Alcohols, Phenols, Ethers, Aldehydes, Ketones, Carboxylic Acids

Preparation, Properties, Reactions, and Important Tests

This section will cover a crucial set of functional groups in organic chemistry: alcohols, phenols, ethers, aldehydes, ketones, and carboxylic acids. Understanding their preparation, physical and chemical properties, characteristic reactions, and identification tests is fundamental for excelling in competitive exams like NEET. We will explore each class of compounds systematically.

1. Alcohols

Alcohols are organic compounds characterized by the presence of one or more hydroxyl (-OH) functional groups attached to a saturated carbon atom. The general formula for a monohydric alcohol is R-OH, where R is an alkyl or substituted alkyl group.

1.1 Classification of Alcohols

Alcohols can be classified based on the number of hydroxyl groups and the nature of the carbon atom to which the -OH group is attached.

1.1.1 Based on the Number of -OH Groups:

• Monohydric Alcohols: Contain one -OH group (e.g., Methanol - CH₃OH, Ethanol - C₂H₅OH).
• Dihydric Alcohols (Glycols): Contain two -OH groups (e.g., Ethane-1,2-diol - HOCH₂CH₂OH).
• Trihydric Alcohols (Glycerol): Contain three -OH groups (e.g., Propane-1,2,3-triol - HOCH₂CH(OH)CH₂OH).
• Polyhydric Alcohols: Contain more than three -OH groups.

1.1.2 Based on the Nature of Carbon Atom:

• Primary (1°) Alcohols: The -OH group is attached to a primary carbon atom (a carbon atom bonded to only one other carbon atom). General formula: RCH₂OH. (e.g., Ethanol).
• Secondary (2°) Alcohols: The -OH group is attached to a secondary carbon atom (a carbon atom bonded to two other carbon atoms). General formula: R₂CHOH. (e.g., Propan-2-ol).
• Tertiary (3°) Alcohols: The -OH group is attached to a tertiary carbon atom (a carbon atom bonded to three other carbon atoms). General formula: R₃COH. (e.g., 2-Methylpropan-2-ol).

1.2 Preparation of Alcohols

Alcohols can be prepared through various methods, depending on the type of alcohol desired.

1.2.1 From Alkenes:

• Acid-Catalyzed Hydration: Alkenes react with water in the presence of an acid catalyst (like H₂SO₄) to form alcohols. This follows Markovnikov's rule, where the -OH group adds to the more substituted carbon.

  RCH=CH₂ + H₂O $\xrightarrow{H^+}$ RCH(OH)CH₃ (Markovnikov addition)

• Hydroboration-Oxidation: This method adds water across the double bond in an anti-Markovnikov fashion, yielding primary alcohols from terminal alkenes. It involves two steps:
  1. Reaction with borane (BH₃) or its derivatives (e.g., diborane B₂H₆) followed by oxidation with hydrogen peroxide (H₂O₂) in alkaline medium.

  RCH=CH₂ $\xrightarrow{1. BH_3/THF}$ $\xrightarrow{2. H_2O_2, OH^-}$ RCH₂CH₂OH (Anti-Markovnikov addition)

• Oxymercuration-Demercuration: This is another method for Markovnikov hydration of alkenes, but it avoids carbocation rearrangements. It involves reaction with mercuric acetate in THF followed by reduction with sodium borohydride (NaBH₄).

  RCH=CH₂ $\xrightarrow{1. Hg(OAc)_2, THF}$ $\xrightarrow{2. NaBH_4, OH^-}$ RCH(OH)CH₃

1.2.2 From Carbonyl Compounds (Aldehydes and Ketones):

Reduction of aldehydes and ketones using reducing agents like lithium aluminium hydride (LiAlH₄) or sodium borohydride (NaBH₄), or catalytic hydrogenation (H₂/Ni, Pt, Pd).

• Aldehydes yield primary alcohols: RCHO $\xrightarrow{[H]}$ RCH₂OH
• Ketones yield secondary alcohols: R₂CO $\xrightarrow{[H]}$ R₂CHOH
• Ester reduction yields primary alcohols: RCOOR' $\xrightarrow{LiAlH_4}$ RCH₂OH + R'OH

Grignard Reagents: Reaction of Grignard reagents (RMgX) with aldehydes and ketones followed by hydrolysis produces alcohols.

• Formaldehyde + RMgX $\xrightarrow{H_2O/H^+}$ RCH₂OH (Primary alcohol)
• Other aldehydes + RMgX $\xrightarrow{H_2O/H^+}$ RCH(R')OH (Secondary alcohol)
• Ketones + RMgX $\xrightarrow{H_2O/H^+}$ R₂C(R')OH (Tertiary alcohol)

1.2.3 From Carboxylic Acids and their Derivatives:

• Reduction of carboxylic acids and their esters using strong reducing agents like LiAlH₄.

