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The electronic effect is imposed by electrons present in the chemical bonds. In other words, it is an effect via which the reactivity of the compound in one part is controlled by the electron attraction or repulsion produced in another part of the molecule. It is known to influence the three significant parts of the molecule – reactivity, properties, and structure. Generally, the electronic effects are categorized into different types: resonance, electromeric effect, inductive effect, and hyperconjugation. Here is a detailed look at the different effects and how they influence the molecule.
Electromeric Effect
Electromeric effects are often seen in organic compounds that have multiple bonds present in them. However, this isn’t a permanent effect. The effect arises when an attacking reagent is brought near the organic compound. When pi electrons are ultimately shared between the participating atoms under the influence of an attacking reagent and form a dipole in the molecule, it is called electromeric effects. However, one condition must be followed to observe the effect: the presence of at least one multiple bond. The effect stays as long as the attacking reagent remains present. The polarized molecule regains its original state as soon as the attacking reagent gets removed.
Subdivisions Of Electromeric Effects
The electromeric effect is further divided into two main types. The two classifications are based on how and in which direction the electron pair is transferred.
- +E effect.
- -E effect.
+E Effect – +E effect is observed when the electron pair present in the bond gets transferred to the unique reagent. This effect occurs when the attacking reagent is an electrophile. During the effect, the π electrons are shifted to the positively charged atoms.
-E Effect- When the electrons are transferred away from the attacking reagent, during this type of bonding, the attacking reagent is attached to the positively charged species during this type of bonding. One significant difference between the +E effect and the -E effect is that -E effects occur when an attacking reagent is a nucleophile.
Resonance
Resonance is varied structures of a molecule or ion that differ only in the position of electrons. In other words, structural resonance represents different correct forms of a molecule that are all required to represent and define the properties of the structure. C–C single and C=C double bonds are the perfect example of resonance. Additionally, the resonating structures formed from the actual structure possess more energy.
Subdivisions Of Resonance Effects
There are two types of resonance effects +R or –R.
+R effect – during the positive resonance effect, the electrons are displayed away from the atom or the substituent group. Additionally, the resonating structures formed from the +R effect possess high electron density at a few places. A few examples of this type of effect are halogen,–OH,–OR,–OCOR,–NH2–NHR,–NR2–NHCOR.
-R effect – Opposite to the positively charged resonance effect, this effect occurs when the electrons are displayed from the atom or substituent group. During this effect, the structures represent positive charges at different locations. A few examples of the – R effect are COOH,–CHO,>C=0,–CN,–NO2.
Hyperconjugation
Hyperconjugation, also known as no bond resonance, involves the overlapping or interaction of empty p- orbital or Π – orbital and filled σ-bond orbital of an adjacent carbon atom. It is a permanent effect in which sigma electrons of C–H single bonds are involved in the delocalisation of electrons. The stability of the structure with hyper conjugative effect is directly related to the number of hyperconjugation hydrogen present in it. In other words, hyperconjugation is a way that explains the stability of the alkyl radicals. Additionally, it occurs in alkenes and alkyl arenes.
Inductive Effect
Being a permanent effect, the inductive effect is polarized in which electron density is uneven between the two, unlike atoms. Meaning, the electron density is high where the atoms have high electronegativity. And is less where the atoms have low electronegativity. It is a distance-dependent phenomenon in which the bond is polarized. The effect occurs when an electron-withdrawing or electron releasing group is attached. The bond that shows the inductive effect is represented by C-C-C-C-C1-X, in which X is an electronegative element. When the electronegative atom is attached, it pulls the electron from the nearest carbon atom. Similar to this case, the C1 atom, due to electron deficiency, pulls the electron towards it and makes the C2 get a positive charge. The same goes until the positive charge ultimately becomes nil.
Subdivisions Of Inductive Effects
Although the effect is permanent yet the other effects overshadow it. Here are the two categories in which the inductive effect is divided.
-I effect – This is observed when an electron-withdrawing group is attached to the molecule. The place where the highly electronegative atom is attached has a high electron density compared to the other place. Some of the examples of electron-withdrawing groups include nitro (-NO2), halogen, cyano (-CN), carboxy (-COOH), aryloxy (-OAr), etc.
+I effect – Opposite to the above effect, the +I effect is observed when an electron releasing or donating group is attached to the molecule. One of the best examples of electron releasing or donating groups is the alkyl group. Thus, this effect is observed when fewer electronegative atoms are near it.
Conclusion
Chemical reactions between atoms or molecules are executed to lose or gain electrons and become stable. In any covalent bond or other types of bonds, the electronic effects occur, influencing the electron distribution. This is a way to stabilize the unstable atoms or molecules that can occur due to electron shifting between the species. So these were general types of electronic effects that can occur in the molecule, and the main is resonance.
