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CHEMISTRY
CARBOXYLIC ACIDS & ACID DERIVATIVES
Introduction
Compounds containing the carboxyl group are distinctly acidic and are called carboxylic acids.
O
R – C – O – HCarboxylic acid
There have general formula CnH2nO2
Carboxylic acid derivatives are compounds with functional groups that can be converted to carboxylic
acids by a simple acidic or basic hydrolysis. The most important acid derivatives are esters, amides,
nitriles, acid halides and anhydrides.
O
R – C – X
acid halide
O O
R – C – O – C – R
anhydride
O
R – C – O – R'
ester
O
R – C – NH2
amide
R – C Nnitrile
Esters and amides are particularly common in nature. For example, isoamyl acetate found in ripe bananas
and geranyl acetate is found in the oil of roses, geraniums and many other flowers. N, N-diethyl-meta-
toluamide (DEET) is one of the best insect repellents known and penicillin G is one of the antibiotics that
revolutionized modern medicine.
O
O – C – CH3
isoamyl acetate(banana oil)
O
O – C – CH 3
geranyl acetate(geranium oil)
C
O
N(CH CH )2 3 2
N, N-diethyl-meta-toluamide
H C3H C3H C3H C3H C3H C3H C3H C3H C3H C3H C3
O
O
PhCH – C – NH2
N
S
COOH
CH3
CH3
Penicillin G
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IUPAC Nomenclature of Acid and Acid derivatives:-
Table- 1
O
O
O
O O
O
O
O
O
O
O
O
O
O
O
O
O
Compound IUPAC Name
(1) H – C – OH
(2) CH – C – OH3
(3) CH – CH – C – OH3
Ethanoic acid
Methanoic acid
2-Cyclohexylpropanoic acid
3-Oxo-2-propylbutanoic acid(4) CH CCH – C – OH3
CH CH CH2 2 3
(5) CH – CH – CH – C – OH2 2 2
(6) CH CH CH – – C – OH3 2 CH2
NH2
4-Aminobutanoic acid
Ph
(7) CH – CH – CH – C – OH3 2
(8) CH – C – F3
(9) CH – C – Cl3 CH –2
(10) CH – CH – C – Br 3 CH –2
CH3
3-Phenylpentanoic acid
3-Methylbutanoic acid
Ethanoylfluoride
Propanoylchloride
Br
3-Bromobutanoylbromide
(11) – C – Cl Cyclopentanecarbonyl
chloride
(12) CH – C – O – C – CH3 3
(13) CF – C – O – C – CF3 3
Ethanoic anhydride
Trifluoroethanoic anhydride
(14) O
O
1,2-Benzenedicarboxylicanhydride
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O O
O
O
O
O
O
CH – C – O – C – H3
CH – C – O – C – CF3 3CH2Trifluoroethanoic propanoicanhydride
Ethanoic methanoicanhydride
Cyclopropane carbonitrile
3-Cyanopentanoic acid
C – OCH CH32
CN
Ethyl o-cyanobenzoate
–C N
CH – CH – CH – CH – COOH3 2 2
–CN
C – NH2
C – H
2-Formylcyclohexanecarboxamide
CH – CH – CH – C N3 2
OH2-Hydroxybutane nitrile
Dicarboxylic acids
If the substituent is a second carboxyl group, we have a dicarboxylic acid. For example :
acidopanedioicPr acidMalonic
COOHHOOCCH2
acidedioictanBuacidSuccinic
COOHCHHOOCCH 22
acidcHexanedioiacid Adipic
COOHCHCHCHHOOCCH 2222
acidedioictanBromopen2acidricBromoglutaBr
|CHCOOHCHHOOCCH 22
acidedioictanpenDimethyl3,3acidtaricDimethyglu,
3
22
3
CH
|COOHCCHHOOCCH
|CH
acidedioictanpenDichloro4,2acidutaricDichlorogl, ClCl
||CHCOOHHOOCCHCH2
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Physical properties of acids and acid derivatives
(1) First three members are colourless pungent smelling liquid. The next three members also colurless oilyliquid with unpleasant smell. Higher member (> 7) are colourless waxy solids.(2) Boiling points:The boiling point of carboxylic acids are higher than that of alcohols, ketones or aldehydes of similar molecular weight.
