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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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Halogenation of Alkenes02:46

Halogenation of Alkenes

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Halogenation is the addition of chlorine or bromine across the double bond in an alkene to yield a vicinal dihalide. The reaction occurs in the presence of inert and non-nucleophilic solvents, such as methylene chloride, chloroform, or carbon tetrachloride.
Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the π electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate.
15.5K
Radical Halogenation: Thermodynamics01:34

Radical Halogenation: Thermodynamics

3.8K
The thermodynamic favorability of a reaction is determined by the change in Gibbs free energy (ΔG). ΔG has two components- enthalpy (ΔH) and entropy (ΔS). The entropy component is negligible for alkane halogenation because the number of reactants and product molecules are equal. In this case, the ΔG is governed only by the enthalpy component. The most crucial factor that determines ΔH is the strength of the bonds. ΔH can be determined by comparing the energy...
3.8K
Electrophilic Addition to Alkynes: Hydrohalogenation02:35

Electrophilic Addition to Alkynes: Hydrohalogenation

9.9K
Electrophilic addition of hydrogen halides, HX (X = Cl, Br or I) to alkenes forms alkyl halides as per Markovnikov's rule, where the hydrogen gets added to the less substituted carbon of the double bond. Hydrohalogenation of alkynes takes place in a similar manner, with the first addition of HX forming a vinyl halide and the second giving a geminal dihalide.
9.9K
Radical Substitution: Hydrogenolysis of Alkyl Halides with Tributyltin Hydride01:26

Radical Substitution: Hydrogenolysis of Alkyl Halides with Tributyltin Hydride

1.8K
Radical substitution reactions can be used to remove functional groups from molecules. The hydrogenolysis of alkyl halides is one such reaction, where the weak Sn–H bond in tributyltin hydride reacts with alkyl halides to form alkanes. Here, the reagent Bu3SnH yields tributyltin halide as a byproduct.
The bonds formed in this reaction are stronger than the bonds broken, making it energetically favorable. The reaction follows a radical chain mechanism similar to radical halogenation...
1.8K
Acid-Catalyzed α-Halogenation of Aldehydes and Ketones01:21

Acid-Catalyzed α-Halogenation of Aldehydes and Ketones

3.7K
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 halogen to form a...
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Recent Advances in Halogen-Metal Exchange Reactions.

Baosheng Wei1, Yi-Hung Chen2, Paul Knochel3

  • 1College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan China.

Accounts of Chemical Research
|July 2, 2024
PubMed
Summary
This summary is machine-generated.

New halogen-metal exchange reactions enable the synthesis of functionalized organometallic reagents. Advances include stereoselective synthesis of chiral compounds and efficient exchanges with magnesium, zinc, and lanthanide metals.

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Area of Science:

  • Organic Chemistry
  • Organometallic Chemistry

Background:

  • Halogen-metal exchange is crucial for synthesizing functionalized organometallic reagents.
  • Developing new exchange reagents with improved reactivity and compatibility is an ongoing objective.

Purpose of the Study:

  • To outline recent advances in halogen-metal exchange reactions, focusing on new reagents and applications.
  • To highlight breakthroughs in stereoselective synthesis, efficient metal exchanges, and novel metal applications.

Main Methods:

  • Stereoretentive iodine/lithium exchange on secondary alkyl iodides.
  • Development of lithium alkoxide-complexed reagents for faster halogen-magnesium and halogen-zinc exchanges.
  • Establishment of halogen-lanthanide exchanges and kinetic studies.
  • Implementation of bromine/sodium exchange in continuous flow using a soluble reagent.

Main Results:

  • Synthesis of nonstabilized chiral secondary alkyllithium reagents for stereoselective synthesis.
  • Highly efficient and rapid halogen-magnesium and halogen-zinc exchanges in toluene.
  • Introduction of lanthanide metals into halogen-metal exchange reactions.
  • Successful bromine/sodium exchange in continuous flow, enhancing organosodium reagent utility.

Conclusions:

  • Upgrading exchange reagents drives innovation in halogen-metal exchange reactions and their synthetic applications.
  • Continued research into new exchange reactions, especially with novel metals, is essential for advancing synthetic chemistry.