Aldehydes, Ketones and Carboxylic AcidsClass 12 Chemistry Notes

Aldehydes, Ketones and Carboxylic Acids

Organic compounds containing a carbon-oxygen double bond ($>\text{C}=\text{O}$), known as the carbonyl group, are central to organic chemistry. This group is one of the most important functional groups.

• In aldehydes, the carbonyl group is bonded to a carbon and a hydrogen atom. The general formula is R-CHO.
• In ketones, the carbonyl group is bonded to two carbon atoms. The general formula is R-CO-R'.
• In carboxylic acids, the carbonyl carbon is bonded to a hydroxyl group (-OH). This combined functional group, -COOH, is called a carboxyl group.

These compounds are widespread in nature and industry. They are found in fabrics, flavourings, plastics, and drugs. Many have pleasant fragrances and are used in perfumes and food products. For example:

• Vanillin (from vanilla beans)
• Salicylaldehyde (from meadowsweet)
• Cinnamaldehyde (from cinnamon)

Nomenclature and Structure of Carbonyl Group

Nomenclature of Aldehydes and Ketones

There are two main systems for naming aldehydes and ketones:

(a) Common Names

• Aldehydes: The common names of aldehydes are usually derived from the common names of the corresponding carboxylic acids. The ending "-ic acid" is replaced with "-aldehyde". For example, the aldehyde corresponding to acetic acid is acetaldehyde.
• The position of substituents on the carbon chain is indicated by Greek letters ($\alpha, \beta, \gamma, \delta$, etc.), where the $\alpha$-carbon is the carbon atom directly attached to the aldehyde group.
  • Example: $\text{CH}_3\text{CHO}$ (Acetaldehyde), $\text{C}_6\text{H}_5\text{CHO}$ (Benzaldehyde)
• Ketones: The common names are derived by naming the two alkyl or aryl groups attached to the carbonyl group, followed by the word "ketone".
• The position of substituents is indicated by Greek letters ($\alpha, \alpha', \beta, \beta'$, etc.), starting from the carbons next to the carbonyl group.
• Some ketones have historical names, like acetone for dimethyl ketone. Alkyl phenyl ketones are often named as "-phenone".
  • Example: $\text{CH}_3\text{COCH}_3$ (Acetone), $\text{C}_6\text{H}_5\text{COCH}_3$ (Acetophenone)

(b) IUPAC Names

• Aldehydes: The IUPAC name is derived from the corresponding alkane by replacing the ending "-e" with "-al". The carbon chain is numbered starting from the aldehyde carbon as C-1.
• When an aldehyde group is attached to a ring, the suffix "-carbaldehyde" is used.
  • Example: $\text{CH}_3\text{CHO}$ is Ethanal. Cyclohexane with a -CHO group is Cyclohexanecarbaldehyde.
• Ketones: The IUPAC name is derived from the corresponding alkane by replacing the ending "-e" with "-one". The carbon chain is numbered from the end that is nearer to the carbonyl group.
  • Example: $\text{CH}_3\text{COCH}_2\text{CH}_3$ is Butan-2-one.

Here are some examples of common and IUPAC names:

Structure of the Carbonyl Group

• The carbonyl carbon atom is $sp^2$-hybridised. It forms three sigma ($\sigma$) bonds.
• The geometry around the carbonyl carbon is trigonal planar, with bond angles of approximately $120^\circ$.
• The fourth valence electron of carbon forms a pi ($\pi$) bond with an oxygen p-orbital.
• The carbon-oxygen double bond is polar because oxygen is more electronegative than carbon. This creates a partial positive charge ($\delta+$) on the carbon and a partial negative charge ($\delta-$) on the oxygen.
• This polarity makes the carbonyl carbon an electrophilic (Lewis acid) center and the carbonyl oxygen a nucleophilic (Lewis base) center.
• The high polarity is also explained by resonance, involving a neutral structure (A) and a dipolar structure (B).