CH – C – OHacetic acid,
bp 118ºC
3
O
CH OH1-propanol
bp 97ºC
3CH CH2 2
CH
bp 49ºC
3CH CHPropionaldehyde
3
O
The high boiling points of carboxylic acids is the result of formation of a stable hydrogen-bonded dimer.
R – C C – R
O ----- H – O
O – H ----- O
hydrogen bonded acid dimer
Carboxylic acids have higher boiling points than corresponding molucular mass alcohols because of -
(i) –OH bond in carboxylic acid is more polar than alcohol due to the presence of group.
(ii) Carboxylic acid molecules are held together by two H-bonds.
Esters and acid chlorides have boiling points near those of the unbranched alkanes with similar molecular weights.Nitriles also have higher boiling points than esters and acid chlorides of similar molecular weight. This effectresults from a strong dipolar association between adjacent cyano groups.
R – C N :δ –δ+ δ –δ
+: N C – R (dipolar association of nitriles)
These acid derivatives contain highlypolar carbonyl groups, but the polarity of the carbonyl group has only asmall effect on the boiling points.
×
×
×
×
× ×
×
×
× ×
acidchlorides
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
N,N-dimethyl 3° amides
methylesters
1° aocohol
acids
20 60 100 140 180
n-alkanes
200
300
100
0
–100
Molecular weight
B
o i l i n g p o i n t ( º C )
nitriles
1° amides
N-methyl 2°amides
CH –C–NH3 2
O
CH –C–OH3
O
CH CH CH OH3 2 2
O
CH CH –C N3 2
CH CH CH CH3 2 2 3
Examples(MW 55 – 60) bp(ºC)
222
118
9797
H–C–OCH3 32
0
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(3) Melting points :
Acids containing more than 8 carbon atoms are generally solids, unless they contain double bonds. Thepresence of double bonds (especially cis double bond) in a long chain impedes the formation of a stablecrystal lattice resulting in a lower boiling point.
O
CH – (CH ) – C – OH3 2 16
Stearic acid, mp 70ºC
OC = C C = C
H H H H
CH (CH )3 2 4CH2 (CH ) – C – OH2 7
linoleic acid mp –5ºCMelting point of carboxylic acids : There is no regular pattern in melting point of carboxylic acid (up to 10carbon atoms) having even number of C atoms are higher than neighbouring members having odd number of C atoms because carboxylic acid and methyl group in even members lie in opposite side of zig-zag carbonchain hence they fit better into crystal lattice resulting in higher melting points.Vice-versa is observed in caseof carboxylic acid having odd no. of carbon atoms.
Amides have surprisingly high boiling points and melting points compared with other compounds of similar molecular weight. Primary and secondary amides participate in strong hydrogen bonding.
C
R N
R'
R'
:O:
. .C
R N
R'
R'
:O:
+
. . –
C = N
C = N
O
R
R
H
H
H
H
+
+
–
– –...
O O. . . C
R
N – H+
H
Hydrogen bonding
–
R'
N
R'
C
R
O+
+
– – +O
C
R
N
R'
R'
+
Intermolecular attraction
Strong hydrogen bonding between molecules of primary and secondary amides also results in unusually
high melting points.
O
H – C – N
CH3
CH3
dimethylformamide(DMF) mp –61ºC
O
CH – C – N3
H
CH3
N-methylacetamidem.p. 28ºC
O
CH CN3CH2
H
H
Propionamidemp 79ºC
(4) Solubility:
Carboxylic acids form hydrogen bonds with water and the lower molecular - weight carboxylic acids (upto4 carbon atoms) are miscible with water.
Acid derivatives (esters, acid chlorides, anhydrides, nitriles and amides) are soluble in common organicsolvents such as alcohols, ethers, chlorinated alkanes and aromatic hydrocarbons. Acid chlorides and
anhydrides cannot be used in nucleophilic solvents such as H2O and alcohols, because they react withthese solvents.
