Question
Read the passage given below and answer the following questions from 1 to 5.
A reagent that brings an electron pair to the reactive site is called a nucleophile (Nu:) i.e., nucleus seeking and the reaction is then called nucleophilic. A reagent that takes away an electron pair from reactive site is called electrophile (E+) i.e., electron seeking and the reaction is called electrophilic.
Electron Displacement Effects in Covalent Bonds The electron displacement in an organic molecule may take place either in the ground state under the influence of an atom or a substituent group or in the presence of an appropriate attacking reagent. The electron displacements due to the influence of an atom or a substituent group present in the molecule cause permanent polarlisation of the bond. Inductive effect and resonance effects are examples of this type of electron displacements. Temporary electron displacement effects are seen in a molecule when a reagent approaches to attack it. This type of electron displacement is called electrometric effect or polarisability effect.
Inductive Effect When a covalent bond is formed between atoms of different electronegativity, the electron density is more towards the more electronegative atom of the bond. Such a shift of electron density results in a polar covalent bond. Bond polarity leads to various electronic effects in organic compounds. Let us consider cholorethane $(CH_3CH_2Cl)$ in which the C–Cl bond is a polar covalent bond. It is polarised in such a way that the carbon-1 gains some positive charge $(\delta+)$ and the chlorine some negative charge $(\delta-)$ The fractional electronic charges on the two atoms in a polar covalent bond are denoted by symbol (delta) and the shift of electron density is shown by an arrow that points from$(\delta+)$ to $(\delta-)$ end of the polar bond.
In turn carbon-1, which has developed partial positive charge $(\delta+)$draws some electron density towards it from the adjacent C-C bond. Consequently, some positive charge$(\delta\delta+)$develops on carbon-2 also, where $(\delta\delta+)$ symbolises relatively smaller positive charge as compared to that on carbon – 1. In other words, the polar C – Cl bond induces polarity in the adjacent bonds. Such polarisation of σ- bond caused by the polarisation of adjacent $σ-$bond is referred to as the inductive effect.
Resonance Structure There are many organic molecules whose behaviour cannot be explained by a single Lewis structure. An example is that of benzene. Its cyclic structure containing alternating C–C single and C=C double bonds shown is inadequate for explaining its characteristic properties.
As per the above representation, benzene should exhibit two different bond lengths, due to C–C single and C=C double bonds. However, as determined experimentally benzene has a uniform C–C bond distances of 139 pm, a value intermediate between the C–C single(154 pm) and C=C double (134 pm) bonds. Thus, the structure of benzene cannot be represented adequately by the above structure. Further, benzene can be represented equally well by the energetically identical structures I and II.
Therefore, according to the resonance theory the actual structure of benzene cannot be adequately represented by any of these structures, rather it is a hybrid of the two structures (I and II) called resonance structures. The resonance structures (canonical structures or contributing structures) are hypothetical and individually do not represent any real molecule. They contribute to the actual structure in proportion to their stability.
Resonance Effect The resonance effect is defined as ‘the polarity produced in the molecule by the interaction of two π-bonds or between a π-bond and lone pair of electrons present on an adjacent atom’. The effect is transmitted through the chain. There are two types of resonance or mesomeric effect designated as R or M effect. (i) Positive Resonance Effect (+R effect) In this effect, the transfer of electrons is away from an atom or substituent group attached to the conjugated system. This electron displacement makes certain positions in the molecule of high electron densities. This effect in aniline is shown as : (ii) Negative Resonance Effect (- R effect) This effect is observed when the transfer of Electrons is towards the atom or substituent Group attached to the conjugated system. For Example in nitrobenzene this electron Displacement can be depicted as : The atoms or substituent groups, which represent +R or –R electron displacement effects are as follows: +R effect: – halogen, –OH, –OR, –OCOR, –NH2, –NHR, –NR2, –NHCOR, – R effect: $– COOH, –CHO, > C = O, – CN, – NO_2$ The presence of alternate single and double bonds in an open chain or cyclic system is termed as a conjugated system. These systems often show abnormal behaviour. The examples are 1,3- butadiene, aniline and nitrobenzene etc. In such systems, the π-electrons are delocalised and the system develops polarity.
Electromeric Effect (E effect) It is a temporary effect. The organic compounds having a multiple bond (a double or triple bond) show this effect in the presence of an attacking reagent only. It is defined as the complete transfer of a shared pair of π-electrons to one of the atoms joined by a multiple bond on the demand of an attacking reagent. The effect is annulled as soon as the attacking reagent is removed from the domain of the reaction. It is represented by E and the shifting of the electrons is shown by a curved arrow ( ). There are two distinct types of electromeric effect.
a) Positive Electrometric Effect (+E effect)- In this effect the π−electrons of the multiple bond are transferred to that atom to which the reagent gets attached. For example
b) Negative Electromeric Effect (–E effect) -In this effect the $\pi-$ electrons of the multiple bond are transferred to that atom to which the attacking reagent does not get attached. For example: When inductive and electromeric effects operate in opposite directions, the electomeric effect predominates.
A reagent that brings an electron pair to the reactive site is called a nucleophile (Nu:) i.e., nucleus seeking and the reaction is then called nucleophilic. A reagent that takes away an electron pair from reactive site is called electrophile (E+) i.e., electron seeking and the reaction is called electrophilic.
