Reaction Intermediates

Reaction Intermediates

Contents

•       Heterocyclic fission

•       Homolytic fission

•       Bond dissociation energy

•       Nucleophiles

•       Electrophiles

•       Nucleophiles and electrophiles

•       Carbocation

•       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

•       Nucleophiles

•       Electrophiles

•       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 

AB+CD à AC+BD

•       Elimination reaction 

Aà B + C

•       Addition reaction A + B à C

•       Rearrangement reactions A à B

Definitions

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

Nucleophile

•       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

Electrophile

•       Electron loving species

Types

•       Neutral, Examples: BF_3, 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

•       Carbocation

•       Carbanion

•       Free radicals

•       Carbenes

•       nitrenes

Carbocation

•       Electron deficient

•        Positively charged reaction intermediate

•        Formed by heterolysis of covalent bond

•       Generation  

Classification

Primary carbocation       1 alkyl substituent bonded to positively charged carbon

Secondary carbocation 2 alkyl substituents

Tertiary carbocation       3 alkyl substituents

Examples

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

•       Hyperconjugation

•       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

Carbanion

•       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

Stability

•       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

•       Carbenes  are neutral species

•       Carbon atom  with two bonds and two electrons

•       R₂C:

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

Nitrenes

•       Nitrogen analogue of carbene

•       Electron deficient species

•       Nitrogen has six electrons

•       RN:

•       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

Summary

•       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