Skip to content
AS Level Chemistry 9701
16. Hydroxy compounds
Written by: Pranav I
Formatted by: Pranav I
Index
16.1 The homologous series of alcohols
- Organic molecules containing the hydroxyl group, -OH
- General formula: CnH2n+1OH
- Naming: add ‘-ol’ to the alkane stem
Classifying alcohols
- Primary alcohol: an alcohol in which the carbon atom bonded to the -OH group is attached to one alkyl group (or carbon atom)
- Secondary alcohol: an alcohol in which the carbon atom bonded to the -OH group is attached to two alkyl groups (or carbon atoms)
- Tertiary alcohol: an alcohol in which the carbon atom bonded to the -OH group is attached to three alkyl groups (or carbon atoms)
Properties of alcohols
- Have higher boiling points than expected due to hydrogen bonding between alcohol molecules
- Soluble in water
Comparing the acidity of alcohols and water
- Ethanol molecules have a very small degree of dissociation in water (position of equilibrium well over to the left)
- The concentration of H+ ions is lower than that in water
- Ethanol is a weaker acid than water
- Electron-donating alkyl group bonded to O– in the ethoxide ion
- The positive inductive effect concentrates more negative charge in the oxygen atom
- More readily accepts H+ while compared to OH–
16.2 Reactions to make alcohols
| # | Reagent | Acts on | Reaction type | Conditions | Example |
|---|---|---|---|---|---|
| 1 | H2O(g) | Alkenes | Addition (electrophilic) | Concentrated H3PO4 catalyst | \[ \text{CH}_2=CH_2 + H_2O \rightarrow{\text{H}_3\text{PO}_4} \text{CH}_3\text{CH}_2\text{OH} \] |
| 2 | KMnO4 | Alkenes | Oxidation | Cold, dilute & acidified | \[ \text{C}_2\text{H}_4 + \text{H}_2\text{O} + [\text{O}] \rightarrow{} \text{HOCH}_2\text{CH}_2\text{OH} \] |
| 3 | NaOH | Haloalkanes | Substitution (nucleophilic) | Aqueous & heat | \[ \text{CH}_3\text{Br} + \text{NaOH (aq)} \rightarrow{\text{Heat}} \text{CH}_3\text{OH} + \text{NaBr} \] |
| 4 | NaBH4 | Aldehydes | Reduction | Aqueous | \[ \text{CH}_3\text{CHO} + 2[\text{H}] \rightarrow{} \text{CH}_3\text{CH}_2\text{OH} \] |
| Ketones | \[ \text{CH}_3\text{COCH}_3 + 2[\text{H}] \rightarrow{} \text{CH}_3\text{CH}(\text{OH})\text{CH}_3 \] | ||||
| 5 | LiAlH4 | Aldehydes | Reduction | Dry ether | \[ \text{CH}_3\text{CHO} + 2[\text{H}] \rightarrow{} \text{CH}_3\text{CH}_2\text{OH} \] |
| Ketones | \[ \text{CH}_3\text{COCH}_3 + 2[\text{H}] \rightarrow{} \text{CH}_3\text{CH}(\text{OH})\text{CH}_3 \] | ||||
| Carboxylic Acids | \[ \text{CH}_3\text{COOH} + 4[\text{H}] \rightarrow{} \text{CH}_3\text{CH}_2\text{OH} + \text{H}_2\text{O} \] | ||||
| 6 | H2O | Esters | Hydrolysis | Acid, aqueous & heat | \[ \text{CH}_3\text{COOCH}_3 + \text{H}_2\text{O} \rightarrow{\text{HCl}} \text{CH}_3\text{COOH} + \text{CH}_3\text{OH} \] |
| 7 | NaOH | Esters | Hydrolysis | Aqueous & heat | \[ \text{CH}_3\text{COOCH}_3 + \text{NaOH (aq)} \rightarrow{} \text{CH}_3\text{COONa} + \text{CH}_3\text{OH} \] |
16.3 Reactions of the alcohols
Combustion
- React with excess oxygen in the air to form carbon dioxide and water
- Ethanol burns with a clean blue flame
\[ \text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O} \]
Substitution to form a haloalkane
- Reagent: hydrogen halide → HX(g)
- A halogen atom replaces -OH group in the alcohol → haloalkane produced
✅ HX(g) can be made in situ (in the reaction vessel)
