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Related Concept Videos

π Molecular Orbitals of the Allyl Cation and Anion01:18

π Molecular Orbitals of the Allyl Cation and Anion

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...
Acidity of 1-Alkynes02:42

Acidity of 1-Alkynes


The acidic strength of hydrocarbons follows the order: Alkynes > Alkenes > Alkanes. The strength of an acid is commonly expressed in units of pKa — the lower the pKa, the stronger the acid. Among the hydrocarbons, terminal alkynes have lower pKa values and are, therefore, more acidic. For example, the pKa values for ethane, ethene, and acetylene are 51, 44, and 25, respectively, as shown here.
Acid Halides to Alcohols: LiAlH4 Reduction01:19

Acid Halides to Alcohols: LiAlH4 Reduction

Acid halides are reduced to alcohols in the presence of a strong reducing agent like lithium aluminum hydride.
The mechanism proceeds in three steps. First, the nucleophilic hydride ion attacks the carbonyl carbon of the acid halide to form a tetrahedral intermediate. Next, the carbonyl group is re-formed, and the halide ion departs as a leaving group, generating an aldehyde. A second nucleophilic attack by the hydride yields an alkoxide ion, which, upon protonation, gives a primary alcohol as...
Electrophilic Addition to Alkynes: Halogenation02:38

Electrophilic Addition to Alkynes: Halogenation

Introduction
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.
Acid Halides to Alcohols: Grignard Reaction01:15

Acid Halides to Alcohols: Grignard Reaction

Organomagnesium halides, commonly known as Grignard reagents, convert acid halides to tertiary alcohols. The reaction requires two equivalents of the Grignard reagent and proceeds via a ketone intermediate.
Grignard reagents are a source of carbanions and function as nucleophiles. The mechanism begins with the nucleophilic attack by the carbanion at the carbonyl carbon of the acid halide to form a tetrahedral intermediate. Next, the carbonyl group is re-formed, and the halide ion departs,...
π Molecular Orbitals of the Allyl Radical01:27

π Molecular Orbitals of the Allyl Radical

Allyl radicals are three-carbon conjugated systems. They are readily formed as intermediates in halogenation reactions of alkenes involving the addition of halogen to the allylic carbon instead of the double bond. As seen in allyl cations and anions, each of the three sp2-hybridized carbon atoms in allyl radicals has an unhybridized p orbital. These orbitals combine to give three π molecular orbitals.
The allyl systems have identical molecular orbitals but differ in the number of π electrons.

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A Protocol for Safe Lithiation Reactions Using Organolithium Reagents
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Published on: November 12, 2016

Gas phase and solution structures of 1-methoxyallenyllithium.

Darryl D Dixon1, Marcus A Tius, Lawrence M Pratt

  • 1Department of Chemistry, University of Hawaii, 2545 The Mall, Honolulu, Hawaii 96822, USA.

The Journal of Organic Chemistry
|June 22, 2010
PubMed
Summary

Computational and NMR studies revealed the aggregation behavior of 1-methoxyallenyllithium in solution. The species exists in a dimer-tetramer equilibrium, contrasting with predicted hexamer formation in the gas phase.

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Accessing Valuable Ligand Supports for Transition Metals: A Modified, Intermediate Scale Preparation of 1,2,3,4,5-Pentamethylcyclopentadiene

Published on: March 20, 2017

Area of Science:

  • Organometallic Chemistry
  • Computational Chemistry
  • Spectroscopy

Background:

  • Organolithium compounds are crucial in organic synthesis.
  • Understanding the aggregation state of organolithiums in solution is key to controlling their reactivity.
  • 1-methoxyallenyllithium is a specific organolithium species with potential synthetic applications.

Purpose of the Study:

  • To elucidate the solution-phase aggregation structure of 1-methoxyallenyllithium.
  • To compare aggregation behavior in the gas phase versus solution.
  • To provide insights into the structural dynamics of this organometallic species.

Main Methods:

  • Density Functional Theory (DFT) calculations for gas-phase structures and aggregation energies.
  • (13)Carbon Nuclear Magnetic Resonance ((13)C NMR) spectroscopy in Tetrahydrofuran (THF) solution.
  • Analysis of experimental NMR chemical shifts correlated with computational data.

Main Results:

  • Gas-phase calculations predicted a stable hexameric aggregate for 1-methoxyallenyllithium.
  • NMR data and computational analysis supported an equilibrium between dimer and tetramer species in THF solution.
  • Calculated aggregation energies and chemical shifts were consistent with the observed solution behavior.

Conclusions:

  • The aggregation state of 1-methoxyallenyllithium differs significantly between the gas phase and THF solution.
  • A dimer-tetramer equilibrium governs the structure of 1-methoxyallenyllithium in solution.
  • This study highlights the importance of solvent effects on organolithium aggregation.