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Chemistry Notes for class 12 Chapter 12 Aldehydes, Ketones and Carboxylic Acids In aldehydes, the carbonyl group ( )C=O) is bonded to carbon and 

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MODULE - 7Chemistry

Notes Chemistry of OrganicCompounds29ALDEHYDES, KETONESANDCARBOXYLICACIDSI

n the last lesson, you studied about organic compounds containing functional groupshaving carbon-oxygen single bond. There are other classes of organic compounds in whichthe functional group contains the carbon- oxygen double bond. The examples of theseclasses of compounds being carbonyl compounds such asaldehydes andketones as wellascarboxylic acids and theirderivatives. These organic compounds are very importantboth in the industry and in the synthesis of other organic compounds. Therefore, theirstudy forms an important part of the organic chemistry. Let us study the chemistry ofthese classes of compounds in detail.ObjectivesAfter reading this lesson, you should be able togive IUPAC names of aldehydes and ketones;describe the general methods of preparation of aldehydes and ketones;discuss the trends in physical properties of the aldehydes and ketones in the light ofthe polar nature of the carbonyl group;explain important reactions exhibited by aldehydes and ketones;distinguish between aldehydes and ketones on the basis of certain reactions and testsbased on them;give IUPAC names of carboxylic acids;explain general methods of preparation of carboxylic acids;discuss the physical properties and their trends for simple monocarboxylic acids;describe important reactions exhibited by carboxylic acids;explain the preparation and some interconversion reactions of carboxylic acidderivatives, andhighlight the importance of aldehydes, ketones and carboxylic acids.

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NotesChemistry of OrganicCompounds29.1 Aldehydes and KetonesYou have some familiarity with these classes of compounds from previous lessons. Thesecompounds are also referred to ascarbonyl compounds and have-C-O-- functionalitypresent in them. These compounds exist widely in nature and are responsible for theflavour and aroma of many foods. They are also important industrially both as reagents insynthesis and as solvents.Aldehydes have at least one hydrogen atom bonded to the carbonyl group, the other groupmay be either a hydrogen or an alkyl (or aryl) group. Inketones, the carbonyl group isbonded to two alkyl or aryl groups. The two groups bonded to a ketone may be similar ordifferent resulting in a symmetrical or an unsymmatrical ketone, respectively.

You must be familiar with vanilin and camphor. Their structures are given below. You cansee that they contain an aldehyde and a keto functional group, respectively.

29.1.1 Nomenclature of Aldehydes and KetonesIn the IUPAC system of nomenclature, aliphaticaldehydes are named asalkanals. Thefinal-e in the name of the corresponding alkane is substituted by -al.Some common examples of aldehydes and their names are given below :

Note that when the -CHO group is attached to a ring, then the compound is called acarbaldehyde. 220

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Chemistry of OrganicCompoundsRemember that the carbonyl carbon of the aldehydes is present at the end of the chain andis assigned 1 position. Therefore, it is not necessory to specify its position in the name ofthe aldehyde. The examples given below illustrate this point.Ketonesare named asalkanonesin the IUPAC namenclature. Their names are obtainedby replacing final -e in the name of alkane by -one. The carbon chain is numbered in sucha way that the carbonyl group gets the lowest number. Some examples of ketones arementioned below :

33CHCCHOO323CHCHCCHO

Cyclohexanone Butan-2-one(Ethyl methyl ketone)Propanone(Acetone)1234

322CHCCHCHCH=OPent-4-en-2-onePentan 3 one(Diethyl ketone)- -3223CHCHCCHCHO12345

29.1.2 Preparation of Aldehydes and KetonesYou have already studied most of the methods used in the synthesis of aldehydes andketones in the previous lesson. Let us now refresh them.1. Oxidation of Primary and Secondary AlcoholsFrom the last lesson, you know that primary alcohols can be oxidised to aldehydes andsecondary alcohols can be oxidised to ketones.2. Ozonolysis of AlkenesThis reaction has been discussed in lesson 26. The products obtained are aldehydes orketones depending upon the structure of the starting alkene.3. Hydration of AlkynesHydration of alkynes can give an aldehyde or a ketone. Markovnikov"s hydration yieldsketones whereas anti-Markovnikov"s hydration gives aldehydes.

