introduction to reaction mechanism

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Chapter 1 Introduction to organic reactions 1

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In organic chemistry, various reactions take place and requires us to know which is which.

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Chapter 1 Introduction to organic

reactions

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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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Reaction categoriesSubstitutionsAdditionsEliminationsRearrangements

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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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Carbocation rearrangement in dehydration of 3,3-Dimethyl-2-butanol

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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

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Electrophiles and nucleophilesReact to form a bond with a carbon atom

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Question

Each of the following carbocations has the potential to rearrange to a more stable one.

Write the structure of the rearranged carbocation and use curved arrows to show how it is formed

(a) CH3CH2+CH2

(b) (CH3CH2)3C+CH2