Revision: Class 12 >> Organic Compounds Containing Oxygen NEET (UG) Organic Compounds Containing Oxygen

Organic compounds containing carbon-oxygen double bond, i.e. \[\mathrm{>C=O}\] group, are known as carbonyl compounds.

• Alcohols: Compounds with one or more –OH groups attached directly to a carbon chain. General formula: C₂H₂ₙ₊₁OH.
• Phenols: Compounds where –OH group is directly bonded to an aromatic (benzene) ring.
• Ethers: Compounds with general formula R–O–R'. If R = R', it is a symmetrical ether; if R ≠ R', it is an unsymmetrical ether.

Types of Alcohols

Classification of Alcohols

Based on number of —OH groups

Based on hybridisation of carbon bearing —OH (Monohydric only):

• Alcohols: O atom is sp³ hybridised; two bond pairs + two lone pairs; bent structure.
• Phenols: –OH directly on benzene ring; lone pair on O delocalised into ring → more acidic than alcohols.
• Ethers: O is sp³ hybridised. Two O–C sigma bonds + two lone pairs. Structure similar to water molecule. Bent/angular shape.

• Hydroboration–Oxidation — This method of preparing alcohols from alkenes follows Anti-Markovnikov's rule; though the product seems as Markovnikov's, it is equivalent to anti-Markovnikov's addition.
• Lucas Reagent Test — On adding Lucas reagent: a primary alcohol turns turbid only on heating, a secondary alcohol turns turbid slowly without heating, and a tertiary alcohol turns turbid immediately without heating.
• Grignard Reagent Preparation of Alcohols — Formaldehyde (HCHO) + RMgX → 1° alcohol (R–CH₂OH); aldehyde (R'CHO) + RMgX → 2° alcohol; ketone (R'COR'') + RMgX → 3° alcohol.
• Preparation of Phenol — Phenol can be prepared from chlorobenzene (NaOH/623 K/150 atm – Dow Process), benzene sulphonic acid (NaOH/573 K), cumene (O₂/cobalt naphthenate/423 K – commercial method), and aniline (NaNO₂–HCl/273 K, then H₂O/Δ – diazotisation).
• Acidic Character — In aqueous medium, phenols show a weak acidic character, while alcohols are neutral.

• Intermolecular Forces — Alcohols and phenols are polar; –OH groups form strong hydrogen bonding.
• Boiling Point — Increases with molecular mass; decreases with branching (n-butyl > isobutyl > sec-butyl > tert-butyl).
• Solubility — Phenols and lower alcohols (≤3 C) are water-soluble via H-bonding with water.
• Phenol is less soluble than alcohols due to the large hydrophobic benzene ring.
• Lower alcohols are colourless liquids, higher ones become waxy solids.
• Phenols are crystalline solids with a characteristic odour and higher boiling points.
• Kolbe's & Reimer–Tiemann — Phenoxide + CO₂/H⁺ → salicylic acid; + CHCl₃/NaOH → salicylaldehyde (electrophile: CCl₂).
• Oxidation/Reduction of Phenol — Na₂Cr₂O₇/H₂SO₄ → p-benzoquinone; 3H₂/Ni/433 K → cyclohexanol; Zn → benzene.

Methanol (Wood Spirit):

• Produced by catalytic hydrogenation of CO: 

  \[\ce{CO + 2H2  ->[ZnO/Cr2O3, 200-300atm, 573-673K] CH3OH}\]

• Highly poisonous; used as a solvent in paints and varnishes.

Ethanol:

• Produced by fermentation of sugar: 

  \[\ce{C12H22O11 + H2O ->[Invertase] \underset{Glucose}{C6H12O6} + \underset{Fructose}{C6H12O6}}\]

• Used as a solvent and in the preparation of carbon compounds.

Differentiation between Methanol & Ethanol:

• Iodoform test: Ethanol gives yellow ppt (CHI₃); methanol gives no reaction.

• With salicylic acid + H₂SO₄: Methanol forms methyl salicylate (characteristic odour); ethanol gives no specific odour.