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NotesChemistry of OrganicCompounds

You can go through the details of these reactions as discussed in lesson 26.4. Friedal-Crafts AcylationAromatic ketones can be prepared by Friedel-Crafts acylation (alkanoylation) reaction.One example of this reaction is given below:

3CHO3CHOOC3CH3331.CHCOOCOCH,AlCl,CS222.HCl,HOSimilar acylation reaction using ethanoyl chloride was also discussed in lesson 26 undertheelectrophilic substitutionreactions of aromatic hydrocarbons.29.1.3 Structure and Physical PropertiesIn both aldehydes and ketones, the carbonyl carbon and oxygen atoms aresp2 hybridised.Therefore, the groups attached to the carbon atom and oxygen are present in a plane. Thisis shown in Fig. 29.1.

Fig. 29.1 :The structure of the carbonyl functional groupYou can see in the figure that a-bond is formed by the overlap ofp-orbitals of carbonand oxygen atoms. Thep-orbitals are present in a plane perpendicular to the plane of themolecule. Note the presence of two lone pairs of electrons on oxygen atom.You also know that oxygen is more electronegative than carbon. Hence, it attracts theelectrons of the carbon-oxygen double bond (C--O-- bond ) resulting in its appriciablepolarisation.

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Chemistry of OrganicCompoundsThe oxygen atom, thus, acquires a partial negative charge () whereas the carbon atomgets a partial positive charge (). This polar nature of the carbonyl group makes theoxygen atomnucleophilic andbasic while the carbon atom becomeselectrophilic. Thephysical properties and chemical reactions of aldehydes and ketones are a directconsequence of this polarisation.The dipole-dipole attraction between the molecules of aldehydes and ketones results intheir higher boiling points as compared to the hydrocarbons of similar molecular weight.The physical properties of some aldehydes and ketones are given in Table 29.1.Table 29.1 Physical properties of some representative aldehydes and ketonesCompoundm.p. b.p.Water Solubility(K) (K)(%)Methanal18125255Ethanal150294Propanal19232220Butanal1663487.1Benzaldehyde2174520.3Propan-2-one178329Butan-2-one18735325.6Pentan-2-one1953755.5Pentan-3-one2323744.8Acetophenone 29447515Benzophenone 321578-You can see from Table 29.1 that these compounds have appriciable water solubility. Thisis because of the hydrogen bonding possible between the oxygen atom of the aldehyde (orthe ketone) with hydrogen atom of water molecule, as shown in Fig 29.2.

CORHHOHydrogen bonding

R'

Fig. 29.2 :Hydrogen bonding between carbonyl compound and water molecule29.1.4 Reactions of Aldehydes and KetonesIf you examine the structure of a molecule of the carbonyl compound, you will find thatthere are three centres of reactivity in it as shown below :

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NotesChemistry of OrganicCompoundsSince the oxygen atom isnucleophilicin nature, it is attacked by theelectrophiles, whereasthe carbonyl carbon iselectrophilic in nature and hence is attacked bynucleophiles. Thethird site of reactivity is hydrogen atom present at the-carbon atom. It isacidic innature and gives typical reactions which you will study in this section.It is also important to know here that aldehydes are more reactive than ketones. This isbecause of the following two reasons :(i)Aldehydes have only one alkyl group whereas ketones have two. Since the alkylgroups are electron donating in nature, the carbonyl carbon in ketones which is bondedto two alkyl groups, is less positive (electrophilic) as compared to the aldehydic carbonylcarbon. Hence, it is less susceptible to attack by nucleophiles.(ii)The two alkyl groups in ketones also make the carbonyl carbon more crowded ascompared to carbonyl carbon in aldehydes. This factor also makes the aldehydiccarbonyl carbon more accessible for attack by the nucleophiles as compared tocarbonyl carbon of the ketone.With this background in mind, let us now study the reactions of aldehydes and ketones.A. Nucleophilic Addition ReactionsThe general reaction of addition of nucleophiles on the carbonyl group can be representedas follows :Some specific reactions of this category are discussed below :1. Formation of CyanohydrinsCarbonyl compounds react with hydrogen cyanide to yieldcyanohydrins.Note that one more carbon atom is present in the cyanohydrin as compared to the startingcarbonyl compound.Cyanohydrins are useful in the synthesis of carboxylic acids about you will study in thenext section.

