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α-Halogenation of aldehydes and ketones is a reaction involving the substitution of α hydrogens with halogens in the presence of a base.  The reaction begins with the abstraction of  α hydrogen by the base to produce a nucleophilic enolate ion. This intermediate undergoes a subsequent nucleophilic substitution with the halogen to produce a monohalogenated carbonyl compound. If the starting substrate has more than one α hydrogen, it is difficult to stop the reaction...
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Formation of Halohydrin from Alkenes02:41

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An alkene, such as propene, reacts with bromine in the presence of water to yield a halohydrin. Halohydrins contain a halogen and a hydroxyl group attached to adjacent carbons. When the halogen is bromine, it is called a bromohydrin, while a chlorohydrin has chlorine as the halogen.
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ortho–para-Directing Deactivators: Halogens01:24

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Halogens are ortho–para directors. They are more electronegative than carbon. Therefore, as ring substituents, they can withdraw electrons through the inductive effect and deactivate the aromatic ring towards electrophilic substitution. Halogens also have an electron-donating resonance effect on the ring, which influences the orientation of the incoming electrophile. If an electrophile attacks at the ortho or the para position, the halogen donates electrons and stabilizes the intermediate...
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By replacing an α-hydrogen with a halogen, acid-catalyzed α-halogenation of aldehydes or ketones yields a monohalogenated product
In the first step of the mechanism, the acid protonates the carbonyl oxygen resulting in a resonance-stabilized cation, which subsequently loses an α-hydrogen to form an enol tautomer. The C=C bond in an enol is highly nucleophilic because of the electron-donating nature of the –OH group. Consequently, the double bond attacks an electrophilic...
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Structural Properties
Alkyl halides are halogen-substituted alkanes wherein one or more hydrogen atoms of an alkane is replaced by a halogen atom such as fluorine, chlorine, bromine, or iodine. The carbon atom in an alkyl halide is bonded to the halogen atom, which is sp3-hybridized and exhibits a tetrahedral shape.
Unlike alkyl halides, compounds in which a halogen atom is bonded to an sp2 -hybridized carbon atom of a carbon-carbon double bond (C=C) are called vinyl halides. Whereas aryl...
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In the presence of organic peroxides, the addition of hydrogen bromide to an alkene yields the isomer that is not predicted by Markovnikov’s rule. For example, the addition of hydrogen bromide to 2-methylpropene in the presence of peroxides gives 1-bromo-2-methylpropane. This addition reaction proceeds via a free radical mechanism, which reverses the regioselectivity. The free radical reaction mechanism involves three stages: initiation, propagation, and termination.
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A Protocol for Safe Lithiation Reactions Using Organolithium Reagents
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Bases estructurales para la respiración por organohaluros.

Martin Bommer1, Cindy Kunze2, Jochen Fesseler1

  • 1Institut für Biologie, Strukturbiologie/Biochemie, Humboldt-Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany.

Science (New York, N.Y.)
|October 4, 2014
PubMed
Resumen

Las deshalogenasas reductoras, como el PceA, desintoxican contaminantes como el tricloroeteno (TCE) al catalizar el paso final de transferencia de electrones. El análisis estructural revela un cofactor B12 que permite este crucial proceso de eliminación de halo reductor.

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Área de la Ciencia:

  • La bioquímica es la bioquímica.
  • Microbiología Microbiología.
  • Biología Estructural Biología estructural.

Sus antecedentes:

  • Los microorganismos que respiran organohaluros utilizan contaminantes persistentes como el tricloroeteno (TCE) como aceptores de electrones.
  • Las deshalogenasas reductoras catalizan el paso crítico de transferencia de dos electrones en la respiración de organohaluros.

Objetivo del estudio:

  • Para determinar la estructura cristalina de rayos X de PceA, una dehalogenasa clave de Sulfurospirillum multivorans. para determinar la estructura cristalina de rayos X de PceA, una dehalogenasa clave de Sulfurospirillum multivorans. para determinar la estructura cristalina de rayos X de PceA, una dehalogenasa clave de Sulfurospirillum multivorans. para determinar la estructura cristalina de rayos X de PceA, una dehalogenasa clave de Sulfurospirillum multivorans. para determinar la estructura cristalina de rayos X de PceA, una dehalogenasa clave de Sulfurospirillum multivorans. para determinar la estructura cristalina de rayos X de PceA.
  • Para aclarar los mecanismos estructurales que subyacen a la desintoxicación de TCE por PceA.

Principales métodos:

  • Se empleó cristalografía de rayos X para obtener la estructura de PceA.
  • Se determinaron las estructuras de PceA en complejo con tricloroeteno (TCE) y análogos de productos.

Principales resultados:

  • La deshalogenasa arquetípica PceA presenta un cofactor norpseudo-B12 profundamente enterrado dentro de un pliegue de nitrorreductasa.
  • Los complejos estructurales revelan cómo el cofactor de cobalamina facilita la eliminación reductora del halo de TCE.
  • Se identificó un andamio de unión de B12 conservado y una región de captura de sustrato variable.

Conclusiones:

  • La estructura de PceA proporciona información a nivel atómico sobre el mecanismo de la respiración de los organohaluros.
  • La comprensión de la estructura de PceA puede informar estrategias para la biorremediación de contaminantes persistentes.