Ch1_Basic_concepts_Organic_sem1.ppt bv c xvbbb

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1. 1
Basic Concepts
of Organic
Chemistry

2. 2
Carbocations
Carbocation:
•aspeciesinwhichacarbonatomhasonlysixelectronsinits
valenceshellandbearspositivecharge
Carbocationsare:
•classifiedas1°,2°,or3°dependingonthenumberof
carbonsbondedtothecarbonbearingthepositive
charge
•electrophiles;thatis,theyareelectron-loving
•Lewisacids

3. 3
CarbocationStability
•relativestability
•methylandprimarycarbocationsaresounstablethat
theyareneverobservedinsolutionMethyl
cation
(methyl)
Ethyl
cation
(1°)
Isopropyl
cation
(2°)
tert-Butyl
cation
(3°)
Increasing carbocation stability
+ + + +C
H
H
CH
3 CCH
3
CH
3
H
C
CH
3
CH
3
CH
3
CH
H
H

4. 4
Carbocation Stability
•wecanaccountfortherelativestabilityof
carbocationsifweassumethatalkylgroupsbonded
tothepositivelychargedcarbonareelectron
releasingandtherebydelocalizethepositivecharge
ofthecation
•weaccountforthiselectron-releasingabilityofalkyl
groupsby(1)theinductiveeffect,and(2)
hyperconjugation

5. 5H
C
H H
CH
3
C
H H
CH
3
C
H
3C H
CH
3
C
H
3C CH
3
Methyl
Carbocation
Primary
Carbocation
Secondary
Carbocation
Tertiary
Carbocation
LEAST
STABLE
MOST
STABLE
The methyl groups have +I inductive
effects.
Carbon atom is electron deficient (only has 6
electrons in its outer valence).
Thus, extra electron density is ‘pushed’
onto the carbocation, which stabilises the
carbocation.CH
3
C
H
3C CH
3

6. 6
The Inductive Effect
Thepolarization(polarity)ofabondINDUCEDbyby
adjacentpolarbondisknownastheInductiveeffect.
inductive effect of an atom or functional group is a
function of that groups
1). Electronegativity
2). Charge
3). Position within a structure.
Inductive effects refer to those electronic effects
of an atom or functional group can contribute
through single bonds such as saturated (sp3)
carbon atoms

7. 7
The Inductive Effect
Itinvolvesσelectrons.Theσelectronswhichforma
covalentbondareseldomsharedequallybetweenthe
twoatoms.
Thisisbecausedifferentatomshavedifferent
electronegativityvalues,i.e.,differentpowersof
attractingtheelectronsinthebond.
Consequently,electronsaredisplacedtowardsthemore
electronegativeatomintroducingacertaindegreeof
polarityinthebond.
Themoreelectronegativeatomacquiresasmallnegative
charge(δ
-
).
Thelesselectronegativeatomacquiresasmallpositive
charge(δ
+
).

8. 8
Atomsorfunctionalgroupsthatareelectronegativerelativetohydrogen
suchasthehalogens,oxygen,nitrogen,etc.mayhaveanegative
inductiveeffect(-I).Thustheseatomswithdrawelectrondensitythrough
thesinglebondstructureofacompound.Considerthecaseofacetic
acid,chloroaceticacidandtrichloroaceticacidshownbelow.Allthreeof
thesecompoundscanionize(lossofprotonfromthecarboxylOH).The
onlydifferencebetweenthesethreestructuresinthedegreeofchloro
groupsubstitution.Chlorineatomsareelectronegativeandthushavea-I
effect.Thustheycanhelpstabilizeanegativecharge,andenhancethe
ionizationofanacid.
ELECTRONEGATIVITY

9. 9
Bondingorderandcharge:Asmentionedabove,itisimportantto
considerboththeelectronegativityandbondingorderwhen
analyzingtheinductivepotentialofanatom.Forexample,oxygen
inahydroxylgroup(OH)iselectronwithdrawingbyinduction(-I)
becausetheoxygenatomisrelativelyelectronegativeandis
unchargedinthatbondingarrangement.However,oxygeninan
"alkoxide"(O-)structureiselectrondonating(+I)byinduction
becauseinthisbondingorder(asinglebondtooxygen)ithasan
"excess"ofelectrondensity.
Bonding order and charge