Preparation of Aldehydes and Ketones

General Preparation Methods

1. By oxidation of alcohols: Primary alcohols are oxidized to aldehydes, and secondary alcohols are oxidized to ketones.
2. By dehydrogenation of alcohols: Alcohol vapours passed over heavy metal catalysts (like Ag or Cu) yield aldehydes (from primary alcohols) and ketones (from secondary alcohols). This is an industrial method.
3. From hydrocarbons:
   • Ozonolysis of alkenes: Cleavage of alkenes with ozone, followed by reaction with zinc dust and water, gives aldehydes, ketones, or a mixture of both.
   • Hydration of alkynes: Adding water to ethyne in the presence of $\text{H}_2\text{SO}_4$ and $\text{HgSO}_4$ yields acetaldehyde. All other alkynes give ketones in this reaction.

Preparation of Aldehydes

1. From acyl chloride (Rosenmund Reduction): Acyl chlorides are hydrogenated over a catalyst (palladium on barium sulphate) to produce aldehydes. This reaction is called the Rosenmund reduction. $\text{Acyl chloride} \quad \xrightarrow{\text{H}_2, \text{Pd-BaSO}_4} \quad \text{Aldehyde}$
2. From nitriles and esters:
   • Stephen Reaction: Nitriles are reduced to an imine with stannous chloride ($\text{SnCl}_2$) and hydrochloric acid (HCl). The imine is then hydrolysed to give the corresponding aldehyde. $\text{RCN} + \text{SnCl}_2 + \text{HCl} \rightarrow \text{RCH=NH} \xrightarrow{\text{H}_3\text{O}^+} \text{RCHO}$
   • Using DIBAL-H: Nitriles can be selectively reduced to aldehydes using diisobutylaluminium hydride (DIBAL-H) followed by hydrolysis. Esters are also reduced to aldehydes using DIBAL-H.
3. From hydrocarbons (Aromatic Aldehydes):
   • Etard Reaction: Chromyl chloride ($\text{CrO}_2\text{Cl}_2$) oxidizes the methyl group of toluene (or its derivatives) to a chromium complex, which on hydrolysis gives benzaldehyde.
   • Using Chromic Oxide ($\text{CrO}_3$): Toluene reacts with chromic oxide in acetic anhydride to form benzylidene diacetate, which is then hydrolysed to benzaldehyde.
   • Side Chain Chlorination: Chlorination of toluene in the presence of sunlight gives benzal chloride ($\text{C}_6\text{H}_5\text{CHCl}_2$), which upon hydrolysis yields benzaldehyde. This is a commercial method.
   • Gatterman-Koch Reaction: Benzene or its derivatives are treated with carbon monoxide (CO) and HCl in the presence of anhydrous aluminium chloride ($\text{AlCl}_3$) or cuprous chloride (CuCl) to form benzaldehyde.

Preparation of Ketones

1. From acyl chlorides: Acyl chlorides react with dialkylcadmium ($\text{R}_2\text{Cd}$), which is prepared from a Grignard reagent, to produce ketones. $2\text{R'COCl} + \text{R}_2\text{Cd} \rightarrow 2\text{R'COR} + \text{CdCl}_2$
2. From nitriles: Treating a nitrile with a Grignard reagent, followed by hydrolysis, yields a ketone.
3. From benzene (Friedel-Crafts Acylation): Benzene or a substituted benzene reacts with an acid chloride or anhydride in the presence of anhydrous aluminium chloride ($\text{AlCl}_3$) to form the corresponding ketone.

Solution

(i) $\text{C}_5\text{H}_5\text{NH}^+\text{CrO}_3\text{Cl}^-$ (PCC - Pyridinium chlorochromate) (ii) Anhydrous $\text{CrO}_3$ (iii) $\text{CrO}_2\text{Cl}_2$ followed by hydrolysis (Etard reaction) or $\text{CrO}_3$ in acetic anhydride followed by hydrolysis. (iv) Diisobutylaluminium hydride (DIBAL-H) followed by hydrolysis. (v) PCC (vi) Ozonolysis ($\text{O}_3$) followed by reaction with $\text{H}_2\text{O}$ and Zn dust.

