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Organic Chemistry – Some Basic Principles and Techniques - Electronic Displacements (Inductive, Electromeric, Resonance, Hyperconjugation)

Grade 11CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Inductive Effect (II effect): It is the permanent displacement of σ\sigma electrons along a saturated carbon chain due to the difference in electronegativity between atoms. It is classified into −I-I (electron-withdrawing) like −NO2-NO_2, −CN-CN, −F-F and +I+I (electron-donating) like −CH3-CH_3, −C2H5-C_2H_5.

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Electromeric Effect (EE effect): A temporary effect involving the complete transfer of a shared pair of π\pi-electrons to one of the atoms joined by a multiple bond in the presence of an attacking reagent. It is +E+E if electrons move toward the reagent and −E-E if they move away.

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Resonance (Mesomeric) Effect (RR or MM effect): The polarity produced in a molecule by the interaction of two π\pi-bonds or between a π\pi-bond and a lone pair of electrons. +R+R effect involves electron donation into the conjugated system (e.g., −OH-OH, −NH2-NH_2), while −R-R involves electron withdrawal (e.g., −NO2-NO_2, >C=O>C=O).

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Hyperconjugation: Also known as 'no-bond resonance' or the Baker-Nathan effect. It involves the delocalization of σ\sigma-electrons of a C−HC-H bond of an alkyl group directly attached to an atom of an unsaturated system or to an atom with an unshared pp-orbital (like a carbocation).

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Stability of Carbocations: Stability increases with the number of alkyl groups attached to the positively charged carbon due to +I+I effect and hyperconjugation: Tertiary>Secondary>Primary>MethylTertiary > Secondary > Primary > Methyl.

📐Formulae

Stability order of carbocations:(CH3)3C+>(CH3)2CH+>CH3CH2+>CH3+Stability\ order\ of\ carbocations: (CH_3)_3C^+ > (CH_3)_2CH^+ > CH_3CH_2^+ > CH_3^+

Decreasing order of −I effect:−NH3+>−NO2>−CN>−SO3H>−CHO>−F>−Cl>−Br>−IDecreasing\ order\ of\ -I\ effect: -NH_3^+ > -NO_2 > -CN > -SO_3H > -CHO > -F > -Cl > -Br > -I

Decreasing order of +I effect:−O−>−COO−>(CH3)3C−>(CH3)2CH−>−CH2CH3>−CH3Decreasing\ order\ of\ +I\ effect: -O^- > -COO^- > (CH_3)_3C- > (CH_3)_2CH- > -CH_2CH_3 > -CH_3

Number of hyperconjugative structures=n+1 (where n is the number of α-hydrogens)Number\ of\ hyperconjugative\ structures = n + 1 \text{ (where } n \text{ is the number of } \alpha\text{-hydrogens)}

💡Examples

Problem 1:

Arrange the following carboxylic acids in increasing order of their acidity: CH3COOHCH_3COOH, ClCH2COOHClCH_2COOH, Cl2CHCOOHCl_2CHCOOH, Cl3CCOOHCl_3CCOOH.

Solution:

CH3COOH<ClCH2COOH<Cl2CHCOOH<Cl3CCOOHCH_3COOH < ClCH_2COOH < Cl_2CHCOOH < Cl_3CCOOH

Explanation:

The acidity of carboxylic acids increases with the presence of electron-withdrawing groups (−I-I effect). Chlorine is an electronegative element that exerts a −I-I effect, which stabilizes the carboxylate anion (RCOO−RCOO^-) by dispersing the negative charge. As the number of ClCl atoms increases, the −I-I effect strengthens, making the acid stronger.

Problem 2:

Why is the TertiaryTertiary butyl carbocation (CH3)3C+(CH_3)_3C^+ more stable than the EthylEthyl carbocation CH3CH2+CH_3CH_2^+?

Solution:

Due to greater hyperconjugation and +I+I effect in (CH3)3C+(CH_3)_3C^+.

Explanation:

In (CH3)3C+(CH_3)_3C^+, there are 99 α\alpha-hydrogens available for hyperconjugation, whereas in CH3CH2+CH_3CH_2^+, there are only 33 α\alpha-hydrogens. More hyperconjugative structures lead to greater delocalization of the positive charge, resulting in higher stability. Additionally, three methyl groups provide a stronger +I+I effect than one methyl group.

Problem 3:

Identify the type of electronic displacement in the reaction of H+H^+ with CH2=CH2CH_2=CH_2.

Solution:

+E+E effect (Positive Electromeric effect).

Explanation:

When the attacking reagent is an electrophile (H+H^+), the π\pi-electrons of the double bond are transferred to the carbon atom to which the reagent gets attached. This is defined as the +E+E effect.