  RCOOH $\xrightarrow{LiAlH_4}$ RCH₂OH

  RCOOR' $\xrightarrow{LiAlH_4}$ RCH₂OH + R'OH

1.2.4 From Alkyl Halides:

Nucleophilic substitution of alkyl halides with aqueous alkali (NaOH or KOH) or moist silver oxide (Ag₂O). This method is suitable for primary and secondary alkyl halides. Tertiary alkyl halides undergo elimination predominantly.

RX + NaOH(aq) $\rightarrow$ ROH + NaX

1.3 Properties of Alcohols

1.3.1 Physical Properties:

• Boiling Points: Alcohols have higher boiling points than corresponding alkanes, haloalkanes, and ethers of similar molecular masses. This is due to intermolecular hydrogen bonding between alcohol molecules. Boiling points increase with increasing molecular size and decrease with branching.
• Solubility: Lower alcohols (methanol, ethanol, propanol) are soluble in water due to their ability to form hydrogen bonds with water molecules. Solubility decreases with an increase in the size of the hydrophobic alkyl chain.

1.3.2 Chemical Properties (Reactions):

The reactions of alcohols are primarily due to the polar nature of the C-O and O-H bonds.

1.3.2.1 Reaction involving cleavage of O-H bond:

These reactions involve the acidic nature of the hydrogen atom of the hydroxyl group.

• Reaction with Active Metals: Alcohols react with active metals like sodium (Na), potassium (K), and aluminium (Al) to liberate hydrogen gas and form alkoxides.

  2ROH + 2Na $\rightarrow$ 2RONa + H₂

  Acidity order: Phenols > Tertiary Alcohols > Secondary Alcohols > Primary Alcohols > Water > Acetylene > Ammonia > Alkanes.

  Mnemonic for Acidity Order: Think of it like a "Hot Tea Party For Very Annoyed Squirrels And Apes".
  Hot (H₂O)
  Tea (Tertiary Alcohol)
  Party (Phenol)
  For (Formaldehyde)
  Very (Vinyl Alcohol)
  Annoyed (Ammonia)
  Squirrels (Secondary Alcohol)
  And (Alkanes)
  Apes (Acetylene)
  
  *Note: This mnemonic is a simplified representation and the actual acidity order is more nuanced. The generally accepted order for simple alcohols/phenols is: Phenols > Primary > Secondary > Tertiary. Water's acidity is comparable to primary alcohols.
  Corrected order for simple alcohols/phenols: Phenols > Primary > Secondary > Tertiary. Water's acidity is often between primary and secondary.
• Esterification: Alcohols react with carboxylic acids in the presence of an acid catalyst (like concentrated H₂SO₄ or HCl) to form esters. This is a reversible reaction.

  RCOOH + R'OH $\rightleftharpoons$ RCOOR' + H₂O (Fischer Esterification)

• Reaction with Acyl Halides and Acid Anhydrides: Alcohols react with acyl halides and acid anhydrides to form esters, usually in the presence of a base like pyridine.

  RCOCl + R'OH $\xrightarrow{Pyridine}$ RCOOR' + HCl

  (RCO)₂O + R'OH $\xrightarrow{Pyridine}$ RCOOR' + RCOOH

1.3.2.2 Reaction involving cleavage of C-O bond:

These reactions are characteristic of alcohols and involve the breaking of the bond between carbon and oxygen.

• Dehydration: Alcohols undergo dehydration to form alkenes (at high temperatures, ~443 K, with excess acid) or ethers (at lower temperatures, ~413 K, with less acid).
  • Formation of Alkenes: Elimination of a water molecule from adjacent carbon atoms.

    CH₃CH₂OH $\xrightarrow{Conc. H_2SO_4, 443K}$ CH₂=CH₂ + H₂O

    Order of dehydration: Tertiary > Secondary > Primary alcohols.

  • Formation of Ethers: Intermolecular dehydration.

    2CH₃CH₂OH $\xrightarrow{Conc. H_2SO_4, 413K}$ CH₃CH₂OCH₂CH₃ + H₂O

    This method is suitable for preparing symmetrical ethers from primary alcohols.

• Reaction with Hydrogen Halides (HX): Alcohols react with hydrogen halides to form alkyl halides. The reactivity order of HX is HI > HBr > HCl.

  ROH + HX $\rightarrow$ RX + H₂O

  Order of reactivity of alcohols: Tertiary > Secondary > Primary.

  Lucas Test: This test is used to distinguish between primary, secondary, and tertiary alcohols based on their reactivity with anhydrous zinc chloride (ZnCl₂) and concentrated hydrochloric acid (HCl).

  • Tertiary alcohols react immediately to give turbidity (cloudiness).
  • Secondary alcohols react in 5-10 minutes.
  • Primary alcohols do not react at room temperature but react upon heating.

• Oxidation: The products of oxidation depend on the type of alcohol and the oxidizing agent used.
  • Primary Alcohols: Can be oxidized to aldehydes using mild oxidizing agents like Pyridinium Chlorochromate (PCC) or Pyridinium Dichromate (PDC). Strong oxidizing agents like acidified KMnO₄ or K₂Cr₂O₇ oxidize them to carboxylic acids.

