Reaction Intermediates

Reaction Intermediates


       Heterocyclic fission

       Homolytic fission

       Bond dissociation energy



       Nucleophiles and electrophiles


       Stability of carbocation

Learning Objectives

At the end of this lecture, student will be able to

       Define the  terms

        Heterocyclic fission

       Homolytic fission

       Bond dissociation energy



       List examples for nucleophiles and electrophiles

       Define the term  carbanion  ,free radicals , carbenes and nitrenes

       Discuss the generation and fate of carbanions and free radicals

       Compare the stabilities of  different carbanions

       Compare the stabilities of different free radicals

Different Types of Reaction

       Substitution reaction 


       Elimination reaction 

Aà B + C

       Addition reaction A + B à C

       Rearrangement reactions A à B


Bond dissociation energy

       Measure of the strength in a chemical bond

       It is defined as the standard enthalpy change when a bond is cleaved 

Heterolytic fission

       If a bond were to split unevenly (one atom getting both electrons, and the other none), ions would be formed

       The atom that got both electrons would become negatively charged

       While the other one would become positive

Homolytic fission

       If the bond breaks in such a way that each fragment  gets one electron

       Free radicals are formed


       Nucleus loving

       Electron rich

       Attacking reagents

Types of nucleophile

       Negative nucleophile

       Neutral nucleophile

       Negative  nucleophile, Examples: Cl‾, OH‾, CN‾, OR‾, H‾, X‾, SH‾

        Neutral nucleophile, Examples: NH3, H2O, ROH, RSH, ROR


       Electron loving species


       Neutral, Examples: BF3, AlCl3

       Positive electrophiles, Examples: Carbocation  Br+

Reaction Intermediates

       Reactions in organic chemistry proceed in more than one step

       Via one more short lived reactive intermediate

       Starting material à Intermediate à Products

       Specific sequence in which bonds are made and broken

       Reactants are converted into products



       Free radicals




       Electron deficient

        Positively charged reaction intermediate

        Formed by heterolysis of covalent bond



Primary carbocation       1 alkyl substituent bonded to positively charged carbon

Secondary carbocation 2 alkyl substituents

Tertiary carbocation       3 alkyl substituents


CH3+         CH3—CH2

Stability of Carbocations

       Inductive effect

       Hyperconjugative effect

Inductive effect

       G → electron releasing group like alkyl group → stabilizes carbocation

       G → electron withdrawing group like nitro group → destabilizes carbocation


       Order of stability of carbocations


Rearrangement of Carbocations

       Driving force - stability

       1° carbonium ion  rearrange to a 2° or 3° carbonium ion

           CH3-CH2-CH2-CH2+ ———→ CH3-CH2-CH+-CH3


                             CH3                                           CH3

                              ǀ                                                 ǀ

             CH3-CH2-CH-CH2+ ———→ CH3-CH2-C-CH3

       2° carbonium ion rearrange to 3° carbonium ion


                                 CH3                                     CH3

                                 ǀ                                           ǀ

                         CH3-C-CH-CH3 ———→ CH3-C-C-H

                                     ǀ     +                                     +    ǀ

                              CH3                                     CH3

       Rearrangement takes place by 1,2-shift

       Migration of hydrogen  - hydride shift

       Migration of alkyl group   - alkyl shift

1,2 – hydride shift

1,2 – methyl shift


       Carbon atom bearing negative charge

       Typically nucleophile and basic in nature

Stability of Carbanions

       Basicity and nucleophilicity of carbanions are determined by the substituents on carbon

       Inductive effect Electronegative atoms adjacent to the charge will stabilize the charge;

       Extent of conjugation of the anion. Resonance effects can stabilize the anion

       Conjugation  with unsaturated bond

       Order of stability

       Because of fact that greater s-character, closer are the electrons to the nucleus and hence of lower energy 

 Fate of Carbanions

       Addition reaction

Reaction intermediates

       Of the five, only carbanions have complete octet around carbon

Free Radical

       Species  which is having odd or unpaired electron

       Highly reactive species


       Hyperconjugative effect

       Resonance effect

       Relative stabilities of free radicals

       Benzyl >allyl >tert > sec > primary > methyl

Generation of Free Radicals

       Free radicals are formed from molecules by breaking a bond so that each fragment keeps one electron

       Energy necessary to break the bond is supplied in two ways

       Thermal cleavage or  photochemical cleavage of  covalent bonds

       Reactions of free radicals either give stable products or lead to other radicals

Fate of Free Radicals

       Radical –radical interaction

       Radical –molecule interaction (propagation reactions)

       Addition to multiple bonds


       Carbenes  are neutral species

       Carbon atom  with two bonds and two electrons


Generation of carbenes

       In a elimination, a carbon loses a group without its electron pair, usually a proton, and then a group with its pair, usually a halide ion

       Formation of dichlorocarbene by treatment of chloroform with base

       Photolysis of ketene

Fate of carbenes

       Addition to carbon-carbon double bonds

       Addition to aromatic systems- usually with ring enlargement

       Carbene reacts with methane to give ethane and propane


       Nitrogen analogue of carbene

       Electron deficient species

       Nitrogen has six electrons


       Too reactive for isolation under ordinary conditions

Generation of nitrenes

       Breakdown of certain double bond compounds

       Photolytic or thermal decomposition of azides

Fate of nitrenes

       Addition to carbon-carbon double bonds

       Rearrangements of alkyl nitrene


       Products of homolytic fission are free radicals

       Products of heterocyclic fission are ions

       Intermediates are highly unstable

       They are formed in a rection

       Carbocations, carbanions, free radicals, carbene and nitrenes are   reaction intermediates

       Carbanions are negatively charged species

       Carbanions are formed by heterolytic fission of bonds

       Carbanions  undergo addition reactions

       Tertiary carbanion is the least stable anion

       Free radicals are neutral species and possess an odd electron

       Free radicals are formed by homolytic  fission  of bonds

       Free radicals are formed by Thermal cleavage or  photochemical cleavage of  covalent bonds

       Free radicals take part in Radical –molecule interaction (propagation reactions) and addition to multiple bonds

       Benzyl free radical is the most stable free radical

       Carbon with two electrons and two bonds are called as carbenes

       Nitrogen analogue of carbene is called as nitrene

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