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Physical Properties of Carboxylic AcidsTable -2
IUPAC name Common Name Formula mp bp Solubility
(ºC) (ºC) (g/100 g H2O)
Methanoic formic HCOOH 8 101 (miscible)
Ethanoic acetic CH3COOH 17 118
Propanoic propionic CH3CH2COOH –21 141
2-Propenoic acrylic H2C=CH–COOH 14 141
Butanoic butyric CH3(CH2)2COOH –6 163
2-Methylpropanoic isobutyric (CH3)2CHCOOH –46 155 23
Trans-2-butenoic crotonic CH3 –CH=CH–COOH 71 185 8.6
Pentanoic valeric CH3(CH2)3COOH –34 186 3.7
3-Methylbutanoic isovaleric (CH3)2CHCH2COOH –29 177 5
2,2-Dimethylpropanoic pivalic (CH3)2C–COOH 35 164 2.5
Hexanoic caproic CH3(CH2)4COOH –4 206 1.0
Octanoic caprylic CH3
(CH2
)6
COOH 16 240 0.7
Decanoic capric CH3(CH2)8COOH 31 269 0.2
Physical Properties of Acid DerivativesTable -3
Compound Name mp (ºC) bp (ºC) Water
Solubility
CH3COCl Ethanoylchloride
(CH3CO)2O Ethanoic anhydride
CH3COOH Ethanoic acid 17 118
CH3CONH2 Ethanamide 222
CH –C–OCH CH3 2 3
O
Ethyl acetate – 83 77 10%
H–C–N(CH )3 2
O
Dimethylformamide (DMF) – 61 153 miscible
CH –C–N(CH )3 3 2
O
Dimethylacetamide (DMA) – 20 165 miscible
CH3 –C N Acetonitrile – 45 82 miscible
Methods of preparation of carboxylic ac ids
1. Synthesis of carboxylic acids by the carboxylation of grignard reagents
RMgX + O = C = Oether
Dry OMgXCR
||O
X)OH(Mg
OH/H 2
OHCR||O
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etanChlorobu2Cl| CHCHCHCH 323
1. Mg / diethyl ether
2. CO2
3. H O3
+
%)8676(acidoictanMethylbu2
HCO|
CHCHCHCH
2
323
1. Mg / diethyl ether
2. CO2
3. H O3
+
Ex.
3
3
CH|
MgBr CHCH
H/OH)ii(
CO)i(
2
2
)acidpropanoicmethyl2(acidIsobutyricCH|
COOHCHCH
3
3
2. Synthesis of Carboxylic acids by the hydrolysis of nitriles
Mechanism :
+ + H+ heat+
Hydrolysis of cyanides (Acid catalysed) :
Ex. DMSO
NaCN CH CN2
Benzyl cyanide (92%)
CH COH2
Phenylacetic acid (77%)
||O
Ex. CH CCH CH CH3 2 2 3
|OH
|
CN
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Note: (1) Alkyl cyanides needed for the purpose can easily be prepared from the corresponding alkyl halides withalcoholic KCN or NaCN.
R – Cl + KCN R – C N + KCl(2) This reaction is used to ascend the series having one carbon atom more than the corresponding alkylhalides which are prepared from alcohol on treating with phosphorus halide.
ROH + PX5 R – X + POX3 + HX(3) This hydrolysis of alkyl cyanide provides a useful method to get carboxylic acid having one carbon atommore than the original alkyl halide and alcohols.
3. By oxidation of alkylbenzenes - aromatic acids are produced.
OH/H)ii(
¯ OH/KMnO
2
4
OH/H)ii(
¯ OH/KMnO)i(
2
4
Ex.42
722
SOH
OCr K
Chemical Reactions
1. Acidic Strength :
Acidity of carboxylic acids:-
R – C – OH
O
-R – C – O
O(I)
+ H
(i) -
R – C – O
O
(I) exists as two equivalent canonical structures I(A) and I(B). This ion is resonance stablised
and resonance hybrid structure is I(C).-
R – C
O
O
I(A)
-R – C
O
O
I(B)
R – C
O
O
I(C)
(ii) -
R – C – O
O
ion is more stable due to resonance, hence carboxylic acids are acidic in nature.
(iii) Electron withdrawing group (–I effect) stablises the anion and hence, increases acidic nature.
