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Related Concept Videos

Alkyl Halides02:45

Alkyl Halides

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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Sulfides are the sulfur analog of ethers, just as thiols are the sulfur analog of alcohol. Like ethers, sulfides also consist of two hydrocarbon groups bonded to the central sulfur atom. Depending upon the type of groups present, sulfides can be symmetrical or asymmetrical. Symmetrical sulfides can be prepared via an SN2 reaction between 2 equivalents of an alkyl halide and one equivalent of sodium sulfide.
π Molecular Orbitals of the Allyl Cation and Anion01:18

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An allyl group is a three-carbon conjugated system where the sp³-hybridized allylic carbon is bonded to a CH=CH2 group via a single bond. Allyl anions can be obtained by treating propene with a strong base that can deprotonate methyl groups. Allyl cations are formed as intermediates during substitution reactions involving allylic halides. In both cases, the hybridization of the allylic carbon changes from sp3 to sp2, giving rise to a carbon chain with three sp2-hybridized carbons, each with an...
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The radical dimerization of ketones or aldehydes gives vicinal diols through a pinacol coupling reaction. However, the behavior of titanium metals used for the reaction as a source of electrons is unusual. When the reaction is carried out in the presence of titanium, diols can be isolated at low temperatures. Else titanium further reacts with diols, forming alkenes through the McMurry reaction.
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The polymerization process that involves carbanion as an intermediate is called anionic polymerization. It is also a type of addition or chain-growth polymerization. Anionic polymerization gets initiated by a strong nucleophile such as an organolithium or a Grignard reagent. The most commonly used initiator for anionic polymerization is butyl lithium. Monomers involved in anionic polymerization must possess a vinyl group bonded to one or two electron-withdrawing groups. For instance,...
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Malonic ester synthesis is a method to obtain α substituted carboxylic acids from ꞵ-diesters such as diethyl malonate and alkyl halides.

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Methylaluminoxane Reactivities and Anionic Structures: From Small Oligomers to Large Sheets.

Xiaotong Mao1, Munmun Bharti1, Scott Collins1

  • 1Department of Chemistry and Sustainable Technology, University of Eastern Finland, Joensuu Campus, Yliopistokatu 7, FI-80100, Joensuu, Finland.

Chemistry (Weinheim an Der Bergstrasse, Germany)
|January 23, 2025
PubMed
Summary

Computational chemistry reveals the true structures of methylaluminoxane (MAO) species. Higher-level calculations show MAO prefers 3-coordinate oxygen and 4-coordinate aluminum, impacting their ionization pathways and stability.

Keywords:
DFTESI-MSmethylaluminoxaneoctamethyltrisiloxanesheet

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Area of Science:

  • Organometallic Chemistry
  • Computational Chemistry
  • Materials Science

Background:

  • Methylaluminoxane (MAO) is a crucial cocatalyst in olefin polymerization.
  • Understanding the precise structures and reactivity of small MAO species is essential for catalyst design.
  • Previous studies have been limited by computational accuracy in describing MAO structures.

Purpose of the Study:

  • To elucidate the accurate structures and reactivity of small methylaluminoxane (MAO) species.
  • To investigate the ionization pathways and stability of MAO anions.
  • To develop a stability metric correlating computational results with experimental observations.

Main Methods:

  • Density Functional Theory (DFT) with M06-2X functional.
  • High-level ab initio calculations (MP2 and CCSD(T)).
  • Analysis of ionization pathways and stability metrics.

Main Results:

  • DFT calculations artificially stabilize structures with 4-coordinate oxygen.
  • Higher-level calculations confirm a preference for 3-coordinate oxygen and 4-coordinate aluminum centers in neutral MAO.
  • Anion formation occurs via methide or Me2Al+ abstraction, with the latter dominant for larger sheet structures.
  • A comprehensive stability metric successfully explains observed anion distributions in ESI-MS.

Conclusions:

  • Accurate computational methods are critical for understanding MAO structure and reactivity.
  • The proposed stability metric provides a predictive tool for MAO behavior.
  • Reactivity trends in larger MAO species, including recently characterized structures, warrant further investigation.