Aldehydes, Ketones and Carboxylic Acids - Notes

Aldehydes, Ketones, and Carboxylic Acids

Objectives

After studying this unit, you should be able to:

• Write the common and IUPAC names of aldehydes, ketones, and carboxylic acids.
• Write the structures of compounds containing carbonyl and carboxyl groups.
• Describe important methods of preparation and reactions of these compound classes.
• Correlate physical properties and chemical reactions of aldehydes, ketones, and carboxylic acids with their structures.
• Explain the mechanism of selected reactions of aldehydes and ketones.
• Understand factors affecting the acidity of carboxylic acids and their reactions.
• Describe the uses of aldehydes, ketones, and carboxylic acids.

Introduction

• Carbonyl compounds are essential in organic chemistry, found in fabrics, flavorings, plastics, and drugs.
• This unit focuses on organic compounds with a carbon-oxygen double bond (>C=O), known as the carbonyl group.
• Aldehydes: Carbonyl group bonded to a carbon and a hydrogen atom.
• Ketones: Carbonyl group bonded to two carbon atoms.
• Carboxylic acids: Carbonyl group bonded to a carbon or hydrogen and a hydroxyl $(–OH)$ group.
• Amides and acyl halides: Carbonyl group attached to carbon or hydrogen and nitrogen of $-NH_2$ moiety or to halogens.
• Esters and anhydrides: Derivatives of carboxylic acids.

8.1 Nomenclature and Structure of Carbonyl Group

• Aldehydes, ketones, and carboxylic acids are widespread in plants and animals.
• They play roles in biochemical processes and add fragrance and flavor.
• Examples: vanillin (vanilla beans), salicylaldehyde (meadow sweet), cinnamaldehyde (cinnamon).
• Used in food products, pharmaceuticals, solvents (e.g., acetone), adhesives, paints, resins, perfumes, plastics, and fabrics.

8.1.1 Nomenclature

I. Aldehydes and Ketones

• Two systems of nomenclature:
  • Common names
  • IUPAC names

(a) Common Names

• Aldehydes: Derived from carboxylic acid names by replacing "-ic acid" with "aldehyde."
• Names reflect Latin or Greek terms for the original source.
• Substituent locations use Greek letters: $\, \beta, \gamma, \delta$, etc. ($\alpha$-carbon is directly linked to the aldehyde group).

(b) IUPAC Names

• Aliphatic aldehydes and ketones: Derived from alkanes by replacing "-e" with "-al" (aldehydes) or "-one" (ketones).
• Aldehydes: Longest carbon chain numbered from the aldehyde carbon.
• Ketones: Numbering starts from the end nearer to the carbonyl group.
• Substituents are prefixed alphabetically with position numerals.
• Cyclic ketones: Carbonyl carbon is numbered as one.
• Aldehyde group attached to a ring: Suffix "carbaldehyde" is added after the cycloalkane name.
• Aromatic aldehydes: Simplest is benzenecarbaldehyde (benzaldehyde is also accepted by IUPAC).

8.1.2 Structure of the Carbonyl Group

• Carbonyl carbon is $sp^2$-hybridized, forming three sigma bonds.
• The fourth valence electron is in a p-orbital, forming a $\pi$-bond with oxygen.
• Oxygen has two non-bonding electron pairs.
• Carbonyl carbon and attached atoms lie in the same plane; $\pi$-electron cloud is above and below this plane.
• Bond angles are approximately $120^\circ$, expected for a trigonal planar structure.
• The carbon-oxygen double bond is polarized due to oxygen's higher electronegativity.
• Carbonyl carbon is electrophilic (Lewis acid); carbonyl oxygen is nucleophilic (Lewis base).
• Carbonyl compounds have substantial dipole moments and are more polar than ethers.
• High polarity explained by resonance involving neutral and dipolar structures.

8.2 Preparation of Aldehydes and Ketones

8.2.1 Preparation of Aldehydes and Ketones

1. By Oxidation of Alcohols

   • Primary alcohols yield aldehydes, and secondary alcohols yield ketones.

2. By Dehydrogenation of Alcohols

   • Suitable for volatile alcohols; used industrially.
   • Alcohol vapors passed over heavy metal catalysts (Ag or Cu).
   • Primary alcohols give aldehydes; secondary alcohols give ketones.

3. From Hydrocarbons

   (i) By Ozonolysis of Alkenes:

   • Ozonolysis followed by reaction with zinc dust and water gives aldehydes, ketones, or a mixture.
     (ii) By Hydration of Alkynes:
   • Water addition to ethyne with $H<em>2SO</em>4$ and $HgSO_4$ gives acetaldehyde.
   • Other alkynes yield ketones.

