Aldehydes, Ketones, and Carboxylic Acids - Comprehensive Notes

Aldehydes, Ketones, and Carboxylic Acids

Objectives

• Nomenclature: Naming aldehydes, ketones, and carboxylic acids using common and IUPAC names.
• Structure: Drawing structures of carbonyl and carboxyl group-containing compounds.
• Preparation: Describing methods of preparation and reactions of aldehydes, ketones, and carboxylic acids.
• Properties: Correlating physical properties and chemical reactions with their structures.
• Mechanism: Explaining mechanisms of selected reactions of aldehydes and ketones.
• Acidity: Understanding factors affecting carboxylic acid acidity and their reactions.
• Uses: Describing uses of aldehydes, ketones, and carboxylic acids.

Introduction

• Carbonyl compounds are significant in organic chemistry, found in fabrics, flavorings, plastics, and drugs.
• This unit focuses on compounds with a carbon-oxygen double bond (>C=O), 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 carbon or hydrogen and a hydroxyl group (-OH).
• Amides and Acyl Halides: Carbonyl compounds where carbon is bonded to carbon/hydrogen and nitrogen of -NH2 moiety or to halogens, respectively.
• Esters and anhydrides are carboxylic acid derivatives.

Importance

• Aldehydes, ketones, and carboxylic acids are widespread in plants and animals.
• They play roles in biochemical processes and contribute fragrances and flavors.
• Examples: vanillin (vanilla beans), salicylaldehyde (meadowsweet), cinnamaldehyde (cinnamon).
• Used in food products and pharmaceuticals for flavor.
• Used as solvents (e.g., acetone) and in preparing adhesives, paints, resins, perfumes, plastics, fabrics.

Nomenclature and Structure of Carbonyl Group

Nomenclature

Aldehydes and Ketones

• Two systems: common names and IUPAC names.

Common Names

• Most aldehyde common names are derived from corresponding carboxylic acids by replacing "-ic acid" with "aldehyde."
• Names reflect Latin or Greek terms for the original source.
• Substituent location in the carbon chain indicated by Greek letters (α, β, γ, δ, etc.).
• α-carbon: directly linked to the aldehyde group.

Ketones

• Ketone common names are derived by naming the two alkyl or aryl groups bonded to the carbonyl group.
• Substituent locations indicated by Greek letters (α, α', β, β', etc.) starting from carbons next to the carbonyl group (indicated as ```). Some ketones have historical common names; dimethyl ketone is acetone.
• Alkyl phenyl ketones: acyl group name as a prefix to "phenone."

IUPAC Names

• Open-chain aliphatic aldehydes and ketones: derive from corresponding alkanes by replacing "-e" with "-al" and "-one," respectively.
• Aldehydes: longest carbon chain numbered from the aldehyde carbon.
• Ketones: numbering begins from the end nearer to the carbonyl group.
• Substituents prefixed alphabetically with numerals indicating positions.
• Cyclic ketones: carbonyl carbon is numbered as one.
• Aldehyde group attached to a ring: suffix "carbaldehyde" added after the cycloalkane name.
• Numbering starts from the carbon attached to the aldehyde group.
• Simplest aromatic aldehyde (benzene ring with aldehyde group): "benzenecarbaldehyde," but benzaldehyde is also accepted.
• Other aromatic aldehydes are named as substituted benzaldehydes.

Structure of the Carbonyl Group

• Carbonyl carbon atom is $sp^2$-hybridized, forming three sigma (σ) bonds.
• Fourth valence electron remains in its p-orbital, forming a π-bond with oxygen by overlap with oxygen's p-orbital.
• Oxygen atom has two non-bonding electron pairs.
• Carbonyl carbon and the three attached atoms lie in the same plane.
• π-electron cloud is above and below this plane.
• Bond angles are approximately 120°, expected of a trigonal planar structure.
• 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 (A) and dipolar (B) structures.

Preparation of Aldehydes and Ketones

General Methods

1. Oxidation of Alcohols

   • Primary alcohols yield aldehydes; secondary alcohols yield ketones.

2. Dehydrogenation of Alcohols

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

3. From Hydrocarbons

   • (i) Ozonolysis of Alkenes:

     • Ozonolysis followed by reaction with zinc dust and water gives aldehydes, ketones, or mixtures depending on the alkene's substitution pattern.

   • (ii) Hydration of Alkynes:

     • Addition of water to ethyne in the presence of $H<em>2SO</em>4$ and $HgSO_4$ gives acetaldehyde.
     • Other alkynes give ketones.

