chapter 20 carboxylic acid derivatives nucleophilic acyl substitution
TRANSCRIPT
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Chapter 20Chapter 20Carboxylic Acid DerivativesCarboxylic Acid Derivatives
Nucleophilic Acyl SubstitutionNucleophilic Acyl Substitution
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The key to this chapter is the next The key to this chapter is the next slide.slide.
It lists the various carboxylic acids in It lists the various carboxylic acids in order of decreasing reactivity toward order of decreasing reactivity toward their fundamental reaction type their fundamental reaction type (nucleophilic acyl substitution).(nucleophilic acyl substitution).
The other way to read the list is in The other way to read the list is in order of increasing stabilization of the order of increasing stabilization of the carbonyl group.carbonyl group.
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OO
RCClRCCl
RCOR'RCOR'
OO
RCNR'RCNR'22
OO
CarboxylicCarboxylicAcidAcid
DerivativesDerivativesRCOCRRCOCR
OO OO
Acyl chlorideAcyl chloride
AnhydrideAnhydride
EsterEster
AmideAmide
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OO
RCClRCCl
RCOR'RCOR'
OO
RCNR'RCNR'22
OO
RCOCRRCOCR
OO OO
Acyl chlorideAcyl chloride
AnhydrideAnhydride
EsterEster
AmideAmide
DecreasingDecreasingReactivityReactivity
(Nucleophilic Acyl(Nucleophilic Acyl
Substitution)Substitution)
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OO
RCClRCCl
RCOR'RCOR'
OO
RCNR'RCNR'22
OO
RCOCRRCOCR
OO OO
Acyl chlorideAcyl chloride
AnhydrideAnhydride
EsterEster
AmideAmide
IncreasingIncreasingstabilizationstabilization
of C=Oof C=O
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20.120.1
Nomenclature of Carboxylic Acid DerivativesNomenclature of Carboxylic Acid Derivatives
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Acyl HalidesAcyl Halides
RCRC
OO
XX
name the acyl group and add the word name the acyl group and add the word chloridechloride, , fluoridefluoride, , bromidebromide, or , or iodideiodide
as appropriateas appropriate
acyl chlorides are, by far, the most frequently encountered of the acyl halidesacyl chlorides are, by far, the most frequently encountered of the acyl halides
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Acyl HalidesAcyl Halides
CHCH33CClCCl
OO
acetyl chlorideacetyl chloride
3-butenoyl chloride3-butenoyl chloride
OO
HH22CC CHCHCHCH22CClCCl
OO
CBrCBrFF pp-fluorobenzoyl bromide -fluorobenzoyl bromide
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Acid AnhydridesAcid Anhydrides
when both acyl groups are the same, name the when both acyl groups are the same, name the
acid and add the word acid and add the word anhydrideanhydride
when the groups are different, list the names of the when the groups are different, list the names of the
corresponding acids in alphabetical order and add corresponding acids in alphabetical order and add
the word the word anhydrideanhydride
RCOCR'RCOCR'
OO OO
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Acid AnhydridesAcid Anhydrides
acetic anhydrideacetic anhydride
benzoic anhydridebenzoic anhydride
benzoicbenzoic heptanoicheptanoic anhydride anhydride
CHCH33COCCHCOCCH33
OO OO
CC66HH55COCCCOCC66HH55
OO OO
CC66HH55CCOOC(CHC(CH22))55CHCH33
OO OO
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EstersEsters
name as name as alkyl alkanoatesalkyl alkanoates
cite the alkyl group attached to oxygen first (R')cite the alkyl group attached to oxygen first (R')
name the acyl group second; substitute the suffixname the acyl group second; substitute the suffix
-ate-ate for the for the -ic -ic ending of the corresponding acidending of the corresponding acid
RCOR'RCOR'
OO
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EstersEsters
CHCH33COCOCHCH22CHCH33
OO
ethylethyl acetate acetate
methylmethyl propanoate propanoate
2-chloroethyl 2-chloroethyl benzoate benzoate
OO
CHCH33CHCH22COCOCHCH33
COCOCHCH22CHCH22ClCl
OO
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Amides having an NHAmides having an NH22 group group
identify the corresponding carboxylic acididentify the corresponding carboxylic acid
replace the replace the -ic acid -ic acid or or -oic acid -oic acid ending by -amide.ending by -amide.