8.2.2 Preparation of Aldehydes

1. From Acyl Chloride (Acid Chloride)

   • Acyl chloride hydrogenated over palladium on barium sulfate catalyst (Rosenmund reduction).

2. From Nitriles and Esters

   • Nitriles reduced to imines with stannous chloride in the presence of hydrochloric acid (Stephen reaction).
     • Hydrolysis of imines gives aldehydes.
   • Alternatively, nitriles are reduced by diisobutylaluminium hydride (DIBAL-H) to imines, followed by hydrolysis.
   • Esters are also reduced to aldehydes with DIBAL-H.

3. From Hydrocarbons

   • Aromatic aldehydes (benzaldehyde and its derivatives) are prepared from aromatic hydrocarbons.

   (i) By Oxidation of Methylbenzene:

   *   Strong oxidizing agents oxidize toluene derivatives to benzoic acids.
   *   Oxidation can be stopped at the aldehyde stage using suitable reagents.
   *   **(a) Use of chromyl chloride (CrO2Cl2):**
       *   Chromyl chloride oxidizes the methyl group to a chromium complex, which on hydrolysis gives benzaldehyde (Etard reaction).
   *   **(b) Use of chromic oxide (CrO3):**
       *   Toluene or substituted toluene converted to benzylidene diacetate using chromic oxide in acetic anhydride.
       *   Hydrolysis yields benzaldehyde.
   

   (ii) By side chain chlorination followed by hydrolysis

   *   Side chain chlorination of toluene gives benzal chloride, hydrolyzed to benzaldehyde. Commercial method for benzaldehyde manufacture.
   

   (iii) By Gatterman – Koch reaction

   *   Benzene or its derivatives treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminum chloride or cuprous chloride yields benzaldehyde or substituted benzaldehyde.
   

8.2.3 Preparation of Ketones

1. From Acyl Chlorides

   • Acyl chlorides react with dialkylcadmium (prepared from cadmium chloride and Grignard reagent) to give ketones.

2. From Nitriles

   • Nitrile treated with Grignard reagent followed by hydrolysis yields a ketone.

3. From Benzene or Substituted Benzenes

   • Benzene or substituted benzene treated with acid chloride in the presence of anhydrous aluminum chloride affords the corresponding ketone (Friedel-Crafts acylation reaction).

8.3 Physical Properties

• Methanal is a gas at room temperature; ethanal is a volatile liquid; other aldehydes and ketones are liquid or solid.
• Boiling points of aldehydes and ketones are higher than hydrocarbons and ethers of comparable molecular masses due to dipole-dipole interactions.
• Boiling points are lower than alcohols due to the absence of intermolecular hydrogen bonding.
• Lower members (methanal, ethanal, propanone) are miscible with water due to hydrogen bonding.
• Solubility decreases with increasing alkyl chain length.
• Aldehydes and ketones are fairly soluble in organic solvents (benzene, ether, methanol, chloroform, etc.).
• Lower aldehydes have sharp, pungent odors; larger molecules have less pungent, more fragrant odors.
• Many natural aldehydes and ketones are used in perfumes and flavoring agents.

8.4 Chemical Reactions

• Aldehydes and ketones undergo similar chemical reactions due to the carbonyl group.

1. Nucleophilic Addition Reactions

• Aldehydes and ketones undergo nucleophilic addition reactions, unlike electrophilic addition in alkenes.
  (i) Mechanism of nucleophilic addition reactions

  • A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group perpendicularly to the plane of $sp^2$ hybridized orbitals.
  • Hybridization changes from $sp^2$ to $sp^3$, producing a tetrahedral alkoxide intermediate.
  • The intermediate captures a proton from the reaction medium to give the neutral product.
  • Net result: addition of $Nu^–$ and $H^+$ across the carbon-oxygen double bond.
    (ii) Reactivity

• Aldehydes are generally more reactive than ketones due to steric and electronic reasons.

  • Sterically: Two large substituents in ketones hinder the nucleophile approach.
  • Electronically: Two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively in ketones.

(iii) Some important examples of nucleophilic addition and nucleophilic addition-elimination reactions:

**(a) Addition of Hydrogen Cyanide (HCN):**
    *   Aldehydes and ketones react with HCN to yield cyanohydrins.
    *   Catalyzed by a base to generate $CN^-$, a stronger nucleophile.
    *   Cyanohydrins are useful synthetic intermediates.

