Alkanols (commonly called alcohols) are organic compounds that contain one or more hydroxyl (–OH) functional groups attached to a saturated alkyl chain. They are derived from alkanes by replacing one hydrogen atom with a hydroxyl group. The general formula of monohydric alkanols is CₙH₂ₙ₊₂O. Alkanols show hydrogen bonding, which strongly influences their physical and chemical properties.

The IUPAC naming of alkanols follows systematic rules to ensure consistency. The steps are:

1. Select the longest carbon chain containing the hydroxyl group.

2. Number the carbon chain starting from the end nearer to the –OH group.

3. Replace the “–e” in the parent alkane name with –ol, e.g., methane → methanol.

4. Indicate the position of the hydroxyl group using a number before the name, e.g., butan-2-ol.

5. Name and number all substituents (alkyl groups, halogens, etc.) where necessary.

Examples:

•          H   
           |   
       H - C - OH
           |   
           H   
      CH₃OH (Methanol)       
      

•          H   H
           |   |
       H - C - C - OH
           |   |
           H   H
      CH₃CH₂OH (Ethanol)
      

•          H   H   H
           |   |   |
       H - C - C - C - H
           |   |   |
           H   OH  H
      CH₃CH(OH)CH₃ (Propan-2-ol)
      

•          H   CH₃ H   H
           |   |   |   |
       H - C - C - C - C - H
           |   |   |   |
           H   OH  H   H
        CH₃C(CH₃)(OH)CH₂CH₃ 
        (2 - methyl butan-2-ol)
      

•          H   H   H   H
           |   |   |   |
       H - C - C - C = C 
           |   |       |
           H   OH      H
         CH₃CH(OH)CH=CH₂
        3-buten-2-ol (alkenol)
  
      

Alkanols (alcohols) can be prepared in the laboratory through several classical organic synthesis routes. These methods typically involve reduction, substitution, or hydration processes, depending on the starting material. Below are the major laboratory methods, each accompanied by an explanation and representative equations.

1. Hydrolysis of Alkyl Halides

This method involves the nucleophilic substitution of an alkyl halide by aqueous alkali (e.g., KOH or NaOH). The halogen atom is replaced by the hydroxyl group to form the corresponding alkanol. This route is especially suitable for preparing primary alcohols.

R–X  +  KOH(aq)  →  R–OH  +  KX
(where X = Cl, Br, I)

Example:
CH₃CH₂Cl + KOH(aq) → CH₃CH₂OH + KCl

2. Acid-Catalysed Hydration of Alkenes

Alkenes react with water in the presence of concentrated sulphuric acid or phosphoric acid catalysts to form alkanols. According to Markovnikov’s rule, the –OH group attaches to the carbon bearing more hydrogen atoms in an unsymmetrical alkene.

CH₂=CH₂ + H₂O  →(H⁺)→  CH₃–CH₂OH   (ethene to ethanol)

3. Reduction of Aldehydes and Ketones

Aldehydes and ketones are reduced to alkanols using reducing agents such as sodium borohydride (NaBH₄) or lithium aluminium hydride (LiAlH₄). Aldehydes produce primary alcohols, while ketones produce secondary alcohols.

Aldehyde + 2[H] → Primary alkanol
Ketone + 2[H] → Secondary alkanol

Example:
CH₃CHO + 2[H] → CH₃CH₂OH

4. Reduction of Carboxylic Acids and Esters

Carboxylic acids and esters can be reduced to primary alkanols using strong reducing agents such as LiAlH₄. This method is particularly useful for producing methanol and ethanol from simple acids.

RCOOH + 4[H] → RCH₂OH + H₂O
RCOOR' + 4[H] → RCH₂OH + R'OH

5. Fermentation of Sugars

Though more of an industrial or semi-laboratory method, fermentation is used in school laboratories to prepare ethanol. Natural sugars are broken down anaerobically by enzymes (zymase) present in yeast.