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Chemistry of OrganicCompounds2. Formation of HemiacetalsAldehydes and ketones react with alcohols to givehemiacetals.Hemi in Greek meanshalf. Hemiacetals have an -OH and an -OR group attached to the same carbon atom intheir molecules.When an excess of the alcohol is used, a second molecule of the alcohol reacts to give anacetal.

COH+3CHOH3CHCHO

3OCH3CHCHEthanalan Acetal

OH a Hemiacetal3OCH

3OCH3CHHCl (g)CH OH3Methanol

C+3CHOH33CHCCHO

3OCH3CHCHPropanonean Acetal

OH a Hemiacetal3OCH

3OCH3CH3CH3CHHCl (g)CH OH3MethanolNote that an acetal has two -OR groups attached to the same carbon atom.Acetals are stable in basic solutions and are 'therefore" used as protecting groups foraldehydes and ketones. Acetals can be converted back to the carbonyl compounds bytreating them with dilute acids because of the reversible nature of the above reaction.3. Formation of AlcoholsGrignard reagents (RMgX) react with aldehydes and ketones to give alcohols as shownbelow :

32CHCHMgBrO

a Primary alcoholH-C-OH23CHCHHH-C-H+Ethylmagnesium bromideMethanal

32CHCHMgBr3CHC-HO

a Secondary alcohol-C-23CHCHHCHOH+Ethyl magnesium bromideEthanal3

32CHCHMgBr+3CHC-CH3O

a Tertiary alcohol-C-23CHCH3CH3CHOHEthyl magnesiumbromidePropanoneYou have already studied these reactions under the preparation of alcohols in lesson 28.

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NotesChemistry of OrganicCompoundsB. Addition-Elimination or Condensation Reactions1. Reaction with Ammonia and its DerivativesAldehydes and ketones react with ammonia and primary amines to giveimines which arecompounds having carbon-nitrogen double bond.

It appears that during the above reaction a molecule of water is lost from the primaryamine and the carbonyl compound. The reactions with other derivatives of ammonia aregiven below:CH O + H NOH2HydroxylamineCH N-OH + H O2an OximeC O + H NNH2Hydrazinea HydrazoneCH3C NNH2CH32

CH CH CH O + H NNH3 2 2NO2NO2

2, 4-DinitrophenzylhydrazineCH CH CH NNH3 2NO2NO2

2, 4-DinitrophenylhydrazoneThe compounds formed above are relatively insoluble solids and have characteristic meltingpoints. These compounds can be prepared for the unknown aldehyde or ketone and theirmelting points can be determined. These melting points are matched with the derivativesof already known aldehydes and ketones listed in standard tables and the carbonyl compoundis thus identified.C. De-oxygenation ReactionsDe-oxygenation reactions are reactions involving removal of oxygen. Aldehydes and ketonescan be reduced to the corresponding alkanes by the following two reactions:1. Wolff-Kishner ReductionWhen an aldehyde or a ketone is heated in a basic solution of hydrazine in a high-boiling

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Chemistry of OrganicCompoundsalcohol, then the carbonyl group gets converted to a methylene (--2CH) group.

2. Clemmensen ReductionIt is carried out in acidic medium using amalgamated zinc and hydrochloric acid.D. Oxidation of AldehydesUnlike ketones, aldehydes can be easily oxidised to carboxylic acids using a variety ofoxidising agents. These reagents can be chromic acid, chromium trioxide, permanaganateor silver oxide. You have already read about oxidation with some of these reagents.Silverions selectively oxidise -CHO group. This forms the basis ofTollen"s test. It involvesthe addition of a mixture of aqueous silver nitrate and aqueous ammonia which is knownasTollen"s reagent to the carbonyl compound. Tollen"s reagent contains [Ag(NH3)2]+complex ion. If an aldehyde is present, it gets oxidised to the carboxylic acid whereas theAg ions are reduced to form silver metal which gets deposited on the walls of the testtube and this gives a mirror like shining appearance.Aldehydes are also oxidised byFehling solution, which contain2Cu (cupric) ionscomplexed with tartarate ions as the oxidant. These2Cuions are reduced by the aldehydesin alkaline medium to give a brick red precipitate of cuprous oxide.E. Reactions at carbonThe hydrogen in aldehydes and ketones is quite acidic and can be easily abstrated bya strong base.The resulting anion can stabilise by resonance as shown above. It is called anenolate ion.On protonation, it gives anenol.