C
O
OX
Ex. F – CH2 – COOH > Cl – CH2COOH > Br – CH2COOH > I – CH2COOH
Ex. Cl – C – COOH
Cl
Cl
> Cl – CH – COOH
Cl
> Cl – CH2COOH > CH3COOH
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(iv) Electron releasing group (+ I effect) destablises the anion and hence decreases acidic nature.
C
O
O
X
Ex. HCOOH > CH3COOH > CH3 – CH2 – COOH
Ex.COOH
COOH>
COOH
COOHCH2 >
CH2 – COOH
CH2 – COOH
Ex. Relative acid strength is:-
RCOOH > HOH > ROH > HC CH > NH3 > RHNote Acidity of acids is compared by compairing stability of conjugate base.
2. Reaction involving removal of proton from –OH group.
(i) Action with blue litmus : All carboxylic acids turn blue litmus red.(ii) Reaction with metals :
2 CH3COOH + 2Na + H2
2CH3COOH + Zn + H2
(iii) Reaction with alkalies :
CH3COOH + NaOH CH3COONa + H2O(iv) Reaction with carbonates and bicarbonates :
2CH3COOH + Na2CO3 2CH3COONa + CO2 + H2OCH3COOH + NaHCO3 CH3COONa + CO2 + H2OReaction of carboxylic acid with aqueous sodium carbonate solution produces brisk efferuescence.However most phenols do not produce effervescence. Therefore, the reaction may be used to distinguishbetween carboxylic acids and phenols.
(v) Reaction with grignard reagent :
R–CH2MgBr+RCOOH R–CH3 + RCOOMgBr
Note: A stronger acid displaces a weaker acid from the salt of the weaker acid.Ex. CH3COOH (Stronger acid) + CH3ONa CH3COONa + CH3 –OH (Weaker Acid)Ex. CH3COOH (stronger acid) + NaHCO3 CH3COONa + H2CO3 (Weaker acid) H2O + C O2
(lab. test of carboxylic acid)
3. Reaction involving replacement of –OH group
(i) Formation of acid chlorides :
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Ex. + SOCl2 + SO2 + HCl
Ex. + PCl5 + POCl3 + HCl
(ii) Fisher Esterification
Carboxylic acid react with alcohol to form esters through a condensation reaction known as esterification.
General Reaction :
+ R – OH + H2O
Specific Examples:
+ CH3CH2 –OH
+ CH3 –OH
Mechanism : (Acid catalysed esterification)
If we follow the forward reactions in this mechanism, wehave the mechanism forthe acid catalysedesterificationof an acid. If however, we follow the reverse reactions, we have the mechanism for the acid catalysedhydrolysis of an ester. Acid catalysed ester hydrolysis.
which resut we obtain will depend on the condition we choose. If we want to esterify an acid, we use anexcess of the alcohol and, if possible remove the water as it is formed. If we want to hydrolyse an ester, weuse a large excess of water that is we reflux the ester with dilute aqueous HCl or dilute aqueous H 2SO4.
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(iii) Formation of amides :
In fact amides can not be prepared from carboxylic acids and amines unless the ammonium salt is heatedstrongly to dehydrate it. This is not usually a good method of preparing amides.
(iv) Formation of acid anhydride :
4. Decarboxylation reactions :
(i) Soda-lime decarboxylation :
General reaction :
In this reaction carbanion intermediate is formed.Rate of reaction depends upon the stability of carbanion intermediate.Electron with drawing group at R–COOH will increases the rate of decarboxylation.
Ex.
Rate of decarboxylation. I > II > III > IV
(ii) (a) Decarboxylation of -keto carboxylic acids:
Acids whose molecules have a carbonyl group one carbon removed from the carboxylic acid group, called-keto acids, decarboxylate readily when they are heated to 100–150ºC.
There are two reasons for ease of decarboxylation.When the acid estelf decarboxylates, it can do so through a six membered cyclic trensition state :
This reaction produces an enol directly and avoids an anionic intermediate. The enol then tautomerises to amethyl ketone.
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When the carboxylate anion decarboxylates, it forms a resonance stabilized enolate anion.