    RCH₂OH $\xrightarrow{PCC}$ RCHO $\xrightarrow{KMnO_4/H^+}$ RCOOH

  • Secondary Alcohols: Are oxidized to ketones using both mild and strong oxidizing agents.

    R₂CHOH $\xrightarrow{[O]}$ R₂CO

  • Tertiary Alcohols: Are generally resistant to oxidation under mild conditions. Under drastic conditions (strong oxidizing agents and heating), they undergo C-C bond cleavage and oxidation.
  Oxidation Order: Primary $\xrightarrow{mild [O]}$ Aldehyde $\xrightarrow{strong [O]}$ Carboxylic Acid
  Secondary $\xrightarrow{[O]}$ Ketone
  Tertiary $\rightarrow$ No reaction (mild) / Cleavage (strong)
• Dehydrogenation: Primary and secondary alcohols, when heated with copper or silver catalyst at high temperatures (around 573 K), undergo dehydrogenation to form aldehydes and ketones, respectively. Tertiary alcohols undergo dehydration.

  Primary alcohol $\xrightarrow{Cu, 573K}$ Aldehyde + H₂

  Secondary alcohol $\xrightarrow{Cu, 573K}$ Ketone + H₂

1.4 Important Tests for Alcohols

• Lucas Test: (Discussed above) Distinguishes between 1°, 2°, and 3° alcohols.
• Sodium Metal Test: All alcohols react with sodium metal to liberate H₂ gas. This is a general test for the presence of an -OH group.
• Esterification Test: Alcohols react with carboxylic acids to form esters, which often have characteristic fruity smells. This can be used for identification.
• Oxidation Test:
  • Reaction with acidified K₂Cr₂O₇: Primary alcohols turn the orange solution green (formation of Cr³⁺). Secondary alcohols also give the same result. Tertiary alcohols do not react.
  • Reaction with Fehling's solution/Tollens' reagent: Aldehydes (formed from primary alcohols) can be detected by these reagents, but ketones (formed from secondary alcohols) cannot.

2. Phenols

Phenols are organic compounds in which a hydroxyl (-OH) group is directly attached to an aromatic carbon atom of a benzene ring. The simplest phenol is C₆H₅OH.

2.1 Preparation of Phenols

• From Halobenzenes: Fusion of chlorobenzene with sodium hydroxide (NaOH) at high temperature (around 623 K) and pressure.

  C₆H₅Cl + 2NaOH $\xrightarrow{623K, 300 atm}$ C₆H₅ONa + NaCl + H₂O

  C₆H₅ONa + H₂O $\xrightarrow{H^+}$ C₆H₅OH + NaOH

• From Benzene Sulfonic Acid: Reaction of benzene sulfonic acid with molten NaOH followed by acidification.

  C₆H₆ $\xrightarrow{Conc. H_2SO_4}$ C₆H₅SO₃H $\xrightarrow{NaOH (fusion)}$ C₆H₅ONa $\xrightarrow{H^+}$ C₆H₅OH

• From Diazonium Salts: Hydrolysis of diazonium salts by warming with aqueous acid. This is a versatile method for preparing substituted phenols.

  C₆H₅N₂⁺Cl^- $\xrightarrow{H_2O, \Delta}$ C₆H₅OH + N₂ + HCl

• Cumene Process (Industrial Method): Oxidation of cumene (isopropylbenzene) followed by acid-catalyzed rearrangement.

  Benzene + Propene $\xrightarrow{H_3PO_4}$ Cumene $\xrightarrow{O_2}$ Cumene Hydroperoxide $\xrightarrow{H^+/H_2O}$ Phenol + Acetone

2.2 Properties of Phenols

2.2.1 Physical Properties:

• Phenol is a colorless crystalline solid with a characteristic odor. It is sparingly soluble in water but soluble in organic solvents.
• Phenols have higher boiling points than benzene and comparable molecular mass alcohols due to intermolecular hydrogen bonding.

2.2.2 Chemical Properties (Reactions):

Phenols exhibit reactions due to the polar O-H bond and the activation of the aromatic ring by the -OH group.

2.2.2.1 Acidity of Phenols:

Phenols are weakly acidic due to the resonance stabilization of the phenoxide ion. The negative charge is delocalized into the benzene ring.

C₆H₅OH + NaOH $\rightarrow$ C₆H₅ONa + H₂O

Phenols are more acidic than alcohols but less acidic than carboxylic acids. Electron-withdrawing groups (like -NO₂) on the benzene ring increase acidity, while electron-donating groups (like -CH₃) decrease acidity.

• p-Nitrophenol is more acidic than phenol.
• o-Nitrophenol and p-Cresol (methylphenol) show specific effects due to resonance and inductive effects.

2.2.2.2 Electrophilic Substitution Reactions:

The -OH group is an activating and ortho-, para- directing group. Phenols undergo electrophilic substitution reactions readily, often without a Lewis acid catalyst.