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NotesChemistry of OrganicCompoundsThus, keto form and enol form are in equilibrium. This is also known asketo-enoltautomerism.Other reactions feasible due to the presence of-hydrogen are as follows:1. HalogenationKetones having an-hydrogen atom react readily with halogens and-haloketones areobtained as the product. The reaction is promoted both by acids and bases.2 2 2 2( X Cl, Br orI)OH23CHCCHO23BrCHCCHHBr+23Br,CHCOOH2HO,343KPropanone BromopropanoneIn presence of the base, multiple halogenation occurs to give the trihalo product.3NaX+COHCHH+COXCXX+23HO trihaloketone

23XNaOH+The trihalo group is a good leaving group and the trihalo ketone reacts with OH- whichfinally gives a carboxylate ion and ahaloform.++ HO-OCCarboxylate ionCOXCXXCXXXHHaloform-trihaloketone

O-This reaction is called thehaloform reaction after the name of the product.If iodine is used as the halogen, then we getiodoform(3CHI) as the product. The iodoformis a bright yellow solid having a characterstic melting point. This reaction, thus, forms thebasis of theiodoform test. Thus, methyl ketones give a positive iodoform test. You hadstudied the iodoform formation in lesson 27 also.2. Aldol CondensationAldehydes having hydrogen atom on reaction with dil. NaOH givealdols. The reactionis illustrated below byusing ethanal as the example.OC3CHH+3HCCHOOHC3CHH2CHCHOEthanalEthanal3-Hydroxybutanal(an Aldol)

2HO,278KNaOH

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Chemistry of OrganicCompounds

Note that the product contains both thealdehyde and thealcoholfunctional groups andtherefore, it is called analdol. The aldol addition product on heating undergoes dehydrationto give an,- unsaturated aldehyde which is a condensation product.

This complete sequence of reactions is calledaldol condensation.Aldol condensation is also possible with ketones. Can you now think of a little more complexsituation? What will be the products of aldol condensation when two diffrent aldehydeshaving hydrogen atoms are used as reactants. In this case, the reaction is called acrossed-aldol condensation.This is left as an exercise for you. There is a hint of course.Suppose, the two aldehyde molecules are represented by A and B; then condensation canoccur between two molecules of the same aldehyde or diffrent aldehydes. Thus, the productsobtained would be the following types:A-A, B-B, A-B and B-A.With this background in mind, you can now proceed to write the aldol addition products ofethanal and propanal.Intext Questions 29.11.Classify the following as aldehydes or ketones and give their IUPAC names :(i)3CH CHO(ii)3 2 3CH COCH CH(iii)COCH3 (iv)2 3OHCCH CH...................................................................................................................................2.How will you prepare propanone from propyne ?...................................................................................................................................3.Why are aldehydes more reactive than ketones towards nucleophilic addition reactions?...................................................................................................................................4.Write the general structure for the following :(i) a cyanohydrin(ii) an acetal(iii) a hemiacetal...................................................................................................................................5.How can you convert the carbonylC O group toCH2 group ?...................................................................................................................................6.What is an aldol ?...................................................................................................................................

