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After studying this Unit, you will be able to
- write the common and IUPAC names of aldehydes, ketones and carboxylic acids;
- write the structures of the compounds containing functional groups namely carbonyl and carboxyl groups;
- describe the important methods of preparation and reactions of these classes of compounds;
- correlate physical properties and chemical reactions of aldehydes, ketones and carboxylic acids, with their structures;
- explain the mechanism of a few selected reactions of aldehydes and ketones;
- understand various factors affecting the acidity of carboxylic acids and their reactions;
- describe the uses of aldehydes, ketones and carboxylic acids.
Objectives
Carbonyl compounds are of utmost importance to organic chemistry. They are constituents of fabrics, flavourings, plastics and drugs.
UnitUnitUnit UnitUnit
AldehydesAldehydesAldehydes,AldehydesAldehydes,,,, KKKKKeeeeetonestonestonestonestones
andandandandand CarboxylicCarboxylicCarboxylicCarboxylicCarboxylic
AAAcidsAAcidscidscidscids
AldehydesAldehydesAldehydesAldehydesAldehydes,,,,, KKKKeKeeetonesetonestonestonestones
andandandandand CarboxylicCarboxylicCarboxylicCarboxylicCarboxylic
AAAAAcidscidscidscidscids
In the previous Unit, you have studied organic compounds with functional groups containing carbon- oxygen single bond. In this Unit, we will study about the organic compounds containing carbon-oxygen double bond (>C=O) called carbonyl group, which is one of the most important functional groups in organic chemistry. In aldehydes, the carbonyl group is bonded to a carbon and hydrogen while in the ketones, it is bonded to two carbon atoms. The carbonyl compounds in which carbon of carbonyl group is bonded to carbon or hydrogen and oxygen of hydroxyl moiety (-OH) are known as carboxylic acids, while in compounds where carbon is attached to carbon or hydrogen and nitrogen of -NH 2 moiety or to halogens are called amides and acyl halides respectively. Esters and anhydrides are derivatives of carboxylic acids. The general formulas of these classes of compounds are given below:
Chemistry 228
Aldehydes, ketones and carboxylic acids are widespread in plants and animal kingdom. They play an important role in biochemical processes of life. They add fragrance and flavour to nature, for example, vanillin (from vanilla beans), salicylaldehyde (from meadow sweet) and cinnamaldehyde (from cinnamon) have very pleasant fragrances.
Nomenclature
I. Aldehydes and ketones Aldehydes and ketones are the simplest and most important carbonyl compounds. There are two systems of nomenclature of aldehydes and ketones. (a) Common names Aldehydes and ketones are often called by their common names instead of IUPAC names. The common names of most aldehydes are derived from the common names of the corresponding carboxylic acids [Section 8.6.1] by replacing the ending – ic of acid with aldehyde. At the same time, the names reflect the Latin or Greek term for the original source of the acid or aldehyde. The location of the substituent in the carbon chain is indicated by Greek letters a, b, g, d, etc. The a-carbon being the one directly linked to the aldehyde group, b- carbon the next, and so on. For example
8.18.18.18.18.1 Nomenclature and Structure of Carbonyl GroupNomenclature and Structure of Carbonyl GroupNomenclature and Structure of Carbonyl GroupNomenclature and Structure of Carbonyl GroupNomenclature and Structure of Carbonyl Group
They are used in many food products and pharmaceuticals to add flavours. Some of these families are manufactured for use as solvents (i.e., acetone) and for preparing materials like adhesives, paints, resins, perfumes, plastics, fabrics, etc.
Chemistry 230
Aldehydes HCHO Formaldehyde Methanal CH 3 CHO Acetaldehyde Ethanal (CH 3 ) 2 CHCHO Isobutyraldehyde 2-Methylpropanal
g-Methylcyclohexanecarbaldehyde 3-Methylcyclohexanecarbaldehyde
CH 3 CH(OCH 3 )CHO a-Methoxypropionaldehyde 2-Methoxypropanal CH 3 CH 2 CH 2 CH 2 CHO Valeraldehyde Pentanal CH 2 =CHCHO Acrolein Prop-2-enal
Phthaldehyde Benzene-1,2-dicarbaldehyde
m -Bromobenzaldehyde 3-Bromobenzaldehyde
Ketones CH 3 COCH 2 CH 2 CH 3 Methyl n -propyl ketone Pentan-2-one (CH 3 ) 2 CHCOCH(CH 3 ) 2 Diisopropyl ketone 2,4-Dimethylpentan-3-one
a-Methylcyclohexanone 2-Methylcyclohexanone
(CH 3 ) 2 C=CHCOCH 3 Mesityl oxide 4-Methylpent-3-en-2-one
Table 8.1: Common and IUPAC Names of Some Aldehydes and Ketones
Structure Common name IUPAC name
The common and IUPAC names of some aldehydes and ketones are given in Table 8.1.
