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Hello, Future Chemist! Understanding Alcohols (Hydroxy Compounds)

Welcome to the world of alcohols! This chapter is one of the most central topics in organic chemistry because the hydroxyl ($\text{-OH}$) group is a stepping stone to many other functional groups. If you can master how alcohols are made and how they react, you unlock vast pathways for organic synthesis!

Don't worry if this seems tricky at first. Alcohols involve classifications and specific conditions that must be memorized. We will break down these concepts step-by-step, focusing on clarity and easy recall.

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1. Structure and Classification of Alcohols

1.1 What is an Alcohol?

Alcohols are organic compounds that contain the hydroxyl functional group ($\text{-OH}$) attached to a saturated carbon atom (an $\text{sp}^3$ hybridized carbon).
The general formula is $\text{R-OH}$, where R is an alkyl group.

1.2 Classification: Primary, Secondary, and Tertiary Alcohols

Alcohols are classified based on the number of alkyl (R) groups attached to the carbon atom that is bonded directly to the $\text{-OH}$ group (the carbinol carbon).

  • Primary Alcohol (\(1^{\circ}\)): The carbinol carbon is attached to one alkyl group and two hydrogen atoms.
    Example: Ethanol ($\text{CH}_3\text{CH}_2\text{OH}$) or Propan-1-ol.

  • Secondary Alcohol (\(2^{\circ}\)): The carbinol carbon is attached to two alkyl groups and one hydrogen atom.
    Example: Propan-2-ol ($\text{CH}_3\text{CH(OH)}\text{CH}_3$).

  • Tertiary Alcohol (\(3^{\circ}\)): The carbinol carbon is attached to three alkyl groups and zero hydrogen atoms.
    Example: 2-methylpropan-2-ol.
Memory Aid: P, S, T - The number of friends!

Think of the $\text{-OH}$ carrying carbon atom. How many other carbons (alkyl groups) is it 'friends' with?
$\text{1 friend} = 1^{\circ}$ (Primary)
$\text{2 friends} = 2^{\circ}$ (Secondary)
$\text{3 friends} = 3^{\circ}$ (Tertiary)

Quick Review: Key Functional Group
The functional group that defines alcohols is the hydroxyl group ($\text{-OH}$). This group is responsible for almost all the characteristic chemical properties we will study.

2. Physical Properties and Hydrogen Bonding

2.1 Boiling Points

Compared to alkanes or haloalkanes of similar molecular mass, alcohols have significantly higher boiling points. Why?

The $\text{-OH}$ group contains a highly electronegative oxygen atom bonded to a hydrogen atom ($\text{O-H}$). This creates strong permanent dipoles and allows for hydrogen bonding between alcohol molecules.

  • Hydrogen bonds are the strongest type of intermolecular force (IMF).
  • More energy (heat) is required to overcome these strong forces, leading to a higher boiling point.

Analogy: If dispersion forces (van der Waals) are sticky tape, hydrogen bonds are superglue!

2.2 Solubility in Water

Small alcohol molecules (like methanol, ethanol, propanol) are completely miscible (soluble) in water.

  • Water molecules ($\text{H}_2\text{O}$) also exhibit strong hydrogen bonding.
  • Alcohols can form hydrogen bonds with water molecules. This strong attraction is enough to overcome the hydrogen bonds between water molecules and between alcohol molecules.
  • As the carbon chain length increases, the hydrocarbon (non-polar) part becomes dominant. The solubility decreases rapidly.
Did you know?
The hydroxyl group ($\text{-OH}$) makes the alcohol molecule polar, while the alkyl chain (R) is non-polar. When the chain gets too long (typically $C_4$ or $C_5$ and above), the non-polar end wins the fight, and the alcohol becomes poorly soluble.

3. Preparation of Alcohols (Synthesis Routes)

The syllabus requires you to recall six key methods for producing alcohols:

3.1 From Alkenes (Addition Reactions)

(a) Hydration of Alkenes (Electrophilic Addition of Steam):

Alkenes react with steam to form alcohols. This is the industrial method.

  • Reagents: Steam ($\text{H}_2\text{O(g)}$)
  • Conditions: High temperature, High pressure, $\text{H}_3\text{PO}_4$ catalyst (or $\text{H}_2\text{SO}_4$)

(b) Reaction of Alkenes with Cold Dilute Acidified $\text{KMnO}_4$ (Oxidation):

This method adds two $\text{-OH}$ groups across the double bond, forming a diol (a molecule with two $\text{-OH}$ groups).