• Halogenation:
  • With bromine water (aqueous solution): Phenol reacts with bromine water to give a white precipitate of 2,4,6-tribromophenol.

    C₆H₅OH + 3Br₂ (aq) $\rightarrow$ 2,4,6-C₆H₂Br₃OH (ppt) + 3HBr

  • In non-polar solvents (like CS₂): ortho- and para- substituted products are formed.

    C₆H₅OH + Br₂ (CS₂) $\rightarrow$ o-bromophenol + p-bromophenol + HBr

• Nitration:
  • With dilute nitric acid: Gives a mixture of o-nitrophenol and p-nitrophenol. o-nitrophenol is steam volatile due to intramolecular hydrogen bonding.

    C₆H₅OH + HNO₃ (dilute) $\rightarrow$ o-nitrophenol + p-nitrophenol

  • With concentrated nitric acid: Gives 2,4,6-trinitrophenol (Picric acid).

    C₆H₅OH + 3HNO₃ (conc.) $\rightarrow$ 2,4,6-C₆H₂(NO₂)₃OH + 3H₂O

• Sulfonation: Phenol reacts with concentrated sulfuric acid. At lower temperatures (~288 K), it gives o-hydroxybenzenesulfonic acid, and at higher temperatures (~373 K), it gives p-hydroxybenzenesulfonic acid.

  C₆H₅OH + H₂SO₄ (conc.) $\xrightarrow{288K}$ o-hydroxybenzenesulfonic acid

  C₆H₅OH + H₂SO₄ (conc.) $\xrightarrow{373K}$ p-hydroxybenzenesulfonic acid

• Friedel-Crafts Alkylation and Acylation: Phenols undergo these reactions readily.
• Reimer-Tiemann Reaction: Reaction of phenol with chloroform (CHCl₃) in the presence of a strong base (like NaOH or KOH) introduces an aldehyde group (-CHO) at the ortho position, forming salicylaldehyde.

  C₆H₅OH + CHCl₃ + NaOH $\rightarrow$ o-hydroxybenzaldehyde (Salicylaldehyde)

• Kolbe's Reaction (Kolbe-Schmitt Reaction): Reaction of sodium phenoxide with carbon dioxide (CO₂) under pressure (~125 atm) and at high temperature (~400 K) followed by acidification gives salicylic acid (o-hydroxybenzoic acid).

  C₆H₅ONa + CO₂ $\xrightarrow{pressure, \Delta}$ Sodium salicylate $\xrightarrow{H^+}$ Salicylic acid

2.2.2.3 Other Reactions:

• Etherification: Phenols react with alkyl halides in the presence of a base (like NaOH or K₂CO₃) to form phenyl ethers (Williamson Ether Synthesis).

  C₆H₅OH + RX + NaOH $\rightarrow$ C₆H₅OR + NaX + H₂O

• Reaction with Acid Chlorides/Anhydrides: Forms esters (similar to alcohols).

2.3 Important Tests for Phenols

• Ferric Chloride Test: Phenols react with neutral ferric chloride (FeCl₃) solution to give a characteristic color (violet, blue, or green) due to the formation of a complex. This is a sensitive test for the phenolic -OH group.

  C₆H₅OH + FeCl₃ $\rightarrow$ Colored complex

• Bromine Water Test: As described above, formation of a white precipitate of 2,4,6-tribromophenol.
• Azo Coupling Test: Phenols couple with diazonium salts in alkaline solution to form colored azo compounds.
• Liebermann's Nitroso Reaction: Phenols react with nitrous acid (HNO₂) to give a characteristic color.

3. Ethers

Ethers are organic compounds in which an oxygen atom is bonded to two alkyl or aryl groups. The general formula is R-O-R' or Ar-O-R.

3.1 Classification of Ethers

• Simple Ethers: Both alkyl/aryl groups are the same (e.g., Diethyl ether - CH₃CH₂OCH₂CH₃).
• Mixed Ethers: The alkyl/aryl groups are different (e.g., Ethyl methyl ether - CH₃OCH₂CH₃).

3.2 Preparation of Ethers

• Williamson Ether Synthesis: This is the most common and versatile method. It involves the reaction of an alkoxide (sodium or potassium alkoxide) with a primary alkyl halide or sulfate. It is an S_N2 reaction.

  R-ONa + R'-X $\rightarrow$ R-O-R' + NaX (where R' is primary alkyl halide)

  Limitation: If a secondary or tertiary alkyl halide is used, elimination (E2) reactions become predominant, leading to alkenes instead of ethers.

• From Alcohols (Intermolecular Dehydration): Heating primary alcohols with concentrated sulfuric acid at a lower temperature (~413 K). This method is suitable for preparing symmetrical ethers.

  2RCH₂OH $\xrightarrow{Conc. H_2SO_4, 413K}$ RCH₂OCH₂R + H₂O

• From Alkenes: Reaction of alkenes with alcohols in the presence of an acid catalyst (like H₂SO₄) yields ethers. This follows Markovnikov's rule.

