introduction to organic chemistry 2018 moreburton.chem.ox.ac.uk/introduction-to-organic-4.pdf ·...

Introduction to Organic Chemistry 1

IntroductiontoOrganicChemistry2018– 7 lecturesHandout1- Bonding

DrJonathanBurton

http://burton.chem.ox.ac.uk/teaching.html

◼ OrganicChemistry J.Clayden,N.Greeves,S.Warren◼ StereochemistryataGlance J.Eames&J.M.Peach◼ Themajorityoforganicchemistrytextbookshavegoodchaptersonthetopicscoveredbytheselectures

◼ ElielStereochemistryofOrganicCompounds(advancedreferencetext)

[email protected]

C C C

H

H

H

H

C C C

H

H

H

H


CourseOutlineThesesevenlectureswillfocusonbonding,stereochemistry,andanintroductiontomechanism(curlyarrows).Alloftheseareasarecrucialtobeingabletorationalise,andmoreimportantlypredict,whatwillhappenwhenorganicmoleculesaretreatedwithanyofaplethoraofreagentsusedbyorganicchemistsandchemistsingeneral.Aspectsofbondingwillreceivemuchmorein-depthtreatmentsinboththe‘inorganic’and‘physicalandtheoretical’lecturecoursesthroughoutthedegreeandherewewillmeetsimple,butusefulviewsofbonding,thatarefitforourpurposes.‘Curlyarrow’mechanismswillbemetinvirtuallyalloftheorganicchemistrylecturecoursesandthiscoursewillintroducethekeybasicprinciplesfordrawingreasonablecurlyarrowmechanisms.Uniquely,thiscoursewillprovidevirtuallyallofthecontentyourequireregardingstereochemistry.Stereochemistryischemistryinthree-dimensionsanddevelopingskillstobeabletomanipulatemoleculesin3-Dinone’sownheadwillproveimportant.Practisingorganicchemistsfrequentlybuild‘ball-and-stick’modelsofmoleculestobetteraidvisualisationandacheap,plasticmolecularmodelkitcanbeveryusefulwhenworkingthoughstereochemicalproblemsandwillbeusefulthroughoutyourdegree– adecentonecanbepurchasedhere:https://www.cochranes.co.uk/show_product.asp?id=399&catid=44

SynopsisBonding– Lewisstructuresandbonding,formalcharge,basicVSEPR,hybridisation,molecularorbitals,structuresofsomefunctionalgroups,drawingorganicmolecules,skeletalrepresentations

Stereochemistry– conformation,stereochemical definitions,opticalactivity,configuration,chirality,enantiomersanddiastereomers,CIPrules,compoundswithmorethanonestereocentre,meso compounds,racemisation,planarandhelicalchirality,resolution,relativeandabsoluteconfiguration

Mechanism– acidbasereactions,curlyarrows,delocalisation


◼ Bohratomicmodel- electronsmovinginspecificorbits,s,p,d,f.

◼ LewisBondingTheory (1916)– historicanduseful

◼ Bonding=achievementofcompletelyfilled(orempty)valenceshellsbysharing(covalentbonds)ortransferofelectrons(ionicbonds)betweenatoms

◼ Lewisbondingformsasimplebasisforthequalitativeunderstandofthestructuresandpropertiesoforganicmolecules

Atoms

Quantumnumber

max.no.ofelectrons inshell

1 2e 1H1s1

2He1s2

2 8e 3Li2s1

4Be2s2

5B2s2,2p1

6C2s2,2p2

7N2s2,2p3

8O2s2,2p4

9F2s2,2p5

10Ne2s2,2p6

3 8e 11Na 12Mg 13Al 14Si 15P 16S 17Cl 18Ar

•

•

atomicnumber=numberofprotons

••••••

••••••••

•••••••••••••

••••••••

••

electronsinoutershell(valenceshell)

◼ youwillmeetmoresophisticatedtheoriesofbonding– particularlymolecularorbitaltheory– asthecourseprogresses

Gilbert.N.Lewis


◼ Electronegativity(c chi)– measureoftherelativeabilityofanatomtoattractvalenceelectronstoitself(PaulingScale1932,empirical)

◼ thefollowingorderofelectronegativities canbeusefultoremember:F>O>Cl>N

◼ usuallyoccurbetweenatomsatoppositeendsoftheperiodictablei.e.betweenatomsthathaveverydifferentelectronegativities

◼ theatomsinvolvedinionicbondingachievefilledshellsbythetransferofelectrons(usually1or2electrons)