• Williamson Synthesis (most important): R–O–Na + X–R' → R–O–R' + NaX. Primary alkyl halide is preferred (SN2 mechanism; 2° or 3° alkyl halide gives elimination).

• Acid-catalysed dehydration of alcohols: 

  \[\ce{2R - OH ->[H2SO4, 413K] R - O - R + H2O}\]

  (works best for symmetrical ethers)

• From alcohols by catalytic dehydration: 

  \[\ce{2C2H5OH ->[Al2O3, 513-523K] C2H5 - O - C2H5 + H2O}\]

• Alkoxy mercuration-demercuration: \[\begin{array}{cc}
  \phantom{}\ce{CH3 - CH = CH2 + C2H5OH + Hg(OCOCF3)2 -> CH3 - CH - CH2 - HgOCOCF3 ->[NaBH4/OH^{-}] CH3 - CH - CH3}\\
  \phantom{................................................................................}|\phantom{.....................................................................}|\phantom{.}\\
  \phantom{............................................................................................}\ce{OC2H5}\phantom{...........................................................}\ce{O-C2H5}\phantom{.}
  \end{array}\]

• Colourless liquids (except dimethyl ether and diethyl ether, which are gases).
• Polar due to bent structure (like a water molecule).
• Low boiling point due to the absence of H-bonding between ether molecules.
• Slightly soluble in water due to H-bonding with water; more soluble in organic solvents.
• Structure: O is sp³ hybridised; two sp³ orbitals form O–C sigma bonds; two sp³ orbitals have lone pairs.

• Methods of preparation of ethers: Acid-catalysed dehydration of alcohols (conc. H₂SO₄, 443 K); catalytic dehydration (Al₂O₃, 250°C); Williamson synthesis (alkyl halide + sodium alkoxide, Sₙ2); reaction of alkyl halides with dry Ag₂O.
• Preparation of Diethyl Ether (Simple Ether): From ethanol using conc. H₂SO₄ / H₃PO₄ at 413 K; or by Williamson's synthesis from C₂H₅ONa + BrCH₂CH₃ under heat.
• Reactions of Diethyl Ether: O₂ (long contact) → peroxide; dil. H₂SO₄ → 2 C₂H₅OH; PCl₅ → C₂H₅OH + C₂H₅Cl; hot HI → C₂H₅I + C₂H₅OH; excess HI → 2 C₂H₅I.
• Preparation of Anisole (Mixed Ether): CH₃Br + sodium phenoxide (C₆H₅ONa) → Methyl phenyl ether (Anisole) on heating.
• Reactions of Anisole: HI (398 K) → phenol + CH₃I; Br₂/CH₃COOH → p-bromoanisole (major) + o-bromoanisole (minor); conc. HNO₃ + conc. H₂SO₄ → 4-nitroanisole (major) + 2-nitroanisole (minor); CH₃Cl/AlCl₃ → 4-methoxytoluene (major) + 2-methoxytoluene (minor); CH₃COCl/AlCl₃ → 4-methoxyacetophenone (major) + 2-methoxyacetophenone (minor).

• Ethers are generally very unreactive (no H-bonding between ether molecules).
• When excess HX is added → C–O bond cleaves → alkyl halides.
• Reactivity of HX: HI > HBr > HCl
• If 1° or 2° alkyl groups: Smaller alkyl group forms alkyl iodide. (e.g., C₂H₅–O–CH₃ + HI → C₂H₅OH + CH₃I)
• If one alkyl group is 3°: Forms tertiary alkyl halide (SN1 pathway).

Reaction with conc. HI:

• With excess HI: Both groups convert to iodo compounds
• e.g., \[\ce{C2H5OC2H5 + HI ->[Cold] C2H5I + C2H5OH}\]

Substitution Reactions in Aromatic Ether: The alkoxу group in ether activates the aromatic ring at ortho and para positions for electrophilic substitution. Common electrophilic substitution reactions are halogenation, Friedel-Crafts reaction, etc.

