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Base-Promoted α-Halogenation of Aldehydes and Ketones00:51

Base-Promoted α-Halogenation of Aldehydes and Ketones

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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

ortho–para-Directing Deactivators: Halogens

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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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Acid-Catalyzed α-Halogenation of Aldehydes and Ketones01:21

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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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Alkyl Halides02:45

Alkyl Halides

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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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Regioselectivity of Electrophilic Additions-Peroxide Effect02:35

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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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Structural basis for organohalide respiration.

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
Summary

Reductive dehalogenases, like PceA, detoxify pollutants such as trichloroethene (TCE) by catalyzing the final electron transfer step. Structural analysis reveals a B12 cofactor enabling this crucial reductive haloelimination process.

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Area of Science:

  • Biochemistry
  • Microbiology
  • Structural Biology

Background:

  • Organohalide-respiring microorganisms utilize persistent pollutants like trichloroethene (TCE) as electron acceptors.
  • Reductive dehalogenases catalyze the critical two-electron transfer step in organohalide respiration.

Purpose of the Study:

  • To determine the x-ray crystal structure of PceA, a key dehalogenase from Sulfurospirillum multivorans.
  • To elucidate the structural mechanisms underlying TCE detoxification by PceA.

Main Methods:

  • X-ray crystallography was employed to obtain the structure of PceA.
  • Structures of PceA in complex with trichloroethene (TCE) and product analogs were determined.

Main Results:

  • The archetypal dehalogenase PceA features a deeply buried norpseudo-B12 cofactor within a nitroreductase fold.
  • Structural complexes reveal how the cobalamin cofactor facilitates reductive haloelimination of TCE.
  • A conserved B12-binding scaffold and a variable substrate-capturing region were identified.

Conclusions:

  • The structure of PceA provides atomic-level insights into the mechanism of organohalide respiration.
  • Understanding PceA's structure can inform strategies for bioremediation of persistent pollutants.