Electronic effect

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(Redirected from
Polar effect
)
This article is about the influences on the chemical structure, reactivity, or properties of a molecule. For electric audio effects, see Effects unit.

An electric effect influences the

steric effect.[1] In organic chemistry, the term stereoelectronic effect is also used to emphasize the relation between the electronic structure and the geometry (stereochemistry
) of a molecule.

The term polar effect is sometimes used to refer to electronic effects, but also may have the more narrow definition of effects resulting from non-conjugated substituents.[2]

Types

Redistributive effects

Induction is the redistribution of electron density through a traditional sigma bonded structure according to the electronegativity of the atoms involved. The inductive effect drops across every sigma bond involved limiting its effect to only a few bonds.

electron lone pairs
with respect to the pi-system. Electronic effects can be transmitted throughout a pi-system allowing their influence to extend further than induction.

In the context of electronic redistribution, an electron-withdrawing group (EWG) draws electrons away from a reaction center. When this center is an electron rich carbanion or an alkoxide anion, the presence of the electron-withdrawing substituent has a stabilizing effect. Similarly, an electron-releasing group (ERG) or electron-donating group (EDG) releases electrons into a reaction center and as such stabilizes electron deficient carbocations.

In

deactivating groups
. Resonance electron-releasing groups are classed as activating, while Resonance electron-withdrawing groups are classed as deactivating.

Non-redistributive effects

antibonding π orbital or an antibonding sigma orbital to give an extended molecular orbital that increases the stability of the system.[3] Hyperconjugation can be used to explain phenomena such as the gauche effect and anomeric effect
.

Diels-Alder reaction
, among others.

coulombic interactions of atoms that hold like charges
.

Dioxygen is a triplet molecule, since the two unpaired electrons allow for three spin states. The reaction of a triplet molecule with a singlet molecule is spin-forbidden
in quantum mechanics. This is the major reasons there is a very high reaction barrier for the extremely thermodynamically favorable reaction of singlet organic molecules with triplet oxygen. This kinetic barrier prevents life from bursting into flames at room temperature.

Electronic spin states are more complex for transition metals. To understand the reactivity of transition metals, it is essential to understand the concept of d electron configuration as well as high-spin and low-spin configuration. For example, a low-spin d8 transition metal complex is usually square planar substitutionally inert with no unpaired electrons. In contrast, a high-spin d8 transition metal complex is usually octahedral, substitutionally labile, with two unpaired electrons.

degenerate
electronic ground state will undergo a geometrical distortion that removes that degeneracy. This has the effect of lowering the overall energy. The Jahn–Teller distortion is especially common in certain transition metal complexes; for example, copper(II) complexes with 9 d electrons.

Trans influence
is the influence that a ligand in a square or octahedral complex has on the bond to the ligand trans to it. It is caused by electronic effects, and manifests itself as the lengthening of the trans bonds and as an effect on the overall energy of the complex.

Comparison with steric effects

The structure, properties, and reactivity of a molecule are dependent on straightforward bonding interactions including

ionic bonds, hydrogen bonds, and other forms of bonding. This bonding supplies a basic molecular skeleton that is modified by repulsive forces generally considered steric effects
. Basic bonding and steric effects are at times insufficient to explain many structures, properties, and reactivity. Thus, steric effects are often contrasted and complemented by electronic effects, implying the influence of effects such as induction, conjunction, orbital symmetry, electrostatic interactions, and spin state. There are more esoteric electronic effects but these are among the most important when considering chemical structure and reactivity.

Special computational procedure was developed to separate steric and electronic effects of an arbitrary group in the molecule and to reveal their influence on structure and reactivity.[4]

References

  1. ISBN 0-13-035471-6
    .
  2. ^ "polar effect". IUPAC Gold Book. IUPAC. Retrieved 2024-02-06.
  3. ISBN 0-534-07968-7
  4. doi:10.1002/ejic.200700850
    .
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