27 ALDEHYDES, KETONES AND CARBOXYLIC ACIDS
MODULE - 7Aldehydes, Ketones and Carboxylic AcidsChemistry of OrganicCompounds27NotesALDEHYDES, KETONES ANDCARBOXYLIC ACIDSIn the last lesson, you studied about organic compounds containing functionalgroups having carbon-oxygen single bond. There are other classes of organiccompounds in which the functional group contains the carbon- oxygen doublebond. The examples of these classes of compounds being carbonyl compoundssuch as aldehydes and ketones as well as carboxylic acids and their derivatives.These organic compounds are very important both in the industry and in thesynthesis of other organic compounds. Therefore, their study forms an importantpart of the organic chemistry. Let us study the chemistry of these classes ofcompounds in detail.OBJECTIVESAfter reading this lesson, you should be able tozgive IUPAC names of aldehydes and ketones;zdescribe the general methods of preparation of aldehydes and ketones;discuss the trends in physical properties of the aldehydes and ketones in thelight of the polar nature of the carbonyl group;explain important reactions exhibited by aldehydes and ketones;distinguish between aldehydes and ketones on the basis of certain reactionsand tests based on them;give IUPAC names of carboxylic acids;explain general methods of preparation of carboxylic acids;zzzzzzz306discuss the physical properties and their trends for simple monocarboxylicacids;describe important reactions exhibited by carboxylic acids;CHEMISTRY
Aldehydes, Ketones and Carboxylic Acidszexplain the preparation and some interconversion reactions of carboxylicacid derivatives, andzhighlight the importance of aldehydes, ketones and carboxylic acids.MODULE - 7Chemistry of OrganicCompounds27.1 ALDEHYDES AND KETONESYou have some familiarity with these classes of compounds from previouslessons. These compounds are also referred to as carbonyl compounds and haveNotes—O—C— functionality present in them. These compounds exist widely in natureand are responsible for the flavour and aroma of many foods. They are alsoimportant industrially both as reagents in synthesis and as solvents.Aldehydes have at least one hydrogen atom bonded to the carbonyl group, theother group may be either a hydrogen or an alkyl (or aryl) group. In ketones,the carbonyl group is bonded to two alkyl or aryl groups. The two groupsbonded to a ketone may be similar or different resulting in a symmetrical oran unsymmatrical ketone, respectively.You must be familiar with vanilin and camphor. Their structures are given below.You can see that they contain an aldehyde and a keto functional group,respectively.27.1.1 Nomenclature of Aldehydes and KetonesIn the IUPAC system of nomenclature, aliphatic aldehydes are named asalkanals. The final -e in the name of the corresponding alkane is substitutedby -al.CHEMISTRY307
MODULE - 7Chemistry of OrganicCompoundsAldehydes, Ketones and Carboxylic AcidsSome common examples of aldehydes and their names are given below :NotesNote that when the -CHO group is attached to a ring, then the compound iscalled a carbaldehyde.Remember that the carbonyl carbon of the aldehydes is present at the end ofthe chain and is assigned 1 position. Therefore, it is not necessory to specifyits position in the name of the aldehyde. The examples given below illustratethis point.Ketones are named as alkanones in the IUPAC namenclature. Their names areobtained by replacing final -e in the name of alkane by -one. The carbon chainis numbered in such a way that the carbonyl group gets the lowest number. Someexamples of ketones are mentioned below :308CHEMISTRY
Aldehydes, Ketones and Carboxylic Acids27.1.2 Preparation of Aldehydes and KetonesMODULE - 7Chemistry of OrganicCompoundsYou have already studied most of the methods used in the synthesis of aldehydesand ketones 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 toaldehydes and secondary alcohols can be oxidised to ketones.Notes2. Ozonolysis of AlkenesThis reaction has been discussed in lesson 26. The products obtained arealdehydes or ketones depending upon the structure of the starting alkene.3. Hydration of AlkynesHydration of alkynes can give an aldehyde or a ketone. Markovnikov’s hydrationyields ketones whereas anti-Markovnikov’s hydration gives aldehydes.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:Similar acylation reaction using ethanoyl chloride was also discussed in lesson26 under the electrophilic substitution reactions of aromatic hydrocarbons.CHEMISTRY309