or
3-Bromobenzenecarbaldehyde
231 Aldehydes, Ketones and Carboxylic Acids
The carbonyl carbon atom is sp^2 -hybridised and forms three sigma (s) bonds. The fourth valence electron of carbon remains in its p -orbital and forms a p-bond with oxygen by overlap with p -orbital of an oxygen. In addition, the oxygen atom also has two non bonding electron pairs. Thus, the carbonyl carbon and the three atoms attached to it lie in the same plane and the p-electron cloud is above and below this plane. The bond angles are approximately 120° as expected of a trigonal coplanar structure (Figure 8.1).
8.1.2 Structure of the Carbonyl Group
π
Fig.8.1 Orbital diagram for the formation of carbonyl group
The carbon-oxygen double bond is polarised due to higher electronegativity of oxygen relative to carbon. Hence, the carbonyl carbon is an electrophilic (Lewis acid), and carbonyl oxygen, a nucleophilic (Lewis base) centre. Carbonyl compounds have substantial dipole moments and are polar than ethers. The high polarity of the carbonyl group is explained on the basis of resonance involving a neutral (A) and a dipolar (B) structures as shown.
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8.1 Write the structures of the following compounds. (i) a-Methoxypropionaldehyde (ii) 3-Hydroxybutanal (iii) 2-Hydroxycyclopentane carbaldehyde (iv) 4-Oxopentanal (v) Di-sec. butyl ketone (vi) 4-Fluoroacetophenone
Some important methods for the preparation of aldehydes and ketones are as follows:
1. By oxidation of alcohols Aldehydes and ketones are generally prepared by oxidation of primary and secondary alcohols, respectively (Unit 7, Class XII). 2. By dehydrogenation of alcohols This method is suitable for volatile alcohols and is of industrial application. In this method alcohol vapours are passed over heavy metal catalysts (Ag or Cu). Primary and secondary alcohols give aldehydes and ketones, respectively (Unit 7, Class XII). 3. From hydrocarbons ( i ) By ozonolysis of alkenes: As we know, ozonolysis of alkenes followed by reaction with zinc dust and water gives aldehydes,
8.2.1 Preparation of Aldehydes and Ketones
8.28.28.28.28.2 PreparationPreparationPreparationPreparationPreparation ofofofofof AldehydesAldehydesAldehydesAldehydesAldehydes
andandandandand KetonesKetonesKetonesKetonesKetones
233 Aldehydes, Ketones and Carboxylic Acids
This reaction is called Etard reaction. (b) Use of chromic oxide (CrO 3 ): Toluene or substituted toluene is converted to benzylidene diacetate on treating with chromic oxide in acetic anhydride. The benzylidene diacetate can be hydrolysed to corresponding benzaldehyde with aqueous acid.
(iii) By Gatterman – Koch reaction When benzene or its derivative is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride or cuprous chloride, it gives benzaldehyde or substituted benzaldehyde.
(ii) By side chain chlorination followed by hydrolysis Side chain chlorination of toluene gives benzal chloride, which on hydrolysis gives benzaldehyde. This is a commercial method of manufacture of benzaldehyde.
This reaction is known as Gatterman-Koch reaction.
1. From acyl chlorides Treatment of acyl chlorides with dialkylcadmium, prepared by the reaction of cadmium chloride with Grignard reagent, gives ketones.