10. 10
The strength of the inductive effect produced by a particular atom
or functional group is dependent on it's position within a structure.
For example, the further from the site of ionization, the lower the
inductive effect. This is illustrated in the example below where the
acid with the chlorine atom positioned on a carbon atom nearer the
reaction site (OH) is more acidic that the acid where the chlorine
atom is positioned further away.
Bonding position

11. 11
The Inductive Effect
Considerthecarbon-chlorinebond.Aschlorineis
moreelectronegative,itwillbecomenegatively
chargedwithrespecttothecarbonatom.
Structure(1)indicatestherelativechargesonthetwoatoms.
In(2),thearrowheadplacedinthemiddleofthebondindicates
thedirectioninwhichtheelectronsaredrawn.
In(3),themoreheavilyshadedpartshowstheregioninwhich
theelectrondensityisgreatest.
(1) (2) (3)

12. 12
The Inductive Effect
Theinductiveeffect(IEffect)referstothepolarity
producedinamoleculeasaresultofhigherelectro
negativityofoneatomcomparedtoanother.
Thecarbon-hydrogenbondisusedasastandard.Zero
effectisassumedinthiscase.
Atomsorgroupswhichloseelectronstowardacarbon
atomaresaidtohavea+IEffect.Suchgroupswillbe
referredtoaselectron-releasing.
Thoseatomsorgroupswhichdrawelectronsaway
fromacarbonatomaresaidtohavea–IEffect.Such
groupswillbereferredtoaselectron-attracting.

13. 13
The Inductive Effect
Somecommonatomsorgroupswhichcause–I
EffectGroups(Electron-attracting)are:
NO
2>-CN>COOH>F>Cl>Br>I>OH>C
6H
5–
&atomsorgroupswhichcause+IEffectGroups
(Electron-releasing):
(CH
3)
3C–>(CH
3)
2CH–>CH
3CH
2–>CH
3–

14. 14
The Inductive Effect
Tertiaryalkylgroupsexertgreater+Ieffectthan
secondarywhichinturnexertagreatereffectthan
primary.
Aninductiveeffectisnotconfinedtothepolarizationof
onebond.Itistransmittedalongachainofcarbon
atoms,althoughittendstobeinsignificantbeyondthe
secondcarbon.
TheinductiveeffectofC
1uponC
2issignificantlyless
thantheeffectofthechlorineatomonC
1.Theinductive
effectresultsinapermanentstateofthemoleculeand
canbeobservedpractically

15. 15
Applications of inductive effect
IE have the following applications
1.Strength of an acid
Commonly the strength of an acid cab be
calcualtedfor the pKavalue. Greater the pKa
less the strong will be the acid. The pKais
related to the electron donating and electron
drawing group. For example

16. 16
Strength of an acid
Formic acid
Acetic acid
Intheaboveacidsformicacidismorestrongthanaceticacid.
Becauseinaceticacid,withcarbonlycarbonEDG(CH
3)isattached.
Incaseofelectrondonationbymethyltheirwillbeabundanceof
electron(-vecharge)onCorbonlycarbon.Forthestabilityof
CorbonlycarbonEWGisrequiredbutCH
3isEDG.Inaceticacid
duetoelectronabundanceoncarbonlycarbonitwillnoteasilygive
H
+
ions.LessthepotentialofreleasingH
+
ionslessstongwillbe
theacid.

17. 17
Strength of an acid
in case of FORMIC ACID there is attached EWG (H) with carbonyl
carbon. In case of EWG their will be electron deficiency on the next
carbon. This electron deficient carbon (+ve charge) will stabilize by
sharing of electron from oxygen and so H ions will be easily
released.
Other examples
Acetic acid (CH3 is EDG)
Chloroacetic acid (Cl is EWG)
Di-chloroacetic acid (2 Cl are EWG)
Tri chloracetic acid (3 Cl are EWG)
ALL chloro acetic acids are stronger than acetic acid.

