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Carbocations are one of the reaction intermediates formed during several nucleophilic substitutions or elimination reactions. A carbocation is an electron-deficient species with the central carbon atom having six electrons and three bonded atoms. The central carbon in a carbocation is sp2 hybridized with trigonal planar geometry. It has an empty p orbital perpendicular to the plane of the structure that can accept electrons. Thus, carbocations act as strong electrophiles and may react with any...
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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...
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Conjugated dienes have lower heats of hydrogenation than cumulated and isolated dienes, making them more stable. The enhanced stabilization of conjugated systems can be understood from their π molecular orbitals.
The simplest conjugated diene is 1,3-butadiene: a four-carbon system where each carbon is sp2-hybridized and has an unhybridized p orbital that contains an unpaired electron. According to molecular orbital theory, atomic orbitals combine to form molecular orbitals such that the...
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UV–Vis Spectroscopy: Woodward–Fieser Rules01:29

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UV–Visible absorption spectra of conjugated dienes arise from the lowest energy π → π* transitions. The light-absorbing part of the molecule is called the chromophore, and the substituents directly attached to the chromophore are called auxochromes. A strong correlation exists between the absorption maxima, λmax, and the structure of a conjugated π system. The Woodward–Fieser rules predict the value of λmax for a...
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The mathematical expression known as the wave function, ψ, contains information about each orbital and the wavelike properties of electrons in an isolated atom. When atoms are bound together in a molecule, the wave functions combine to produce new mathematical descriptions that have different shapes. This process of combining the wave functions for atomic orbitals is called hybridization and is mathematically accomplished by the linear combination of atomic orbitals. The new orbitals that...
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Radicals: Electronic Structure and Geometry01:07

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This lesson delves into the geometry of a radical, which is influenced by the electronic structure of the molecule. The principle is similar to that of a lone pair, where the unpaired electron influences the geometry at the radical center.
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Hyperconjugation in Carbocations, a BLW Study with DFT approximation.

Zakaria Alamiddine1, Stéphane Humbel1

  • 1Centrale Marseille, Aix Marseille Université, CNRS, iSm2 UMR 7313 Marseille, France.

Frontiers in Chemistry
|May 3, 2014
PubMed
Summary

Hyperconjugation significantly stabilizes carbocations by delocalizing electrons onto the positively charged carbon atom. This effect, quantified using the Block Localized Wavefunction (BLW) method, is comparable to conjugation effects in stabilizing organic molecules.

Keywords:
carbocationconjugationhyperconjugationsiliconvalence bond

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Area of Science:

  • Computational Chemistry
  • Organic Chemistry

Background:

  • Carbocations are key intermediates in organic reactions.
  • Understanding their stability is crucial for predicting reaction pathways.
  • Hyperconjugation is a known stabilizing effect in carbocations.

Purpose of the Study:

  • To quantify the hyperconjugation effect in various carbocations.
  • To compare the stabilizing energy of hyperconjugation with conjugation.
  • To investigate specific effects like the beta-silicon effect.

Main Methods:

  • Density Functional Theory (DFT) calculations using the B3LYP/6-311G(d) level of theory.
  • Block Localized Wavefunction (BLW) method to isolate and quantify hyperconjugation.
  • Evaluation of hyperconjugation as energy gained by electron delocalization onto the C(+) atom.

Main Results:

  • Hyperconjugation provides significant stabilization to carbocations.
  • The magnitude of hyperconjugation was found to be comparable to conjugation effects in many systems.
  • The beta-silicon effect was successfully computed, demonstrating the method's versatility.

Conclusions:

  • Hyperconjugation is a critical factor in carbocation stability.
  • The BLW method provides a reliable way to quantify this effect.
  • Computational insights can guide the understanding of organic reaction mechanisms.