Atoms

H2.2

Li1.0

Be1.6

B2.0

C2.6

N3.0

O3.4

F4.0

Na0.9

Mg1.3

Al1.6

Si1.9

P2.2

S2.6

Cl3.2

Br2.9

I2.6

increasingelectronegativity

increasin

gelectron

egativity

◼ examplesLi F Na Cl

◼ IonicBonds

3Li+(2s1)

9F(2s2,2p5)

3Li+ 9F–

(2s02s2,2p6)

LinusPauling


◼ thesharedpairisthereforenotequallydistributedbutdisplacedtowardsF– thisisbondpolarisationbytheinductiveeffectleadingtoapermanentdipole

Covalentbonds

◼ generallybetweenatomsofsimilarelectronegativity– sharingofelectrons

H+(1s1)

H(1s1)

• • H H••

covalentbond◼ eachHatomhas2electronsarounditi.e.1st quantumnumbershellisfull

1H+(1s1)

9F(2s2,2p5)

• ••H F

•••••••••••

••

2electronsaroundH

8electronsaroundF

◼ bothatomshavefilledshells

◼ inHFtheatomshavealargedifferenceinnuclearchargeandhenceelectronegativity

H Fd+ d-

◼ theGreekletterd (delta)isusedtoindicateapartialcharge

◼ organicmoleculesareheldtogetherbycovalentbonds

◼ carbonisinthemiddleoftheperiod,withnogreatdifferenceinelectonegativitywithotherelements

◼ verydifficulttoformstableclosedshellbyionisationi.e.C4+ andC4–

Energy

1s2 (core)

2s2

2p2


◼ formalchargeonatom=numberofvalenceelectrons– (numberoflonepairelectrons+½bondingpairelectrons)

◼ CH4 - methane

◼ bondsformedbythecombinationof4Hatoms(1s1)and1Catom(2s2,2p2)

4H+(1s1)

C(2s2,2p2)

•••••

H

C

H

HH ••

••

••••

C– 8electronsH– 2electrons

◼ electronegativityofcarbonandhydrogenaresimilarcc =2.6,cH =2.2,◼ bondsslightlypolarised

d+

d-

◼ NH3 - ammonia

3H+(1s1)

N(2s2,2p3)

•••••••N

H

HH ••

••••

N– 8electronsH– 2electrons

lonepair– notinvolvedincovalentbond

2H+(1s1)

O(2s2,2p4)

••••••••O HH •

•••••

O– 8electronsH– 2electrons

2lonepairs

RulesforLewisstructures

◼ line=sharedpair=covalentbond– nomoredrawingdotsforelectronsinbonds

◼ =unsharedpairorlonepair••

◼ for2nd rowelementsLi→Fthesumofthenumberofbondsplusthenumberoflonepairscannotexceed4i.e.nomorethan8electronsaroundtheatom– Lewisoctetrule

◼ H2O- water


◼moregenerally,wheneveranatomhas8electronsaroundit,andhasonemorebondthanitsneutralstate,itwillbepositivelycharged,andonefewerbondsitwillbenegative

◼ formalcharge– chargeassignedtoeachatominamoleculeassumingtheelectronsinthebondsaresharedequallybetweenatomsregardlessofelectronegativity– bookkeepingtooltodenoterelationshipbetweenthebondingelectronsandthose‘formally’belongingtoeachatom– ausefulformalismbutmustbeinterpretedwithcare

◼ inthehydroniumiontheformalpositivechargeisdrawnonoxygen

◼ inrealitytheoxygen,beingmoreelectronegativethanhydrogen,doesnothaveapositivechargeandthepositivechargeisspreadoutoverthe3hydrogenatoms

◼ formalchargeonatom =numberofvalenceelectrons– (numberoflonepairelectrons+numberofbonds)

+H+••NH

H

H H

N

H

HH 5– (0+8/2)=+1

Nvalenceelectrons

lonepairelectrons

bondingelectrons

NH

H

H

O forN5– (0+8/2)=+1

forO6– (6+2/2)=-1•••••• O HH +H+ O HH

H

forO6– (2+6/2)=+1◼ (organic)chemistsdonotworkouttheformalchargeonatoms,theyjustrememberthatwheneveroxygenhas8electronsaroundit,andhas1,2,or3bonds,itisnegative,neutralorpositiverespectively

H O O HH O HH

H

NH2 NH3 NH4

O HH

H

hydroniumion


◼ attraction– eachelectronsees2positivenuclei()