C₆H₁₂O₆  →(yeast/zymase)→  2C₂H₅OH  +  2CO₂

6. Hydrolysis of Esters (Saponification)

Esters are heated with aqueous alkalis such as NaOH or KOH. This yields the sodium or potassium salt of the acid and the corresponding alkanol. Acidification regenerates the free acid.

RCOOR' + NaOH → RCOONa + R'OH

7. Preparation Using Grignard Reagents

Grignard reagents react with carbonyl compounds, followed by hydrolysis, to form various classes of alkanols. Formaldehyde forms primary alkanols, aldehydes form secondary alkanols, and ketones form tertiary alkanols.

RMgX + HCHO → RCH₂OMgX →(H⁺)→ RCH₂OH    (primary alcohol)
RMgX + R'CHO → RR'CHOMgX →(H⁺)→ RR'CHOH   (secondary)
RMgX + R'COR'' → RR'R''COMgX →(H⁺)→ RR'R''COH (tertiary)

Physical Properties

• Lower alkanols (methanol–propanol) are colorless, volatile liquids.

• Boiling points increase with molecular mass due to hydrogen bonding.

• Lower alkanols are miscible with water; solubility decreases as carbon chain increases.

• They have higher boiling points compared to alkanes and alkenes of similar mass.

Chemical Properties

• Combustion to form CO₂ and H₂O.

    R - OH + O₂ → CO₂ and H₂O
         

• Oxidation to aldehydes, ketones, or carboxylic acids. The oxidation of a primary alkanol produces alkanal and then the corresponding alkanoic acid, the oxidation of a secondary alkanol produces alkanone. A tertiary alkanol is not oxidized because the carbon atom bearing the -OH has no free hydrogen atom

• Esterification with organic acids to form esters. The reaction of alkanol and alkanoic acid in the presence of Conc. H₂SO₄ as catalyst produces alkanoates.

   R - OH + R'COOH → RCOOR' + H₂O
    

• Reaction with sodium to form alkoxides and hydrogen gas.

    R - OH + Na  → RCONa + H₂
         

1. Based on Number of Hydroxyl Groups

• Monohydric Alkanols – contain one –OH group. e.g., CH₃OH, C₂H₅OH.

           H   H   H
           |   |   |
       H - C - C - C - H
           |   |   |
           H   OH  H
      CH₃CH(OH)CH₃ (Propan-2-ol)
      

• Dihydric Alkanols (Glycols) – contain two –OH groups. e.g., HO–CH₂–CH₂–OH (ethane-1,2-diol).

            H   H
            |   |
       OH - C - C - OH
            |   |
            H   H
      CH₂OHCH₂OH (Ethan-1,2-diol)
      

• Trihydric Alkanols (Glycerols) – contain three –OH groups. e.g., propane-1,2,3-triol.

      H  - C - CH₂OH
           |
       H - C - CHOH
           |
       H - C - CH₂OH
       propane-1,2,3-triol (glycerol)   
      

2. Based on the Number of Alkyl Groups Attached to the Carbon Bearing the –OH

• Primary (1°) Alkanols – the carbon carrying –OH is attached to one alkyl group. Example: CH₃–CH₂–OH (ethanol).

           H
           |
       R - C - OH
           |
           H  
      Primary alkanol
      
           H
           |
       H - C - OH
           |
           H  
      CH₃OH (Methanol)
         

• Secondary (2°) Alkanols – the carbon carrying –OH is attached to two alkyl groups. Example: CH₃–CH(OH)–CH₃ (propan-2-ol).

           R'
           |
       R - C - OH
           |
           H  
      Secondary alkanol
      
             H
             |
       CH₃ - C - OH
             |
             CH₃  
      CH₃CH(OH)(CH₃) propan-2-ol
         

• Tertiary (3°) Alkanols – the carbon carrying –OH is attached to three alkyl groups. Example: (CH₃)₃C–OH (2-methylpropan-2-ol).