8.2.3 Preparation of Ketones
Chemistry 234
2. From nitriles Treating a nitrile with Grignard reagent followed by hydrolysis yields a ketone.
Give names of the reagents to bring about the following transformations: (i) Hexan-1-ol to hexanal (ii) Cyclohexanol to cyclohexanone (iii) p -Fluorotoluene to (iv) Ethanenitrile to ethanal p -fluorobenzaldehyde (v) Allyl alcohol to propenal (vi) But-2-ene to ethanal
(i) C 5 H 5 NH+CrO 3 Cl-(PCC) (ii) Anhydrous CrO 3 (iii) CrO 3 in the presence (iv) (Diisobutyl)aluminium of acetic anhydride/ hydride (DIBAL-H)
- CrO 2 Cl 2 2. HOH (v) PCC (vi) O 3 /H 2 O-Zn dust
Example 8.1Example 8.1 Example 8.1Example 8.1Example 8.
SolutionSolutionSolutionSolutionSolution
(C 6 H CH ) Cd + 2 CH 5 2 2 3 COCl
CH 3
NO 2
1.CrO Cl 2 2 2.H 3 O+
(iii) C C H Hg
2+ , H SO H C 3 2 4 (iv)
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8.2 Write the structures of products of the following reactions;
(i) (ii)
3. From benzene or substituted benzenes When benzene or substituted benzene is treated with acid chloride in the presence of anhydrous aluminium chloride, it affords the corresponding ketone. This reaction is known as Friedel-Crafts acylation reaction.
Chemistry 236
Since aldehydes and ketones both possess the carbonyl functional group, they undergo similar chemical reactions.
1. Nucleophilic addition reactions Contrary to electrophilic addition reactions observed in alkenes, the aldehydes and ketones undergo nucleophilic addition reactions. (i) Mechanism of nucleophilic addition reactions A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp^2 hybridised orbitals of carbonyl carbon (Fig. 8.2). The hybridisation of carbon changes from sp^2 to sp^3 in this process, and a tetrahedral alkoxide intermediate is produced. This intermediate captures a proton from the reaction medium to give the electrically neutral product. The net result is addition of Nu–^ and H+ across the carbon oxygen double bond as shown in Fig. 8.2.
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8.3 Arrange the following compounds in increasing order of their boiling points. CH 3 CHO, CH 3 CH 2 OH, CH 3 OCH 3 , CH 3 CH 2 CH 3
Fig.8.2: Nucleophilic attack on carbonyl carbon
Would you expect benzaldehyde to be more reactive or less reactive in nucleophilic addition reactions than propanal? Explain your answer. The carbon atom of the carbonyl group of benzaldehyde is less electrophilic than carbon atom of the carbonyl group present in propanal. The polarity of the carbonyl group is reduced in benzaldehyde due to resonance as shown below and hence it is less reactive than propanal.
ExampleExampleExample ExampleExample 8.38.38.38.38.
SolutionSolutionSolutionSolutionSolution
(ii) Reactivity Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in former.
8.48.48.4 8.48.4 ChemicalChemicalChemicalChemicalChemical
ReactionsReactionsReactionsReactionsReactions
237 Aldehydes, Ketones and Carboxylic Acids
(iii) Some important examples of nucleophilic addition and nucleophilic addition-elimination reactions: (a) Addition of hydrogen cyanide (HCN) : Aldehydes and ketones react with hydrogen cyanide (HCN) to yield cyanohydrins. This reaction occurs very slowly with pure HCN. Therefore, it is catalysed by a base and the generated cyanide ion (CN-) being a stronger nucleophile readily adds to carbonyl compounds to yield corresponding cyanohydrin. Cyanohydrins are useful synthetic intermediates. (b) Addition of sodium hydrogensulphite: Sodium hydrogensulphite adds to aldehydes and ketones to form the addition products. The position of the equilibrium lies largely to the right hand side for most aldehydes and to the left for most ketones due to steric reasons. The hydrogensulphite addition compound is water soluble and can be converted back to the original carbonyl compound by treating it with dilute mineral acid or alkali. Therefore, these are useful for separation and purification of aldehydes. (c) Addition of Grignard reagents: (refer Unit 7, Class XII). (d) Addition of alcohols: Aldehydes react with one equivalent of monohydric alcohol in the presence of dry hydrogen chloride to yield alkoxyalcohol intermediate, known as hemiacetals, which further react with one more molecule of alcohol to give a gem -dialkoxy compound known as acetal as shown in the reaction. Ketones react with ethylene glycol under similar conditions to form cyclic products known as ethylene glycol ketals. Dry hydrogen chloride protonates the oxygen of the carbonyl compounds and therefore, increases the electrophilicity of the carbonyl carbon facilitating
239 Aldehydes, Ketones and Carboxylic Acids
reduction] or with hydrazine followed by heating with sodium or potassium hydroxide in high boiling solvent such as ethylene glycol (Wolff-Kishner reduction).