  • Reagents: Cold, dilute, acidified Potassium Manganate(VII) ($\text{KMnO}_4$)
  • Observation: The purple $\text{KMnO}_4$ solution is decolourised.

3.2 From Halogenoalkanes (Nucleophilic Substitution)

Halogenoalkanes react with aqueous alkali to replace the halogen atom with an $\text{-OH}$ group.

  • Reagents: Aqueous Sodium Hydroxide ($\text{NaOH(aq)}$) or Potassium Hydroxide ($\text{KOH(aq)}$)
  • Conditions: Heat (warming).
  • Mechanism: Nucleophilic Substitution ($\text{S}_\text{N}1$ or $\text{S}_\text{N}2$ depending on the class of haloalkane).
  • Equation Example: $\text{R-X} + \text{OH}^- \longrightarrow \text{R-OH} + \text{X}^-$

3.3 From Carbonyls and Carboxylic Acids (Reduction)

Reduction means adding hydrogen, often represented by $[\text{H}]$.

(d) Reduction of Aldehydes and Ketones:

  • Product: Aldehydes reduce to primary alcohols; Ketones reduce to secondary alcohols.
  • Reagents: Sodium borohydride ($\text{NaBH}_4$) or Lithium aluminium hydride ($\text{LiAlH}_4$).
    (Note: $\text{NaBH}_4$ is generally preferred for carbonyls as it is milder and safer.)

(e) Reduction of Carboxylic Acids:

  • Product: Carboxylic acids reduce to primary alcohols.
  • Reagents: Only the strong reducing agent Lithium aluminium hydride ($\text{LiAlH}_4$) is used for this purpose.

3.4 From Esters (Hydrolysis)

Esters can be broken down (hydrolysed) back into an alcohol and a carboxylic acid (or a carboxylate salt).

  • Acid Hydrolysis: $\text{Ester} + \text{H}_2\text{O} \rightleftharpoons \text{Carboxylic Acid} + \text{Alcohol}$ (Reversible)
  • Alkaline Hydrolysis (Saponification): $\text{Ester} + \text{OH}^- \longrightarrow \text{Carboxylate Salt} + \text{Alcohol}$ (Irreversible)
  • Conditions for both: Dilute acid or dilute alkali, and heat.
Key Takeaway for Preparation:
Preparation methods are often categorized by the type of reaction: Addition (alkenes), Substitution (haloalkanes), or Reduction (carbonyls/acids). Memorize the specific reagents required for each route!

4. Chemical Reactions of Alcohols

Alcohols undergo a wide range of reactions, including combustion, substitution, elimination, and oxidation. The most important reactions involve the cleavage of either the $\text{C-O}$ bond or the $\text{O-H}$ bond.

4.1 Oxidation: The Key Distinguishing Reaction

Oxidation is the most important chemical characteristic of alcohols, as it allows us to distinguish between primary, secondary, and tertiary alcohols.

General Reagents: Acidified Potassium Dichromate(VI) ($\text{K}_2\text{Cr}_2\text{O}_7$) or Acidified Potassium Manganate(VII) ($\text{KMnO}_4$). The $[\text{O}]$ symbol is often used to represent the oxidizing agent.

(i) Primary Alcohols (\(1^{\circ}\)):

Primary alcohols can be oxidized in two stages:

  1. To Aldehydes: Requires distillation to remove the aldehyde as soon as it forms, preventing further oxidation.
    \(\text{RCH}_2\text{OH} + [\text{O}] \xrightarrow{\text{acidified } \text{K}_2\text{Cr}_2\text{O}_7, \text{ distill}} \text{RCHO} + \text{H}_2\text{O}\)
  2. To Carboxylic Acids: Requires refluxing (heating in a sealed system) so the oxidizing agent remains in contact with the alcohol and aldehyde for a prolonged time.
    \(\text{RCH}_2\text{OH} + 2[\text{O}] \xrightarrow{\text{acidified } \text{K}_2\text{Cr}_2\text{O}_7, \text{ reflux}} \text{RCOOH} + \text{H}_2\text{O}\)

(ii) Secondary Alcohols (\(2^{\circ}\)):

Secondary alcohols are oxidized to ketones. Ketones are much harder to oxidize further, so distillation or refluxing yields the same product (a ketone).