            R'
            |
       R" - C - OH
            |
            R  
      Tertiary alkanol
      
             CH₃
             |
       CH₃ - C - OH
             |
             CH₃
     2 - methyl propan-2-ol
       

Alkanols have a wide range of uses in industry, laboratories, households, and as fuels. Their applications arise from properties such as flammability, ability to dissolve many organic compounds, hydrogen bonding, and reactivity with acids and oxidizing agents.

• As Fuels and Fuel Additives: Lower alkanols like methanol and ethanol burn cleanly and are used as fuels or blended with petrol to improve combustion and reduce emissions.

• As Solvents: Ethanol, methanol, and propanols dissolve many organic compounds and are used in pharmaceuticals, perfumes, paints, dyes, and inks.

• In the Manufacture of Chemicals: Alkanols serve as starting materials for producing aldehydes, ketones, esters, acids, and other industrial chemicals.

• In Medicine and Sterilization: Ethanol and isopropanol are used as antiseptics and disinfectants because they kill microorganisms by protein denaturation.

• As Beverages (Ethanol only): Ethanol is the active ingredient in alcoholic drinks such as beer, wine, and spirits.

• In the Manufacture of Perfumes and Cosmetics: Ethanol acts as a solvent and carrier for fragrances and helps them evaporate easily.

• As Antifreeze Agents: Ethane-1,2-diol (ethylene glycol) is used in vehicle radiators to lower the freezing point of water and prevent icing.

• In the Production of Plastics and Polymers: Methanol and ethanol are used in the synthesis of formaldehyde and other intermediates important in plastics manufacture.

• As Cleaning Agents: Surgical spirit and rubbing alcohol (70% isopropanol) are used for cleaning surfaces and medical equipment.

• In the Production of Esters: Alkanols react with organic acids to form esters used in flavorings, solvents, and synthetic fragrances.

• Sodium Metal Test

This test detects the presence of the hydroxyl (–OH) group. Alcohols react with sodium metal to produce hydrogen gas. A colourless solution of sodium alkoxide is formed. The reaction is vigorous in lower alkanols.

2ROH + 2Na → 2RONa + H₂↑

• Lucas Test

Lucas reagent (conc. HCl + anhydrous ZnCl₂) is used to distinguish primary, secondary, and tertiary alkanols based on the rate of formation of the insoluble alkyl chloride layer.

• Tertiary alkanols: turn cloudy immediately (within seconds).
• Secondary alkanols: turn cloudy within 5–10 minutes.
• Primary alkanols: remain clear (no reaction at room temperature).

This is one of the most reliable tests for classification of alkanols.

• Ceric Ammonium Nitrate Test

When an alkanol is added to ceric ammonium nitrate solution, a colour change from red to: yellow indicates the presence of the hydroxyl group. It is a general test for alcohols.

• Chromic Acid (Jones) Test

This test distinguishes primary and secondary alkanols from tertiary alkanols . Primary and secondary alkanols are oxidized and produce a green colour due to chromium(III). Tertiary alkanols do not oxidize and the orange colour remains unchanged.

Primary:   RCH₂OH → RCHO → RCOOH
Secondary: R₂CHOH → R₂C=O
Tertiary:  No reaction

• Iodoform Test

This test detects ethanol and alkanols that contain the CH₃–CH(OH)– group. When warmed with iodine and NaOH, a yellow precipitate of iodoform (CHI₃) forms.

CH₃CH₂OH + I₂ + NaOH → CHI₃(s) + HCOONa + 2NaI + 2H₂O

• Oxidation Test

Primary and secondary alkanols undergo oxidation with acidified KMnO₄ or K₂Cr₂O₇, showing colour changes:

• Primary: oxidised to aldehydes, then carboxylic acids.
• Secondary: oxidised to ketones.
• Tertiary: no oxidation → no colour change.