Physical Properties

• State: Methanal is a gas, ethanal is a volatile liquid, and other aldehydes and ketones are liquids or solids at room temperature.
• Boiling Points: Aldehydes and ketones have higher boiling points than hydrocarbons and ethers of similar molecular masses. This is due to dipole-dipole interactions between the polar carbonyl groups. However, their boiling points are lower than those of corresponding alcohols because they cannot form intermolecular hydrogen bonds with each other.
  • Order of boiling points: Alkane < Ether < Aldehyde/Ketone < Alcohol
• Solubility: Lower aldehydes and ketones (like methanal, ethanal, propanone) are miscible with water because they can form hydrogen bonds with water molecules. Solubility decreases as the length of the alkyl chain increases. They are soluble in organic solvents like benzene, ether, and methanol.
• Odour: Lower aldehydes have sharp, pungent odours. As the molecular size increases, the odour becomes more fragrant. Many are used in perfumes and flavourings.

Solution

The molecular masses are all similar (72-74).

1. Butan-1-ol ($\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}$): Has the highest boiling point due to extensive intermolecular hydrogen bonding.
2. Butanal ($\text{CH}_3\text{CH}_2\text{CH}_2\text{CHO}$): Is a polar molecule with dipole-dipole attractions, so its boiling point is next highest.
3. Ethoxyethane ($\text{H}_5\text{C}_2\text{-O-}\text{C}_2\text{H}_5$): Is weakly polar.
4. n-Butane ($\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3$): Is nonpolar and only has weak van der Waals forces, giving it the lowest boiling point.

Increasing order of boiling points: $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3 < \text{H}_5\text{C}_2\text{-O-}\text{C}_2\text{H}_5 < \text{CH}_3\text{CH}_2\text{CH}_2\text{CHO} < \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}$

Chemical Reactions

Aldehydes and ketones undergo similar reactions due to the presence of the carbonyl group.

Nucleophilic Addition Reactions

This is the most characteristic reaction of aldehydes and ketones.

• Mechanism: A nucleophile ($\text{Nu}^-$) attacks the electrophilic carbonyl carbon. The hybridization of the carbon changes from $sp^2$ to $sp^3$, forming a tetrahedral alkoxide intermediate. This intermediate then picks up a proton from the reaction medium to form the final product.
• Reactivity: Aldehydes are generally more reactive than ketones in nucleophilic addition reactions. This is due to two main reasons:
  1. Steric Reasons: Ketones have two relatively large alkyl/aryl groups attached to the carbonyl carbon, which hinder the approach of the nucleophile. Aldehydes have only one such group and a small hydrogen atom.
  2. Electronic Reasons: The two alkyl groups in a ketone are electron-donating, which reduces the positive charge (electrophilicity) on the carbonyl carbon, making it less attractive to nucleophiles.

Important Nucleophilic Addition Reactions:

• Addition of Hydrogen Cyanide (HCN): Forms cyanohydrins.
• Addition of Sodium Hydrogensulphite ($\text{NaHSO}_3$): Forms crystalline bisulphite addition products. This reaction is useful for the separation and purification of aldehydes.
• Addition of Alcohols:
  • Aldehydes react with one equivalent of alcohol to form a hemiacetal, and with a second equivalent to form an acetal.
  • Ketones react with diols (like ethylene glycol) to form cyclic ketals.
• Addition of Ammonia and its Derivatives: Aldehydes and ketones react with compounds of the type $\text{H}_2\text{N-Z}$ in a reversible, acid-catalysed reaction. This is an addition-elimination reaction.

Reduction

• Reduction to Alcohols: Aldehydes and ketones are reduced to primary and secondary alcohols, respectively, using reducing agents like sodium borohydride ($\text{NaBH}_4$) or lithium aluminium hydride ($\text{LiAlH}_4$).
• Reduction to Hydrocarbons: The carbonyl group ($>\text{C}=\text{O}$) can be completely reduced to a methylene group ($-\text{CH}_2-$).
  • Clemmensen Reduction: Uses zinc-amalgam (Zn-Hg) and concentrated hydrochloric acid (HCl).
  • Wolff-Kishner Reduction: Uses hydrazine ($\text{NH}_2\text{NH}_2$) followed by heating with a strong base (like KOH) in a high-boiling solvent (like ethylene glycol).

Oxidation

Aldehydes and ketones differ significantly in their oxidation reactions.