**(b) Addition of Sodium Hydrogensulphite:**
    *   Sodium hydrogensulphite adds to aldehydes and ketones to form addition products.
    *   Equilibrium lies to the right for most aldehydes and to the left for most ketones due to steric reasons.
    *   The addition compound is water-soluble and can be converted back to the original carbonyl compound with dilute mineral acid or alkali.
    *   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
    *   Hemiacetals react further with one more molecule of alcohol to give a gem-dialkoxy compound known as acetal.
    *   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 increases the electrophilicity of the carbonyl carbon facilitating the nucleophilic attack of ethylene glycol.
    *   Acetals and ketals are hydrolyzed with aqueous mineral acids to yield corresponding aldehydes and ketones respectively.

**(e) Addition of Ammonia and Its Derivatives:**

    *   Nucleophiles (ammonia and its derivatives $H_2N-Z$) add to the carbonyl group.
    *   The reaction is reversible and acid-catalyzed.
    *   Equilibrium favors product formation due to rapid dehydration of the intermediate to form >C=N-Z.
    *   $Z$ = Alkyl, aryl, OH, $NH_2, C_6H_5NH, NHCONH_2$, etc.

2. Reduction

(i) Reduction to Alcohols:

• Aldehydes and ketones are reduced to primary and secondary alcohols, respectively, using sodium borohydride $(NaBH<em>4)$ or lithium aluminium hydride $(LiAlH</em>4)$, or by catalytic hydrogenation.

(ii) Reduction to Hydrocarbons:
* The carbonyl group is reduced to a $CH_2$ group using zinc-amalgam and concentrated hydrochloric acid (Clemmensen reduction) or hydrazine followed by heating with sodium or potassium hydroxide in a high boiling solvent such as ethylene glycol (Wolff-Kishner reduction).

3. Oxidation

• Aldehydes differ from ketones in their oxidation reactions; aldehydes are easily oxidized to carboxylic acids.
• Oxidizing agents: nitric acid, potassium permanganate, potassium dichromate, Tollens’ reagent, and Fehling’s reagent.
• Ketones require vigorous conditions (strong oxidizing agents, elevated temperatures) that involve carbon-carbon bond cleavage to yield a mixture of carboxylic acids with fewer carbon atoms.
• Mild oxidizing agents distinguish aldehydes from ketones:

(i) Tollens’ Test:
* Aldehyde warmed with freshly prepared ammoniacal silver nitrate solution (Tollens’ reagent) produces a bright silver mirror due to the formation of silver metal.
* Aldehydes are oxidized to corresponding carboxylate anions in alkaline medium.

(ii) Fehling’s Test:
* Fehling's reagent comprises two solutions: Fehling solution A (aqueous copper sulphate) and Fehling solution B (alkaline sodium potassium tartarate, Rochelle salt).
* The two solutions are mixed in equal amounts before test.
* Heating an aldehyde with Fehling’s reagent gives a reddish-brown precipitate.
* Aldehydes are oxidized to corresponding carboxylate anions. Aromatic aldehydes do not respond to this test.

(iii) Oxidation of Methyl Ketones by Haloform Reaction:

*   Methyl ketones are oxidized by sodium hypohalite to sodium salts of corresponding carboxylic acids with one fewer carbon atom.
*   The methyl group is converted to haloform.
*   The carbon-carbon double bond, if present in the molecule, is unaffected.
*   Iodoform reaction with sodium hypoiodite is used for detection of $CH_3CO$ group or $CH_3CH(OH)$ group which produces $CH_3CO$ group on oxidation.

4. Reactions due to $α$-hydrogen

(Acidity of $α$-hydrogens of aldehydes and ketones):

• Aldehydes and ketones undergo reactions due to the acidic nature of $\alpha$-hydrogen.

• Acidity is due to the electron-withdrawing effect of the carbonyl group and resonance stabilization of the conjugate base.
  (i) Aldol Condensation:

  • Aldehydes and ketones having at least one $\alpha$-hydrogen undergo a reaction in the presence of dilute alkali as catalyst to form $\beta$-hydroxy aldehydes (aldol) or $\beta$-hydroxy ketones (ketol), respectively.

  • Aldol and ketol readily lose water to give $\alpha, \beta$-unsaturated carbonyl compounds, which are aldol condensation products.
    (ii) Cross Aldol Condensation:

  • Aldol condensation carried out between two different aldehydes and / or ketones is called cross aldol condensation.

