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In organic chemistry, various reactions take place and requires us to know which is which.
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Lesson outcomeAt the end of this lecture, students should be
able to:Define reaction mechanismCategorize different types of reactionsDefine homolytic and heterolytic bond
cleavageIdentify Electrophiles and Nucleophiles
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Reaction MechanismA description of events that take place on a
molecular level as reactants become productsFinal analysis of proposed reaction
mechanism must be consistent with experimental observations
Example, SN2 reaction:
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Substitution ReactionOne group replaces another groupCharacteristic reactions of saturated
compounds eg. alkanes, alkyl halides, aromatic compounds
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3
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Addition ReactionAll parts of the adding reagent appear in the
product, two molecules become oneCharacteristic of compounds with multiple
bonds, eg. alkene
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Elimination ReactionsOne molecule loses the elements of another
small moleculeIt is opposite of addition reactionseg E2, elimination of hydrogen halide
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RearrangementA molecule undergoes a reorganization of its
constituent parts. Eg, the positions of the double bond or methyl groups changed.
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Using curved arrows in illustrating reactionsShow the direction of electron flow in a
reaction mechanismPoint from the source of electron pair to the
atom receiving the pairShow flow of electrons from a site of higher
electron density to a site of lower electron density
Never show movement of atoms. Atoms are assumed to follow the flow of electrons
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Homolysis and heterolysis of covalent bonds
Heterolytic bond cleavageWhen a bond breaks, one fragment takes away
both electrons of the bond, leaving the other fragment with empty orbital
It is ionic reaction that produces ionsRequires bond to be polarized (differing
electronegativities of atoms joined by the bond)Assisted by a molecule with unshared electron
pair that form a bond to one of the atoms
A : B A+ + B-
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Heterolysis of bonds to carbonHeterolysis of a carbon bond can lead to
either : Carbocation - a positive charge on carbonCarbanion - a negative charge on carbon
A carbocation is electron deficient as it has only 6 electrons, is sp2 hybridized and has an empty p orbital
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Homolysis and heterolysis of covalent bonds
Homolytic bond cleavageWhen a bond breaks, each fragment takes
one of the electrons of the bondHomolysis produces fragments with unpaired
electrons called radicals
A : B A
. + B .
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Bronsted-Lowry Acids and BasesAn acid is a proton donorA base is a proton acceptorAn acid-base reaction involves the transfer of
a single proton from one species to anotherA Brønsted–Lowry acid will donate a proton
to a Brønsted–Lowry base
OH H + O
H H H O OH H
H
++
acid base
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Lewis acids and bases
B
F
F FN
H
H HB N
F H
F HHF
A Lewis acid is an electron-pair acceptor
A Lewis base is an electron-pair donor
Lewis acid Lewis base
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Electrophiles and nucleophilesElectrophiles:‘electron-loving’Reagents that seek electrons so as to achieve a
stable shell of electrons like that of noble gasCarbocations and all Lewis acids are
electrophiles- species are attracted toward a negative center eg H+, ZnCl2
Carbon atoms that are electron-poor but are not carbocations are also electrophiles (eg δ+C=Oδ-)
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Electrophiles and nucleophilesNucleophiles‘nucleus loving’Is a Lewis base that seeks a positive center
such as positively charged carbon atomCarbanions are Lewis bases (nucleophiles)Relative nucleophilicity in polar solvents
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Nucleophilicity versus basicityBasicity is measure of a reagent’s ability to
accept a proton in an acid-base reaction.Relative base strengths are determined by
comparing the relative positions of their equilibria in acid-base reaction, such as degree of ionization in water
I- Br- Cl- ROH H2O -CN -OH -OR
Increasing basicity
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Nucleophilicity is a measure of a reagent’s ability to cause a substitution reaction.
The relative nucleophilicities of reagents are determined by their relative rates of reaction in substitution reaction, eg substitution reaction with bromomethane.
H2O ROH Cl- Br- -OH –OR I- -CN
Nucleophilicity versus basicity
Increasing nucleophilicity