MODULE - 7Chemistry of OrganicCompoundsAldehydes, Ketones and Carboxylic Acids27.1.3 Structure and Physical PropertiesIn both aldehydes and ketones, the carbonyl carbon and oxygen atoms are sp2hybridised. Therefore, the groups attached to the carbon atom and oxygen arepresent in a plane. This is shown in Fig. 27.1.NotesFig. 27.1 : The structure of the carbonyl functional groupYou can see in the figure that a π -bond is formed by the overlap of p-orbitalsof carbon and oxygen atoms. The p-orbitals are present in a plane perpendicularto the plane of the molecule. Note the presence of two lone pairs of electronson oxygen atom.You also know that oxygen is more electronegative than carbon. Hence, it—— ——attracts the electrons of the carbon-oxygen double bond (CO bond )resulting in its appriciable polarisation.The oxygen atom, thus, acquires a partial negative charge (δ ) whereas thecarbon atom gets a partial positive charge (δ ) . This polar nature of the carbonylgroup makes the oxygen atom nucleophilic and basic while the carbon atombecomes electrophilic. The physical properties and chemical reactions ofaldehydes and ketones are a direct consequence of this polarisation.The dipole-dipole attraction between the molecules of aldehydes and ketonesresults in their higher boiling points as compared to the hydrocarbons of similarmolecular weight. The physical properties of some aldehydes and ketones aregiven in Table 27.1.Table 27.1 Physical properties of some representative aldehydes and ketones310Compoundm.p.(K)b.p.(K)Water Solubility(%)Methanal18125255Ethanal150294 Propanal19232220CHEMISTRY
MODULE - 7Aldehydes, Ketones and Carboxylic -one178329 8–Chemistry of OrganicCompoundsNotesYou can see from Table 27.1 that these compounds have appriciable watersolubility. This is because of the hydrogen bonding possible between the oxygenatom of the aldehyde (or the ketone) with hydrogen atom of water molecule,as shown in Fig 27.2.Fig. 27.2 : Hydrogen bonding between carbonyl compound and water molecule27.1.4 Reactions of Aldehydes and KetonesIf you examine the structure of a molecule of the carbonyl compound, you willfind that there are three centres of reactivity in it as shown below :Since the oxygen atom is nucleophilic in nature, it is attacked by theelectrophiles, whereas the carbonyl carbon is electrophilic in nature and henceis attacked by nucleophiles. The third site of reactivity is hydrogen atom presentat the α - carbon atom. It is acidic in nature and gives typical reactions whichyou will study in this section.It is also important to know here that aldehydes are more reactive than ketones.This is because of the following two reasons :(i) Aldehydes have only one alkyl group whereas ketones have two. Since thealkyl groups are electron donating in nature, the carbonyl carbon in ketoneswhich is bonded to two alkyl groups, is less positive (electrophilic) ascompared to the aldehydic carbonyl carbon. Hence, it is less susceptible toattack by nucleophiles.CHEMISTRY311
MODULE - 7Chemistry of OrganicCompoundsAldehydes, Ketones and Carboxylic Acids(ii) The two alkyl groups in ketones also make the carbonyl carbon morecrowded as compared to carbonyl carbon in aldehydes. This factor alsomakes the aldehydic carbonyl carbon more accessible for attack by thenucleophiles as compared to carbonyl carbon of the ketone.With this background in mind, let us now study the reactions of aldehydes andketones.NotesA. Nucleophilic Addition ReactionsThe general reaction of addition of nucleophiles on the carbonyl group can berepresented as follows :Mechanism of Nucleophilic AdditionUnder basic conditions, the nucleophile attacks at the carbonyl group yields atetrahedral intermediate. Subsequent to this, the protonation from a solvent(H2O or alcohol) takes place. This is shown below:However, in acidic additions, in the first step the protonation of the carbonyloxygen takes place.The nucleophile then attacks in the second step to carbonyl carbon and theaddition process is completed.Some specific reactions of this category are discussed below :312CHEMISTRY