18. 18
Substitution of electrophilein Benzene
Electrophile are electronloving groups it will easily
move 2ward nucleuphile (+ve charge). In case of
reaction between nitro group and Toluene, due to
the presence of methyl with benzene ring, which
is electron releasing group will increase the
electronic density on ortho and para position.

19. 19
The Mesomeric Effect
Itinvolvesπelectronsofdoubleandtriplebonds.
TheMesomericeffect(Meffect)referstothepolarity
producedinamoleculeasaresultofinteraction
betweentwoπbondsoraπbondandlonepairof
electrons.Theeffectistransmittedalongachainina
similarwayasareinductiveeffects.
TheMesomericeffectisofgreatimportancein
conjugatedcompounds.(inwhichthecarbonatoms
arelinkedalternatelybysingleanddoublebonds).In
suchsystems,theπelectronsgetdelocalizedasa
consequenceofMesomericeffect,givinganumber
ofresonancestructuresofthemolecule.

20. 20
The Mesomeric Effect
Consideracarbonylgroup(>C=O).Theoxygen
atomismoreelectronegativethanthecarbon
atom.Asaresult,theπelectronsofthecarbon-
oxygendoublebondgetdisplacedtowardthe
oxygenatom.Thisgivesthefollowingresonance
structures:

21. 21
The Mesomeric Effect
Themesomericeffectisrepresentedbyacurved
arrow.Theheadofthearrowindicatesthe
movementofapairofπelectrons.Ifthecarbonyl
groupisconjugatedwithacarbon-carbondouble
bond,theabovepolarizationwillbetransmitted
furtherviatheπelectrons.

22. 22
In a system involving resonance the distribution of
electron density is different from the system that
does not involve resonance. For example, in
ammonia where resonance is absent, the
unshared pair of electrons is located on the
nitrogen atom, however if one of the H atom is
replaced by benzene ring the electron pain of N
is delocalized over the ring and resulting the
decrease of electron density on the N atom and
the corresponding increase f electron density on
the ring.

23. 23

24. 24
This decrease in electron density at one position
and the corresponding increase elsewhere is
called the RESONACE EFFECT OR MESOMERIC
EFFECT. Thus –NH2 group in aniline donated
electrons to the ring by the resonance effect or
mesomeric effect.

25. 25
The Mesomeric Effect
Themesomericeffectliketheinductiveeffectmay
bepositiveornegative.
Atomsorgroupswhichloseelectronstowarda
carbonatomaresaidtohavea+MEffect.
Atomsorgroupswhichdrawelectronsawayfrom
acarbonatomaresaidtohavea–MEffect
Somecommonatomsorgroupswhichcause
(a)+MEffectare:
Cl,Br,I,NH
2,NR
2,OH,OCH
3
&
(b)–MEffectare:
NO
2,CN,>C=O

26. 26
The Mesomeric Effect
The+Meffectofthebromineatomis:
The-MeffectoftheNitrogroupis:

27. 27
Significance of MesomericEffects
Themesomericeffecthasanappreciableinfluenceonthe
physicalpropertiesandthechemicalreactivityofthe
organiccompounds.Forexamplecomparetheacidityif
phenolwiththatofethanol,theacidityofbothistheresultof
thedissociationofO-Hbondyetphenol(pKa=10)ismore
acidicthatethanol(pKa=17).
CH
3CH
2OH + H
2O CH
3CH
2O
-
+ H
3O
+
Ph-OH + H
2O Ph-O
-
+ H
3O
+
The enhanced acidity of phenol can be attributed to the –ve charge
distribution which is not possible in ethoxide ion. Phenol therfore
has much tendency to lose proton and behave as an acid.

28. 28
Nitro group further enhances the acidity of
phenol particularly in the ortho and para position
because it further delocalizes the negative
charge over to the nitro group and increasing the
number of contributing structures.