Alphatic acids that do undergo successful decarboxylation have certain functional groups or double or triplebonds in the or positions.
(iii) Kolbe’s electrolysis
2RCOOK + 2HOH isElectrolysR – R + 2CO2 + H2 + 2KOH
Mechanism : R CO2K R CO2 – + K+
At Anode : R CO2 – R
2CO + e – (oxidation)
(I)
R 2CO R + CO2
(II)
R + R R – R
If n is the number of carbon atoms in the salt of carboxylic acid, the alkane formed has 2(n–1) carbon atoms.
Ex. 2CH3 – COOK + 2H2O isElectrolysCH3CH3 + 2CO2 + H2 + 2KOH.
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(iv) Hunsdiecker Reaction (Bromo-decarboxylation) :R–COOAg + Br 2 R–Br + CO2 + AgBr
Mechanism :
Step 1 : R.COOAg + X2 +AgX
Step 2 :
Step 3 :
Step 4 :
Although bromine is the most often used halogen, chlorine and iodine have also been used.
When iodine is the reagent, the ratio between the reactant is very important and determines the product
A 1 : 1 ratio of salt to iodine gives alkyl halide, as above. A 2 : 1 ratio, however gives the ester RCOOR. This
is called simonini reaction and sometimes used to prepare carboxylic ester.
5. HVZ Reaction (Halogenation of aliphatic acids and Substituted acids)
In the presence of a small amount of phosphorus, aliphatic carboxylic acids react smoothly with chlorine or
bromine to yield a compound in which -hydrogen has been replaced by halogen. This is the Hell-Volhard-
Zelinsky reaction. Because of its regioselectivity-only alpha halogenation-and the readiness with which it
takes place, it is of considerable importance in synthesis.
CH3COOH P,Cl2
ClCH2COOH P,Cl2 Cl2CHCOOH
P,Cl2Cl3CCOOH
The halogen of these halogenated acids ungergoes nucleophilic displacement and elimination much as it
does in the simpler alkyl halides. Halogenation is therefore the first step in the conversion of a carboxylic
acid into many important substituted carboxylic acids.
acidenatedlogha An
Br |
RCHCOOH
+ large excess of NH3
acidomina AnNH|
RCHCOOH
2
Br |
NaOHRCHCOOH
OH|
RCHCOONa H
acidhydroxy AnOH|
RCHCOOH
Br |
)alc(KOHCHCOOHRCH2 RCH = CHCOO¯ H
aciddunsaturate, AnCHCOOHRCH
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Summary of reactions of carboxylic acids :
Carboxylic cid Derivatives
Closely related to the carboxylic acids and to each other are a number of chemical families known asfunctional derivatives of carboxylic acids : acid chlorides, anhydrides, amides, and esters, Thesederivatives are compounds in which the —OH of a carboxyl group has been replaced by —CI,—OOCR, —NH2, or —OR .̀
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Acid chloride Anhydride Amide Ester
They all contain the acyl group,
Like the acid to which it is related, an acid derivative maybe aliphatic or aromatic, substituted or unsubstituted;whatever the structure of the rest of the molecule, the properties of the functional group remain essentilly thesame.Characteristic reaction of acid deerivatives (Nucleophilic acyl substitution) :
Nucleophilic acyl substitution usually takes place by an addition-elimination mechanism.The incomingnucleophile adds to the carbonyl to form a tetrasubstituted intermediate with a tetrahedral carbon.
The tetrahedral intermediate formed when a nucleophile attacks the carbonyl carbon of a carboxylic acid
derivative is not stable and cannot be isolated.
A pair of nonbonding electrons on the oxygen reforms the p bond, and either or is eliminated with its
bonding electrons. Whether or is eliminated depends on their relative basicities. The weaker base ispreferentially eliminated because the weaker the base, the better it is a leaving group.
Thus carboxylic acid derivative will undergo a nucleophilic acyl substitutionreaction provided that theincomingnucleophile is a stronger base than the group that is to be replaced. If the incoming nucleophile and the groupattached to acyl group in the starting material have similar basicities, the tetrahedral intermediate can
expect either group with similar ease. A mixture of starting material and substitution product will result.