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 corresponding imine with stannous chloride in the presence of hydrochloric acid, followed by hydrolysis to aldehyde (Stephen reaction).
   • Alternatively, nitriles selectively reduced by diisobutylaluminum hydride (DIBAL-H) to imines, followed by hydrolysis.
   • Esters are also reduced to aldehydes with DIBAL-H.

3. From Hydrocarbons

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

     • (i) By Oxidation of Methylbenzene:

       • Strong oxidizing agents oxidize toluene to benzoic acids, but oxidation can be stopped at the aldehyde stage.

         • (a) Use of Chromyl Chloride ($CrO<em>2Cl</em>2$):

           • Chromyl chloride oxidizes the methyl group to a chromium complex, which on hydrolysis gives benzaldehyde (Etard reaction).

         • (b) Use of Chromic Oxide ($CrO_3$):

           • Toluene or substituted toluene converted to benzylidene diacetate with chromic oxide in acetic anhydride, followed by hydrolysis to benzaldehyde with aqueous acid.

     • (ii) By Side Chain Chlorination Followed by Hydrolysis:

       • Side-chain chlorination of toluene gives benzal chloride, yielding benzaldehyde upon hydrolysis. This is a commercial method.

     • (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 gives benzaldehyde or substituted benzaldehydes.

Preparation of Ketones

1. From Acyl Chlorides

   • Acyl chlorides treated with dialkylcadmium, prepared from cadmium chloride and Grignard reagent, gives ketones.

2. From Nitriles

   • Treating a nitrile 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).

Physical Properties of Aldehydes and Ketones

• Methanal is a gas at room temperature; ethanal is a volatile liquid; other aldehydes and ketones are liquid or solid.
• Boiling points higher than hydrocarbons and ethers of comparable molecular masses due to dipole-dipole interactions.
• Boiling points lower than alcohols of similar molecular masses due to 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).
• Lower aldehydes have sharp, pungent odors; larger molecules have less pungent, more fragrant odors.
• Many are used in blending perfumes and flavoring agents.

Chemical Reactions of Aldehydes and Ketones

Nucleophilic Addition Reactions

• Aldehydes and ketones undergo nucleophilic addition reactions.

Mechanism

• 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.

Reactivity

• Aldehydes are generally more reactive than ketones due to steric and electronic reasons.
• Sterically, two large substituents in ketones hinder nucleophile approach.
• Electronically, alkyl groups reduce carbonyl carbon's electrophilicity more in ketones.

Important Examples

• (a) Addition of Hydrogen Cyanide (HCN):

  • Yields cyanohydrins, catalyzed by a base to generate the cyanide ion ($CN^−$), a stronger nucleophile.
  • Cyanohydrins are useful synthetic intermediates.

• (b) Addition of Sodium Hydrogensulphite ($NaHSO_3$):

  • Forms addition products; the equilibrium favors aldehydes but shifts to the left for most ketones due to steric reasons.
  • Water-soluble addition compounds can be converted back to the original carbonyl compound with dilute mineral acid or alkali; useful for separation and purification.

• (c) Addition of Grignard Reagents:

  • (Refer to 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.
  • Ketones react with ethylene glycol under similar conditions to form cyclic products known as ethylene glycol ketals.
  • Dry hydrogen chloride protonates oxygen increasing the electrophilicity of the carbonyl carbon.
  • Acetals and ketals are hydrolyzed with aqueous mineral acids to yield corresponding aldehydes and ketones respectively.

• (e) Addition of Ammonia and its Derivatives:

  • Nucleophiles such as ammonia and its derivatives ($H_2N-Z$) add to the carbonyl group of aldehydes and ketones. Acidity catalyzes the reaction, which is reversible.
  • The equilibrium favors product formation due to rapid dehydration of the intermediate, leading to the formation of >C=N-Z ($Z$ = Alkyl, aryl, OH, $NH<em>2$, $C</em>6H<em>5NH$, $NHCONH</em>2$, etc.).

Reduction

• (i) Reduction to Alcohols:

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

• (ii) Reduction to Hydrocarbons:

  • Carbonyl group reduced to $CH_2$ group with zinc-amalgam and concentrated hydrochloric acid (Clemmensen reduction) or with hydrazine followed by heating with sodium or potassium hydroxide in high-boiling solvent such as ethylene glycol (Wolff-Kishner reduction).

Oxidation

• Aldehydes are easily oxidized to carboxylic acids by common oxidizing agents (nitric acid, potassium permanganate, potassium dichromate, etc.).
• Mild oxidizing agents, like Tollens’ and Fehling’s reagents, also oxidize aldehydes.
• Ketones are generally oxidized under vigorous conditions (strong oxidizing agents, elevated temperatures), involving carbon-carbon bond cleavage to give carboxylic acids with fewer carbon atoms.