29. 29
Nitrogroupsstabilizethephenolateionbyresonance
electronwithdrawalthatallowsthenegativechargetobemovedto
anelectronegativeoxygenatominthenitrogroupwhenthenitro
groupisortho-orpara-tothe-OHgroup.Themorenitrogroups
thereareinthesepositions,thegreaterthestabilizationofthe
phenolateandthemoreacidicthephenol.

30. 30
Thebasicityofanimeisverysensitiveto
resonanceeffect.Forexampleanilineisaweaker
basethanaliphaticaminesbecausetheelectron
pairontheNatomwhichisresponsibleforthe
basicstrengthoftheaminesisdelocalizedover
thearomaticringinanilineandisnotavailable
forprotonationtothesameextentasinthecase
ofaliphaticamineswheresuchdelocalizationis
notpossible.

31. 31
Hyperconjugation
Therelativestabilityofvariousclassesofcarbonium
ionsmaybeexplainedbythenumberofno-bond
resonancestructuresthatcanbewrittenforthem.
Suchstructuresarearrivedatbyshiftingthe
bondingelectronsfromanadjacentC–Hbondtothe
electron-deficientcarbon.Inthisway,thepositive
chargeoriginallyoncarbonisdispersedtothe
hydrogen.Thismannerofelectronreleaseby
assumingno-bondcharacterintheadjacentC–H
bondiscalledHyperconjugationorNo-Bond
Resonance.

32. 32
HyperconjugationH
H
H
H
H H
H
H
H
H
ethyl carbocation

33. 33
Hyperconjugation
Themorehyperconjugationstructures(no-bondresonancestructures)
thatcanbewrittenforaspecies,themorestableisthespecies.For
example,
(1) Ethyl carbonium ion is stabilized by three hyperconjugation structures:
(2)Isopropylcarboniumionisstabilizedbysixhyperconjugationstructures.
(3)t-Butylcarboniumionisstabilizedbyninehyperconjugationstructures.

34. 34
Hyperconjugation
Thus,thefollowingorderofstabilityholds:
Ingeneral,resonanceeffects(mesomericeffects)are
moreimportantthanhyperconjugationeffects.
Theallylandbenzylcarboniumionsaremorestablethan
mostalkylcarboniumionsbecausetheformerare
stabilizedbyresonancewhilethelatterarestabilizedby
hyperconjugation.

35. 35
Conjugation
Adieneissaidtobeconjugatedwhenitsdoublebondsarenot
directlynexttoeachother,butratherseparatedbyasinglebond
inbetweenthem(CH
2=CH-CH=CH
2).
Conjugateddienesareparticularlystableduetothedelocalization
ofthepielectronsalongsp
2
hybridizedorbitals,andtheyalsotend
toundergoreactionsatypicalofdoublebondchemistry.For
instance,chlorinecanaddto1,3-butadiene(CH
2=CH-CH=CH
2)to
yieldamixtureof3,4-dichloro-1-butene(ClCH2-CHCl-CH=CH
2)
and1,4-dichloro-2-butene(ClCH
2-CH=CH-CH
2Cl).Theseare
knownas1,2additionand1,4addition,respectively.1,2-additionis
favoredinmildreaction(irreversible)conditions(thekinetically
preferredproduct)and1,4-additionisfavoredinharsherreaction
(reversible)conditions(whichresultsinthethermodynamically
preferredproduct).

36. 36
The Resonance
Anumberoforganiccompoundscannotbeaccurately
representedbyonestructure.Forexample,benzeneis
ordinarilyrepresentedas:
Thisstructurehasthreecarbon-carbonsinglebondsand
threecarbon-carbondoublebonds.However,ithasbeen
determinedexperimentallythatallcarbon-carbonbondsin
benzeneareidenticalandhavethesamebondlength(1.42Å).
Furthermore,thecarbon-carbonbondlengthof(1.42Å)is
intermediatebetweenthenormalcarbon-carbondouble-bond
length(1.33Å)andthenormalcarbon-carbonsingle-bond
length(1.52Å).Actuallytwoalternativestructures(1and2)
canbewrittenforbenzene:
1
2

37. 371.33 Å 1.52 Å
1.42 Å 1.42 Å

38. 38
The Resonance
Thesetwostructuresdifferonlyinthepositionof
electrons.Neither(1)nor(2)isacorrect
representationofbenzene.Theactualstructureof
benzeneliessomewhere betweenthesetwo
structures.
Thisphenomenoninwhichtwoormorestructures
canbewrittenforacompoundwhichinvolve
identicalpositionsofatomsiscalledResonance.