(i)
(ii)
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(iii)
(iv)
(v)
Condition for acyl nucleophilic substitution reaction :
(i) L must be better leaving group than , i.e., basicity of Nu should be more than that of
(ii) must be a strong enough nucleophilic to attack RCOL.
(iii) Carbonyl carbon must be enough electrophilic to react with .
(A) Acid halides
Methods of preparation of Acyl halides
(i) RCOOH + PCl5 RCOCl + POCl3 + HCl
(ii) 3RCOOH + PCl3 3RCOCl + H3PO3
(iii) RCOOH + SOCl2 Pyridine
RCOCl + SO2 + HCl
Ex. 3CH3COONa + PCl3 Distil
chloride Acetyl333 PONaCOClCH3
Ex.
benzoate.Sod356 POClCOONaHC2 Distil
chlorideBenzoylNaPONaClCOClHC2 356
Chemical Reactions(1) Reaction with carboxylic acids
Acyl chlorides react with carboxylic acids to yield acid anhydrides. When this reaction is used for preparativepurposes, a weak organic base such as pyridine is normally added. Pyridine is a catalyst for the reactionand also acts as a base to neutralize the hydrogen chloride that is formed.
+ +
CH (CH ) CCl3 2 5
Heptanoylchloride
O
+
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(2) Reaction with alcohols
Acyl chlorides react with alcohols to form esters. The reaction is typically carried out in the presence of pyridine.
+ +
+
(3) Reaction with ammonia and amines
+ + + +
(4) Hydrolysis
Acyl chlorides react with water to yield carboxylic acids. In base, the acid is converted to its carboxylatesalt. The reaction has little preparative value because the acyl chloride is nearly always prepared from thecarboxylic acid rather than vice versa.
+
water
OH2 +
+
water
OH2 +
(5) Reaction of acid halide with organometallic
(a) with Grignard reagent
(b) Reaction with Gilmann reagent
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(6) Reduetion of acid halides
(a) Reduction by LiAIH4
(b) Reduction with H2 /Pd / BaSO4 (Rosenmund reduction)
Summary of reactions of acid halide
(B) Acid amidesMethods of preparation of acids amides
1. By reaction of esters with ammonia and amines
Ex. + +
Ammonia is more nucleophilic than water, making it possible to carry out this reaction using aqueousammonia.
Ex.
Methyl 2-methylpropenoate
H C = C – COCH2 3
|
||
CH3
O
+
2-Methylpropenamide(75%)
H C = C – CNH2 2
|
||
CH3
O
+
Amines, which are substituted derivatives of ammonia, react similarly :
Ex. + +
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Ex. + +
Ex. + +
Ex. + +
2. From acid halidesRCOCl + 2NH3 RCONH2 + NH4Cl
3. From anhydride
(RCO)2O+2NH3 RCONH2 + RCOONH4
4. From esters
RCOOR + NH3 RCONH2 + ROH
5. From ammonium salt of carboxylic acid
RCOONH4 RCONH2 + H2O
43COONHCH 23CONHCH
6. From cyanides
R – C N + H2ONaOHOHor
HCl.Conc
22 R – CONH2
OHNCCH 23 42SOH.Conc23 CONHCH
7. +
Chemical Reactions1. Hoffmann rearrangement
In the Hofmann rearrangement an unsubstituted amide is treated with sodium hydroxide andbromine to givea primary amine that has one carbon fewer than starting amide
General reaction.
+ NaOH + Br 2 R – N = C = O R – NH2
isocyanate
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Mech :
OH
CO2R– NH
R – NH2
(2) Hydrolysis of amides
+ +
In acid, however, the amine is protonated, giving an ammonium ion, R2 :
+ +
In base the carboxylic acid is deprotonated, giving a carboxylate ion :
+ +
The acid-base reactions that occur after the amide bond is broken make the overall hydrolysis irreversible.In both cases the amine product is protonated in acid ; the carboxylic acid is deprotonated in base.
Ex.
422 SOH/OH+
Ex. +
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Summary of reaction of amide:
Ex.
Ans.