  CH₂=CH₂ + C₂H₅OH $\xrightarrow{H^+}$ CH₃CH(OC₂H₅)CH₃ (Markovnikov addition)

• From Aryl Halides (Less Common): Reaction of sodium phenoxide with alkyl halides (Williamson Synthesis).

3.3 Properties of Ethers

3.3.1 Physical Properties:

• Lower ethers (like diethyl ether, dimethyl ether) are colorless, volatile liquids with characteristic pleasant odors.
• They are sparingly soluble in water due to the inability to form strong hydrogen bonds with water molecules (they can act as H-bond acceptors but not donors).
• Their boiling points are much lower than those of alcohols of comparable molecular mass due to the absence of intermolecular hydrogen bonding.

3.3.2 Chemical Properties (Reactions):

Ethers are relatively unreactive due to the strong C-O bonds and the absence of acidic hydrogen atoms.

• Cleavage of C-O bond by Acids: Ethers react with strong acids like HI, HBr, and HCl to form alkyl halides. The reaction proceeds via protonation of the ether oxygen followed by nucleophilic attack by the halide ion.

  R-O-R' + HX $\rightarrow$ RX + R'OH (if R' is primary or secondary)

  R-O-R' + 2HX $\rightarrow$ RX + R'X + H₂O (if excess acid is used or if R and R' are tertiary/benzylic)

  With HI, the reaction is:
  CH₃OCH₃ + 2HI $\rightarrow$ 2CH₃I + H₂O
  C₂H₅OCH₃ + HI $\rightarrow$ C₂H₅I + CH₃OH (followed by CH₃OH + HI $\rightarrow$ CH₃I + H₂O)
  Reactivity Order: Tertiary alkyl > Allylic/Benzylic > Secondary alkyl > Primary alkyl.
  Acid Reactivity Order: HI > HBr > HCl.

  Shortcut for Ether Cleavage with HI/HBr: Identify the alkyl group attached to oxygen that is most likely to form a stable carbocation (tertiary, allylic, benzylic). That group will form the alkyl halide. The other group forms the alcohol, which is then further converted to the alkyl halide by the excess acid.
• Electrophilic Substitution on Aromatic Ring (for Aryl Ethers): Aryl ethers undergo electrophilic substitution reactions similar to phenols, but the -OR group is less activating than -OH.
  • Nitration: Forms o- and p-nitro derivatives.
  • Halogenation: Forms o- and p-halo derivatives.
  • Friedel-Crafts Reactions: Can undergo alkylation and acylation.
• Autoxidation: Ethers, especially in the presence of air and light, can form explosive peroxides. Diethyl ether and isopropyl ether are particularly prone to this.

  RCH₂-O-CH₂R + O₂ $\rightarrow$ RCH(OOH)-O-CH₂R (Peroxide formation)

  Caution: Always test for peroxides before distilling ethers.

4. Aldehydes and Ketones

Aldehydes and ketones are organic compounds containing the carbonyl group (C=O).

• Aldehydes: Have the carbonyl group at the end of a carbon chain, bonded to at least one hydrogen atom. General formula: RCHO (where R can be H, alkyl, or aryl).
• Ketones: Have the carbonyl group within a carbon chain, bonded to two alkyl or aryl groups. General formula: RCOR' (where R and R' are alkyl or aryl groups).

4.1 Preparation of Aldehydes and Ketones

4.1.1 Preparation of Aldehydes:

• From Primary Alcohols: Oxidation using mild oxidizing agents like PCC or PDC, or dehydrogenation over copper.

  RCH₂OH $\xrightarrow{PCC}$ RCHO

  RCH₂OH $\xrightarrow{Cu, 573K}$ RCHO + H₂

• From Alkynes:
  • Hydration of terminal alkynes in the presence of mercuric sulfate (HgSO₄) and dilute sulfuric acid gives aldehydes (Markovnikov addition).

    RC≡CH + H₂O $\xrightarrow{HgSO_4, H_2SO_4}$ [R-C(OH)=CH₂] $\rightarrow$ RCOCH₃ (Ketone)
    For formaldehyde: CH≡CH + H₂O $\rightarrow$ CH₃CHO (Acetaldehyde)

• Rosenmund Reduction: Catalytic hydrogenation of acid chlorides using a poisoned catalyst (e.g., Pd/BaSO₄ poisoned with sulfur or quinoline).

  RCOCl + H₂ $\xrightarrow{Pd/BaSO_4, S}$ RCHO + HCl

• Stephen Reduction: Reduction of nitriles using stannous chloride (SnCl₂) and HCl, followed by hydrolysis.

  RCN + SnCl₂ + HCl $\rightarrow$ [RCH=NH₂]⁺Cl^- $\xrightarrow{H_2O}$ RCHO

• From Hydrocarbons: Oxidation of methyl groups attached to benzene rings (e.g., Toluene to Benzaldehyde using CrO₂Cl₂ - Etard reaction, or controlled oxidation).