Aldehydes, Ketones and Carboxylic Acids1. Formation of CyanohydrinsMODULE - 7Chemistry of OrganicCompoundsCarbonyl compounds react with hydrogen cyanide to yield cyanohydrins.NotesNote that one more carbon atom is present in the cyanohydrin as compared tothe starting carbonyl compound.Cyanohydrins are useful in the synthesis of carboxylic acids about you will studyin the next section.2. Formation of HemiacetalsAldehydes and ketones react with alcohols to give hemiacetals. Hemi in Greekmeans half. Hemiacetals have an –OH and an –OR group attached to the samecarbon atom in their molecules.When an excess of the alcohol is used, a second molecule of the alcohol reactsto give an acetal.Note 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 groupsfor aldehydes and ketones. Acetals can be converted back to the carbonylcompounds by treating them with dilute acids because of the reversible natureof the above reaction.3. Formation of AlcoholsGrignard reagents (RMgX) react with aldehydes and ketones to give alcoholsas shown below :CHEMISTRY313
MODULE - 7Aldehydes, Ketones and Carboxylic AcidsChemistry of OrganicCompoundsNotesYou have already studied these reactions under the preparation of alcohols inlesson 28.B. Addition-Elimination or Condensation Reactions1. Reaction with Ammonia and its DerivativesAldehydes and ketones react with ammonia and primary amines to give imineswhich are compounds having carbon-nitrogen double bond.It appears that during the above reaction a molecule of water is lost from theprimary amine and the carbonyl compound. The reactions with other derivativesof ammonia are given below:314CHEMISTRY
Aldehydes, Ketones and Carboxylic AcidsMODULE - 7Chemistry of OrganicCompoundsNotesThe compounds formed above are relatively insoluble solids and have characteristicmelting points. These compounds can be prepared for the unknown aldehydeor ketone and their melting points can be determined. These melting points arematched with the derivatives of already known aldehydes and ketones listed instandard tables and the carbonyl compound is thus identified.C. De-oxygenation ReactionsDe-oxygenation reactions are reactions involving removal of oxygen. Aldehydesand ketones can be reduced to the corresponding alkanes by the following tworeactions:1. Wolff-Kishner ReductionWhen an aldehyde or a ketone is heated in a basic solution of hydrazine in ahigh-boiling alcohol, then the carbonyl group gets converted to a methylene( CH 2 ) 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 avariety of oxidising agents. These reagents can be chromic acid, chromiumtrioxide, permanaganate or silver oxide. You have already read about oxidationwith some of these reagents. Silver ions selectively oxidise —CHO group. Thisforms the basis of Tollen’s test. It involves the addition of a mixture of aqueoussilver nitrate and aqueous ammonia which is known as Tollen’s reagent to theCHEMISTRY315
MODULE - 7Chemistry of OrganicCompoundsAldehydes, Ketones and Carboxylic Acidscarbonyl compound. Tollen’s reagent contains [Ag(NH3)2] complex ion. If analdehyde is present, it gets oxidised to the carboxylic acid whereas the Ag ionsare reduced to form silver metal which gets deposited on the walls of the testtube and this gives a mirror like shining appearance.NotesAldehydes are also oxidised by Fehling solution, which contain Cu 2 (cupric)ions complexed with tartarate ions as the oxidant. These Cu 2 ions are reducedby the aldehydes in alkaline medium to give a brick red precipitate of cuprousoxide.E. Reactions at α carbonThe α hydrogen in aldehydes and ketones is quite acidic and can be easilyabstrated by a strong base.The resulting anion can stabilise by resonance as shown above. It is called anenolate ion. On protonation, it gives an enol.Thus, keto form and enol form are in equilibrium. This is also known as ketoenol tautomerism.Other reactions feasible due to the presence of α- hydrogen are as follows:1. HalogenationKetones having an α- hydrogen atom react readily with halogens andα- haloketones are obtained as the product. The reaction is promoted both byacids and bases.316CHEMISTRY