Mild Oxidizing Agents

• (i) Tollens’ Test:

  • Warming an aldehyde with freshly prepared ammoniacal silver nitrate solution (Tollens’ reagent) produces a bright silver mirror due to the formation of silver metal.
  • Aldehydes oxidized to corresponding carboxylate anion in alkaline medium.

• (ii) Fehling’s Test:

  • Fehling reagent comprises Fehling solution A (aqueous copper sulphate) and Fehling solution B (alkaline sodium potassium tartarate, Rochelle salt).
  • Mixed in equal amounts before test; heating an aldehyde with Fehling’s reagent gives a reddish-brown precipitate.
  • Aromatic aldehydes do not respond to this test.

Oxidation of Methyl Ketones by Haloform Reaction

• Aldehydes and ketones with at least one methyl group linked to the carbonyl carbon atom (methyl ketones) are oxidized by sodium hypohalite to sodium salts of corresponding carboxylic acids, having one carbon atom less.
• Methyl group converted to haloform; the carbon-carbon double bond, if present, is unaffected.
• Iodoform reaction with sodium hypoiodite detects $CH<em>3CO$ or $CH</em>3CH(OH)$ groups that produce $CH_3CO$ group on oxidation.

Reactions due to α-Hydrogen

Acidity of α-Hydrogens of Aldehydes and Ketones

• Aldehydes and ketones undergo reactions due to the acidic nature of α-hydrogen, due to the strong electron-withdrawing effect of the carbonyl group and resonance stabilization of the conjugate base.

Aldol Condensation

• Aldehydes and ketones with at least one α-hydrogen undergo reactions in the presence of dilute alkali as catalyst to form β-hydroxy aldehydes (aldol) or β-hydroxy ketones (ketol), respectively. The aldol and ketol readily lose water to give α,β-unsaturated carbonyl compounds which are aldol condensation products.

Cross Aldol Condensation

• Carried out between two different aldehydes and/or ketones, resulting in a mixture of four products if both contain α-hydrogen atoms.
• Ketones can also be used as one component in cross aldol reactions.

Other Reactions

Cannizzaro Reaction

• Aldehydes without an α-hydrogen atom undergo self-oxidation and reduction (disproportionation) on heating with concentrated alkali.
• One molecule of aldehyde is reduced to alcohol, while another is oxidized to carboxylic acid salt.

Electrophilic Substitution Reaction

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

Uses of Aldehydes and Ketones

• Aldehydes and ketones are used as solvents, starting materials, and reagents in chemical industry.
• Formaldehyde is used as formalin (40%) solution to preserve biological specimens and prepare bakelite, urea-formaldehyde glues, and other polymeric products.
• Acetaldehyde is used as a starting material in the manufacture of acetic acid, ethyl acetate, vinyl acetate, polymers, and drugs.
• Benzaldehyde is used in perfumery and dye industries.
• Acetone and ethyl methyl ketone are common industrial solvents.
• Many aldehydes and ketones (butyraldehyde, vanillin, acetophenone, camphor) are valued for odors and flavors.

Nomenclature and Structure of Carboxyl Group

Nomenclature

• Carbon compounds containing a carboxyl functional group, (-COOH) are called carboxylic acids.
• Carboxylic acids can be aliphatic (RCOOH) or aromatic (ArCOOH).
• Higher members of aliphatic carboxylic acids (C12 – C18) 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.
• Many carboxylic acids are known by their common names, ending with the suffix "-ic acid" and derived from Latin or Greek names.
• In the IUPAC system, aliphatic carboxylic acids are named by replacing the ending “-e” in the name of the corresponding alkane with “- oic acid."
• In numbering the carbon chain, the carboxylic carbon is numbered one.
• The position of –COOH groups are indicated by the arabic numeral before the multiplicative prefix.

Structure of Carboxyl Group

• In carboxylic acids, bonds to the carboxyl carbon lie in one plane and are separated by about 120°.
• Carboxylic carbon is less electrophilic than carbonyl carbon because of resonance.

Methods of Preparation of Carboxylic Acids

1. From Primary Alcohols and Aldehydes

   • Primary alcohols are readily oxidized to carboxylic acids with common oxidizing agents such as 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).

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 oxidized to the carboxyl group, irrespective of length.
   • Primary and secondary alkyl groups are oxidized, while tertiary groups are not affected.
   • Suitably substituted alkenes are also oxidized to carboxylic acids with these oxidizing 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 can stop 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 corresponding carboxylic acids after acidification with mineral acid.
   • Grignard reagents and nitriles can be prepared from alkyl halides.
   • These methods are useful for converting alkyl halides into corresponding carboxylic acids with one carbon atom more.