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NotesChemistry of OrganicCompounds29.2 Carboxylic AcidsYou already know that carboxylic acids contain acarboxyl (COOH) funtional group.They are most widely distributed in nature and are also industrially important chemicals.Acetic acid in the form of vinegar is produced in large quantities. It is also a very importantbuilding block in complex biological molecules. You must have also heard aboutfattyacids which are long chain aliphatic acids derived from the hydrolysis of fats and oils.Stearic acid is a fatty acid containing a long chain of eighteen carbon atoms.29.2.1 NomenclatureSeveral carboxylic acids have been known since long and their common names are basedon their sources. However, in the IUPAC nomenclature, carboxylic acids are named bychoosing the longest carbon chain containing theCOOH group. The final-e in the nameof the alkane is replaced by-oic acid.While numbering the carbon chain, theCOOHcarbon is always given number 1 as shown below :5 4 3 2 1O||C C C C C OH The other groups and substituents are numbered and named according to the usual rulesof nomenclature which you have already studied.Some common carboxylic acids and their names are given below :O||HCOHMethanoic acid(Formic acid)3O||CHCOH)Ethanoic acid(Acetic acidCarboxylic acids containing two carboxyl groups are calleddicarboxylic acids. They arenamed by addingdioic acidas a suffix to the name of the corresponding hydrocarbon.Both the carboxyl carbon atoms are numbered as a part of the main chain. Note that inthis case, final-e of the alkane isnot dropped.1 2OO|| ||HOCCOHEthanedioicacid(Oxalicacid)2O O||||HOCCH COHPropanedioicacid(Malonicacid)4 3 2 12 2HOOCCH CH COOHButane-1,4-dioicacid(Succinicacid)

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Chemistry of OrganicCompounds29.2.2 Preparation of Carboxylic AcidsThe following methods are generally used for the synthesis of carboxylic acids. You havealready studied some of these methods in the earlier lessons.1. Oxidation of AlkenesAlkenes on oxidation with hot alkaline4KMnO yield carboxylic acids.431KMnO , OH2 H ORCH=CHR RCOOH R COOH 2. Oxidation of Alcohols and AldehydesYou have read in the last lesson and the previous section of this lesson that alcohols andaldehydes can be oxidized to carboxylic acids using a variety of oxidising agent. You canrefer back to the details of these reactions.3. Oxidation of AlkylbenzenesPrimary and secondary alkyl groups attached to the benzene ring can be oxidised, usingalkaline4KMnO, to the carboxyl group.

Acidified sodium dichromate can also be used for this oxidation.

4. Carbonation of Grignard ReagentsGrignard reagents (RMgX) react with carbon dioxide to give magnesium carboxylateswhich on acidification yield carboxylic acids.2O||RMgX+CO R+HCO Mg X RCOOH 2+23Mg 1CO3 2 3 2 3 2Et O2 H OCH CH Cl CH CH MgCl CH CH COOH ChloroethaneGrignard reagentPropanoic acid(an alkylhalide)

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NotesChemistry of OrganicCompoundsNote that there is an increase of one carbon atom in the carboxylic acid as compared tothe starting alkyl halide.5. Hydrolysis of Nitriles and CyanohydrinsAlkyl halidescan be converted to nitriles which on hydrolysis yield carboxylic acids havingone more carbon atom than the starting alkyl halide.Br2 2CH CHNaCNBr NC1,2-Dibromoethane2 2CH CH+3H O2 2CN HOOCCH CH COOHButanedioicacidButanedinitrileCyanohydrins obtained from aldehydes also yield 2-hydroxycarboxylic acids on hydrolysis.3 2OHOH| |CH CHC+3H O3 2CN CH CHCCOOH| |H H2-Hydroxybutanenitrile 2-Hydroxybutanoicacid29.2.3 Structure and Physical PropertiesSimilar to the aldehydes and ketones, the carboxyl carbon atom is2sp hybridised. Thus,the three atoms attached to this carbon which lie in the same plane with an approximatebond angle of 120º between them, see Fig. 29.3.

CCOHO120º120º120ºFig. 29.3 :Structure of the carboxyl groupCarboxylic acids form hydrogen bonds because of the presence of polar carbonyl andhydroxyl groups. Most carboxylic acids exist in dimeric form in which two carboxylic acidmolecules are held together by two hydrogen bonds. This is shown below :

R-CO H-OO-H OC-RHydrogen bond

Dimer of a carboxylic acidIntermolecular hydrogen bonding is in fact responsible for high melting and boiling pointsof carboxylic acids. The melting and boiling points of some carboxylic acids are listed in