4.1.2 Preparation of Ketones:

• From Secondary Alcohols: Oxidation using oxidizing agents like K₂Cr₂O₇/H₂SO₄ or KMnO₄/H₂SO₄, or dehydrogenation over copper.

  R₂CHOH $\xrightarrow{[O]}$ R₂CO

  R₂CHOH $\xrightarrow{Cu, 573K}$ R₂CO + H₂

• From Nitriles: Reaction of nitriles with Grignard reagents followed by hydrolysis.

  RCN + R'MgX $\rightarrow$ R-C(=NMgX)-R' $\xrightarrow{H_2O/H^+}$ RCOR'

• From Carboxylic Acids:
  • Heating calcium salts of carboxylic acids.

    (RCOO)₂Ca $\xrightarrow{\Delta}$ RCOR + CaCO₃

  • Reaction of acid chlorides with organocadmium reagents (R'₂Cd).

    2RCOCl + (R')₂Cd $\rightarrow$ 2RCOR' + CdCl₂

• From Alkenes: Ozonolysis of alkenes followed by reductive workup.

  RCH=CHR' $\xrightarrow{O_3}$ Ozonide $\xrightarrow{Zn/H_2O}$ RCHO + R'CHO
  RR'C=CR''R''' $\xrightarrow{O_3}$ Ozonide $\xrightarrow{Zn/H_2O}$ RCOR' + R''COR'''

  Hydration of internal alkynes (Markovnikov addition).

• Friedel-Crafts Acylation: Reaction of aromatic compounds with acid chlorides or anhydrides in the presence of a Lewis acid catalyst (like AlCl₃).

  ArH + RCOCl $\xrightarrow{AlCl_3}$ ArCOR + HCl

4.2 Properties of Aldehydes and Ketones

4.2.1 Physical Properties:

• Lower aldehydes and ketones are gases or volatile liquids with pungent odors. Higher ones are liquids or solids with pleasant odors.
• Solubility in water decreases with increasing molecular size due to the increasing hydrophobic nature of the alkyl/aryl groups. Lower members are soluble due to hydrogen bonding with water.
• Boiling points are higher than corresponding alkanes/ethers but lower than alcohols of similar molecular mass, as they cannot form intermolecular hydrogen bonds among themselves.

4.2.2 Chemical Properties (Reactions):

The carbonyl group (C=O) is polar, with the carbon atom being partially positive ($\delta^+$) and the oxygen atom being partially negative ($\delta^-$). This polarity makes the carbonyl carbon susceptible to nucleophilic attack.

4.2.2.1 Nucleophilic Addition Reactions:

These are the characteristic reactions of aldehydes and ketones. The reaction involves the addition of a nucleophile across the C=O double bond.

• Addition of Hydrogen Cyanide (HCN): Forms cyanohydrins.

  RCHO + HCN $\rightarrow$ RCH(OH)CN
  R₂CO + HCN $\rightarrow$ R₂C(OH)CN

  Cyanohydrins are important intermediates for synthesizing $\alpha$-hydroxy acids and amino acids.

• Addition of Sodium Bisulfite (NaHSO₃): Forms crystalline bisulfite addition products, which helps in the purification and isolation of aldehydes and ketones.

  RCHO + NaHSO₃ $\rightleftharpoons$ RCH(OH)SO₃Na
  R₂CO + NaHSO₃ $\rightleftharpoons$ R₂C(OH)SO₃Na

• Addition of Grignard Reagents: As discussed in alcohol preparation.
• Addition of Alcohols: Forms hemiacetals and acetals (with aldehydes) or hemiketals and ketals (with ketones). These reactions are important in the protection of carbonyl groups.

  RCHO + R'OH $\rightleftharpoons$ Hemiacetal $\xrightarrow{R'OH}$ Acetal + H₂O

• Addition of Ammonia and its Derivatives: React with ammonia, primary amines, hydroxylamine, hydrazine, phenylhydrazine, and semicarbazide to form imines, Schiff bases, oximes, hydrazones, phenylhydrazones, and semicarbazones, respectively. These are often used for characterization.

  RCHO + NH₂OH $\rightarrow$ Oxime
  RCHO + C₆H₅NHNH₂ $\rightarrow$ Phenylhydrazone

4.2.2.2 Reduction:

• Reduction to Alcohols: Using reducing agents like NaBH₄, LiAlH₄, or catalytic hydrogenation (H₂/Ni, Pt, Pd). Aldehydes give primary alcohols, and ketones give secondary alcohols.
• Clemmensen Reduction: Reduction of the carbonyl group to a methylene group (-CH₂-) using amalgamated zinc (Zn-Hg) and concentrated HCl. This is effective for aromatic ketones.

  ArCOR $\xrightarrow{Zn-Hg, HCl}$ ArCH₂R

• Wolff-Kishner Reduction: Reduction to a methylene group by heating the carbonyl compound with hydrazine (N₂H₄) in the presence of a strong base (like KOH or NaOH) in a high-boiling solvent (like ethylene glycol).