39. 39
The Resonance
Theactualstructureofthemoleculeissaidtobea
ResonanceHybridofvariouspossiblealternativestructures.
ThealternativestructuresarereferredtoastheResonance
StructuresorCanonicalForms.Adoubleheadedarrow(↔)
betweentheresonancestructuresisusedtorepresentthe
resonancehybrid.Thusinthecaseofbenzene,(1)and(2)
representtheresonancestructures.Actualstructureofthe
moleculemayberepresentedashybridofthesetwo
resonancestructures,orbythesinglestructuralformula(3).
Itshouldbeclearlyunderstoodthattheresonance
structures(1)and(2)arenotactualstructuresofthe
benzenemolecule.Theyexistonlyintheory.Noneofthese
structuresadequatelyrepresentsthemolecule.Inresonance
theory,weviewthebenzenemolecule(whichisofcoursea
realentity)asbeinghybridofthesetwohypothetical
resonancestructures.

40. 40
The Resonance Energy
Theresonancehybridismorestablethananyoneofthevarious
resonancestructures.Thedifferenceinenergybetweenthehybrid
andthemoststableresonancestructureisknownasthe
ResonanceEnergy.Resonanceenergycanbedeterminedbythe
differencebetweenthecalculatedandexperimentalheatsof
combustion(energygivenoffasheatwhenonemoleofcompound
isburned)ofthecompound.
Forexample,ithasbeencalculatedthatthehypotheticalstructure
(1)or(2)wouldhaveaheatofcombustionof797Kcal/mole.The
measuredvaluefortheheatofcombustionofbenzeneis759
Kcal/mole.Therefore,theresonanceenergyofbenzeneis(797–
759)Kcal/mol.Thebenzeneissaidtobe"stabilised"bya
resonanceenergyof38Kcal/mole

41. 41
The Resonance
Anotherspeciesthatisnotcorrectlyrepresentedbyasingle
structureistheacetateion.Asinthecaseofbenzene,
acetateionisahybridoftworesonancestructures.Both
carbon-oxygenbondsintheacetateionareidenticaland
havethesamebondlength(1.26Å).Thecarbon-oxygen
bondlengthof1.26Åisintermediatebetweenthenormal
carbon-oxygendouble-bondlength(1.20Å)andthenormal
carbon-oxygensingle-bondlength(1.43Å).

42. 42
The Resonance (Governing Rules)
1.Resonanceoccurswheneveramoleculecanberepresentedbytwo
ormorestructuresdifferingonlyinthearrangementofelectrons,
withoutshiftinganyatoms.Resonanceonlyinvolvesthe
delocalizationofelectrons.
2.Resonancestructuresarenotactualstructuresforthemolecule.
Theyarenonexistentandhypothetical.
3.Resonancestructuresareinterconvertiblebyoneoraseriesof
short electron-shifts. For example,

43. 43
The Resonance (Governing Rules)
4.Resonancehybridrepresentstheactualstructureofthe
molecule.Thestructureoftheresonancehybridis
intermediatebetweenthevariousresonancestructuresand
isnotamixtureofthem.
5.Resonancehybridisrepresentedbyadoubleheadedarrow
(↔).Thisshouldnotbeconfusedwiththetwoarrows( )
usedtodenoteequilibriumbetweentwodifferent
compounds.
6.Resonancehybridismorestablethananyofitscontributing
forms(resonancestructures).
7.Resonancealwaysincreasesthestabilityofamoleculeand
lessensitsreactivity.