H H

◼ physicaloriginofbondingforces(simplified)– iftwoatomsformabond,thebondedstatemustbemorestablethentheseparatedatoms

◼ incovalentbondstherepulsionbetweentheelectronsandbetweenthenucleiisbalancedbytheattractionoftheelectronstothepositivenucleiatthestableinternuclear distance– thebondlength

◼ lonepairs,a.k.a.non-bondedpairs,formallyinteractwithonlyonepositivenucleus,hencetheirinterelectron repulsionisgreaterthanforabondpair– theyarehigherinenergyandmoreavailableforbonding

••NH

H

H

electron-electronrepulsioninlonepair

◼ reactionswhichreducethiselectron-electronrepulsionarefavoured

+H+••NH

H

H H

N

H

HH

◼ intheproductthelonepairelectronsarenowabondpairandsee2positivenuclei◼ thisistheLewisbasicityoflonepairs- frequentlythereactivesiteofamoleculetowardselectrophiles

◼ Lewisstructuresareusefulinpredictingthebondinginmoleculesbuttheytellusnothingoftheshapeofthemolecules

H

C

H

HH C CH

H H

HC C HH C NH H2C C CH2

O

C HH

methane ethene(ethylene)

ethyne(acetylene)

hydrogencyanide

allene formaldehyde(methanal)

◼ H2

+1

-1

+1

-1

◼ repulsion – electron-electronandnucleus-nucleus()


HH

N

H

HH

H

H

◼ theshapesofthevastmajorityoforganiccompoundsaredeterminedbyrepulsionbetweenelectronsinthevalanceshell(VSEPR- valenceshellelectronpairrepulsion)

◼ i.e.bondpair– bondpairrepulsionsbondpair– lonepairrepulsionsbondpair– bondpairrepulsions

◼ geometryofmoleculesisdeterminedbyminimisingtheaboveinteractions

◼ formethane,CH4,thereare4bondingpairsofelectronsandtheyrepeleachothertobeasfarapartaspossibleleadingtothetetrahedralstructureofmethane

◼ tetrahedralgeometry,109.5° H-C-Hbondangle

109.5°

1.09Å

(1Å =10-10 m)

◼ withammoniatherearestillfour‘groups’toplacearoundthecentralnitrogenatom– 3bondpairsand1lonepairNH3 isthereforebasedonatetrahedron– itsshapeisdescribedaspyramidal

HH

N

H

◼ 107° H-C-HbondangleN–Hbondlength=1.01Å

HH

H

C

H

107°

1.01Å

HO

H

◼ similarlywithwatertherearestillfour‘groups’toplacearoundthecentraloxygenatom– 2bondpairsand2lonepairs– waterH2Oisthereforealsobasedonatetrahedron– itsshapeisdescribedasbent

104°

0.96Å◼ bondanglesdecreaseintheorderCH4,NH3,H2Obecause:lonepair/lonepairrepulsionsaregreaterthanlonepair/bondpairrepulsionswhicharegreaterthanbondpair/bondpairrepulsions

HO

H


◼ someexamples

◼ BF3◼ planar120° bondangleB– 6electrons(nolonepair)F– 8electrons

3F+(2s2,2p5)

B(2s2,2p1)

F BF

FF B

F

F

◼ CH3+

H CH

HH C

H

H ◼ planar120° bondangle,C– 6electrons(nolonepair),H– 2electrons

◼ QuantumNumbersandAtomicOrbitals

◼ atomicorbitals– mathematicalfunctions(wavefunctions)relatedtotheprobabilityoffindinganelectroninaparticularregionofspace– fororganicchemistrywewillgenerallybeconcernedwithsandp-orbitals

◼ atomicorbitalscomeinsetsassociatedwiththeprincipalquantumnumbern =1,2,3,…

◼ eachrowoftheperiodictableindicatesadifferentprincipalquantumnumber

◼ anatom’shighestprincipalquantumnumberdeterminesthevalenceshelloftheatom- onlytheseorbitalsaregenerallyinvolvedinbonding

◼ forhydrogentheprincipalquantumnumberis1givingrisetothefamiliar1sorbital

◼ forcarbontheprincipalquantumnumberis2givingrisetothefamiliar2sandthree2porbitals

s p


◼ ‘s’and‘p’orbitals

s

x

y

z

x

y

z

py

x

y

z

px

x

y

z

pz

◼ s-orbitals- sphericalelectrondensitydecreasesinmovingawayfromthenucleus

◼ therearethreep-orbitals,px,py andpz,orthogonaltoeachother

nucleus

◼ p-orbitalshavephase(sign)andanodeatthenucleus(wherethewavefunctionchangessign)– theelectrondensityiszeroatanode