Electron Displacement Effects in Covalent Bonds The electron displacement in an organic molecule may take place either in the ground state under the influence of an atom or a substituent group or in the presence of an appropriate attacking reagent. The electron displacements due to the influence of an atom or a substituent group present in the molecule cause permanent polarlisation of the bond. Inductive effect and resonance effects are examples of this type of electron displacements. Temporary electron displacement effects are seen in a molecule when a reagent approaches to attack it. This type of electron displacement is called electrometric effect or polarisability effect.
Inductive Effect When a covalent bond is formed between atoms of different electronegativity, the electron density is more towards the more electronegative atom of the bond. Such a shift of electron density results in a polar covalent bond. Bond polarity leads to various electronic effects in organic compounds. Let us consider cholorethane $(CH_3CH_2Cl)$ in which the C–Cl bond is a polar covalent bond. It is polarised in such a way that the carbon-1 gains some positive charge $(\delta+)$ and the chlorine some negative charge $(\delta-)$ The fractional electronic charges on the two atoms in a polar covalent bond are denoted by symbol (delta) and the shift of electron density is shown by an arrow that points from$(\delta+)$ to $(\delta-)$ end of the polar bond.
In turn carbon-1, which has developed partial positive charge $(\delta+)$draws some electron density towards it from the adjacent C-C bond. Consequently, some positive charge$(\delta\delta+)$develops on carbon-2 also, where $(\delta\delta+)$ symbolises relatively smaller positive charge as compared to that on carbon – 1. In other words, the polar C – Cl bond induces polarity in the adjacent bonds. Such polarisation of σ- bond caused by the polarisation of adjacent $σ-$bond is referred to as the inductive effect.
Resonance Structure There are many organic molecules whose behaviour cannot be explained by a single Lewis structure. An example is that of benzene. Its cyclic structure containing alternating C–C single and C=C double bonds shown is inadequate for explaining its characteristic properties.
As per the above representation, benzene should exhibit two different bond lengths, due to C–C single and C=C double bonds. However, as determined experimentally benzene has a uniform C–C bond distances of 139 pm, a value intermediate between the C–C single(154 pm) and C=C double (134 pm) bonds. Thus, the structure of benzene cannot be represented adequately by the above structure. Further, benzene can be represented equally well by the energetically identical structures I and II.
Therefore, according to the resonance theory the actual structure of benzene cannot be adequately represented by any of these structures, rather it is a hybrid of the two structures (I and II) called resonance structures. The resonance structures (canonical structures or contributing structures) are hypothetical and individually do not represent any real molecule. They contribute to the actual structure in proportion to their stability.
Resonance Effect The resonance effect is defined as ‘the polarity produced in the molecule by the interaction of two π-bonds or between a π-bond and lone pair of electrons present on an adjacent atom’. The effect is transmitted through the chain. There are two types of resonance or mesomeric effect designated as R or M effect. (i) Positive Resonance Effect (+R effect) In this effect, the transfer of electrons is away from an atom or substituent group attached to the conjugated system. This electron displacement makes certain positions in the molecule of high electron densities. This effect in aniline is shown as : (ii) Negative Resonance Effect (- R effect) This effect is observed when the transfer of Electrons is towards the atom or substituent Group attached to the conjugated system. For Example in nitrobenzene this electron Displacement can be depicted as : The atoms or substituent groups, which represent +R or –R electron displacement effects are as follows: +R effect: – halogen, –OH, –OR, –OCOR, –NH2, –NHR, –NR2, –NHCOR, – R effect: $– COOH, –CHO, > C = O, – CN, – NO_2$ The presence of alternate single and double bonds in an open chain or cyclic system is termed as a conjugated system. These systems often show abnormal behaviour. The examples are 1,3- butadiene, aniline and nitrobenzene etc. In such systems, the π-electrons are delocalised and the system develops polarity.
Electromeric Effect (E effect) It is a temporary effect. The organic compounds having a multiple bond (a double or triple bond) show this effect in the presence of an attacking reagent only. It is defined as the complete transfer of a shared pair of π-electrons to one of the atoms joined by a multiple bond on the demand of an attacking reagent. The effect is annulled as soon as the attacking reagent is removed from the domain of the reaction. It is represented by E and the shifting of the electrons is shown by a curved arrow ( ). There are two distinct types of electromeric effect.
a) Positive Electrometric Effect (+E effect)- In this effect the π−electrons of the multiple bond are transferred to that atom to which the reagent gets attached. For example
b) Negative Electromeric Effect (–E effect) -In this effect the $\pi-$ electrons of the multiple bond are transferred to that atom to which the attacking reagent does not get attached. For example: When inductive and electromeric effects operate in opposite directions, the electomeric effect predominates.
- A reagent that brings an electron pair to the reactive site is called a …
- nucleophile
- electrophile
- amphoteric
- amphophillic
- A reagent that takes away an electron pair from reactive site is called ..
- nucleophile
- electrophile
- amphoteric
- amphophillic
- The … effect is defined as the polarity produced in the molecule by the interaction of two π-bonds or between a π-bond and lone pair of electrons present on an adjacent atom.
- hindrance
- inductive
- resonance
- hyperconjunction
- –OH group, represent … electron displacement effect.
- M+
- M-
- R-
- R+
- – COOH group, represent … electron displacement effect.
- M+
- M-
- R-
- R+