44. 44
The Hydrogen Bonding
Abondformedbetweenafunctionalgroup(H-A)andanotheratom(B)or
groupwithinthesamemoleculeordifferentmoleculeiscalledHydrogen
bonding.ORHydrogenbondingisanattractiveforcewhichoccursinany
compoundwhosemoleculescontainO–HorN–Hbonds(asinwater,alcohols,
acids,amines,andamides)oranyENatom.TheO–Hbond,forexample,isa
highlypolarbond.Oxygenismoreelectronegativethanhydrogenandpullsthe
bondingelectronsclosertoit.Asaresultofthisdisplacement,theoxygenatom
acquiresasmallnegativecharge(δ–)andthehydrogenatomasmallpositive
charge(δ+).
AdjacentmoleculesofthecompoundcontaininganO–Hbondwillbeattractedto
eachotherbymeansoftheseoppositecharges.Thisforceofattractionisknown
astheHydrogenBond.Usuallyahydrogenbondisrepresentedbyadottedline.

45. 45
The Hydrogen Bonding
Consider the following HB
H
2O –H
20
NH
3-H
20
In all the above examples we saw that in all cases the two E.N atoms
are linked due to H atoms. Besides this the strength of H-Bond will
depend on the value of electronegativity for examples in H-F the H-
Bond is most stronger as compared to HCl, HBr and HI. This is only
due to EN. The order of Hydrogen bond strength in haloges atoms
will be decreased from TOP to the BOTTOM.
H----F
H----Cl
H-----Br
H-----I

46. 46
The Hydrogen Bonding
Types of Hydrogen bonding
There are two type of Hydrogen bonding
1. Intermolecular HB
2. Intramolecular BH
1. INTERMOLECULAR HB
Intermolecular HB exist between two SAME
molecules or DIFFERENT molecules.
Examples
HF-HF
CH
3-O-CH
3(Dimethly ether) and H
20

47. 47
The Hydrogen Bonding
2.INTRAMOLECULAR HB
IntramolecularHBocurresWITHINTHESAMEmoleculeandsometimeit
iscalledasINTERNALHB.
ForexampleSALICYLICACID
SometimestheintramolecularHBresultsintheformationofa2nd
psudoringlikeinsalicyladehyde.Theformationofanextraringis
knowasCHELATION(holdingofaHatombetweentwoatomsofthe
samemolecule).IncaseofchelationtheHatomfindsitselfamember
ofasixmemberring.

48. 48
The Hydrogen Bonding
Energy of HB
TheHBismuchweakerthanordinarycovalentbond.ThestrengthoftheHB
areintherangeof8–42kj/mol.IngeneralthestrengthofHBincreases
withtheacidityofhydrogeninH-AandthebasicityofB.Forexamplethe
HBenergyforHF-HF,H
2O-H
2O,NH
3-NH
3is41.84,29.29and8.37kj/mol
respectively.IncaseofFluorine(strongbase)averylowpolarization
occurebecauseofitselectronsbeingclosetoandtightlyheldbythe
nucleus,formastrongerHB.
IftheHatom(presentinstrongacid)istoostrongacidicandtheacceptor
atomistoostrongbasic,theHatomwillshfitasaprotontoforma
covalentbondwiththeacceptoratominasimpleacidbasereaction.

49. 49
The Hydrogen Bonding
Thestrengthsofhydrogenbonds(5to10Kcalperbond)aremuchless
thanthestrengthsofordinarycovalentbonds.However,theyhaveavery
significanteffectonthephysicalproperties(boilingpoints,solubility)of
organiccompounds.
EffectonBoilingPoints:
Itisunderstandablethatsubstanceshavingnearlythesamemolecular
weights,havethesameboilingpoint.Theboilingpointsofalkanesand
ethersofcomparablemolecularweightsarenotfarapart,buttheboiling
pointsofalcoholshavingalmostequalmolecularweightsareconsiderably
higher.
CH
3--CH
2---CH
3 CH
3—O—CH
3 CH
3—CH
2—OH
Propane Dimethylether Ethanol
(MW44;bp–45°C) (MW46;bp–25°C) (MW46;bp+78°C)
Thiscanbeexplainedonthebasisofhydrogenbonding.Ethanolforms
hydrogenbonds.Extraenergyintheformofheatisrequiredtobreakthe
hydrogenbondsholdingthemoleculestogetherbeforeitcanbevolatilized.
Propaneanddimethyletherdonotformhydrogenbondsand,therefore,
havelowboilingpoints.