RCRCNNHH22
OO
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Amides having an NHAmides having an NH22 group group
CHCH33CCNNHH22
OO
acetamideacetamide
3-methylbutanamide3-methylbutanamide
OO
(CH(CH33))22CHCHCHCH22CCNNHH22
CCNNHH22
OO
benzamidebenzamide
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Amides having substituents on NAmides having substituents on N
name the amide as beforename the amide as before
precede the name of the amide with the name of precede the name of the amide with the name of
the appropriate group or groupsthe appropriate group or groups
precede the names of the groups by the letter precede the names of the groups by the letter N- N-
(standing for nitrogen and used as a locant)(standing for nitrogen and used as a locant)
RCRCNNHHR'R'
OO
andand RCRCNNR'R'22
OO
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Amides having substituents on NAmides having substituents on N
CHCH33CCNNHHCHCH33
OO
NN--methylmethylacetamideacetamide
NN--isopropylisopropyl--NN--methylmethylbutanamidebutanamide
CCNN(CH(CH22CHCH33))22
OO
NN,,NN--diethyldiethylbenzamidebenzamide
OO
CHCH33CHCH22CHCH22CCNNCH(CHCH(CH33))22
CHCH33
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NitrilesNitriles
add the suffix add the suffix -nitrile -nitrile to the name of the parent to the name of the parent
hydrocarbon chain (including the triply bonded hydrocarbon chain (including the triply bonded
carbon of CN)carbon of CN)
or: replace the or: replace the -ic acid -ic acid or or -oic acid -oic acid name of the name of the
corresponding carboxylic acid by corresponding carboxylic acid by -onitrile-onitrile
or: name as an or: name as an alkyl cyanide alkyl cyanide (functional class (functional class
name)name)
RCRC NN
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NitrilesNitriles
CHCH33CC NNethanenitrileethanenitrileor: acetonitrileor: acetonitrileor: methyl cyanideor: methyl cyanide
CC66HH55CC NN benzonitrilebenzonitrile
NNCC
CHCH33CHCHCHCH33 2-methylpropanenitrile2-methylpropanenitrileor: isopropyl cyanideor: isopropyl cyanide
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20.220.2
Structure of Carboxylic Acid DerivativesStructure of Carboxylic Acid Derivatives
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Electron Delocalization and the Carbonyl GroupElectron Delocalization and the Carbonyl Group
The main structural feature that distinguishes acyl The main structural feature that distinguishes acyl
chlorides, anhydrides, esters, and amides is the chlorides, anhydrides, esters, and amides is the
interaction of the substituent with the carbonyl interaction of the substituent with the carbonyl
group. It can be represented in resonance terms group. It can be represented in resonance terms
as:as:
RCRC
OO
XX••••
••••••••
RCRC
OO
XX••••
••••••••••••
++RCRC
OO
XX
••••••••••••
++
––––
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Electron Delocalization and the Carbonyl GroupElectron Delocalization and the Carbonyl Group
The extent to which the lone pair on X can be The extent to which the lone pair on X can be
delocalized into C=O depends on:delocalized into C=O depends on:
1) the electronegativity of X1) the electronegativity of X
2) how well the lone pair orbital of X interacts 2) how well the lone pair orbital of X interacts
with the with the orbital of C=O orbital of C=O
RCRC
OO
XX••••
••••••••
RCRC
OO
XX••••
••••••••••••
++RCRC
OO
XX
••••••••••••
++
––––
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Orbital overlaps in carboxylic acid derivativesOrbital overlaps in carboxylic acid derivatives
orbital of carbonyl grouporbital of carbonyl group
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Orbital overlaps in carboxylic acid derivativesOrbital overlaps in carboxylic acid derivatives
lone pair orbitallone pair orbital
of substituentof substituent
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Orbital overlaps in carboxylic acid derivativesOrbital overlaps in carboxylic acid derivatives
electron pair of substituent delocalized into electron pair of substituent delocalized into
carbonyl carbonyl orbitalorbital
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acyl chlorides have the least stabilized carbonylacyl chlorides have the least stabilized carbonyl
groupgroup
delocalization of lone pair of Cl into C=O group isdelocalization of lone pair of Cl into C=O group is
not effective because C—Cl bond is too longnot effective because C—Cl bond is too long
Acyl ChloridesAcyl Chlorides
••••
CC
OO
RR
ClCl•••• ••••
••••••••
••••
CC
OO
RR
ClCl••••
•••• ••••••••
++
––
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RCClRCCl
OO
least stabilized C=Oleast stabilized C=O
most stabilized C=Omost stabilized C=O
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lone pair donation from oxygen stabilizes thelone pair donation from oxygen stabilizes the