• Aldehydes: Are easily oxidised to carboxylic acids by common oxidising agents ($\text{KMnO}_4$, $\text{K}_2\text{Cr}_2\text{O}_7$, etc.) and even by mild ones.
• Ketones: Are generally resistant to oxidation. They are only oxidised under vigorous conditions (strong oxidising agents, high temperatures), which involves the cleavage of carbon-carbon bonds.

Tests to Distinguish Aldehydes from Ketones:

1. Tollens' Test (Silver Mirror Test): When an aldehyde is warmed with Tollens' reagent (ammoniacal silver nitrate solution), the aldehyde is oxidised to a carboxylate anion, and the $\text{Ag}^+$ ions are reduced to metallic silver, which forms a bright silver mirror on the inside of the test tube. Ketones do not give this test. $\text{RCHO} + 2[\text{Ag}(\text{NH}_3)_2]^+ + 3\text{OH}^- \rightarrow \text{RCOO}^- + 2\text{Ag} \downarrow + 2\text{H}_2\text{O} + 4\text{NH}_3$
2. Fehling's Test: When an aliphatic aldehyde is heated with Fehling's reagent (a mixture of aqueous copper sulphate and alkaline sodium potassium tartrate), a reddish-brown precipitate of copper(I) oxide ($\text{Cu}_2\text{O}$) is formed. Aromatic aldehydes and ketones do not respond to this test. $\text{RCHO} + 2\text{Cu}^{2+} + 5\text{OH}^- \rightarrow \text{RCOO}^- + \text{Cu}_2\text{O} \downarrow + 3\text{H}_2\text{O}$

Haloform Reaction: Aldehydes and ketones with at least one methyl group attached to the carbonyl carbon (methyl ketones) are oxidised by sodium hypohalite ($\text{NaOX}$) to form a haloform ($\text{CHX}_3$) and the sodium salt of a carboxylic acid with one less carbon atom. The iodoform test (using iodine and NaOH) produces a yellow precipitate of iodoform ($\text{CHI}_3$) and is used to detect the $\text{CH}_3\text{CO-}$ group.

Reactions due to α-Hydrogen

• Acidity of α-Hydrogens: The hydrogen atoms on the carbon adjacent to the carbonyl group (the $\alpha$-carbon) are acidic. This is because the carbonyl group is strongly electron-withdrawing, and the resulting conjugate base (an enolate ion) is stabilised by resonance.

• Aldol Condensation: Aldehydes and ketones having at least one $\alpha$-hydrogen react in the presence of a dilute alkali (like NaOH) to form $\beta$-hydroxy aldehydes (aldol) or $\beta$-hydroxy ketones (ketol). This is called the Aldol reaction. The aldol/ketol product readily loses water upon heating to form an $\alpha,\beta$-unsaturated carbonyl compound. This overall reaction is the Aldol condensation. $2\text{CH}_3\text{CHO} \quad \xrightarrow{\text{dil. NaOH}} \quad \text{CH}_3\text{CH(OH)CH}_2\text{CHO} \quad \xrightarrow{\Delta} \quad \text{CH}_3\text{CH=CHCHO} + \text{H}_2\text{O}$ (Ethanal) $\quad \quad$ (3-Hydroxybutanal - an aldol) $\quad \quad$ (But-2-enal - condensation product)

• Cross Aldol Condensation: When aldol condensation is carried out between two different aldehydes and/or ketones, it is called a cross aldol condensation. If both reactants have $\alpha$-hydrogens, a mixture of four different products is formed.

Other Reactions

• Cannizzaro Reaction: Aldehydes that do not have an α-hydrogen (e.g., formaldehyde, benzaldehyde) undergo a self-oxidation and reduction reaction (disproportionation) when heated with concentrated alkali. One molecule is reduced to an alcohol, and the other is oxidised to the salt of a carboxylic acid. $2\text{HCHO} + \text{conc. KOH} \rightarrow \text{CH}_3\text{OH} + \text{HCOOK}$ (Formaldehyde) $\quad \quad$ (Methanol) (Potassium formate)

• Electrophilic Substitution Reaction: In aromatic aldehydes and ketones, the carbonyl group is a deactivating and meta-directing group for electrophilic substitution on the benzene ring.

