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

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