Here is a balanced and concise class note on Alkanols for SSS 2 Chemistry:

Alkanols: An Introduction to Organic Compounds

Welcome to the fascinating world of Alkanols! These are a crucial class of organic compounds with many real-world applications. Understanding them will build a strong foundation for future topics in organic chemistry.

Core Concepts

1. What are Alkanols?

Alkanols, commonly known as alcohols, are organic compounds containing a hydroxyl functional group (-OH) attached to a saturated carbon atom. They are derivatives of alkanes where one hydrogen atom has been replaced by a hydroxyl group. Their general molecular formula is CnH2n+1OH.

2. Nomenclature (Naming Alkanols)

Alkanols are named using the IUPAC (International Union of Pure and Applied Chemistry) system. The rules are:

• Identify the longest continuous carbon chain containing the -OH group.
• Number the carbon chain from the end closest to the -OH group.
• Replace the '-e' ending of the corresponding alkane name with '-ol'.
• Indicate the position of the -OH group by a number before the '-ol' suffix (e.g., propan-1-ol).
• For multiple -OH groups, use diol, triol, etc. (e.g., ethane-1,2-diol).
• Other substituents are named and numbered as usual.

Examples:

• CH₃OH: Methanol
• CH₃CH₂OH: Ethanol
• CH₃CH₂CH₂OH: Propan-1-ol
• CH₃CH(OH)CH₃: Propan-2-ol

Image: A 3D space-filling model of an ethanol molecule.

3. Classification of Alkanols

Alkanols can be classified based on two main criteria:

a) Based on the number of -OH groups:

• Monohydric Alkanols: Contain one -OH group (e.g., ethanol).
• Dihydric Alkanols: Contain two -OH groups (e.g., ethane-1,2-diol, also known as ethylene glycol).
• Trihydric Alkanols: Contain three -OH groups (e.g., propane-1,2,3-triol, also known as glycerol).

b) Based on the carbon atom to which the -OH group is attached:

• Primary (1°) Alkanols: The carbon atom bonded to the -OH group is attached to only one other alkyl group (e.g., propan-1-ol).
• Secondary (2°) Alkanols: The carbon atom bonded to the -OH group is attached to two other alkyl groups (e.g., propan-2-ol).
• Tertiary (3°) Alkanols: The carbon atom bonded to the -OH group is attached to three other alkyl groups (e.g., 2-methylpropan-2-ol).

Image: Diagram illustrating primary, secondary, and tertiary classifications of alcohols.

4. Isomerism in Alkanols

Alkanols exhibit various types of isomerism:

• Positional Isomerism: Occurs when the -OH group is attached to different carbon atoms in the same carbon chain (e.g., propan-1-ol and propan-2-ol).
• Chain Isomerism: Occurs when the carbon chain itself is arranged differently (e.g., butan-1-ol and 2-methylpropan-1-ol).
• Functional Group Isomerism: Alkanols can be functional group isomers with ethers (e.g., ethanol and methoxymethane).

5. Physical Properties of Alkanols

• Boiling Point: Alkanols have significantly higher boiling points than alkanes of comparable molecular mass. This is due to the presence of the highly polar -OH group, which allows for strong hydrogen bonding between alkanol molecules.
• Solubility in Water: Lower molecular mass alkanols (like methanol, ethanol, propanol) are miscible (mix completely) with water. This is because they can form hydrogen bonds with water molecules. As the length of the hydrocarbon chain increases, the non-polar part of the molecule becomes more dominant, reducing solubility in water.
• State: Lower alkanols (C1 to C12) are colourless liquids at room temperature. Higher alkanols are waxy solids.

Image: A 2D representation of hydrogen bonding between water molecules, a similar principle applies to alkanols and water.

6. Chemical Properties of Alkanols

Alkanols undergo various reactions due to the presence of the -OH group:

• Combustion: Alkanols burn in air to produce carbon dioxide and water, releasing heat. CH₃CH₂OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)
• Dehydration (Elimination): When heated with concentrated H₂SO₄ (a dehydrating agent), alkanols lose a molecule of water to form alkenes. CH₃CH₂OH $\xrightarrow{\text{Conc. H₂SO₄, 170°C}}$ CH₂=CH₂ + H₂O
• Oxidation: The type of product depends on the class of alkanol and the oxidizing agent.
  • Primary alkanols oxidize to aldehydes, then further to carboxylic acids.
  • Secondary alkanols oxidize to ketones.
  • Tertiary alkanols are generally resistant to oxidation under mild conditions.
• Reaction with Sodium: Alkanols react with active metals like sodium to produce hydrogen gas and sodium alkoxides. 2ROH + 2Na → 2RONa + H₂ (where R is an alkyl group)
• Esterification: Alkanols react with carboxylic acids in the presence of a strong acid catalyst (like H₂SO₄) to form sweet-smelling esters and water. RCOOH + R'OH $\rightleftharpoons$ RCOOR' + H₂O

7. Preparation of Alkanols (Briefly)

• Fermentation of Carbohydrates: Ethanol is commonly produced by the anaerobic fermentation of sugars (from starch-containing crops like maize, cassava) by yeast. C₆H₁₂O₆ (glucose) $\xrightarrow{\text{Yeast}}$ 2CH₃CH₂OH (ethanol) + 2CO₂
• Hydration of Alkenes: Alkenes react with steam in the presence of a catalyst (e.g., H₃PO₄) to form alkanols. CH₂=CH₂ + H₂O $\xrightarrow{\text{Catalyst, Heat, Pressure}}$ CH₃CH₂OH

Image: A simplified diagram showing the process of fermentation, a common method for ethanol production.

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Real-World Example

Ethanol (a common alkanol) is widely used as an antiseptic in hand sanitizers and as a solvent in many perfumes and medicines. It's also blended with petrol to create 'gasohol' as a biofuel, reducing reliance on fossil fuels.

Practice/Reflection Question

Identify two different alkanols that are positional isomers of each other and draw their condensed structural formulas. Briefly explain how their properties might differ due to the position of the -OH group.