carbonyl group of an acid anhydridecarbonyl group of an acid anhydride
the other carbonyl group is stabilized in anthe other carbonyl group is stabilized in an
manner by the lone pairmanner by the lone pair
Acid AnhydridesAcid Anhydrides
••••CCRR
OO•••• ••••
OO••••
CC
OO•••• ••••
RR
OO•••• ••••
••••••••
++
––
CCRR
OO ••••
OO••••
CCRR
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RCOCR'RCOCR'
OO OO
least stabilized C=Oleast stabilized C=O
most stabilized C=Omost stabilized C=O
RCClRCCl
OO
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lone pair donation from oxygen stabilizes thelone pair donation from oxygen stabilizes the
carbonyl group of an estercarbonyl group of an ester
stabilization greater than comparable stabilizationstabilization greater than comparable stabilization
of an anhydrideof an anhydride
EstersEsters
••••••••
++
––
CCRR
OO ••••
OR'OR'••••
OO•••• ••••
••••CCRR OR'OR'
••••
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RCOCR'RCOCR'
OO OORCClRCCl
OO
RCOR'RCOR'
OO
least stabilized C=Oleast stabilized C=O
most stabilized C=Omost stabilized C=O
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lone pair donation from nitrogen stabilizes thelone pair donation from nitrogen stabilizes the
carbonyl group of an amidecarbonyl group of an amide
N is less electronegative than O; therefore, N is less electronegative than O; therefore,
amides are stabilized more than esters and amides are stabilized more than esters and
anhydridesanhydrides
AmidesAmides
••••••••
++
––
CCRR
OO ••••
NR'NR'22
OO•••• ••••
••••CCRR NR'NR'22
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amide resonance imparts significant double-bondamide resonance imparts significant double-bond
character to C—N bondcharacter to C—N bond
activation energy for rotation about C—N bondactivation energy for rotation about C—N bond
is 75-85 kJ/molis 75-85 kJ/mol
C—N bond distance is 135 pm in amides versusC—N bond distance is 135 pm in amides versus
normal single-bond distance of 147 pm in aminesnormal single-bond distance of 147 pm in amines
AmidesAmides
••••••••
++
––
CCRR
OO ••••
NR'NR'22
OO•••• ••••
••••CCRR NR'NR'22
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RCOCR'RCOCR'
OO OORCClRCCl
OO
RCOR'RCOR'
OO
RCRCNR'NR'22
OO
least stabilized C=Oleast stabilized C=O
most stabilized C=Omost stabilized C=O
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very efficient electron delocalization and dispersalvery efficient electron delocalization and dispersal
of negative chargeof negative charge
maximum stabilizationmaximum stabilization
Carboxylate ionsCarboxylate ions
OO•••• ••••
••••CCRR
––OO••••
••••
••••••••––
CCRR
OO ••••
••••••••OO
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RCOCR'RCOCR'
OO OORCClRCCl
OO
RCOR'RCOR'
OO
RCRCNR'NR'22
OO
RCORCO––
OO
least stabilized C=Oleast stabilized C=O
most stabilized C=Omost stabilized C=O
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Reactivity is related to structure: Table 20.1Reactivity is related to structure: Table 20.1
RCOCR'RCOCR'
OO OORCClRCCl
OO
RCOR'RCOR'
OO
RCRCNR'NR'22
OO
StabilizationStabilization Relative rateRelative rate
of hydrolysisof hydrolysis
very smallvery small 10101111
smallsmall 101077
largelarge < 10< 10-2-2
moderatemoderate 1.01.0
The more The more
stabilized the stabilized the
carbonyl group, carbonyl group,
the less reactive the less reactive
it is.it is.
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RCOCR'RCOCR'
OO OORCClRCCl
OO
RCOR'RCOR'
OO
RCRCNR'NR'22
OO
RCORCO––
OO
most reactivemost reactive
least reactiveleast reactive
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Nucleophilic Acyl SubstitutionNucleophilic Acyl Substitution
In general:In general:
OO•••• ••••
CCRR XX
+ H+ HYY
OO•••• ••••
CCRR YY
+ H+ HXX
Reaction is feasible when a less stabilized Reaction is feasible when a less stabilized
carbonyl is converted to a more stabilized carbonyl is converted to a more stabilized
one one (more reactive to less reactive)(more reactive to less reactive)..
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General Mechanism for Nucleophilic Acyl SubstitutionGeneral Mechanism for Nucleophilic Acyl Substitution
involves formation and dissociationinvolves formation and dissociationof a tetrahedral intermediateof a tetrahedral intermediate
OO•••• ••••
CCRR XX
HHYY
CC
RROHOH
XX
YY
OO•••• ••••
CCRR YY
-H-HXX
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RCOCR'RCOCR'
OO OORCClRCCl
OO
RCOR'RCOR'
OO
RCRCNR'NR'22
OO
RCORCO––
OO
most reactivemost reactive
least reactiveleast reactive
a carboxylic acid a carboxylic acid derivative can be derivative can be converted by converted by nucleophilic acyl nucleophilic acyl substitution to any other substitution to any other type that lies below it in type that lies below it in this tablethis table