5. From Acyl Halides and Anhydrides

   • Acid chlorides, when hydrolyzed with water, give carboxylic acids. They are more readily hydrolyzed with aqueous base to give carboxylate ions, which on acidification provide corresponding carboxylic acids.
   • Anhydrides, on the other hand, are hydrolyzed to corresponding acid(s) with water.

6. From Esters

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

Physical Properties of Carboxylic Acids

• 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 their low volatility.
• Carboxylic acids are higher boiling liquids than aldehydes, ketones, and even alcohols of comparable molecular masses due to more extensive association through intermolecular hydrogen bonding.
• Hydrogen bonds are not broken completely even in the vapor phase; most carboxylic acids exist as dimers in the vapor phase or in aprotic solvents.
• Simple aliphatic carboxylic acids having up to four carbon atoms are miscible in water due to the formation of hydrogen bonds with water.
• The solubility decreases with increasing number of carbon atoms.
• Higher carboxylic acids are practically insoluble in water due to the increased hydrophobic interaction of hydrocarbon part.
• Benzoic acid, the simplest aromatic carboxylic acid, is nearly insoluble in cold water.
• Carboxylic acids are also soluble in less polar organic solvents like benzene, ether, alcohol, chloroform, etc.

Chemical Reactions of Carboxylic Acids

Reactions Involving Cleavage of O–H Bond

• (Acidity)

  • Carboxylic acids evolve hydrogen with electropositive metals and form salts with alkalies, similar to phenols. However, unlike phenols, they react with weaker bases such as carbonates and hydrogencarbonates to evolve carbon dioxide.
  • This reaction detects the presence of the carboxyl group in an organic compound.
  • Carboxylic acids dissociate in water to give resonance-stabilized carboxylate anions and hydronium ions.

• For the reaction:

  $RCOOH + H<em>2O \rightleftharpoons RCOO^- + H</em>3O^+$

  $K<em>{eq} = \frac{[RCOO^-][H</em>3O^+]}{[RCOOH][H_2O]}$

  $K<em>a = K</em>{eq}[H<em>2O] = \frac{[RCOO^-][H</em>3O^+]}{[RCOOH]}$

  • pKa = – log Ka
  • Smaller the pKa, the stronger the acid

• Carboxylic acids are weaker than mineral acids, but stronger than alcohols and many simple phenols.

• Carboxylate ion is stabilized by two equivalent resonance structures, while phenoxide ion has non-equivalent resonance structures.

• (Effect of substituents on the acidity of carboxylic acids)

  • Substituents affect the stability of the conjugate base.

  • Electron-withdrawing groups increase acidity by stabilizing the conjugate base through delocalization of the negative charge by inductive and/or resonance effects.

  • Electron-donating groups decrease acidity by destabilizing the conjugate base.

  • Electron withdrawing group (EWG) stabilizes the carboxylate anion and strengthens the acid

  • Electron donating group (EDG) destabilizes the carboxylate anion and weakens the acid

  • Groups increasing acidity: Ph < I < Br < Cl < F < CN < NO2 < CF3

  • Direct attachment of phenyl or vinyl increases acidity due to the greater electronegativity of $sp^2$ hybridized carbon.

  • Electron-withdrawing groups on the phenyl of aromatic carboxylic acids increase acidity; electron-donating groups decrease acidity.

Reactions Involving Cleavage of C–OH Bond

1. Formation of Anhydride

   • Carboxylic acids heated with mineral acids such as H2SO4 or with P2O5 give corresponding anhydride.

2. Esterification

   • Carboxylic acids are esterified with alcohols or phenols in the presence of a mineral acid such as concentrated H2SO4 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 replaced by chlorine atom on treating with $PCl<em>5$, $PCl</em>3$ or $SOCl<em>2$. Thionyl chloride ($SOCl</em>2$) is preferred because the other two products are gaseous.

4. Reaction with Ammonia

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

Reactions Involving –COOH Group

1. Reduction

   • Carboxylic acids are reduced to primary alcohols by lithium aluminum 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).

Substitution Reactions in the Hydrocarbon Part

1. Halogenation:

   • Carboxylic acids having an α-hydrogen are halogenated at the α-position on treatment with chlorine or bromine in the presence of small amount of red phosphorus to give α-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 however, do not undergo Friedel-Crafts reaction (because the carboxyl group is deactivating and the catalyst aluminium chloride (Lewis acid) gets bonded to the carboxyl group).

Uses of Carboxylic Acids

• Methanoic acid is used in 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.