◼ theorderofoccupancyofatomicorbitalsforagroundstateconfigurationfollowstheAufbau principle

1s

2s 2p

3s 3p 3d

4s 4p 4d 4f

5s 5p 5d 5f

◼ groundstateelectronicconfigurationi) orbitalsfilledlowestenergytohighestenergyfollowingthearrowsfromtop

tobottomi.e.1s,2s,2p,3s,3p,4s,3d,4p,etc.ii) maximumtwoelectronsineachorbitalwhichmusthaveoppositespins(Pauli

exclusionprinciple)iii) emptyorbitalsofequalenergy(e.g.3x2porbitals)takeoneelectroneach

beforepairingoccurs(Hund’sruleofmaximummultiplicity)


◼MolecularOrbitals(simplified– anorganicchemistryview)◼molecularorbitalsareformedfromcombiningatomicorbitals– wewilllookatH2

◼ overlapoftwohydrogen1satomicorbitalsleadstotwomolecularorbitals,onelowerinenergy,andonehigherinenergy,thantheoriginalatomicorbitals

+

+

◼ constructiveoverlapofatomicorbitalsleadsto bondingmolecularorbital

H1s H1s

◼ destructiveoverlapofatomicorbitalsleadsto anti-bondingmolecularorbital(internuclear node)

◼molecularorbitaldiagramforH2

Energy

H1s H1s

(s*)antibondingmolecularorbital(empty)

(s)bondingmolecularorbital(full)

◼ electronsarefedintothelowestenergymolecularorbitalfirst

◼ energyreleasedincombiningtwohydrogenatomstogiveH2isthebondstrength=432kJmol-1

◼ bondingmolecularorbitalinH2 hascylindricalsymmetryandistermedaσ-orbital(sigmaorbital)– c.f.– s-atomicorbital◼ inH2 thisorbitalcanbeviewedasthe(Lewis)bondandiscalledaσ-bondalthoughtheelectrondensityisdistributedoverthewholemolecule

s

s*


◼ combinationsofp-orbitals(withallorbitals,thestrongestbondingoccurswhenmaximumoverlapof(atomic)orbitalsoccurs)

◼ inasimilarmannerwecancombinep-orbitalstomakebondingandantibondings-molecularorbitals

p-orbital p-orbital

p-orbital p-orbital

◼ destructiveoverlapofatomicorbitalsleadsto anti-bonding s*molecularorbital– internuclear node

◼ constructiveoverlapofatomicorbitalsleadsto a bondingsmolecularorbital- oftendrawnassotheprovenanceofthemolecularorbitalisclear

s*

s

◼ p-bonding– p-orbitalscanalsooverlapedgeonwhichleadstop-bonds– weakerthanthecorrespondings-bondsastheorbitaloverlapisnotasgood

+ p*◼ destructiveoverlapofatomicorbitalsleadstoan anti-bonding p*molecularorbital– internuclear node

+p

◼ constructiveoverlapofatomicorbitalsleadstoa bonding

p molecularorbital- oftendrawnassotheprovenanceofthemolecularorbitalisclear

s bond

p bondasinethene

+

+


◼ organicchemistsgenerallydrawLewisstructureswherethelineconnectingtheatomsrepresentsalocalised2centre,2-electronbond.Howcanwerationalisethiswithamolecularorbitalviewofbondinginvolvingoverlapoforbitals?

◼ returningtoCH4 (methane)

HH

H

H

2centre,2electronbond

◼methane– CH4 ◼ carbongroundstate1s2,2s2,2p2 – the1selectronsarecoreleavingthe2sand2pvalenceelectronsforbonding

Energy

1s2 (core)

2s2

2p2

x

y

z

2py

x

y

z

2px

x

y

z

2pz

2pz

2s

x

y

◼ the2sorbitalisspherical(non-directional)andthethree2porbitalspointalongtheirrespectiveaxes(i.e.at90° tooneanother)

◼ howcansandpatomicorbitalsaccommodatethetetrahedralstructureofmethane(andthestructuresofmanyorganicmolecules)?

