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Organic molecules, the majority of which are made up of a combination of the six elements carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulphur, are involved in these electronic aspects. However, the limited number of building blocks does not prohibit organic compounds from exhibiting a wide range of physical and chemical properties. Organic chemistry’s fine differentiation of diverse compounds is critical for the biological functions of molecules and generates a vast range of reactions. Differences in electron behaviour when atoms other than carbon and hydrogen join in molecular bonds account for some of the variations in organic chemistry. The three compounds showed above, for example, have similar formula units and structures but react extremely differently due to these electrical variables. Delocalization effects occur when the electron cloud for a given bond expands to more than two atoms within the molecule due to variations in electronegativity.
Introduction
The electronic effect refers to the influence of electrons located in chemical bonds within the atoms of a molecule. The electronic effect is defined as the process by which the reactivity of a substance in one part of the molecule is controlled by electron repulsion or attraction in another part of the molecule.
The reactivity, characteristics, and structure of a chemical are all affected by an electrical effect. The term ‘stereo electronic effect’ emphasises the relationship between the compound’s electronic structure and geometry.
Inductive, mesomeric (or resonance) effect, electromeric effect, and hyperconjugation effect are the four primary forms of electronic effects. The electromeric effect is transient, whereas the others are permanent and manifest as the molecule’s dipole moment. Hydrocarbons, on the other hand, are non-polar. However, functional groups, or compounds with heteroatoms (atoms other than carbon) or groups, are polar. We’ll look at how the presence of a heteroatom/group introduces polarity in organic compounds while studying all of these impacts. It is in the context of this group that numerous types of impacts on the remaining half of the molecule are known to occur.
Inductive Effect
The difference in electronegativity of atoms linked together causes the inductive effect. If the electronegativities of two atoms differ, the link between them is polarised. The development of partial charges + and, which have effects on surrounding bonds at a relatively short distance, is caused by the polarisation of the bond. This impact is no longer detectable after four bonds, according to popular belief. It could be electron repulsion (atoms less electronegative than carbon: Mg, Al, etc.) or electron withdrawal (atoms more electronegative than carbon: O, N, F, etc). (we are dealing, here, with the bonding of different atoms to carbon).
The weakening of the bonds between a heteroatom (O, N, S…) and a hydrogen atom is one of the outcomes of this inductive action. An electron-withdrawing group weakens the O-H bond of an acid but increases the basicity of a nitrogen atom in an amine by lowering the electron density in the free doublet.
Electromeric Effect
Multiple bonds are made up of two bonds: – and -. Because the electrons in a – bond are loosely bound, they are easily polarisable. When a double or triple bond is attacked by an attacking reagent, the two electrons are entirely transferred to one of the atoms. The polarity formed in a multiple bonded molecule as it is approached by a reagent is referred to as the electromeric effect (E effect).
The electromeric effect is depicted by a curving arrow and is denoted by the letter E. The – electron is totally transported to B in the above illustration of the Electromeric action. As a result, A gains a positive charge and B gains a negative charge. The electromeric effect is a transient phenomenon that occurs only when a reagent is present. The molecule is returned to its native electronic state when the attacking reagent is withdrawn.
When the electron displacement is towards the attacking reagent, the electromeric effect is said to be +E, and when it is away from the attacking reagent, it is said to be –E.
Resonance
For some molecules, there exist multiple proper Lewis structures. One example is ozone (O3). The complex is a chain of three oxygen atoms, and the centre oxygen atom must make a single bond with one terminal oxygen and a double bond with the other terminal oxygen in order to minimise charges while giving each atom an octet of electrons.
The positioning of the double bond in the Lewis structure is completely arbitrary, and each option is equally accurate. The resonance forms are the several right ways of sketching the Lewis structure.
Because single bonds are normally longer than double bonds, a beginning chemistry student could ask if ozone has two distinct length bonds based on the resonance forms. The ozone molecule, on the other hand, is fully symmetrical, with bonds of the same length. None of the resonance forms accurately represent the molecule’s real structure. Rather, the electrons that would form a double bond have their negative charge delocalized, or spread uniformly across the three oxygen atoms. The real structure is a composite, with bonds that are shorter than predicted for single bonds but longer than double bonds.
Hyperconjugation
Hyperconjugation is the delocalization of -electrons or a lone pair of electrons into adjacent -orbital or p-orbital. It happens when a -bonding orbital or an orbital with a lone pair overlaps with an adjacent -orbital or p-orbital. It’s also known as the “Baker-Nathan effect” or “no bond resonance.”
Hyperconjugation requires the presence of an a-CH group or a lone pair on an atom next to a sp2 hybrid carbon or other atoms such as nitrogen or oxygen. Alkenes, alkyl carbocations, alkyl free radicals, nitro compounds containing -hydrogen, and so on are examples.
Conclusion
The -I Effect (pronounced “minus I effect”) occurs when atoms/groups that are more electronegative than carbon obtain a tiny negative charge and pull the electrons from the carbon chain towards themselves.
The +I Effect (pronounced “plus I effect”) occurs when atoms/groups that are more electropositive than carbon receive a small positive charge and push the electrons of the carbon chain away from themselves.
The atoms/groups in which a lone pair (or electrons of negative charge) is conjugated with a double or triple bond are electron donors, gaining a formal positive charge in the resonating structure as a result of the process, and are known to exert the +M/+R Effect.
The -M/-R Effect is caused by atoms/groups that are in conjugation with a double or triple bond and are electron-withdrawing, gaining a formal negative charge in the resonating structure in the process.