In IUPAC system in aldehyde the suffix ‘e’ of alkane is replaced by ‘al’, e.g.,
CH₃—CH₂—CH=O; Propanal

\[ \underset{\text{2-methylpropanal}}{\mathrm{CH}_3 - \underset{\underset{\displaystyle \mathrm{CH}_3}{|}}{\mathrm{CH}} - \mathrm{CH} = \mathrm{O}} \]

In ketones, the suffix ‘e’ of alkane is replaced by ‘one’.

For example,

\[ \underset{\text{Butan-2-one}}{\mathrm{CH}_3 - \mathrm{CH}_2 - \overset{\displaystyle \mathrm{O}}{\overset{||}{\mathrm{C}}} - \mathrm{CH}_3} \quad \quad \underset{\text{Propanone (Acetone)}}{\mathrm{CH}_3\mathrm{COCH}_3} \]

• Carbon is sp² hybridised (trigonal planar, bond angle ≈ 120°).
• C = O bond consists of one σ bond and one π bond.
• Oxygen is more electronegative, so it pulls electron density towards itself →
  C gets a partial positive charge (δ⁺) and O gets a partial negative charge (δ⁻).
• This makes the carbonyl group polar.
• Hence, the carbon atom becomes electrophilic and is susceptible to nucleophilic attack.

Preparation of aliphatic aldehydes and ketones 

By oxidation of alcohols:

\[\ \begin{array}{r@{\;}c@{\;}l} \mathrm{R} & & \\ & \backslash & \\ & & \mathrm{CH}-\mathrm{OH} \\ & / & \\ \mathrm{R}' & & \end{array} + [\mathrm{O}] \xrightarrow[\text{Or KMnO}_4]{\text{K}_2\text{Cr}_2\text{O}_7/\text{H}_2\text{SO}_4} \begin{array}{r@{\;}c@{\;}l} \mathrm{R} & & \\ & \backslash & \\ & & \mathrm{C}=\mathrm{O} \\ & / & \\ \mathrm{R}' & & \end{array} + \mathrm{H}_2\mathrm{O}\]

When,

• R' = H then 1° alcohol to aldehyde. 
• R' = alkyl group then 2º alcohol to ketone.

By dehydrogenation of alcohols:

\[ \begin{array}{r@{\;}c@{\;}l} \mathrm{R} & & \\ & \backslash & \\ & & \mathrm{CH}-\mathrm{OH} \\ & / & \\ \mathrm{R}' & & \end{array} \xrightarrow[\text{573 K}]{\text{Cu}} \begin{array}{r@{\;}c@{\;}l} \mathrm{R} & & \\ & \backslash & \\ & & \mathrm{C}=\mathrm{O} \\ & / & \\ \mathrm{R}' & & \end{array} + \mathrm{H}_2 \]

When

• R' = H then 1° alcohol to aldehyde.
• R' = alkyl group the 2° alcohol to ketone.

By acid chloride:

\[ \mathrm{R} - \overset{\displaystyle \mathrm{O}}{\overset{||}{\mathrm{C}}} - \mathrm{Cl} + \mathrm{H}_2 \xrightarrow[\text{Rosenmund Reduction}]{\text{Pd}-\text{BaSO}_4} \underset{\text{Aldehyde}}{\mathrm{R} - \overset{\displaystyle \mathrm{O}}{\overset{||}{\mathrm{C}}} - \mathrm{H}} + \mathrm{HCl} \]

2RMgX + CdCl₂ → R₂Cd + 2MgXCl

\[\ 2\mathrm{R}' - \overset{\displaystyle \mathrm{O}}{\overset{||}{\mathrm{C}}} - \mathrm{Cl} + \mathrm{R}_2\mathrm{Cd} \longrightarrow 2\mathrm{R}' - \underset{\text{Ketone}}{\overset{\displaystyle \mathrm{O}}{\overset{||}{\mathrm{C}}}} - \mathrm{R} + \mathrm{CdCl}_2 \]