3. Oxidation Aldehydes differ from ketones in their oxidation reactions. Aldehydes are easily oxidised to carboxylic acids on treatment with common oxidising agents like nitric acid, potassium permanganate, potassium dichromate, etc. Even mild oxidising agents, mainly Tollens’ reagent and Fehlings’ reagent also oxidise aldehydes.
Ketones are generally oxidised under vigorous conditions, i.e., strong oxidising agents and at elevated temperatures. Their oxidation involves carbon-carbon bond cleavage to afford a mixture of carboxylic acids having lesser number of carbon atoms than the parent ketone.
The mild oxidising agents given below are used to distinguish aldehydes from ketones: (i) Tollens’ test: On warming an aldehyde with freshly prepared ammoniacal silver nitrate solution (Tollens’ reagent), a bright silver mirror is produced due to the formation of silver metal. The aldehydes are oxidised to corresponding carboxylate anion. The reaction occurs in alkaline medium.
(ii) Fehling’s test: Fehling reagent comprises of two solutions, Fehling solution A and Fehling solution B. Fehling solution A is aqueous copper sulphate and Fehling solution B is alkaline sodium potassium tartarate (Rochelle salt). These two solutions are mixed in equal amounts before test. On heating an aldehyde with Fehling’s reagent, a reddish brown precipitate is obtained. Aldehydes are oxidised to corresponding carboxylate anion. Aromatic aldehydes do not respond to this test.
Bernhard Tollens (1841-1918) was a Professor of Chemistry at the University of Gottingen, Germany.
Chemistry 240
ExampleExampleExampleExampleExample 8.48.48.48.48.4 An organic compound (A) with molecular formula C 8 H 8 O forms an
orange-red precipitate with 2,4-DNP reagent and gives yellow precipitate on heating with iodine in the presence of sodium hydroxide. It neither reduces Tollens’ or Fehlings’ reagent, nor does it decolourise bromine water or Baeyer’s reagent. On drastic oxidation with chromic acid, it gives a carboxylic acid (B) having molecular formula C 7 H 6 O 2. Identify the compounds (A) and (B) and explain the reactions involved. (A) forms 2,4-DNP derivative. Therefore, it is an aldehyde or a ketone. Since it does not reduce Tollens’ or Fehling reagent, (A) must be a ketone. (A) responds to iodoform test. Therefore, it should be a methyl ketone. The molecular formula of (A) indicates high degree of unsaturation, yet it does not decolourise bromine water or Baeyer’s reagent. This indicates the presence of unsaturation due to an aromatic ring. Compound (B), being an oxidation product of a ketone should be a carboxylic acid. The molecular formula of (B) indicates that it should be benzoic acid and compound (A) should, therefore, be a monosubstituted aromatic methyl ketone. The molecular formula of (A) indicates that it should be phenyl methyl ketone (acetophenone). Reactions are as follows:
SolutionSolutionSolutionSolutionSolution
(iii) Oxidation of methyl ketones by haloform reaction : Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom (methyl ketones) are oxidised by sodium hypohalite to sodium salts of corresponding carboxylic acids having one carbon atom less than that of carbonyl compound. The methyl group is converted to haloform. This oxidation does not affect a carbon-carbon double bond, if present in the molecule. Iodoform reaction with sodium hypoiodite is also used for detection of CH 3 CO group or CH 3 CH(OH) group which produces CH 3 CO group on oxidation.
Chemistry 242
( ii ) Cross aldol condensation: When aldol condensation is carried out between two different aldehydes and / or ketones, it is called cross aldol condensation. If both of them contain a-hydrogen atoms, it gives a mixture of four products. This is illustrated below by aldol reaction of a mixture of ethanal and propanal.
Ketones can also be used as one component in the cross aldol reactions.
