Chapter 5 Organic Reaction Mechanism - Faculty of Sciencemysite.science.uottawa.ca/sgambarotta/sites/default/files/chm 1321... · Chapter 5 Organic Reaction ... 5.2.1 Reactions occur

Chapter5OrganicReaction

MechanismUSINGCURVEDARROWSTOANALYZEREACTION

MECHANISMS

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Considerthereaction:

Theequationtellsuswhat happens,butnothow ithappens.

Reactionmechanism:• theparticularsequenceofelectronmovementsina

chemicalreaction• movementsofelectronsarerepresentedbycurved

arrows


Guidelinesforusingcurvedarrowsinreactionmechanisms:

1. Notethatelectronflowsoccuratornearfunctionalgroups.

2. Usearrowstoshowbondformation.

3. Usearrowstoshowbondbreaking(sometimescalledcleaving).

4. Usearrowstokeeptrackofformalcharges.

5.2.1Reactionsoccuratornearfunctionalgroups


Featurestolookforwhenidentifyingfunctionalgroups:

5.2.1.1p bonds

• p bondsindoubleortriplebondsareweakerandmorereactivethanσ bonds

• morepronetoreaction

5.2.1.2Heteroatoms(atomsthatarenotCorH)

• oftenpossesslonepairs

• electronegativitydifferencerelativetocarbontendstoinducereactions


5.2.1.3Formalcharges

• presenceofformalchargeraisesenergystate


5.2.1.4Unpairedelectrons(radicals)

• unstable,highlyreactive


5.2.1.5Hydrocarbons

• generallyunreactive(scaffold)

• oftenabbreviatedR

5.2.2ExpandingLewisstructuresanddrawingconventions


Whendrawingreactionmechanisms,itishelpfultoexplicitlyshowlonepairs.


Example:

Representationformechanism:

Overallreaction:

5.2.3Curvedarrowsandbondformation


Curvedarrowsshowelectronflow,notmovementofatoms.

Examples:

5.2.4Curvedarrowsandbondbreaking


5.2.5Combiningbondformationwithbondbreaking


Consideracid–basereactionbetweenCH3O− andHCl:


Areactionbetweenneutralmolecules:

5.3CurvedArrowsandFormalCharges



Flowofpairs ofelectronscanchangeformalchargesbyoneunit.

Example:


Example:

Example:

5.4IntramolecularReactions


Intermolecularreactions:• occurbetweentwoormoremolecules

Intramolecularreactions:• occurwithinasinglemolecule


5.4.1RepresentingtheMovementofp BondElectrons


Somereactionsoccurthroughthemovementofp electronsonly.

Itiscommontoshowbondformationinthesereactionswithanarrowthatpointstothespacebetweenatoms.

5.5TheStabilizingEffectofDelocalization



SomemoleculescanberepresentedasdifferentLewisstructuresthathave

• identicalplacementofatoms

• differentplacementofelectrons

NH

H

O

NH

H

O

Example:

and


Thesestructuresarecalledresonancestructures:

NH

H

O

NH

H

O

Actualstructureisablendofalloftheresonanceformsatthesametime.

Moleculesdonotflipbackandforthbetweenforms!

NH

H

O

δ+

δ-

5.6ConstructingResonanceForms



Foramoleculetohaveresonance,atleastoneofthefollowingstructuralfeaturesmustbepresent:

1. ap bondmadeupofatomswithdifferentelectronegativities

Example:


2. ap bonddirectlybesideatleastoneofthefollowingfeatures:

a)pairedorunpairedelectrons

b)atomswithincompleteoctets

c)otherp bonds

d)chargedatomslackingoctetsorcarryinglonepairs

Example:


3. anatomwithanincompleteoctetadjacenttoanatomwithapairofnon-bondingelectrons


Example:Drawingresonancestructuresfora,b-unsaturatedketone


Example:Anionderivedfrom a,b-unsaturatedketone


Example(cont’d.):Drawingtheresonancestructures

5.7EvaluatingResonanceFormContributions



Resonancestructuresusuallydonotcontributeequallytotheactualstructure.

Formswithfavourable electrondistributionarestrongercontributors.

NH

H

O

NH

H

O

strongercontributor(morefavourable electrondistribution)


Highestqualityresonanceformshavethefollowingcharacteristics:

1. themostatomswithfulloctets

2. thefewestnumberofformalcharges

3. a)negativeformalchargeslocatedonthemostelectronegativeatoms

b)positiveformalchargeslocatedonthemostelectropositiveatoms

4. a)likechargesseparatedbythemaximumdistancepossible

b)oppositechargesasclosetogetheraspossible

5.7.1Significantandinsignificantstructures


Basedoncriteriafrompreviousslide,wecanessentiallyruleoutsomecontributors.

Example:


Insignificantformsalsooccurwhenelectronsfrombreakingp bondflowtoanatomwithinappropriateelectronegativity.


Anotherexample:

5.8ResonanceandOrbitalStructure



Theactualstructureofagroupundergoingresonancecanberepresentedbydrawingallp orbitalsinvolvedintheresonance.

5.9PatternsinMechanism



Organiccompoundscanbeconsideredcollectionsoffunctionalgroupsheldtogetherbyascaffoldofcarbonatoms.


Inreactionmechanisms,electronsflow

• fromanareaofhighelectrondensity(lonepairorbond)

• toanareaoflowelectrondensity(incompleteoctetorpositivecharge)


Whenabondisbroken,bondingelectronstendtomovetowardtheatomthatisthestrongestelectronattractor.


Ifadouble-barbarrowpointsawayfromanatom,theformalchargeintegerontheatomincreasesbyoneunit.

Ifarrowpointstowardanatom,theformalchargeintegerontheatomdecreasesby1.

Ifarrowpointsawayfromanatomandanotherarrowpointstowardtheatom,thechargeonthatatomdoesnotchange.

5.10PatternsinResonance



Delocalizationisthemostimportantcontributortostabilization.

Trendsinelectronmovementarepredictable:


Chaptersummary


• Reactionsoccuratornearfunctionalgroups.

• Curvedarrowsareusedtoshowelectronflowinorganicreactions.

• Reactionsmayoccurinter- orintramolecularly.

• Moleculescanbestabilizedbyelectrondelocalization.

• Delocalizationcanbedepictedusingresonancestructures.

• Differentresonancestructureshavedifferentdegreesofcontributionstotheoverallstructure.

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Chapter 5 Organic Reaction Mechanism - Faculty of Sciencemysite.science.uottawa.ca/sgambarotta/sites/default/files/chm 1321... · Chapter 5 Organic Reaction ... 5.2.1 Reactions occur