is added to HCN to increase the concentration of the cyanide ions in the reaction mixture.
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.
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Hidrazina borano: un material de almacenamiento de hidrógeno prometedor.
Thomas Hügle1, Moritz F Kühnel, Dieter Lentz
1Institut für Chemie und Biochemie, Anorganische Chemie, Freie Universität Berlin, Fabeckstrasse 34-36, 14195 Berlin, Germany.
Journal of the American Chemical Society
|May 13, 2009
Resumen
Los investigadores mejoraron el borano de hidrazina.
Área de la Ciencia:
- Ciencia de los materiales Ciencia de los materiales.
- Ingeniería Química Ingeniería Química.
Sus antecedentes:
- El borano hidrazina es un material prometedor para el almacenamiento de hidrógeno.
- Mejorar su cinética y capacidad de liberación de hidrógeno es crucial para aplicaciones prácticas.
Objetivo del estudio:
- Para mejorar las capacidades de almacenamiento de hidrógeno de hidrazina borano.
- Para investigar el efecto de la adición de hidruro de litio en la liberación de hidrógeno.
Principales métodos:
- Adición de una cantidad equimolar de hidruro de litio a hidrazina borano.
- Análisis del contenido de hidrógeno utilizando el porcentaje de peso en H.2).
- Estudiar el comportamiento de liberación de hidrógeno a varias temperaturas.
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Principales resultados:
- La mezcla logró una capacidad de almacenamiento de 14,8 wt % H.
- Se observaron excelentes tasas de liberación de hidrógeno.
- La liberación óptima de hidrógeno se produjo entre 100-150 grados C.
Conclusiones:
- La adición equimolar de hidruro de litio mejora significativamente el rendimiento de almacenamiento de hidrógeno del borano de hidrazina.
- El material modificado ofrece una liberación eficiente de hidrógeno a temperaturas moderadas, adecuado para aplicaciones prácticas.