5. Other Reactions

(i) Cannizzaro Reaction:
* Aldehydes lacking an $\alpha$-hydrogen atom undergo self-oxidation and reduction (disproportionation) when heated with concentrated alkali.
* One molecule of the aldehyde is reduced to alcohol, and another is oxidized to a carboxylic acid salt.
(ii) Electrophilic Substitution Reaction:

• Aromatic aldehydes and ketones undergo electrophilic substitution at the ring as a deactivating and meta-directing group.

8.5 Uses of Aldehydes and Ketones

• In chemical industry aldehydes and ketones are used as solvents, starting materials and reagents for the synthesis of other products.
• Formaldehyde is 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 primarily used 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.

8.6 Nomenclature and Structure of Carboxyl Group

• Carboxylic acids contain a carboxyl functional group, $-COOH$, consisting of a carbonyl group attached to a hydroxyl group.
• May be aliphatic $(RCOOH)$ or aromatic $(ArCOOH)$, depending on the attached group.
• Higher members of aliphatic carboxylic acids $(C<em>{12} – C</em>{18})$, known as fatty acids, occur in natural fats as esters of glycerol.
• Carboxylic acids serve as starting material for several other important organic compounds such as anhydrides, esters, acid chlorides, amides, etc.
• A large number of them are known by their common names. The common names end with the suffix –ic acid and have been derived from Latin or Greek names of their natural sources
• In the IUPAC system, aliphatic carboxylic acids are named by replacing the ending –e in the name of the corresponding alkane with – oic acid.

8.6.1 Nomenclature

• In numbering the carbon chain, the carboxylic carbon is numbered one.
• For naming compounds containing more than one carboxyl group, the alkyl chain leaving carboxyl groups is numbered and the number of carboxyl groups is indicated by adding the multiplicative prefix, dicarboxylic acid, tricarboxylic acid, etc. to the name of parent alkyl chain.
• The position of –COOH groups are indicated by the arabic numeral before the multiplicative prefix. Some of the carboxylic acids along with their common and IUPAC names are listed in Table 8.3.

8.6.2 Structure of Carboxyl Group

• In carboxylic acids, the bonds to the carboxyl carbon lie in one plane and are separated by about $120^\circ$.
• The carboxylic carbon is less electrophilic than carbonyl carbon because of the possible resonance structure.

8.7 Methods of Preparation of Carboxylic Acids

1. From Primary Alcohols and Aldehydes

   • Primary alcohols are oxidized to carboxylic acids using potassium permanganate $(KMnO<em>4)$ in neutral, acidic, or alkaline media, or by potassium dichromate $(K</em>2Cr<em>2O</em>7)$ and chromium trioxide $(CrO_3)$ in acidic media (Jones reagent).
   • Carboxylic acids are also prepared from aldehydes using mild oxidizing agents.

2. From Alkylbenzenes

   • Aromatic carboxylic acids are prepared by vigorous oxidation of alkylbenzenes with chromic acid or acidic or alkaline potassium permanganate.
   • The entire side chain is oxidized to the carboxyl group, irrespective of length.
   • Primary and secondary alkyl groups are oxidized, while tertiary groups are unaffected.
   • Substituted alkenes are also oxidized to carboxylic acids with these reagents.

3. From Nitriles and Amides

   • Nitriles are hydrolyzed 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 give carboxylic acids after acidification with mineral acid.
   • Grignard reagents and nitriles can be prepared from alkyl halides.
   • These methods convert alkyl halides into carboxylic acids with one more carbon atom.
     From acyl halides and anhydrides

5. From Acyl Halides and Anhydrides

   • Acid chlorides hydrolyze with water to give carboxylic acids or with aqueous base to give carboxylate ions, which on acidification provide carboxylic acids.
   • Anhydrides hydrolyze to corresponding acid(s) with water.

6. From Esters

   • Acidic hydrolysis of esters gives carboxylic acids directly, while basic hydrolysis gives carboxylates, which on acidification give carboxylic acids.

8.8 Physical Properties

• Aliphatic carboxylic acids up to nine carbon atoms are colorless liquids at room temperature with unpleasant odors.
• Higher acids are wax-like solids and are practically odorless due to low volatility.
• Carboxylic acids have higher boiling points than aldehydes, ketones, and alcohols of comparable molecular masses due to extensive intermolecular hydrogen bonding.
• Most carboxylic acids exist as dimers in the vapor phase or in aprotic solvents.
• Simple aliphatic carboxylic acids with up to four carbon atoms are miscible in water due to hydrogen bonds with water.
• Solubility decreases with increasing number of carbon atoms.
• Higher carboxylic acids are practically insoluble in water due to increased hydrophobic interaction.
• Benzoic acid is nearly insoluble in cold water.
• Carboxylic acids are soluble in less polar organic solvents like benzene, ether, alcohol, and chloroform.