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Chemistry of OrganicCompoundsTable 29.2. You can also see in the table that the lower members have appreciable solubilityin water. This is also due to the presence of hydrogen bonding between the carboxylic acidmolecule and solvent water molecules.Table 29.2 : Some Physical Properties of Carboxylic AcidsCarboxylic Acidm. p.b. p.Watersolubility(K)(K)gmL-1 of H2O at 298K pKaHCOOH281373·53·753CH COOH289.63914·763 2CH CH COOH2524144·873 2 2CH CH CH COOH2674374·823 2 2 2CH CH CH CH COOH2394604·974·812ClCH COOH336462very soluble2·862Cl CHCOOH283.8465very soluble1·483Cl CCOOH329.3471very soluble0·706 5C H COOH2955230·344·193 6 4-CH C H COOHp4505480·034·366 4-ClC H COOHp5150.0093.982 6 4NO C H COOHp5150.033.41Do not worry about theapK values listed in the last column of the table. We will refer tothem when we discuss the acidic nature of carboxylic acids in the following section.29.2.4 Acidity of Carboxylic AcidsCarboxylic acids are acidic in nature. They dissociate in water according to followingequilibrium to give a proton and the carboxylate ion.OR-C-O-H+H2OOR-C-O+H2O-+Carboxylic acidCarboxylate ionThepKa values of some carboxylic acids are given in the last column of Table 29.2.Remember that thelowerpKaindicates greater acidity. If you compare thesepKa valuewith those of alcohols, you will note that the carboxylic acids are much more acidic thanalcohols. This can be explained on the basis of the anion formed as a result of ionisation.The carboxylate ion obtained by the dissociation of carboxylic acids can be represented asa resonance hybrid of the following two structures :R-COO-:::R-COO-:::Resonance structures of Carboxylate Ion

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NotesChemistry of OrganicCompoundsThese structures show that the negative charge is delocalised over two oxygen atoms.Thus, the carboxylate ion gets stabilised. The greater stability of carboxylate ion facilitatesthe release of proton from the -COOH group.If you compare this situation with the alkoxide ion (RO-) obtained by the dissociation of analcohol molecule, you will see that no such resonance stabilisation is possible in the alkoxideion.Let us now analyse the acid strength of different acids and correlate them with theirstructure. If we examine the first five acids listed in table 29.2, we find that theirpKavalues keep on increasing which means that as we go down, their acid strength decreases.Since the alkyl groups are electron releasing in nature, they make the release of H+difficult and hence decrease the acidity. Thus, ethanoic acid is less acidic than methanoicacid. Therefore, we can say that theelectron-donating substitutents decrease theacidity of carboxylic acids.Let us next see what will be the effect of electron withdrawing substituents such ashalogens and nitro-group on the acidity. The comparison ofpKa values of ethanoic acid(4·76) and chloroethanoic acid (2·86) suggests that chloroethanoic acid is a stronger acidthan ethanoic acid. The chloro substituent hasI effect and pulls the electrons towardsitself which facilitates the release of H+ ions.You can also see below that as the number of halogen groups increases in the carboxylicacid, its acidity increases. This is because they make the release of H+ ion more and moreeasy.H-C-COOHHHCl-C-COOHHHCl-C-COOHHClCl-C-COOHClCl

Ethanoic acid4.76Chloroethanoic acid2.86Dichloroethanoic acid1.48Trichloroethanoic acid0.70Acidity increasespKa<<<

Since the inductive effect decreases with increase in the distance of the group in thecarbon chain, 2-chlorobutanoic acid (pKa 2·86) is more acidic than 3-chlorobutanoic acid(pKa 4.05) which is in turn more acidic than 4-chlorobutanoic acid (pKa 4.50).CH CH CHCOOH3 2Cl

Acidity increases(or decreases in the reverse order)

CH CHCH COOH3 2Cl>CH CH CH COOH2 22Cl>2-Chlorobutanoic acid3-Chlorobutanoic acid4-Chlorobutanoic acid29.2.5 Reactions of Carboxylic AcidsLet us now study the reactions given by carboxylic acids.1. Formation of SaltsCarboxylic acids are completely deprotonated by strong bases such as metal hydroxidesto give salts.