  R₂CO + N₂H₄ $\xrightarrow{KOH, \Delta}$ R₂CH₂ + N₂

4.2.2.3 Oxidation:

• Aldehydes: Are easily oxidized to carboxylic acids by mild oxidizing agents (like Tollen's reagent, Fehling's solution, Benedict's solution, acidified K₂Cr₂O₇, or KMnO₄).

  RCHO $\xrightarrow{[O]}$ RCOOH

• Ketones: Are generally resistant to oxidation under mild conditions. Strong oxidizing agents can cleave C-C bonds adjacent to the carbonyl group, leading to carboxylic acids.

  Ketones with $\alpha$-hydrogens can undergo $\alpha$-cleavage under drastic conditions.
  For unsymmetrical ketones, the "ketone cleavage" rule states that the C-C bond $\alpha$ to the carbonyl group breaks such that the smaller alkyl group is oxidized to CO₂ and a carboxylic acid, while the larger group is oxidized to a ketone. (This rule is not universally applicable and depends on conditions).

4.2.2.4 Reactions involving $\alpha$-Hydrogen Atoms:

Aldehydes and ketones with $\alpha$-hydrogens (hydrogens on the carbon adjacent to the carbonyl group) exhibit acidic $\alpha$-hydrogens and undergo reactions like aldol condensation.

• Aldol Condensation: In the presence of a dilute base or acid, aldehydes and ketones with $\alpha$-hydrogens undergo self-condensation to form $\beta$-hydroxy aldehydes or ketones (aldols), which can then dehydrate to $\alpha,\beta$-unsaturated aldehydes or ketones.

  2RCH₂CHO $\xrightarrow{Dilute\, base}$ RCH₂CH(OH)CH(R)CHO (Aldol) $\xrightarrow{-\,H_2O}$ RCH=C(R)CHO ($\alpha,\beta$-unsaturated aldehyde)

  Crossed Aldol Condensation: Occurs between two different aldehydes/ketones. Can lead to a mixture of products.

• Haloform Reaction: Methyl ketones (ketones with a -COCH₃ group) react with halogens (Cl₂, Br₂, I₂) in the presence of a base to form a haloform (CHCl₃, CHBr₃, CHI₃) and a carboxylate salt.

  CH₃COR + 4X₂ + 8OH^- $\rightarrow$ CX₃^- + RCOO^- + 4X^- + 4H₂O

  This test is positive for compounds that can be oxidized to methyl ketones (e.g., ethanol, propan-2-ol).

4.3 Important Tests for Aldehydes and Ketones

• Tollen's Test (Silver Mirror Test): Aldehydes (but not ketones, except for $\alpha$-keto aldehydes) react with Tollen's reagent (ammoniacal silver nitrate solution) to form a silver mirror on the inner walls of the test tube.

  RCHO + 2[Ag(NH₃)₂]⁺ + 3OH^- $\rightarrow$ RCOO^- + 2Ag (s) + 4NH₃ + 2H₂O

• Fehling's Test: Aldehydes (aliphatic) react with Fehling's solution (alkaline solution of cupric ions complexed with tartrate) to form a red precipitate of cuprous oxide (Cu₂O). Aromatic aldehydes do not give this test.

  RCHO + 2Cu²⁺ + 5OH^- $\rightarrow$ RCOO^- + Cu₂O (s) + 3H₂O

• Benedict's Test: Similar to Fehling's test, using Benedict's solution (alkaline solution of cupric ions complexed with citrate). Used for detecting glucose.
• Sodium Nitroprusside Test: Aldehydes give a violet color with sodium nitroprusside in alkaline solution. Ketones give a red color.
• 2,4-Dinitrophenylhydrazine (2,4-DNP) Test: Both aldehydes and ketones react with 2,4-DNP to form brightly colored precipitates (yellow, orange, or red) of their corresponding 2,4-dinitrophenylhydrazones. This is a general test for carbonyl compounds.
• Schiff's Test: Aldehydes decolorize Schiff's reagent (a colorless solution of fuchsin basic dye treated with SO₂). Ketones do not.
• Haloform Test: Positive for methyl ketones and compounds that can be oxidized to methyl ketones. Forms a yellow precipitate of iodoform (CHI₃).
• Lucas Test: Not applicable for aldehydes and ketones.

5. Carboxylic Acids

Carboxylic acids are organic compounds containing the carboxyl group (-COOH). They are characterized by their acidic nature.

5.1 Classification of Carboxylic Acids

• Aliphatic: RCOOH (e.g., Acetic acid - CH₃COOH).
  • Saturated: Only single bonds in the alkyl chain.
  • Unsaturated: Contain C=C or C≡C bonds.
• Aromatic: ArCOOH (e.g., Benzoic acid - C₆H₅COOH).
• Based on number of -COOH groups: Monocarboxylic, Dicarboxylic, Tricarboxylic, etc.

5.2 Preparation of Carboxylic Acids

• From Primary Alcohols and Aldehydes: Oxidation using strong oxidizing agents like acidified KMnO₄ or K₂Cr₂O₇.