5. Other reactions ( i ) Cannizzaro reaction: Aldehydes which do not have an a-hydrogen atom, undergo self oxidation and reduction (disproportionation) reaction on heating with concentrated alkali. In this reaction, one molecule of the aldehyde is reduced to alcohol while another is oxidised to carboxylic acid salt.
D
D
243 Aldehydes, Ketones and Carboxylic Acids
( ii ) Electrophilic substitution reaction : Aromatic aldehydes and ketones undergo electrophilic substitution at the ring in which the carbonyl group acts as a deactivating and meta -directing group.
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8.4 Arrange the following compounds in increasing order of their reactivity in nucleophilic addition reactions. (i) Ethanal, Propanal, Propanone, Butanone. (ii) Benzaldehyde, p -Tolualdehyde, p -Nitrobenzaldehyde, Acetophenone. Hint: Consider steric effect and electronic effect. 8.5 Predict the products of the following reactions:
(i)
(ii)
(iii)
(iv)
In chemical industry aldehydes and ketones are used as solvents, starting materials and reagents for the synthesis of other products. Formaldehyde is well known as formalin (40%) solution used to preserve biological specimens and to prepare bakelite (a phenol-formaldehyde resin), urea-formaldehyde glues and other polymeric products. Acetaldehyde is used primarily as a starting material in the manufacture of acetic acid, ethyl acetate, vinyl acetate, polymers and drugs. Benzaldehyde is used in perfumery and in dye industries. Acetone and ethyl methyl ketone are common industrial solvents. Many aldehydes and ketones, e.g., butyraldehyde, vanillin, acetophenone, camphor, etc. are well known for their odours and flavours.
8.58.58.58.58.5 Uses ofUses ofUses ofUses ofUses of
AldehydesAldehydesAldehydesAldehydesAldehydes
andandandandand KetonesKetonesKetonesKetonesKetones
245 Aldehydes, Ketones and Carboxylic Acids
Benzoic acid Benzenecarboxylic acid (Benzoic acid)
Phenylacetic acid 2-Phenylethanoic acid
Phthalic acid Benzene-1, 2-dicarboxylic acid
In carboxylic acids, the bonds to the carboxyl carbon lie in one plane and are separated by about 120°. The carboxylic carbon is less electrophilic than carbonyl carbon because of the possible resonance structure shown below:
8.6.2 Structure of Carboxyl Group
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8.6 Give the IUPAC names of the following compounds: (i) Ph CH 2 CH 2 COOH (ii) (CH 3 ) 2 C=CHCOOH
(iii) (^) COOH
CH 3
(iv)
Some important methods of preparation of carboxylic acids are as follows.
1. From primary alcohols and aldehydes Primary alcohols are readily oxidised to carboxylic acids with common oxidising agents such as potassium permanganate (KMnO 4 ) in neutral, acidic or alkaline media or by potassium dichromate (K 2 Cr 2 O 7 ) and chromium trioxide (CrO 3 ) in acidic media (Jones reagent).
8.78.78.78.78.7 MethodsMethodsMethodsMethodsMethods ofofofofof
PreparationPreparationPreparationPreparationPreparation
ofofofofof CarboxylicCarboxylicCarboxylicCarboxylicCarboxylic
AcidsAcidsAcidsAcidsAcids
Jones reagent
Chemistry 246
Carboxylic acids are also prepared from aldehydes by the use of mild oxidising agents (Section 8.4).
2. From alkylbenzenes Aromatic carboxylic acids can be prepared by vigorous oxidation of alkyl benzenes with chromic acid or acidic or alkaline potassium permanganate. The entire side chain is oxidised to the carboxyl group irrespective of length of the side chain. Primary and secondary alkyl groups are oxidised in this manner while tertiary group is not affected. Suitably substituted alkenes are also oxidised to carboxylic acids with these oxidising reagents. 3. From nitriles and amides Nitriles are hydrolysed to amides and then to acids in the presence of H+^ or OH
as catalyst. Mild reaction conditions are used to stop the reaction at the amide stage.
4. From Grignard reagents Grignard reagents react with carbon dioxide (dry ice) to form salts of carboxylic acids which in turn give corresponding carboxylic acids after acidification with mineral acid.
As we know, the Grignard reagents and nitriles can be prepared from alkyl halides (refer Unit 6, Class XII). The above methods