\(\text{RCH(OH)R}' + [\text{O}] \xrightarrow{\text{acidified } \text{K}_2\text{Cr}_2\text{O}_7} \text{RCOR}' + \text{H}_2\text{O}\)

(iii) Tertiary Alcohols (\(3^{\circ}\)):

Tertiary alcohols cannot be oxidized by these mild oxidizing agents. This is because there is no hydrogen atom directly attached to the carbinol carbon to be removed.

Observation (Distinguishing Reaction): When the orange acidified $\text{K}_2\text{Cr}_2\text{O}_7$ is added to an alcohol and heated, if the alcohol is primary or secondary, the solution changes colour from orange to green ($\text{Cr}_2\text{O}_7^{2-} \to \text{Cr}^{3+}$). If the alcohol is tertiary, no colour change occurs.

4.2 Substitution to form Halogenoalkanes

The $\text{-OH}$ group is substituted by a halogen atom ($\text{X}$). This involves breaking the strong $\text{C-O}$ bond.

  • Using Hydrogen Halides ($\text{HX}$):
    Reagents: $\text{HX(g)}$ (e.g., $\text{HBr(g)}$)
    *OR* $\text{KCl}$ (or $\text{KBr}$) and concentrated $\text{H}_2\text{SO}_4$ or concentrated $\text{H}_3\text{PO}_4$ (to generate $\text{HX}$ in situ).

  • Using $\text{PCl}_3$ or $\text{PCl}_5$:
    $\text{R-OH} + \text{PCl}_5 \longrightarrow \text{R-Cl} + \text{HCl} + \text{POCl}_3$
    $\text{PCl}_5$ is a useful test for the $\text{-OH}$ group because it produces $\text{HCl}$ gas (white fumes).

  • Using Thionyl Chloride ($\text{SOCl}_2$):
    $\text{R-OH} + \text{SOCl}_2 \longrightarrow \text{R-Cl} + \text{SO}_2(\text{g}) + \text{HCl}(\text{g})$
    This is a highly useful method because the by-products ($\text{SO}_2$ and $\text{HCl}$) are gases and easily separated, yielding a pure haloalkane.

4.3 Dehydration (Elimination) to form Alkenes

Alcohols lose a molecule of water when heated with a concentrated acid catalyst or certain metal oxide catalysts, forming an alkene.

  • Reagents: Concentrated $\text{H}_2\text{SO}_4$ or concentrated $\text{H}_3\text{PO}_4$.
  • Conditions: High temperature (typically $170^{\circ}\text{C}$ for $\text{H}_2\text{SO}_4$) OR a heated catalyst like $\text{Al}_2\text{O}_3$ (aluminium oxide).
  • Reaction Type: Elimination.

4.4 Ester Formation (Esterification)

Alcohols react with carboxylic acids to form esters. This is a reversible condensation reaction (a molecule of water is eliminated).

  • Reagents: Carboxylic acid ($\text{RCOOH}$)
  • Conditions: Concentrated $\text{H}_2\text{SO}_4$ catalyst and heat (refluxing).
  • Example: Reaction of Ethanol with Ethanoic acid to form Ethyl Ethanoate.

Note: The syllabus also mentions the reaction of alcohols with acyl chlorides to form esters, which is a faster, non-reversible reaction carried out at room temperature (covered later in the A-Level syllabus, Topic 32.1).

4.5 Reaction with Active Metals (Sodium)

Alcohols are weakly acidic and react with highly reactive metals like sodium, releasing hydrogen gas.

  • Reagents: Sodium metal ($\text{Na(s)}$)
  • Observation: Bubbles of hydrogen gas ($\text{H}_2$).
  • Product: An $\text{alkoxide}$ (a salt).
    \(\text{2R-OH} + \text{2Na} \longrightarrow \text{2R-O}^-\text{Na}^+ + \text{H}_2\)

4.6 Combustion

Alcohols burn readily in air or oxygen, producing carbon dioxide and water (complete combustion), making them useful fuels.

