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Halogens

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Group 17 elements, known as halogens, are nonmetals. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine a solid. Astatine is a highly unstable radioactive element, so currently, most of its properties are unknown due to its short half-life. Tennessine is a synthetic element also predicted to be in this group. 
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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 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.
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Hierarchy in the Halogen Activation During Surface-Promoted Ullmann Coupling.

Néstor Merino-Díez1,2,3, Alejandro Pérez Paz4, Jingcheng Li2

  • 1Donostia International Physics Center (DIPC), 20018, San Sebastián, Spain.

Chemphyschem : a European Journal of Chemical Physics and Physical Chemistry
|July 23, 2019
PubMed
Summary

Researchers achieved site-selective Ullmann coupling on gold surfaces, controlling carbon-carbon bond formation. This hierarchical reactivity advances molecular nanostructure synthesis using specific carbon-halogen bonds.

Keywords:
Ullmann couplingdensity functional calculationsgraphene nanoribbonshierarchical synthesisscanning probe microscopy

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Area of Science:

  • Surface science
  • Organic chemistry
  • Nanotechnology

Background:

  • Ullmann coupling is key for surface-supported synthesis of molecular nanostructures.
  • Hierarchical reactivity, using varied halogen activation temperatures, enhances control.
  • Site-selective reactions are crucial for precise molecular assembly.

Purpose of the Study:

  • To investigate site-selective Ullmann coupling on a gold surface.
  • To demonstrate control over carbon-carbon bond formation at specific molecular positions.
  • To understand the factors governing preferential reactivity in halogenated precursors.

Main Methods:

  • Utilized precursor molecules with bromine atoms at two distinct carbon sites.
  • Employed scanning tunneling microscopy (STM) for atomic-scale imaging.
  • Applied core-level photoemission spectroscopy (CLPES) for electronic structure analysis.
  • Performed density functional theory (DFT) calculations for mechanistic insights.

Main Results:

  • Observed a strong preference for Ullmann coupling at one specific carbon-bromine site.
  • Demonstrated site-selective covalent C-C bond formation on Au(111).
  • Experimental and computational data confirmed the preferential reactivity.

Conclusions:

  • Site-selective Ullmann coupling is achievable by exploiting differences in carbon-halogen bond environments.
  • This selectivity offers advanced control for constructing complex molecular architectures.
  • The findings pave the way for designing sophisticated surface-supported reactions.