O||
NHCCHCH 223
(C) Esters
Methods of Preparation
(i)
acid AceticOHHCCOOHCH 523
HOHHCOOCCH 2523
OHCHCOOHHC 356 H OHCOOCHHC 2356
(ii) CH3COCl + C2H5OH Pyridine
CH3COOC2H5 + HCl
Alcohols react with acyl chlorides by nucleophilic acyl substitution to yield esters. These reactions aretypically performed in the presence of a weak base such as pyridine.
+ + +
Ex. +
Ex. C6H5COCl + CH3CH2OH NaOH C6H5COOCH2CH3 + HCl
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Chemical Reactions
1. Acid catalysed hydrolysis of ester (AAc2):
Because H2O and R—OH have approximately the same basicity, it will be eqully easy for tetrahedralimtermediate I to collapse to reform the ester as it will be for tetrahedral intermediate II to collapse to formthe carboxylic acid. Consequently, when the reaction has reached equilibrium, both ester and carboxylicacid will be obtained.
CH3COOH + ROH
Excess water will force the equilibrium to the right.
CH3COOH + ROH
Mechanism:
2. Base-Promoted Hydrolysis of Esters : Saponification (BAc2):
Esters not only undergo acid hydrolysis, they also undergo base-promoted hydrolysis. Base-promotedhydrolysis is sometimes called saponification, from the Latin word sapo, soap. Refluxing an ester withaqueous sodium hydroxide, for example, produces an alcohol and the sodium salt of the acid :
The carboxylate ion is very unreactive toward nucleophilic substitution because it is negatively charged.Base-promoted hydrolysis of an ester, as a result, is an essentially irreversible reaction.
The mechanism for base-promoted hydrolysis of an ester also involves a nucleophilic addition-elimination atthe acyl carbon.Mechanism :
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Evidence for this mechanism comes from studies done with isotopically labeled esters. When ethyl propanoatelabled with 18O in the ether-type oxygen of the ester(below) is subjected to hydrolysis with aqueous NaOHall of the 18O shows up in the ethanol that is produced. None of the 18O appears in the propanoate ion.
This labeling result is completely consistent with the mechanism given above . If the hydroxyide ion hadattacked the alkyl carbon instead of the acyl carbon, the alcohol obtained would not have been labled.Attack
at the alkyl carbon is almost never observed.
Although nucleophilic attack at the alkyl carbon seldom occurs with esters of carboxylic acids, it is thepreferred mode of attack with esters of sulfonic acids (e.g. tosylates and mesylates)
Summary of reaction of esters :
(D) Acid anhydrides
Methods of Preparation of acid anhydrides1. From carboxylic acids
acid AceticHOOCCHCOOHCH 33 ,52OP
anhydride AceticOHCH.CO.O.COCH 233
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Ex. ,52OP
Ex. ,52OP
Ex. ,52OP+
Ex. ,52OP
five or six membered cyclic anhydride are stable
2. From acid and acid halide
CH3COOH+ CH3COCl Pyridine
CH3CO.O.COCH3 +HCl
Ex. CH3COCl+ CH3COONa CH3CO.O.COCH3 + NaCl
Chemical Reactions
(1) Reaction with aromatic compounds (Friedel crafts acylation)
+ ArH +
Ex. + +
(2) Reaction with alcohols
Acid anhydrides react with alcohols to form esters. The reaction may be carried out in the presence of pyridine or it may be catalysed by acids. In the example shown, only one acyl group of acetic anhydridebecomes incorporated into the ester ; the other becomes the acyl group of an acetic acid molecule.
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+ +
Ex. + + CH3COOH
(3) Reaction with ammonia and amines
Acid anhydrides react with ammonia and amines to form amides. Two molar equivalents of amine arerequired. In the example shown, only one acyl group of acetic anhydride becomes incorporated into theamide and the other becomes the acyl group of the amine salt of acetic acid.
+ +
+ + CH3COOH
(4) Hydrolysis Acid anhydrides react with water to yield two carboxylic acid functions. Cyclic anhydrides yield dicarboxylicacids.
+ +
+
7. Heating Effects :
(a) Heating effect on monocarboxylic acid
2R – COOH
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(b) Heating effect on dicarboxylic acid
CH3 – COOH
(c) Heating effects on Hydroxy acids
(1)
– Hydroxy acid
(2) – Hydroxy acid
(3) – Hydroxy acid
Since 4 or 8 membered rings are less stable the refore -Hydroxy acids on heating produce unsaturatedcarboxylic acid.