◼ oneapproach,initiallyproposedbyPauling,istheideaofhybridisation – amethodofcreatingorbitalsthatreproducestheobservedshapesofmolecules

◼ atomicorbitalsarejustmathematicalfunctions,sowecanaddandsubtractthem(aswehavedonetoformsimplemolecularorbitals)◼ fullmolecularorbitalanalysis(2nd yearlecturecourse)isanotherapproach


2s

x

y

z

◼ hybridisation◼mixingthe2sandthree2porbitalsgivesfoursp3 hybridsthatpointtowardsthecornersofatetrahedron

x

y

z

2py

x

y

z

2px

x

y

z

2pz

± ± ±

◼ foursp3 hybridorbitals

sp3 hybridorbitals

◼ thesehybridorbitalscanoverlapwiththehydrogen1sorbitalstoformC–Hs-bondingorbitals(andthecorrespondings*antibondingorbitals)

sp3 hybridorbitalonC

hydrogen1sorbital

+ +

C–Hs bondingorbital– as bond

◼ constructiveoverlapofhybridorbitalwith1satomicorbitalleadstoa bondingsmolecularorbital(s bond)- oftendrawnas

sotheprovenanceofthemolecularorbitalisclear

◼ thefoursp3 hybridorbitalsoncarbonoverlapwiththe1sorbitalsofthefourhydrogenatomstogivemethane

HH

H

HH

H

H

H

hydrogen1sorbitals

◼ bondinghybridmolecularorbitalsinmethanecorrespondingtotheLewisstructureC–Hbondseachbond=435kJmol-1

◼ thecarbonatomissaidtobesp3 hybridised


HH

H HH

H

+

HH

H HH

H

◼ staggeredconformation◼ eclipsedconformation◼ the“end-on”viewsaretermedNewmanprojections

HH

HHH

H

HH

H HH

H

+

◼ ethane– C2H6 – sp3 hybridorbitalscanoverlapwitheachotherasintheC–Cbondinethanetogiveabondingsmolecularorbital

HH

H HH

H

C–Cs orbital(bond)

◼ constructiveoverlapofhybridorbitalsleadsto bondingsmolecularorbital- oftendrawnasbelowsotheprovenanceofthemolecularorbitalisclear

sp3 hybridorbitals

◼ rotationaroundtheC–Caxisdoesnotaltertheoverlapofthesp3 orbitals(theC–Cs orbital)andoccursreadilytogiveavarietyofconformers ofethane– moreofthislater

◼ as hybridorbitalsaredirectionaltheyprovidemuchbetteroverlapwiththe1sorbitalsofthehydrogenatoms

H

H HH

HHH

H

H

H

HH

HH

H HH

H HH

H

HH

H


2s

x

y

z

x

y

z

py

x

y

z

px

x

y

z

pz

◼ hybridisation– sp2

◼mixingthe2sandtwo 2porbitalsgivesthree sp2 hybridsthatleadtotrigonalgeometry(120° bondangles)

±±

◼ threesp2 hybridorbitals(drawninxy plane)

sp2 hybridorbitals

◼ formingthreesp2 hybridsleavesonep-orbitalperpendiculartotheplanecontainingthehybridorbitals

◼ threesp2 hybridorbitalsandperpendicularp-orbitalaroundcentral(carbon)atom

◼ thebondinginethene/ethylene(C2H4)isaresultofoverlapofsp2 hybridisedcarbonatomswitheachothertoformtheC–Cs-bond,andwithfourhydrogen1sorbitalstoformthefourC–Hs-bonds

◼ theremainingtwop-orbitalsoverlaptoformap-bond

C CH

HH

H


H

H H

HH

H H

H

C C

H

H H

H

C C

H

H H

H

C CH

HH

H◼ overlapofthesp2 hybridorbitalsleadstothrees-bonds– twoC–HbondsandoneC–Cbond

◼ overlapoftheremainingperpendicular’p’-orbitalsgivesap-bond

C–Cs-bond

thetwolobesoftheC–Cp-bond

overlapofsp3hybridorbitalsgivesC–Cs-bond

◼ thep-bondleadstoelectrondensityaboveandbelowthemolecularplane

overlapof‘p’orbitalsgivesC–Cp-bond

◼ rotationabouttheC–Caxisisimpossibleasthiswouldreduceoverlapofthep-orbitalsandwouldbreakthep-bond

H

H H

H

◼ asbeforethep-molecularorbital(p-bond)isgenerallydrawnastwoseparatep-atomicorbitalsforclarity

H HHH

H

H

H

◼ rotationaboutC–Caxiswouldbreakthep-bondanddoesnotoccur◼ hencealkenesareconfigurationallystable(i.e.cis andtrans)