50. 50
The Hydrogen Bonding
EffectonWaterSolubility:
Ahydrogen-bondedsubstanceisusuallysolubleinanotherhydrogen-
bondedsubstance.Forexample,alcoholsaresolubleinwaterbutalkanes
arenot.Thisisbecauseanonpolaralkanemoleculecannotbreakintothe
hydrogen-bondedsequenceinwater.Itcannotreplacethehydrogenbonds
thatwouldhavetobebrokentoletitin.
Analcoholmoleculeiscapableofhydrogenbonding.Itcanslipintothe
hydrogenbondedsequenceinwater.Itcanreplacethehydrogenbonds
thatmustbebrokentoletitin.

51. 51
The Hydrogen Bonding
EffectonWaterSolubility:
Thusalcoholsoflowmolecularweightarewatersoluble.However,
whenthealkylgroup(R–)isfourormorecarbonsinlengththe
alkanenatureofthemoleculepredominates,andwatersolubility
fallsoffsharply.Alcoholscontainingmorethansevencarbonsare
insolubleinwater.

52. 52
The Hydrogen Bonding
Effect on volatility
VolatilityincreasebyincreasingintramolecularHB.Incaseof
chelationinsalicylaldehydetheBPislowerexpectedlybecausein
thiscasethemoleculebehavesasmonomerandisthereforeeasy
tovolatilize.Thechelatedsalicylaldehydeboilsat196
o
Candcan
easilyvaporizewhileitsparaormataisomorboilsabove240
o
C
andnotvaporizethroughsteamdistillation.
Incaseofo-nitrophenol
Thesolubilityofo-nitrophenoliswaterislowerascomparedtoits
paraandmataisomerbecauseincaseoforthoisomorthevolatiliy
increasesandsolubilitydecreased.Whileincaseofparaisomor
thevolitilydecreasesandsolubilityinwaterincreasesdueto
fromtionofintermolecularHB.

53. 53
The Hydrogen Bonding
Effect on acidity
O-Hydroxybenzoic acid (salicylic acid) is more
acidic than para position. Because in ortho
isomor the OH group is in a better position to
sabilize the carboxylate ion, formed after
ionization, by chelation.

54. 54
The Steric Effect
theeffectofthestructureofmoleculesontheSTABILITYand
REACTIONofthecompoundiscalledstericeffect.
Stericeffectsarisefromthefactthateachatomwithinamolecule
occupiesacertainamountofspace.Ifatomsarebroughttooclose
together,thereisanassociatedcostinenergyduetooverlapping
electronclouds(PauliorBornrepulsion),andthismayaffectthe
molecule'spreferredshape(conformation)andreactivity.Thesize
aswellastheelectronicproperties(i.e.inductiveandmesomeric
effects)ofthesurroundinggroupsaffectsthestabilityof
carbocations,carbanionsandradicals.Whenbulkysubstituents
surroundacationthereactivityofthecationtonucleophilicattack
isreducedbystericeffects.Thisisbecausethebulkygroupshinder
theapproachofanucleophile.

55. 55
The StericEffect
Whenthesizeofgroupsisresponsibleforreducingthe
reactivityatasitewithinamolecule,thisis
attributedtosterichindrance.Whenthesizeof
groupsisresponsibleforincreasingthereactivityat
asitewithinamolecule,thisisattributedtosteric
acceleration.Sterichindranceorsteric
resistanceoccurswhenthesizeofgroups
withinamoleculepreventschemicalreactions
thatareobservedinrelatedsmaller
molecules.Althoughsterichindranceis
sometimesaproblem,itcanalsobeavery
usefultool,andisoftenexploitedbychemists
tochangethereactivitypatternofamolecule
bystoppingunwantedside-reactions(steric
protection).

56. 56
The Steric Hindrance
Nucleophile approaches from the back side.
It must overlap the back lobe of the C-X sp
3
orbital.