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Chemistry of OrganicCompoundsOCH3C-O-H+NaOHO-CH3C-ONa+H2O+Ethanoic acidSodium ethanoateIt will be interesting to know thatsoaps are sodium salts of long chain carboxylic acidswhich are calledfatty acids.OCH3(CH2)16-C-OH+NaOHO-CH3(CH2)16-C-ONa+H2O+Stearic acidSodium stearate (soap)Caboxylic acids are also deprotonated by the weak bases such as sodium bicarbonate. Inthis reaction, they form sodium salt of the acid, carbon dioxide and water.OO||||RCO3H+NaHCO RC+2 2O Na +H O+COThis reaction is also used as a test for carboxylic acids in the laboratory. The liberation of2CO in the form of bubbles on treatment with3NaHCO indicates a carboxyl functionalgroup in the compound.This test is not given by phenols since they are weaker acids than the carboxylic acids. Hence,the two categories of compounds can be distinguished on the basis of the above test.2. Reduction of Carboxylic AcidsCarboxylic acids are reduced to primary alcohols by lithium aluminium hydride4(LiA lH).3. Hell-Volhard-Zelinski ReactionSimilar to aldehydes and ketones, carboxylic acids undergo halogenation at-carbonatom using2 2Br (or Cl) in the presence of phosphorus or phosphorus trihalide.CH3CH2CH2COOHButanoic acid221 Br ,P2 H OBrCH3CH2CHCOOH2 - Bromobutanoic acid-Haloacids so obtained are useful intermediates in the synthesis of other organiccompounds.4. Synthesis of Acid DerivativesThis is one of the very important reactions of carboxylic acids. The nucleophilic additionto the carboxyl carbon of the carboxylic acids is followed by elimination of the leavinggroup leading to asubstitution product. If you remember the reactions of aldehydesand ketones, the addition of nucleophile is followed by addition of the proton to give anaddition product.

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NotesChemistry of OrganicCompoundsIn case of carboxylic acids, since the substitution takes place at the acyl carbon atom asshown below. It is also known asnucleophilic acyl substitution.O O|| ||RCX :Nu RCNu XHere, X = OH in case of the carboxylic acids andNu can be a halide ion,O||OCR, OR or2NH group leading, respectively tocarboxyl acid halides,anhydrides,estersoramides as the substitution products which are known asderivatives of carboxylic acids because they are derived from carboxylic acids.

(i) Formation of Acid ChloridesCarboxylic acids react with2 3SOCl ,PCl or5PCl to give carboxylic acid chlorides alsoknown asacyl chlorides, as shown below :RO||C2OH SOCl R O||C2Cl SO HCl CarboxylicThionylan acidacidchloridechloride3 3 3O O|| ||R-C-OH PCl 3R CCl H PO PhosphorustrichlorideO O|| ||RC5OH + PCl RC3Cl + POCl + HClPhosphoruspentachloride(ii) Formation of Acid Anhydrides2 52P OH OO||2RCOOH R COO||CRa Carboxylic acid an Acid Anhydride

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MODULE - 7Chemistry

Notes

Chemistry of OrganicCompounds2 52P O3 3 3H OO O|| ||2 CH COOH CH C O C CH Ethanoic anhydrideSince the carboxylic acid anhydrides are formally derived from carboxylic acids by loss ofwater, their names are derived from the corresponding acids by using the wordanhydridein place of the acid. As the anhydride formed in the above reaction is derived from ethanoicacid, it is called ethanoic anhydride.This method is used for thepreparation of symmetrical anhydrides.Carboxylic acid also react with acyl chlorides in the presence of pyridine to give carboxylicacid anhydrides.3 2 2 2 2 3 3 2 2 2 2 3PyridineO O O O|| || || ||CH CH CH COH + ClCCH CH CH CH CH CH C O C CH CH CH Butanoic anhydrideButanoic acid Butanoyl chlorideWe can prepare unsymmetrical anhydrides by this method.Cylic anhydrides are obtained by the dehydration of dicarboxylic acids at higher temperature.

(iii) Formation of EstersCarboxylic acids react with alcohols to form esters.RO||C+HOH + R OH RO||C2OR +H OacidCarboxylic Alcohol EsterNote that the acid catalysed esterification is an equilibrium reaction. The equilibrium canbe shifted to the right side towards products it we are able to remove water or the esterfrom the reaction mixture. Also if we use excess of one reagent, then the equilibrium shiftstowards the right side to give the ester. Normally, we take excess of alcohol and use it asa solvent to carry out esterification.3O||CH C2 42H SO ,Δ3 3 3-H OO||OH + CH OH CH COCHEthanoicacid Methanol Methylethanoate

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