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: 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

       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

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