X


H C C HH HH H

2s

x

y

z

x

y

z

py

x

y

z

px

x

y

z

pz

◼ hybridisation– sp◼mixingthe2sandone 2porbitalsgivestwo sp hybridsthatleadtolineargeometry(180° bondangles)

±

◼ twosp hybridorbitals

◼ formingtwosp hybridsleavestwop-orbitalswhichcanformp-bonds

sp hybridorbitals

◼ thetwosp hybridorbitalsarearranged180° apart◼ thisisthebondingarrangementfoundinethyne (acetylene)withthesp hybridsoverlappingwiththehydrogen1sorbitals(notshown)theremainingporbitalsoverlappingtoformthetwoπ-bondsx

y

zp-orbitals

sp hybridorbitals

sp-hybridsoverlaptoformC–Cs-bond

p-bondformedbyp-orbitaloverlap p-bondformedby

p-orbitaloverlap

C C HH

◼ thetriplebondinacetyleneismadeupfromones-bondandtwoorthogonalp-bonds,thecarbonsaresaidtobesphybridised

◼ viewwithouts-orbitals

C C HH


H

H

◼ aquickmethodtoworkoutthehybridisation ofanatomistocountthenumberofsubstituentsonthatatom(includinglonepairsofelectrons),rememberingthatinthebondedenvironmentfirstrowelementshave8electronsaroundthem(thereareexceptionstothismethodwhichwewillmeetlater)

4substituents=sp3 hybridised,3substituents=sp2 hybridised,2substituents=sp hybridised

◼ allene CH2=C=CH2 – whatisitsmolecularshape?

◼ terminalcarbonsbondedto2hydrogenatoms(s-bonds)anddoublebondedtocentralcarbon(s-bondandp-bond)–� terminalcarbonatomsaresp2 hybridised

◼ centralcarbonbondedto2substituents� haslineargeometry� sp hybridised

C C C

H

H

◼ centralcarbonformsasingles-bondwitheachterminalcarbon(sp-sp2 hybridorbitaloverlap)

sp2 hybridised carbon(c.f.ethene)

sp hybridised carbon(c.f.acetylene)

CC

C

H

H

H

HC C C

H

H

H

H

◼ alternativeperspective◼ nofreerotation◼ CH2 groupsareperpendicular


HO

HH

HN

H

◼ hybridisation canbeusedwithatomsotherthancarbon

◼ the nitrogenatominammoniacanbeviewedassp3 hybridisedascantheoxygenatominwateralthoughtheH-X-Hbondangleisslightlylessthan109° duetolonepair–bondpairrepulsion

N-atomissp3 hybridised

lonepairinsp3 orbitalH

HN

HH

OH

O-atomissp3 hybridised

◼ BF3 – threesubstituentsaroundboron– nolonepair� planar,sp2 hybridised,planarwithemptyp-orbitalonboron

F BF

F

◼ similarlyforamines,alcohols,ethersetc.

H3CH

N

Hmethylamine

CH3O

H

CH2CH3O

CH2CH3ethanol diethylether

◼ CH3+ - threesubstituentsaroundcarbon– nolonepair� planar,sp2 hybridised,planarwithempty

p-orbitaloncarbon– othercarbocations havesimilarplanarstructure

H CH

HH C

H

H H CH

H


N

H

Me

Me

◼ formaldehyde(c.f.bondinginethene)

lonepairsinsp2 orbitalsp2 sp2

C

H

H

OC O

H

H

p-bondformedbyoverlapofadjacentp-orbitals

◼ hybridisation infunctionalgroups(sitesofchemicalreactivity)

◼ bondingandreactivityissimilarinaldehydesandketones

◼ electronegativitydifferenceresultsinpolarisation ofC–Obond

◼ nucleophiles� reactoncarbonandbreaktheweakerp-bond

C O

H

Hd+ d-C O

H

H

Nu

NuO

HH

◼ conversely,lonepairsaresitesofelectrondensity

◼ electrophiles� reactonoxygen

C O

H

H•• H C O

H

HH

OMe

H

MeO

Me

acetaldehyde(ethanal)

acetone(propanone)

◼ imines– planar,lonepairinsp2 orbital

◼ oximes– planar,lonepairinsp2 orbital

N

Me

Me

OH

◼ nitriles– e.g.CH3CN(c.f.bondinginacetylene)

d+ d-H3C C N

lonepairinsp-orbital H3C C N

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