57. 57
Examples
1.The methylation of 2,6-Ditertiary butyl pyridine under high pressure
is not possible as compare to the methylation of 2,6-Dimethyl
pyridine. Because the possibility of the 2
nd
reaction is that methyl
groups are less bulkier than tertiary butyl. in case of SN2 reaction
the rate of reaction is inversely proportional to bulkeness of the
attached groups.
2.Penicillin
Penicillin is an antibiotic with the following chemical formula,

58. 58
In the above structure R is related to the chemical
activity of penicillin. If in place of R there is
Benzyl group so this penicillin will be called
benzyl penicillin. Now Benzyl group is not bulky
so the enzyme produced by the bacteria know as
beta lactam or penicillinase will attack on this
penicillin and will rupture the beta lactam ring.

59. 59
In case of
Cloxacillin, the R
ihas been
replaced by the
bulky group and
due to steric
hinderance the
beta lactamase
enzyme can’t
rupture the beta
lactam ring.

60. 60
Keto Enol Tautomerism
(As a general rule enols are unstable)
CC
OH
ol
ene
ENOLS :
( have -OH attached to a double bond)
Think of this combination as unstable.OH
Phenols are not “enols” and they are
very stable (benzene resonance).
NOTE :

61. 61
KetoEnolTautomerism
Natureoftautomerism:
1.Carbonylcompoundswithhydrogensbondedtotheirαcarbonsequilibratewiththeir
correspondingenols.
2.Thisrapidequilibrationiscalledtautomerism,andtheindividualisomersare
tautomers.
3.Unlikeresonanceforms,tautomersareisomers.
4.Despitethefactthatverylittleoftheenolisomerispresentatroomtemperature,
enolsareveryimportantbecausetheyarereactive.Forexample,ethylacetoacetateis
anequilibriummixtureoftheketoandenolform.Atroomtemperature,themixture
contains93%ofketo-formplus6%oftheenol-form.
Mechanismoftautomerism:
1.Inacid-catalyzedenolization,thecarbonylαcarbonisprotonatedtoforman
intermediatethatcanloseahydrogenfromitscarbontoyieldaneutralenol.
2.Inbase-catalyzedenolformation,anacid-basereactionoccursbetweenabaseandan
αhydrogen.
i.Theresultantenolateispotonatedtoyieldanenol.
ii.Protonationcanoccureitheroncarbonoronoxygen.
iii.Onlyhydrogenontheαpositionsofcarbonylcompoundsareacidic.

62. 62
Keto Enol TautomerismCC
H
O
CC
O
H
K
keto enol
For most ketones, the keto form
predominates in the equilibrium

63. 63
Acid-catalyzed Enol Formation
C
C
H
:O:H—A
Keto tautomer
C
C
H
+
:O
H
:A
-C
C
:
:O
H
+
H
Protonation of the carbonyl oxygen
atom by an acid catalyst HAyield a
cation that can be represented by
two resonance structures.
+ HA
C
C
:O
H:
Enol tautomer
Loss of H
+
from the αposition by
reaction with a base A
-
then yields
the enol tautomer and regenerates
HAcatalyst.
Acid-catalyzedenolformation.
TheprotonatedintermediatecanloseH
+
,either
fromtheoxygenatomtoregenerateketotautomer
orfromtheαcarbonatomtoyieldanenol.

64. 64
Base-catalyzed Enol Formation
C
C
H
:O:
Keto tautomer
+ OH
-
C
C
:O
H:
Enol tautomer
-
:OH
:
:
C
C
:O:
I: C
C
:
:O:
-
H—O—H
:
:
Base removes an acidic hydrogen
from the αposition of the carbonyl
compound, yielding an enolate anion
that has two resonance structures.
Protonation of the enolate anion on
the oxygen atom yields an enol and
regenerates the base catalyst.
Base-catalyzedenolformation.
Theintermediateenolateion,aresonance
hybridoftwoforms,canbeprotonatedeither
oncarbontoregeneratethestartingketo
tautomeroronoxygentogiveanenol.