(4)
– Hydroxy acid
(d) Heating effects on esters
R` – COOH to R` – CH = CH2
Mech : R` – CH = CH2 + R – COOH
This reaction follows syn elimination & hoffman product is formed.
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MISCELLANEOUS SOLVED PROBLEMS (MSPS)
1. Select the correct statement about the following compounds I, II, III.
(A) (I) decarboxylates faster than (II) on heating.
(B) Only *CO2 is eliminated on heating of compound (I).(C) Compound (I) eliminates a mixture of CO
2and *CO
2on heating.
(D) The rate of decarboxylation of (II) is faster than (III).Ans. (A)
Sol.
No decarboxylation CO2
CO2 rate of decarboxylation : III > I > II
2. final product is :
(A) (B) (C) (D)
Ans. (B)
Sol.
3. final product is
(A) (B) (C) (D)
Ans. (B)
Sol.
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4. Identify (A), (B), (C) and (D).
C3H5Cl (A) dryether /Mg (B)
H/OH)ii(
CO)i(
2
2 (C) ]O[ C8H12(D)
Saturated
Ans. (A) = ; (B) = ; (C) = ; (D) =
5. Preparation of propanoic acid from ethyl alcohol follows :
Sol. CH3 – CH2OH 5PCl CH3 – CH2 – Cl KCN CH3 – CH2 – CN H/OH2 CH3 – CH2 – COOH
6. Identify (A), (B) and (C).
C3H6Cl2 (A) KCN (B) OH/OH2 (C)
2CO
2-Methylpropanoic acid
Sol.
Cl|
CH –C –CH|Cl
33 KCN
CN|
CH –C –CH|CN
33 OH/OH2
COOH|
CH –C –CH|COOH
332CO
2-Methylpropanoic acid
7. Find the rate of soda-lime decarboxylation.
Sol. Rate of soda-lime decarboxylation. I > II > III > IV > V
8. Identify (A), (B) and (C).
CH3 – CH2 – COOH P/)eqV1(Br 2
(A) KCN
(B) /H/OH2
(C)
Sol. (A)
Br |
COOH—CH—CH3 ; (B)
CN|
COOH—CH—CH3 ; (C) CH3—CH2—COOH
9. Write the structures of (A) C3H7NO which on acid hydrolysis gives acid (B) and amine (C). Acid (B) gives(+)ve silver–mirror test.
Ans. A = or
10. Predict A , B , C , D and E.
Acid (A) B 3 AlCl/Mesitylene
Sol. (A)=CH3COOH; (B) =
OO||||
CH—C—O—C—CH 33 ;
(C) = (D) =
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11. Which of these represents correct reaction ?
(A) NaOD.conc DCOO – + DCH2
OD
(B) + NaOH C(CH2OH)
4 + HCOO –
(excess)
(C) 2Br P
(D) +OH
SOH.conc
2
42
Ans. (A,B,C,D)
Sol. (A) NaOD.conc DCOO – + DCH2 OD (Cannizzaro reaction)
(B) + NaOH C(CH2OH)
4 + HCOO – (Aldol + Cannizzaro reaction)
(excess)
(C) 2Br P (HVZ reaction)
(D) +OH
SOH.conc
2
42
(Esterification reaction)
12. Which are correct against property metioned ?(A) CH
3COCl > (CH
3CO)
2O > C H
3COOEt > CH
3CONH
2 (Rate of hydrolysis)
(B) CH3 –CH
2 –COOH > > (Rate of esterification)
(C) > > (Rate of esterification)
(D) > > Ph–CH2 –COOH (Rate of decarboxylation)
Ans. (A,B)
13. Match the product of column II with the reaction of column I.Column I Column II
(A)
(p) ester with O18
(B) (q)A -diketone with –18OH group
(C) (r) A cyclic anhydride with –18OH group
(D) /OH
(s) A cyclic ester without O
18
Ans. (A) – r ; (B) – s ; (C) – p ; (D) – q.