Uses of Aldehydes and Ketones

• Formaldehyde: Used as a 40% aqueous solution called formalin to preserve biological specimens and to prepare resins like Bakelite.
• Acetaldehyde: Used to manufacture acetic acid, polymers, and drugs.
• Benzaldehyde: Used in the perfume and dye industries.
• Acetone: A common industrial solvent.
• Many others like vanillin and camphor are used for their odours and flavours.

Carboxylic Acids

Compounds containing the carboxyl group (-COOH) are called carboxylic acids. This group is a combination of a carbonyl group ($>\text{C}=\text{O}$) and a hydroxyl group (-OH).

Nomenclature and Structure of Carboxyl Group

Nomenclature

• Common Names: Many carboxylic acids are known by common names derived from their natural sources (e.g., formic acid from ants, acetic acid from vinegar).
• IUPAC Names: In the IUPAC system, the ending "-e" of the corresponding alkane is replaced with "-oic acid". The carboxyl carbon is always numbered as C-1.
  • For compounds with more than one -COOH group, prefixes like "di-", "tri-", etc., are used (e.g., Ethanedioic acid).

Structure of Carboxyl Group

The bonds to the carboxyl carbon lie in one plane with angles of about $120^\circ$. Due to resonance, the lone pair on the hydroxyl oxygen can delocalize with the carbonyl group. This makes the carboxyl carbon less electrophilic than the carbonyl carbon in aldehydes and ketones.

Methods of Preparation of Carboxylic Acids

1. From Primary Alcohols and Aldehydes: Primary alcohols and aldehydes are readily oxidised to carboxylic acids using common oxidising agents like $\text{KMnO}_4$ or $\text{K}_2\text{Cr}_2\text{O}_7$ in an acidic medium.
2. From Alkylbenzenes: Aromatic carboxylic acids are prepared by the vigorous oxidation of alkylbenzenes with agents like chromic acid or alkaline $\text{KMnO}_4$. The entire side chain is oxidised to -COOH, regardless of its length.
3. From Nitriles and Amides: Nitriles are hydrolysed to amides and then to acids in the presence of an acid ($\text{H}^+$) or base ($\text{OH}^-$) catalyst. $\text{R-CN} \xrightarrow{\text{H}^+ \text{ or } \text{OH}^-} \text{R-CONH}_2 \xrightarrow{\text{H}^+ \text{ or } \text{OH}^-} \text{R-COOH}$
4. From Grignard Reagents: Grignard reagents react with carbon dioxide (dry ice) to form salts of carboxylic acids, which give carboxylic acids upon acidification. This method is useful for creating a carboxylic acid with one more carbon atom than the starting alkyl halide. $\text{R-MgX} + \text{CO}_2 \xrightarrow{\text{Dry Ether}} \text{RCOOMgX} \xrightarrow{\text{H}_3\text{O}^+} \text{RCOOH}$
5. From Acyl Halides and Anhydrides: These compounds are hydrolysed with water to give the corresponding carboxylic acids.
6. From Esters: Acidic hydrolysis of esters directly yields carboxylic acids, while basic hydrolysis gives carboxylates, which are then acidified to form carboxylic acids.

Physical Properties

• Boiling Points: Carboxylic acids have significantly higher boiling points than aldehydes, ketones, and even alcohols of comparable molecular masses. This is due to extensive intermolecular hydrogen bonding. Most carboxylic acids exist as stable hydrogen-bonded dimers, even in the vapour phase.
• Solubility: Simple aliphatic carboxylic acids with up to four carbon atoms are miscible with water due to the formation of hydrogen bonds. Solubility decreases as the alkyl chain length increases because the hydrophobic part of the molecule becomes larger.

Chemical Reactions

Reactions Involving Cleavage of O-H Bond (Acidity)

Carboxylic acids are acidic because they can donate a proton to form a resonance-stabilised carboxylate anion.