  RCH₂OH $\xrightarrow{KMnO_4/H^+}$ RCHO $\xrightarrow{KMnO_4/H^+}$ RCOOH

• From Alkanes: Oxidation of alkyl side chains of aromatic hydrocarbons.

  Toluene $\xrightarrow{KMnO_4}$ Benzoic acid

• From Nitriles: Hydrolysis of nitriles in acidic or alkaline medium.

  RCN + 2H₂O $\xrightarrow{H^+/\Delta}$ RCOOH + NH₃

• From Esters: Hydrolysis of esters in acidic or alkaline medium.

  RCOOR' + H₂O $\xrightarrow{H^+/\Delta}$ RCOOH + R'OH

• From Grignard Reagents: Reaction with carbon dioxide followed by acidification.

  RMgX + CO₂ $\xrightarrow{dry\, ether}$ RCOOMgX $\xrightarrow{H^+}$ RCOOH

• From Acid Halides and Anhydrides: Hydrolysis.

  RCOCl + H₂O $\rightarrow$ RCOOH + HCl

  (RCO)₂O + H₂O $\rightarrow$ 2RCOOH

• Haloform Reaction: Methyl ketones and alcohols oxidizable to methyl ketones yield carboxylic acids with one less carbon atom (as carboxylate salts).

5.3 Properties of Carboxylic Acids

5.3.1 Physical Properties:

• Lower aliphatic carboxylic acids (formic acid to butyric acid) are colorless liquids with unpleasant odors. They are miscible with water.
• Higher acids are solids with less pungent odors and are insoluble in water.
• Carboxylic acids exhibit strong intermolecular hydrogen bonding, forming dimers. This leads to higher boiling points compared to alcohols and other compounds of similar molecular mass.

5.3.2 Chemical Properties (Reactions):

Reactions are due to the acidic -OH group and the polar C=O bond.

5.3.2.1 Acidic Nature:

Carboxylic acids are weak acids. They react with bases to form salts and water.

RCOOH + NaOH $\rightarrow$ RCOONa + H₂O

They react with active metals to liberate hydrogen gas.

2RCOOH + 2Na $\rightarrow$ 2RCOONa + H₂

They react with carbonates and bicarbonates to liberate CO₂.

RCOOH + NaHCO₃ $\rightarrow$ RCOONa + H₂O + CO₂

Acidity Order: Carboxylic acids > Phenols > Alcohols.

5.3.2.2 Reactions involving the -OH group of the carboxyl group:

• Esterification: Reaction with alcohols in the presence of an acid catalyst to form esters.

  RCOOH + R'OH $\rightleftharpoons$ RCOOR' + H₂O

• Formation of Acid Halides: Reaction with thionyl chloride (SOCl₂), phosphorus pentachloride (PCl₅), or phosphorus trichloride (PCl₃).

  RCOOH + SOCl₂ $\rightarrow$ RCOCl + SO₂ + HCl

• Formation of Acid Anhydrides: Heating a carboxylic acid with a strong dehydrating agent like P₄O₁₀, or by reacting acid chlorides with sodium salts of carboxylic acids.

  2RCOOH $\xrightarrow{P_4O_{10}, \Delta}$ (RCO)₂O + H₂O

• Formation of Amides: Reaction with ammonia or amines.

  RCOOH + NH₃ $\xrightarrow{\Delta}$ RCONH₂ + H₂O

5.3.2.3 Reactions involving the alkyl/aryl group:

• Hell-Volhard-Zelinsky (HVZ) Reaction: Carboxylic acids with an $\alpha$-hydrogen react with bromine or chlorine in the presence of phosphorus (P) or PCl₃ to give $\alpha$-halo carboxylic acids.

  RCH₂COOH + Br₂ $\xrightarrow{P}$ RCH(Br)COOH + HBr

• Reactions on the aromatic ring (for aromatic carboxylic acids): The -COOH group is deactivating and meta-directing for electrophilic substitution.

5.3.2.4 Reduction:

Carboxylic acids can be reduced to primary alcohols using strong reducing agents like LiAlH₄.

RCOOH $\xrightarrow{LiAlH_4}$ RCH₂OH

5.4 Important Tests for Carboxylic Acids

• Litmus Test: Carboxylic acids turn blue litmus paper red.
• Reaction with Bicarbonates: Effervescence (release of CO₂ gas) with sodium bicarbonate (NaHCO₃) or sodium carbonate (Na₂CO₃) solution.
• Esterification Test: Reaction with an alcohol in the presence of concentrated H₂SO₄ to form an ester with a characteristic fruity smell.
• Silver Mirror Test (for Formic Acid): Formic acid is an aldehyde and a carboxylic acid. It reduces Tollen's reagent to give a silver mirror. Other carboxylic acids do not.

  HCOOH + 2[Ag(NH₃)₂]⁺ + 3OH^- $\rightarrow$ CO₃^2- + 2Ag (s) + 4NH₃ + 2H₂O