From nitriles and esters:

\[ \mathrm{R} - \mathrm{CN} \xrightarrow[\text{(ii) }\mathrm{H}_2\mathrm{O}]{\text{(i) }\mathrm{AlH}(i\text{Bu})_2} \underset{\text{Aldehyde}}{\mathrm{R} - \mathrm{CHO}} \]

\[ \mathrm{CH}_3(\mathrm{CH}_2)_9 - \overset{\displaystyle \mathrm{O}}{\overset{||}{\mathrm{C}}} - \mathrm{OC}_2\mathrm{H}_5 \xrightarrow[\text{(ii) }\mathrm{H}_2\mathrm{O}]{\text{(i) DIBAL-H}} \mathrm{CH}_3(\mathrm{CH}_2)_9 - \underset{\text{Aldehyde}}{\overset{\displaystyle \mathrm{O}}{\overset{||}{\mathrm{C}}} - \mathrm{H}} \]

From hydrocarbons

By ozonolysis:

By hydration:

• Most aldehydes are liquids (except HCHO = gas); ketones of lower order are colourless liquids with a pleasant odour.
• Higher BP than corresponding hydrocarbons but lower than alcohols (no H-bonding between molecules, but dipole-dipole interactions).
• Lower members are soluble in water (H-bonding with water); higher members are insoluble (large alkyl groups).

Addition of HCN:

\[\text{HCN} + \text{OH}^- \rightleftharpoons :\text{CN}^- + \text{H}_2\text{O}\]

Addition of NaHSO₃:

Addition of Grignard reagent:

Addition of alcohols:

\[\text{R}-\text{CHO} + 2[\text{H}] \xrightarrow[\text{or BH}_3]{\text{LiAlH}_4} \text{RCH}_2\text{OH}\]

\[\begin{array}{r@{\;}c@{\;}l r@{\;}c@{\;}l} \ce{R} & & & \ce{R} & & \\ & \backslash & & & \backslash & \\ & & \ce{C=O + H2 ->[Ni or Pt]} & & & \ce{CH2OH} \\ & / & & & / & \\ \ce{R} & & & \ce{R} & & \end{array}\]

Clemmensen reduction:

\begin{array}{r@{\;}c@{\;}l c r@{\;}c@{\;}l} & & & & & & \\ & \backslash & & & & \backslash & \\ & & \ce{C=O} & \xrightarrow[\text{HCl}]{\text{Zn-Hg}} & & & \ce{CH2 + H2O} \\ & / & & & & / & \\ & & & & & & \end{array}

Wolff-Kishner reduction:

\begin{array}{r@{\;}c@{\;}l c r@{\;}c@{\;}l c r@{\;}c@{\;}l} & & & & & & & & & & \\ & \backslash & & & & \backslash & & & & \backslash & \\ & & \ce{C=O} & \xrightarrow[-\ce{H2O}]{\ce{NH2NH2}} & & & \ce{C=NNH2} & \xrightarrow{\text{KOH ethylene Glycol, }\Delta} & & & \ce{CH2 + N2} \\ & / & & & & / & & & & / & \\ & & & & & & & & & & \end{array}

\[\underset{\text{Aldehyde}}{\ce{R-CHO}} + \ce{[O]} \xrightarrow[\text{or }\ce{KMnO4/H2SO4}]{\ce{K2Cr2O7/H2SO4}} \underset{\text{Carboxylic acid}}{\ce{R-COOH}}\]

Aldol condensation:

\[\ce{CH3 - \overset{\displaystyle O}{\overset{||}{C}} - H + H - CH2 - \overset{\displaystyle O}{\overset{||}{C}} - H} \xrightarrow{\text{dil. NaOH}} \ce{H3C - \underset{\underset{\displaystyle OH}{|}}{CH} - \underset{\underset{\displaystyle H}{|}}{CH} - \overset{\displaystyle O}{\overset{||}{C}} - H} \\ \xrightarrow{\text{dil. }\ce{H2SO4}\Delta, \ce{-H2O}} \ce{CH3 - CH = CH - \overset{\displaystyle O}{\overset{||}{C}} - H}\]

Mechanism:

Cross aldol condensation:

\[\begin{array}{ccccccccccccc} & \ce{O} & & \ce{O} & & & & \ce{OH} & & \ce{O} & & & \\ & || & & || & & & & | & & || & & & \\ \ce{C6H5 -} & \ce{C} & \ce{+ HCH2 -} & \ce{C} & \ce{- H} & \overset{\text{dil. NaOH}}{\rightleftharpoons} & \ce{C6H5 -} & \ce{C} & \ce{- CH2 -} & \ce{C} & \ce{- H} & \xrightarrow[\Delta]{\ce{H2O}} & \ce{C6H5CH=CHCHO} \\ & | & & & & & & | & & & & & \text{Cinnamaldehyde} \\ & \ce{H} & & & & & & \ce{H} & & & & & \end{array}\]

Cannizzaro reaction: It is a self-oxidation reduction reaction.

\[\begin{array}{r@{\;}c@{\;}l} \ce{H} & & \\ & \backslash & \\ & & \ce{C=O} \\ & / & \\ \ce{H} & & \end{array} + \begin{array}{r@{\;}c@{\;}l} \ce{H} & & \\ & \backslash & \\ & & \ce{C=O} \\ & / & \\ \ce{H} & & \end{array} + \text{conc. KOH} \xrightarrow{\Delta} \ce{H - \underset{\underset{\displaystyle H}{|}}{\overset{\overset{\displaystyle H}{|}}{C}} - OH} + \ce{H - \overset{\displaystyle O}{\overset{||}{C}} - OK}\]

Haloform Reaction:

The reaction can be used to transform acetyl groups into carboxyl groups or to produce chloroform or iodoform. This reaction has been used in qualitative analysis to indicate the presence of a methyl ketone in which excess iodine is used to halogenate the compound. The product iodoform precipitates as a yellow-coloured substance and has a characteristic odour.

\[\underset{\substack{\text{Acetone} \\ \text{Iodoform Test}}}{\ce{H3C - \overset{\displaystyle O}{\overset{||}{C}} - CH3}} \xrightarrow[4\ce{NaOH}]{3\ce{Cl2}} \underset{\text{Sodium acetate}}{\ce{H3C - \overset{\displaystyle O}{\overset{||}{C}} - O^\ominus Na^\oplus}} + \underset{\text{Chloroform}}{\ce{CHCl3}}\]

• Formaldehyde: Used in making Bakelite (phenol-formaldehyde resin), as a preservative (formalin = 40% HCHO).
• Acetaldehyde: Used in the preparation of acetic acid and ethanol.
• Acetone: Solvent (nail polish remover), used in the manufacture of chloroform.
• Benzaldehyde: Used in perfumes and dyes.

• Carboxylic Acids: Organic compounds with –COOH (carboxyl) functional group.
• IUPAC: Replace 'e' of alkane with 'oic acid' (e.g., HCOOH = Methanoic acid; CH₃COOH = Ethanoic acid).
• Carboxyl group = Carbonyl + Hydroxyl. Carbon is sp² hybridised.
• The conjugate base RCOO⁻ is resonance stabilised → makes carboxylic acid weakly acidic.

• Solubility: Decreases with an increase in the size of the hydrocarbon part.
• Miscibility: Lower carboxylic acids (up to 4 C atoms) are miscible with water due to H-bonding.
• Boiling point: Carboxylic acids have higher B.P. than ketones, aldehydes, and alcohols of comparable molecular mass due to intermolecular H-bonding.
• Order of B.P. (carboxylic acids & aldehydes): Valeric > Butyric > Propionic > Acetic > Formic acid; Hexanal > Pentanal > Butanal > Propanal.
• Order of B.P. (ketones): Hexan-2-one > Pentan-2-one > Butan-2-one > Propanone.

1. Methanoic acid (Formic acid): Leather tanning, dyeing and finishing in textiles.
2. Ethanoic acid (Acetic acid): Manufacturing of rayon and plastic; used as vinegar in cooking.
3. Benzoic acid: Used as a food preservative and in perfumery.
4. Salicylic acid: Used in the preparation of Aspirin (analgesic/antipyretic), Salol, and Oil of Wintergreen (methyl salicylate).

• Aspirin = acetylsalicylic acid; anti-pyretic and pain killer.
• Methyl salicylate = Oil of wintergreen (from methanol + salicylic acid).