The electronic displacements in covalent bonds may occur either in the ground state under the
Channel: Doubtnut
Date: 2020-03-29

The electronic displacements in covalent bonds may occur either in the ground state under the influence of an atom or a substituent group or in presence of an appropriate attacking reagent. As a result of these electron displacements, centres of different electron densities are created and these centres are susceptible to attack by the reagents. These electron displacements occur through inductive electromeric, resonance and hyperconjugation effects. Whereas inductive effect involves displacement of `sigam`-electrons towards the substituent, resonance effect involves delocalization of `pi-`electrons transmitted through the chain and both are permanent effect. Electromeric effect is the complete transfer of a shared pair of `pi`- electrons to one of the atoms joined by a multiple bond on the demand of an attacking reagent. Hyperconjugation effects on the other hand involve delocalization of `sigma`-electrons of `C-H` bond of an alkyl group directly attached to an atom of unsaturated system (i.e., `sigma-pi`-conjugation). Both inductive and hyperconjugation effects can be used to explain the stability of carbocations and free radicals which follow the stability order : `3^(@)gt2^(@)gt1^(@)`. The stability or carbanions, however, follows the reverse order. An organic reaction occurs through making and breaking of bonds. The breaking of a covalent bond may occur either homolytic leading to the formation of free radicals or heterolytic forming positively (carbocations) or negatively (carbanions) charged species. Most of the attacking reagents carry either a positive or a negative charge. The positively charged species with electron deficient centre or neutral species (free radicals, carbenes, nitrene) are collectively called electrophiles, while negatively charged species with electron rich centre or neutral species (like water, alcohol, ammonia, etc.) are called nucleophiles. Out of the following series, the one containing only electrophiles is: