caboxilic acid and its derivatives

CARBOXYLIC ACIDS & ACID DERIVATIVES

By Anand Kumar Rai IIT Kharagpur

Pradeep Agarwal Academy(Taught By IITians.....)

By: Anand Kumar Rai (IITian...) Mob:7840011220

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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 – Xacid halide

O O

R – C – O – C – Ranhydride

O

R – C – O – R'esterRCO R'

O

R – C – NH2

amideRCONH

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 – CH3

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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CHEMISTRY

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 – Br3 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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CHEMISTRY

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 :

acidopanedioicPracidMalonic

COOHHOOCCH2

acidedioictanBuacidSuccinic

COOHCHHOOCCH 22

acidcHexanedioiacidAdipic

COOHCHCHCHHOOCCH 2222

acidedioictanBromopen2acidricBromogluta

Br|CHCOOHCHHOOCCH 22

acidedioictanpenDimethyl3,3acidtaricDimethyglu,

3

22

3

CH|

COOHCCHHOOCCH|CH

acidedioictanpenDichloro4,2acidutaricDichlorogl,

ClCl||CHCOOHHOOCCHCH2



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CHEMISTRY

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 similarmolecular 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 molecularweights.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 highly polar 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

Boili

ng

poin

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

97

97

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

O

C = 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 ofC 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 similarmolecular 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 unusuallyhigh melting points.

O

H – C – NCH3

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 andanhydrides cannot be used in nucleophilic solvents such as H2O and alcohols, because they react withthese solvents.



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Physical Properties of CarboxylicAcidsTable -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)6COOH 16 240 0.7

Decanoic capric CH3(CH2)8COOH 31 269 0.2

Physical Properties ofAcid 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 acids1. 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|

MgBrCHCH

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

OCrK

Chemical Reactions1. 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

O

X

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 > RH

Note 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 + H2O

CH3COOH + NaHCO3 CH3COONa + CO2 + H2O

Reaction 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 distinguish

between 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) + CH3ONaCH3COONa + CH3–OH (Weaker Acid)

Ex. CH3COOH (stronger acid) + NaHCO3 CH3COONa + H2CO3 (Weaker acid) H2O + CO2

(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, we have the mechanism for the acid catalysed esterification

of an acid. If however, we follow the reverse reactions, we have the mechanism for the acid catalysed

hydrolysis 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 an

excess of the alcohol and, if possible remove the water as it is formed. If we want to hydrolyse an ester, we

use a large excess of water that is we reflux the ester with dilute aqueous HCl or dilute aqueous H2SO4.



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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 heated

strongly 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 a

methyl ketone.



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CHEMISTRY

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 triple

bonds in the or positions.

(iii) Kolbe’s electrolysis

2RCOOK + 2HOH isElectrolys

R – 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 isElectrolys

CH3CH3 + 2CO2 + H2 + 2KOH.



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(iv) Hunsdiecker Reaction (Bromo-decarboxylation) :R–COOAg + Br2 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,Cl2

Cl3CCOOH

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.

acidenatedloghaAn

Br|

RCHCOOH

+ large excess of NH3

acidominaAnNH|

RCHCOOH

2

Br|

NaOHRCHCOOH

OH|

RCHCOONa H

acidhydroxyAnOH|

RCHCOOH

Br|

)alc(KOHCHCOOHRCH2 RCH = CHCOO¯ H

aciddunsaturate,AnCHCOOHRCH



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CHEMISTRY

Summary of reactions of carboxylic acids :

Carboxylic Acid DerivativesClosely related to the carboxylic acids and to each other are a number of chemical families known as

functional derivatives of carboxylic acids : acid chlorides, anhydrides, amides, and esters, These

derivatives 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 may be aliphatic or aromatic, substituted or unsubstituted;

whatever the structure of the rest of the molecule, the properties of the functional group remain essentilly the

same.

Characteristic reaction of acid deerivatives (Nucleophilic acyl substitution) :

Nucleophilic acyl substitution usually takes place by an addition-elimination mechanism.The incoming

nucleophile 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 is

preferentially eliminated because the weaker the base, the better it is a leaving group.

Thus carboxylic acid derivative will undergo a nucleophilic acyl substitution reaction provided that the incoming

nucleophile is a stronger base than the group that is to be replaced. If the incoming nucleophile and the group

attached 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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CHEMISTRY

(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

chlorideAcetyl333 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 preparative

purposes, a weak organic base such as pyridine is normally added. Pyridine is a catalyst for the reaction

and 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 carboxylate

salt. The reaction has little preparative value because the acyl chloride is nearly always prepared from the

carboxylic 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 aqueous

ammonia.

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 halides

RCOCl + 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

acetate.Amm43COONHCH

Acetamide23CONHCH

6. From cyanides

R – C N + H2ONaOHOHor

HCl.Conc

22 R – CONH2

OHNCCH 23 42SOH.Conc

23 CONHCH

7. +

Chemical Reactions1. Hoffmann rearrangement

In the Hofmann rearrangement an unsubstituted amide is treated with sodium hydroxide and bromine to give

a primary amine that has one carbon fewer than starting amide

General reaction.

+ NaOH + Br2 R – N = C = O R – NH2

isocyanate



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CHEMISTRY

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) EstersMethods of Preparation

(i)

acidAceticOHHCCOOHCH 523

HOHHCOOCCH 2523

OHCHCOOHHC 356 H

benzoateMethylOHCOOCHHC 2356

(ii) CH3COCl + C2H5OH Pyridine

CH3COOC2H5 + HCl

Alcohols react with acyl chlorides by nucleophilic acyl substitution to yield esters. These reactions are

typically 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 tetrahedral

imtermediate I to collapse to reform the ester as it will be for tetrahedral intermediate II to collapse to form

the carboxylic acid. Consequently, when the reaction has reached equilibrium, both ester and carboxylic

acid 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-promoted

hydrolysis is sometimes called saponification, from the Latin word sapo, soap. Refluxing an ester with

aqueous 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 at

the acyl carbon.

Mechanism :



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Evidence for this mechanism comes from studies done with isotopically labeled esters. When ethyl propanoate

labled with 18O in the ether-type oxygen of the ester(below) is subjected to hydrolysis with aqueous NaOH

all 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 had

attacked 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 the

preferred mode of attack with esters of sulfonic acids (e.g. tosylates and mesylates)

Summary of reaction of esters :

(D) Acid anhydridesMethods of Preparation of acid anhydrides1. From carboxylic acids

acidAceticHOOCCHCOOHCH 33

,52OP

anhydrideAceticOHCH.CO.O.COCH 233



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CHEMISTRY

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 anhydride

becomes 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 are

required. In the example shown, only one acyl group of acetic anhydride becomes incorporated into the

amide 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 dicarboxylic

acids.

+ +

+

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



Since 4 or 8 membered rings are less stable the refore -Hydroxy acids on heating produce unsaturated

carboxylic 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 *CO2is eliminated on heating of compound (I).

(C) Compound (I) eliminates a mixture of CO2and *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(Br2 (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 3AlCl/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– + DCH2OD

(B) + NaOH C(CH2OH)

4+ HCOO–

(excess)

(C) 2BrP

(D) +OH

SOH.conc

2

42

Ans. (A,B,C,D)

Sol. (A) NaOD.conc DCOO– + DCH2OD (Cannizzaro reaction)

(B) + NaOH C(CH2OH)

4+ HCOO– (Aldol + Cannizzaro reaction)

(excess)

(C) 2BrP (HVZ reaction)

(D) +OH

SOH.conc

2

42

(Esterification reaction)

12. Which are correct against property metioned ?(A) CH

3COCl > (CH

3CO)

2O > CH

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 O18

Ans. (A) – r ; (B) – s ; (C) – p ; (D) – q.



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