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Whether solid, liquid, or gas, a substance's state depends on the order and arrangement of its particles (atoms, molecules, or ions). Particles in the solid pack closely together, generally in a pattern. The particles vibrate about their fixed positions but do not move or squeeze past their neighbors. In liquids, although the particles are closely spaced, they are randomly arranged. The position of the particles are not fixed—that is, they are free to move past their neighbors to...
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Properties of Transition Metals02:58

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Transition metals are defined as those elements that have partially filled d orbitals. As shown in Figure 1, the d-block elements in groups 3–12 are transition elements. The f-block elements, also called inner transition metals (the lanthanides and actinides), also meet this criterion because the d orbital is partially occupied before the f orbitals.
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Cooperative Allosteric Transitions01:58

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Cooperative allosteric transitions can occur in multimeric proteins, where each subunit of the protein has its own ligand-binding site. When a ligand binds to any of these subunits, it triggers a conformational change that affects the binding sites in the other subunits; this can change the affinity of the other sites for their respective ligands. The ability of the protein to change the shape of its binding site is attributed to the presence of a mix of flexible and stable segments in the...
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Phase Transitions: Vaporization and Condensation02:39

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The physical form of a substance changes on changing its temperature. For example, raising the temperature of a liquid causes the liquid to vaporize (convert into vapor). The process is called vaporization—a surface phenomenon. Vaporization occurs when the thermal motion of the molecules overcome the intermolecular forces, and the molecules (at the surface) escape into the gaseous state. When a liquid vaporizes in a closed container, gas molecules cannot escape. As these gas phase molecules...
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Some solids can transition directly into the gaseous state, bypassing the liquid state, via a process known as sublimation. At room temperature and standard pressure, a piece of dry ice (solid CO2) sublimes, appearing to gradually disappear without ever forming any liquid. Snow and ice sublimate at temperatures below the melting point of water, a slow process that may be accelerated by winds and the reduced atmospheric pressures at high altitudes. When solid iodine is warmed, the solid sublimes...
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Phase Transitions: Melting and Freezing02:39

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Heating a crystalline solid increases the average energy of its atoms, molecules, or ions, and the solid gets hotter. At some point, the added energy becomes large enough to partially overcome the forces holding the molecules or ions of the solid in their fixed positions, and the solid begins the process of transitioning to the liquid state or melting. At this point, the temperature of the solid stops rising, despite the continual input of heat, and it remains constant until all of the solid is...
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Phase Transitions and Effect of Intermolecular Forces
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Una solución práctica para las tetrasubstituidas definidas estereológicamente

Jianxin Dai1, Minyan Wang1, Guobi Chai1

  • 1Laboratory of Molecular Recognition and Synthesis, Department of Chemistry, Zhejiang University , Hangzhou 310027, Zhejiang, People's Republic of China.

Journal of the American Chemical Society
|February 9, 2016
PubMed
Resumen

Los investigadores desarrollaron un nuevo método para crear olefinas tetrasubstituidas estereodefinidas utilizando reactivos organozinc y 2,3-alenales. Este enfoque ofrece un acceso rápido a las olefinas funcionalizadas por aldehído versátiles con geometría controlada.

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Área de la Ciencia:

  • Química orgánica
  • Química sintética

Sus antecedentes:

  • Las olefinas son compuestos orgánicos cruciales.
  • La síntesis estereoselectiva de las olefinas tetrasubstituidas presenta un desafío significativo en la química orgánica.

Objetivo del estudio:

  • Desarrollar un método práctico y estereoselectivo para la síntesis de olefinas tetrasubstituidas.
  • Para explorar el mecanismo que controla la estereoquímica del nuevo enlace doble.

Principales métodos:

  • Adición conjugada de reactivos orgánicos a los 2,3-alenales.
  • Estudios mecanicistas que incluyen productos intermedios enolados y protonación.
  • Análisis de los intermediarios de 1,3-alcadienol y sus reacciones posteriores.

Principales resultados:

  • Se estableció un nuevo método para la preparación de olefinas estereodefinidas y completamente sustituidas.
  • La protonación de oxígeno regiospecífica de los intermedios enolados dicta la geometría del enlace doble.
  • El proceso produce olefinas tetrasubstituidas funcionalizadas por aldehído versátiles con un alto control estereoquímico.

Conclusiones:

  • El método desarrollado proporciona un acceso eficiente a una amplia gama de olefinas tetrasubstituidas estereodefinidas.
  • El mecanismo de reacción implica una protonación única y un paso de transferencia de hidrógeno concertado.
  • Este trabajo ofrece una herramienta valiosa para los químicos sintéticos que requieren estructuras complejas de olefinas.