• Reaction with Metals and Alkalies: They react with active metals to liberate hydrogen gas and with bases (like NaOH, $\text{Na}_2\text{CO}_3$, $\text{NaHCO}_3$) to form salts. [!note] The reaction with sodium bicarbonate ($\text{NaHCO}_3$), producing effervescence due to the evolution of carbon dioxide ($\text{CO}_2$), is a characteristic test for the carboxyl group.
• Acid Strength ($K_a$ and $pK_a$): The strength of an acid is indicated by its acid dissociation constant, $K_a$, or more conveniently, its $pK_a$ value, where $pK_a = -\log K_a$. A smaller $pK_a$ value indicates a stronger acid.
• Comparison of Acidity:
  • Carboxylic acids are stronger acids than alcohols and phenols.
  • This is because the conjugate base, the carboxylate ion, is highly stabilised by two equivalent resonance structures, where the negative charge is delocalised over two electronegative oxygen atoms.
  • In contrast, the phenoxide ion (conjugate base of phenol) has non-equivalent resonance structures, and the negative charge is delocalised over one oxygen and less electronegative carbon atoms, making it less stable than the carboxylate ion.
• Effect of Substituents:
  • Electron-withdrawing groups (EWGs) (like -Cl, -F, -$\text{NO}_2$, -CN) increase the acidity of carboxylic acids. They stabilise the carboxylate anion by dispersing the negative charge.
  • Electron-donating groups (EDGs) (like -$\text{CH}_3$, -$\text{C}_2\text{H}_5$) decrease the acidity. They destabilise the carboxylate anion by intensifying the negative charge.
  • Order of increasing acidity based on EWG: $\text{CF}_3\text{COOH} > \text{CCl}_3\text{COOH} > \text{NO}_2\text{CH}_2\text{COOH} > \text{FCH}_2\text{COOH} > \text{ClCH}_2\text{COOH} > \text{HCOOH} > \text{C}_6\text{H}_5\text{COOH} > \text{CH}_3\text{COOH}$

Reactions Involving Cleavage of C-OH Bond

1. Formation of Anhydride: Carboxylic acids, on heating with a mineral acid (like $\text{H}_2\text{SO}_4$) or a dehydrating agent (like $\text{P}_2\text{O}_5$), lose a molecule of water to form an acid anhydride.
2. Esterification: Carboxylic acids react with alcohols or phenols in the presence of an acid catalyst (like conc. $\text{H}_2\text{SO}_4$) to form esters. The reaction is reversible. $\text{RCOOH} + \text{R'OH} \quad \xrightarrow{\text{H}^+} \quad \text{RCOOR'} + \text{H}_2\text{O}$
3. Reactions with $\text{PCl}_5$, $\text{PCl}_3$, and $\text{SOCl}_2$: The -OH group of carboxylic acids is replaced by a chlorine atom to form acyl chlorides (RCOCl). Thionyl chloride ($\text{SOCl}_2$) is often preferred because the byproducts are gases ($\text{SO}_2$, HCl) and escape easily.
4. Reaction with Ammonia: Carboxylic acids react with ammonia ($\text{NH}_3$) to form an ammonium salt, which upon heating at high temperature, dehydrates to form an amide.

Reactions Involving the -COOH Group

• Reduction: Carboxylic acids are reduced to primary alcohols using strong reducing agents like lithium aluminium hydride ($\text{LiAlH}_4$) or, more selectively, diborane ($\text{B}_2\text{H}_6$). [!note] Sodium borohydride ($\text{NaBH}_4$) does not reduce the carboxyl group.
• Decarboxylation: The sodium salts of carboxylic acids lose carbon dioxide when heated with sodalime (a mixture of NaOH and CaO) to form hydrocarbons.

Substitution Reactions in the Hydrocarbon Part

• Halogenation (Hell-Volhard-Zelinsky Reaction): Carboxylic acids having an $\alpha$-hydrogen are halogenated at the $\alpha$-position when treated with chlorine or bromine in the presence of a small amount of red phosphorus.
• Ring Substitution: Aromatic carboxylic acids undergo electrophilic substitution. The carboxyl group is deactivating and meta-directing. They do not undergo Friedel-Crafts reactions.

Uses of Carboxylic Acids

• Methanoic Acid (Formic Acid): Used in the rubber, textile, and leather industries.
• Ethanoic Acid (Acetic Acid): Used as a solvent and as vinegar in the food industry.
• Hexanedioic Acid (Adipic Acid): Used in the manufacture of nylon-6,6.
• Sodium Benzoate: Used as a food preservative.
• Higher Fatty Acids: Used to make soaps and detergents.