MODULE - 7Aldehydes, Ketones and Carboxylic AcidsChemistry of OrganicCompounds(X 2 Cl2 , Br2 or I 2 )PropanoneBromopropanoneNotesIn presence of the base, multiple halogenation occurs to give the trihalo product.The trihalo group is a good leaving group and the trihalo ketone reacts withOH– which finally gives a carboxylate ion and a haloform.This reaction is called the haloform reaction after the name of the product.If iodine is used as the halogen, then we get iodoform (CHI3) as the product.The iodoform is a bright yellow solid having a characterstic melting point. Thisreaction, thus, forms the basis of the iodoform test. Thus, methyl ketones givea positive iodoform test. You had studied the iodoform formation in lesson 27also.2. Aldol CondensationAldehydes having α hydrogen atom on reaction with dil. NaOH give aldols.The reaction is illustrated below by using ethanal as the example.Note that the product contains both the aldehyde and the alcohol functionalgroups and therefore, it is called an aldol. The aldol addition product on heatingundergoes dehydration to give an α, β– unsaturated aldehyde which is acondensation product.CHEMISTRY317
MODULE - 7Aldehydes, Ketones and Carboxylic AcidsChemistry of OrganicCompoundsThis complete sequence of reactions is called aldol condensation.NotesAldol condensation is also possible with ketones. Can you now think of a littlemore complex situation? What will be the products of aldol condensation whentwo diffrent aldehydes having α hydrogen atoms are used as reactants. In thiscase, the reaction is called a crossed-aldol condensation. This is left as anexercise for you. There is a hint of course. Suppose, the two aldehyde moleculesare represented by A and B; then condensation can occur between two moleculesof the same aldehyde or diffrent aldehydes. Thus, the products obtained wouldbe the following types:A-A,B-B,A-BandB-A.With this background in mind, you can now proceed to write the aldol additionproducts of ethanal and propanal.3. Cannizzaro ReactionAromatic or aliphatic aldehydes which do not have α-hydrogen, an reaction withconc. KOH/NaOH give one molecule of and alcohol and sodium salt of thecarboxylic acids. This reaction is known as Carnizzaro reaction.In this reaction, one molecule of the aldehyde is oxidised to carboxylic acid salt(sod. or pot. salt) and another molecule of the aldehyde is reduced to alcoholmolecule. For example,318CHEMISTRY
Aldehydes, Ketones and Carboxylic AcidsThis reaction takes place by nucleophilic addition of —OH to an aldehyde togive tetrahedral intermediate. This intermediate expels a hydride ion as theleaving group.MODULE - 7Chemistry of OrganicCompoundsNotesThis hydride ion is accepted by second molecule of aldehydeThus, we can say, that disproportonation has occurred and simultaneousoxidation and reduction has taken place to give one molecule of a carboxylicacid and are molecule of an alcohol from the two molecules of the startingaldehyde. This reaction is limited to those aldehydes only which have nohydrogen on carbon next to the CHO group.INTEXT QUESTIONS 27.11. Classify the following as aldehydes or ketones and give their IUPAC names:(i) CH3CHO(ii) CH3COCH 2 CH3COOH3(iii)CHEMISTRY(iv) OHCCH 2 CH3319
MODULE - 7Chemistry of OrganicCompoundsNotesAldehydes, Ketones and Carboxylic Acids2. How will you prepare propanone from propyne ?3. Why are aldehydes more reactive than ketones towards nucleophilic additionreactions?4. Write the general structure for the following :(i) a cyanohydrin(ii) an acetal(iii) a hemiacetal5. How can you convert the carbonylgroup togroup ?6. What is an aldol?7. Write the equation for cannizzaro reaction using a suitable example.27.2 CARBOXYLIC ACIDSYou already know that carboxylic acids contain a carboxyl ( COOH) funtionalgroup. They are most widely distributed in nature and are also industriallyimportant chemicals. Acetic acid in the form of vinegar is produced in largequantities. It is also a very important building block in complex biologicalmolecules. You must have also heard about fatty acids which are long chainaliphatic acids derived from the hydrolysis of fats and oils. Stearic acid is a fattyacid containing a long chain of eighteen carbon atoms.27.2.1 NomenclatureSeveral carboxylic acids have been known since long and their common namesare based on their sources. However, in the IUPAC nomenclature, carboxylicacids are named by choosing the longest carbon chain containing the COOHgroup. The final -e in the name of the alkane is replaced by -oic acid. Whilenumbering the carbon chain, the COOH carbon is always given number 1 asshown below :The other groups and substituents are numbered and named according to theusual rules of nomenclature which you have already studied.Some common carboxylic acids and their names are given below :OO H C OHMethanoic acid(Formic acid)O CH 3 C OHEthanoic acid(Acetic acid)COHPhenylmethanoic acid(Benzoic acid)Carboxylic acids containing two carboxyl groups are called dicarboxylic acids.320CHEMISTRY