8.9 Chemical Reactions

8.9.1 Reactions Involving Cleavage of O–H Bond

• Carboxylic acids react with electropositive metals to evolve hydrogen and form salts with alkalies, similar to phenols.
• React with weaker bases such as carbonates and hydrogencarbonates to evolve carbon dioxide.
• This reaction detects the presence of the carboxyl group.
• Carboxylic acids dissociate in water to give resonance-stabilized carboxylate anions and hydronium ions.
• $K<em>{eq} = \frac{ [RCOO^-][H</em>3O^+]}{[RCOOH][H_2O]}$
• $K<em>a = K</em>{eq} [H<em>2O] = \frac{ [RCOO^-][H</em>3O^+]}{[RCOOH]}$
  $pKa = – log Ka$
• Smaller the pKa, the stronger the acid

Effect of Substituents on the Acidity of Carboxylic Acids:

• Substituents affect the stability of the conjugate base and thus the acidity of carboxylic acids.
• Electron-withdrawing groups increase acidity by stabilizing the conjugate base through delocalization of the negative charge (inductive and/or resonance effects).
• Electron-donating groups decrease acidity by destabilizing the conjugate base.
• The effect of the following groups in increasing acidity order is
• Ph < I < Br < Cl < F < CN < NO2 < CF3

8.9.2 Reactions Involving Cleavage of C–OH Bond

1. Formation of Anhydride

   • Carboxylic acids heated with mineral acids such as $H<em>2SO</em>4$ or with $P<em>2O</em>5$ give corresponding anhydride.

2. Esterification

   • Carboxylic acids are esterified with alcohols or phenols in the presence of a mineral acid such as concentrated $H<em>2SO</em>4$ or $HCl$ gas as a catalyst.

3. Reactions with $PCl<em>5, PCl</em>3$ and $SOCl_2$

   • The hydroxyl group of carboxylic acids is easily replaced by chlorine when treated with $PCl<em>5, PCl</em>3$ or $SOCl_2$.
   • Thionyl chloride $(SOCl_2)$ is preferred because the other two products are gaseous that escape the reaction mixture, making the purification of the products easier.

4. Reaction with Ammonia

   • Carboxylic acids react with ammonia to give ammonium salt which on further heating at high temperature give amides.

8.9.3 Reactions Involving –COOH Group

1. Reduction

   • Carboxylic acids are reduced to primary alcohols by lithium aluminium hydride or better with diborane.
   • Diborane does not easily reduce functional groups such as ester, nitro, halo, etc. Sodium borohydride does not reduce the carboxyl group.

2. Decarboxylation

   • Carboxylic acids lose carbon dioxide to form hydrocarbons when their sodium salts are heated with sodalime $(NaOH$ and $CaO$ in the ratio of $3 : 1)$.
   • Alkali metal salts of carboxylic acids also undergo decarboxylation on electrolysis of their aqueous solutions and form hydrocarbons having twice the number of carbon atoms present in the alkyl group of the acid (Kolbe electrolysis).

8.9.4 Substitution Reactions in the Hydrocarbon Part

1. Halogenation

   • Carboxylic acids having an $\alpha$-hydrogen are halogenated at the $\alpha$-position on treatment with chlorine or bromine in the presence of a small amount of red phosphorus to give $\alpha$-halocarboxylic acids (Hell-Volhard-Zelinsky reaction).

2. Ring Substitution

   • Aromatic carboxylic acids undergo electrophilic substitution reactions in which the carboxyl group acts as a deactivating and meta-directing group.
   • they do not undergo Friedel-Crafts reaction (because the carboxyl group is deactivating and the catalyst aluminum chloride (Lewis acid) gets bonded to the carboxyl group).

8.10 Uses of Carboxylic Acids

• Methanoic acid is used in the rubber, textile, dyeing, leather and electroplating industries.
• Ethanoic acid is used as a solvent and as vinegar in the food industry.
• Hexanedioic acid is used in the manufacture of nylon-6, 6.
• Esters of benzoic acid are used in perfumery.
• Sodium benzoate is used as a food preservative.
• Higher fatty acids are used for the manufacture of soaps and detergents.