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Organic Chemistry – Some Basic Principles and Techniques - Fundamental Concepts in Organic Reaction Mechanism

Grade 11CBSEChemistry

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

🔑Concepts

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Fission of a Covalent Bond: Organic reactions involve the breaking of bonds. Homolytic fission results in the formation of free radicals (R⋅R \cdot), while Heterolytic fission results in the formation of carbocations (R+R^+) or carbanions (R−R^-).

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Nucleophiles (Nu−Nu^-): Electron-rich species that donate an electron pair to an electron-deficient center. Examples include negative ions like OH−OH^- and CN−CN^-, or neutral molecules with lone pairs like H2OH_2O and NH3NH_3.

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Electrophiles (E+E^+): Electron-deficient species that accept an electron pair from an electron-rich center. Examples include positive ions like H+H^+, Cl+Cl^+, CH3+CH_3^+ and neutral molecules with incomplete octets like BF3BF_3 or AlCl3AlCl_3.

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Inductive Effect (II-effect): A permanent displacement of σ\sigma-electrons along a saturated carbon chain due to the presence of a polar bond. +I+I effect is shown by electron-releasing groups like alkyl groups (−CH3-CH_3, −C2H5-C_2H_5), and −I-I effect is shown by electron-withdrawing groups like −NO2-NO_2, −CN-CN, and −F-F.

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Resonance 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 on an adjacent atom. +R+R effect involves donating electrons (e.g., −OH-OH, −NH2-NH_2), and −R-R effect involves withdrawing electrons (e.g., −COOH-COOH, −CHO-CHO, −NO2-NO_2).

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Electromeric Effect (EE): A temporary displacement of π\pi-electrons in a multi-bonded system at the requirement of an attacking reagent. It is +E+E if electrons shift towards the attacking reagent and −E-E if they shift away.

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Hyperconjugation: Also known as the 'no-bond resonance', 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.

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Stability of Carbocations: The stability follows the order: 3∘>2∘>1∘>methyl3^\circ > 2^\circ > 1^\circ > \text{methyl}. This is explained by the +I+I effect and hyperconjugation of the attached alkyl groups.

📐Formulae

Stability of Carbocations: (CH3)3C+>(CH3)2CH+>CH3CH2+>CH3+\text{Stability of Carbocations: } (CH_3)_3C^+ > (CH_3)_2CH^+ > CH_3CH_2^+ > CH_3^+

Stability of Carbanions: CH3−>CH3CH2−>(CH3)2CH−>(CH3)3C−\text{Stability of Carbanions: } CH_3^- > CH_3CH_2^- > (CH_3)_2CH^- > (CH_3)_3C^-

Order of -I effect: −NO2>−CN>−COOH>−F>−Cl>−Br>−I>−OH>−C6H5\text{Order of -I effect: } -NO_2 > -CN > -COOH > -F > -Cl > -Br > -I > -OH > -C_6H_5

Order of +I effect: (CH3)3C−>(CH3)2CH−>CH3CH2−>CH3−\text{Order of +I effect: } (CH_3)_3C- > (CH_3)_2CH- > CH_3CH_2- > CH_3-

💡Examples

Problem 1:

Between CH3CH2+CH_3CH_2^+ and CH3CH2CH2+CH_3CH_2CH_2^+, which carbocation is more stable and why?

Solution:

CH3CH2CH2+CH_3CH_2CH_2^+ is slightly less stable than a secondary carbocation, but comparing these two specific primary carbocations, CH3CH2+CH_3CH_2^+ is actually very similar. However, if comparing CH3CH2+CH_3CH_2^+ to (CH3)2CH+(CH_3)_2CH^+, the latter is more stable.

Explanation:

Carbocation stability increases with the number of alkyl groups attached to the positive carbon due to the +I+I effect and hyperconjugation. (CH3)2CH+(CH_3)_2CH^+ (2∘2^\circ) has 6 hyperconjugative HH-atoms, while CH3CH2+CH_3CH_2^+ (1∘1^\circ) has only 3.

Problem 2:

Identify the electrophilic center in CH3CHOCH_3CHO.

Solution:

The carbonyl carbon (CC) is the electrophilic center.

Explanation:

In the C=OC=O bond, Oxygen is more electronegative than Carbon. Due to the −I-I and resonance effects, the π\pi-electrons are shifted toward Oxygen, creating a partial positive charge (δ+\delta^+) on Carbon: CH3−Cδ+=Oδ−CH_3-C^{\delta+}=O^{\delta-}. Thus, Carbon is susceptible to nucleophilic attack.

Problem 3:

Which of the following shows the maximum +I+I effect: CH3−CH_3-, CH3CH2−CH_3CH_2-, (CH3)2CH−(CH_3)_2CH-, or (CH3)3C−(CH_3)_3C-?

Solution:

(CH3)3C−(CH_3)_3C- (Tertiary-butyl group).

Explanation:

The Inductive effect is additive. As the number of alkyl groups increases on the central carbon atom, the cumulative electron-releasing power increases.

Fundamental Concepts in Organic Reaction Mechanism Revision - Class 11 Chemistry CBSE