MODULE - 7Aldehydes, Ketones and Carboxylic AcidsThey are named by adding dioic acid as a suffix to the name of the correspondinghydrocarbon. Both the carboxyl carbon atoms are numbered as a part of themain chain. Note that in this case, final- e of the alkane is not dropped.OO HO C C OH1 2Ethanedioic acid(Oxalic acid)OO HOC CH 2 COHPropanedioic acid(Malonic acid)432Chemistry of OrganicCompounds1HOO CC H 2C H 2C OOHButane-1,4-dioic acid(Succinic acid)Notes27.2.2 Preparation of Carboxylic AcidsThe following methods are generally used for the synthesis of carboxylic acids.You have already studied some of these methods in the earlier lessons.1. Oxidation of AlkenesAlkenes on oxidation with hot alkaline KMnO4 yield carboxylic acids.1 KMnO , OH2 H3O4RCH CHR ′ RCOOH R ′COOH 2. Oxidation of Alcohols and AldehydesYou have read in the last lesson and the previous section of this lesson thatalcohols and aldehydes can be oxidized to carboxylic acids using a variety ofoxidising agent. You can refer back to the details of these reactions.3. Oxidation of AlkylbenzenesPrimary and secondary alkyl groups attached to the benzene ring can be oxidised,using alkaline KMnO4 , to the carboxyl group.Acidified sodium dichromate can also be used for this oxidation.CHEMISTRY321
MODULE - 7Aldehydes, Ketones and Carboxylic AcidsChemistry of OrganicCompoundsNotes4. Carbonation of Grignard ReagentsGrignard reagents (RMgX) react with carbon dioxide to give magnesiumcarboxylat
MODULE - 7 Aldehydes, Ketones and Carboxylic Acids Chemistry of Organic Compounds 27.1.3 Structure and Physical Properties In both aldehydes and ketones, the carbonyl carbon and oxygen atoms are sp2 hybridised. Therefore, the groups attached to the carbon atom and oxygen are present in a plane. This is shown in Fig. 27.1.
2. Reduction: (i) Reduction of aldehydes and ketones to primary or secondary alcohol using sodium borohydride or lithium aluminum hydride. (ii) Reduction of aldehydes or ketones to hydrocarbons using Clemmenson reduction or Wolff-Kishner reduction Clemmensen reduction Wolff-Kishner reduction 3. Oxidation: Aldehydes can be easily oxidized to carboxylic acids using nitric acid, potassium
UNIT 12 ALDEHYDES KETONES AND CARBOXYLIC ACIDS 1) What are aldehydes ? 1 Aldehydes are the organic compounds containing carbonyl group,linked with one hydrogen and one alkyl /aryl group. 2) What are carboxylic acids? 1 Carboxylic acids are the organic compounds containing carboxyl(-COOH) group/s 3) Between aldehyde and ketones which one is .
Class XII Chapter 12 – Aldehydes Ketones and Carboxylic Acids Chemistry Page 7 of 41 Website: www.vidhyarjan.com Email: contact@vidhyarjan.com Mobile: 9999 249717 Head Office: 1/3-H-A-2, Street # 6, East Azad Nagar, Delhi-110051 (One Km from ‘Welcome’ Metro Station) Write the IUPAC names of the following ketones and aldehydes.
Aldehydes and Ketones Aldehydes Ketones Carboxylic acids R C H O Carboxylic acid derivatives: Esters Anhydrides Acid halides Amides We begin our study of carbonyl compounds with the study of aldehydes and ketones (the aldehyde/ketone oxidation level). ÐCarbonyl compounds are molecules containing the carbonyl group, C O. These .
Lower members of aldehydes and ketones ( upto C 4) are soluble in water due to H-bonding between polar carbonyl group and water. However, solubility decreases with increase in molecular weight Aromatic aldehydes and ketones are much less soluble than corresponding aliphatic aldehydes and ketones due to large benzene ring.
The lower aldehydes and ketones are soluble in water. Because aldehydes and ketones form hydrogen bonds with water. As the hydrocarbon portion of the molecule increases, the solubility in water decreases rapidly. Aldehydes and ketones with more than six carbons are essentially insoluble in water. CHEM 245 AE 12
Reactions of Aldehydes and Reactions of Aldehydes and Ketones: Ketones: Nucleophilic Additions (Review) ((1188--12)12) Reduction We have already seen that aldehydes and ketones can be reduced to the corresponding alcohols. NaBH 4 or LiAlH 4 can be used. 226
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