\(\text{C}_2\text{H}_5\text{OH} + \text{3O}_2 \longrightarrow \text{2CO}_2 + \text{3H}_2\text{O}\)

Quick Review: Oxidation Conditions
Primary $\to$ Aldehyde: Distillation
Primary $\to$ Carboxylic Acid: Reflux
Secondary $\to$ Ketone: Reflux/Distillation (Ketones don't oxidize further)
Tertiary $\to$ No Reaction

5. Acidity and the Iodoform Reaction

5.1 Comparing Acidity (Alcohol vs. Water)

The $\text{O-H}$ bond in an alcohol is polar, allowing it to act as an acid (donating $\text{H}^+$). However, alcohols are very weak acids.

The syllabus requires you to explain the acidity of alcohols compared with water (Topic 16.1, LO 5). Water is slightly more acidic than most simple alcohols.

  • When an alcohol loses $\text{H}^+$, it forms an alkoxide ion ($\text{R-O}^-$).
  • When water loses $\text{H}^+$, it forms a hydroxide ion ($\text{OH}^-$).
  • Alkyl groups ($\text{R}$) are electron-donating (via the inductive effect). They push electron density towards the negative oxygen atom of the alkoxide ion, making the ion less stable and more reactive.
  • Since the $\text{OH}^-$ ion is more stable than the $\text{R-O}^-$ ion, water is a stronger acid than the alcohol.

Takeaway: Alcohols are weaker acids than water. They can react with strong bases ($\text{Na}$) but generally not with weaker bases (like $\text{NaOH(aq)}$), unlike phenol (an A-Level topic).

5.2 The Iodoform Reaction (Tri-iodomethane Test)

This is a specific test used to deduce the presence of a $\text{CH}_3\text{CH(OH)}–$ group in an alcohol.

  • Target Group: $\text{CH}_3\text{CH(OH)}\text{-R}$ (where R can be $\text{H}$, an alkyl group, or an aryl group).
  • Reagents: Alkaline Iodine ($\text{I}_2\text{(aq)}$) (i.e., $\text{I}_2$ dissolved in $\text{NaOH(aq)}$).
  • Observation: Formation of a bright yellow precipitate of tri-iodomethane ($\text{CHI}_3$).
  • Products: The yellow precipitate ($\text{CHI}_3$) and a carboxylate ion ($\text{RCO}_2^-$).

Example: Ethanol ($\text{CH}_3\text{CH}_2\text{OH}$) gives a positive test because it contains the $\text{CH}_3\text{CH(OH)}–\text{H}$ structure. Propan-2-ol ($\text{CH}_3\text{CH(OH)}\text{CH}_3$) also gives a positive test.

Note: This test is also positive for aldehydes or ketones containing the $\text{CH}_3\text{CO}–\text{R}$ group, as the alkaline iodine first oxidizes the alcohol to the carbonyl compound, and then the test proceeds.

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Summary: Alcohols Checklist

Preparation Methods (Need Reagents/Conditions):

  1. Alkene + $\text{H}_2\text{O(g)}$/$\text{H}_3\text{PO}_4$ catalyst.
  2. Alkene + Cold dilute acidified $\text{KMnO}_4$ (forms diol).
  3. Halogenoalkane + $\text{NaOH(aq)}$/heat ($\text{S}_\text{N}$).
  4. Aldehyde/Ketone + $\text{NaBH}_4$ or $\text{LiAlH}_4$ (Reduction).
  5. Carboxylic Acid + $\text{LiAlH}_4$ (Reduction).
  6. Ester Hydrolysis (Acid/Alkali and heat).

Reactions (Need Product Type and Conditions):

  1. Oxidation ($1^{\circ}$ $\to$ aldehyde/acid; $2^{\circ}$ $\to$ ketone; $3^{\circ}$ $\to$ no reaction).
  2. Substitution (with $\text{HX}$, $\text{PCl}_3/\text{PCl}_5$, $\text{SOCl}_2$ to form haloalkane).
  3. Dehydration (Conc. $\text{H}_2\text{SO}_4$/heat or $\text{Al}_2\text{O}_3$/heat to form alkene).
  4. Esterification (with carboxylic acid and conc. $\text{H}_2\text{SO}_4$ catalyst).
  5. Reaction with $\text{Na(s)}$ (forms alkoxide and $\text{H}_2$).
  6. Combustion (forms $\text{CO}_2$ and $\text{H}_2\text{O}$).

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