- Producing dry HCl gas: NaCl + conc. H2SO4 → HCl + NaHSO4
- Producing dry HBr gas: KBr + conc. H2SO4 → HBr + KHSO4
To understand these reactions better, refer to 11. Group 17 notes
- The reactions listed below are other ways to produce haloalkanes from alcohols
| # | Reagent | Acts on | Reaction type | Conditions | Example |
|---|---|---|---|---|---|
| 1 | SOCl2 | Alcohols | Substitution | Room temperature | \[ \text{CH}_3\text{CH}_2\text{OH} + \text{SOCl}_2 \rightarrow \text{CH}_3\text{CH}_2\text{Cl} + \text{SO}_2 + \text{HCl} \] |
| 2 | PCl5 | Alcohols | Substitution | Room temperature | \[ \text{CH}_3\text{CH}_2\text{OH} + \text{PCl}_5 \rightarrow \text{CH}_3\text{CH}_2\text{Cl} + \text{POCl}_3 + \text{HCl} \] |
| 3 | PCl3 | Alcohols | Substitution | Heat | \[ 3\text{CH}_3\text{CH}_2\text{OH} + \text{PCl}_3 \rightarrow 3\text{CH}_3\text{CH}_2\text{Cl} + \text{H}_3\text{PO}_3 \] |
- In Reaction #3, red phosphorus and Cl2/Br2/I2 are used to produce PCl3, PBr3 or PI3 in situ
Reaction with sodium metal
🚨 The -OH bond breaks in this reaction, not the C-O bond
- Reagent: Na(s) → sodium
- Less vigorous than sodium’s reaction with water (water is more acidic than alcohol)
- The longer the hydrocarbon chain in the alcohol, the less vigorous the reaction with Na(s)
- General reaction: sodium + alcohol → sodium alkoxide + hydrogen gas
\[
\text{C}_2\text{H}_5\text{OH} + 2\text{Na} \rightarrow \text{C}_2\text{H}_5\text{ONa} + \text{H}_2
\]
Esterification
| # | Reagent | Acts on | Reaction type | Conditions | Example |
|---|---|---|---|---|---|
| 1 | Carboxylic acids | Alcohols | Condensation | Concentrated H2SO4 catalyst & heat under reflux | \[ \text{CH}_3\text{COOH} + \text{C}_3\text{H}_7\text{OH} \rightarrow{\text{H}_2\text{SO}_4} \text{CH}_3\text{COOC}_3\text{H}_7 + \text{H}_2\text{O} \] |
Hydrolysis of esters
| # | Reagent | Acts on | Reaction type | Conditions | Example |
|---|---|---|---|---|---|
| 1 | HCl | Esters | Hydrolysis | Dilute & heat under reflux | \[ \text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3\text{COOH} + \text{CH}_3\text{CH}_2\text{OH} \] |
| 2 | NaOH | Esters | Hydrolysis | Heat under reflux | \[ \text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{NaOH} \rightarrow \text{CH}_3\text{COONa} + \text{CH}_3\text{CH}_2\text{OH} \] |
Dehydration
- Type: elimination
- Conditions: (either of)
- Al2O3 catalyst and heat
- Concentrated acid
- General reaction: alcohol → alkene + water
\[
\text{C}_2\text{H}_5\text{OH} \rightarrow{\text{conc. H}_2\text{SO}_4} \text{C}_2\text{H}_4 + \text{H}_2\text{O}
\]
Oxidation
- Reagent: K2Cr2O7 → potassium dichromate(VI) solution
- Conditions: acidified, heat, and reflux (or distill off)
- Acidified K2Cr2O7 solution
- Orange in colour due to dichromate(VI) ions, Cr2O72-(aq)
- Acts as an oxidizing agent
- Reduced to chromium(III) ions, Cr3+(aq) to form a green solution
- Note: the reaction mixture has to be warmed before the oxidation takes place
- Primary, secondary, and tertiary alcohols behave differently in this reaction
- Tertiary alcohols → solution remains orange when warmed (NO REACTION TAKES PLACE)
- Secondary alcohols → oxidized to form a ketone + solution turns green
\[
\text{CH}_3\text{CH(OH)}\text{CH}_3 + [O] \rightarrow \text{CH}_3\text{CO}\text{CH}_3 + \text{H}_2\text{O}
\]
- Primary alcohols → oxidized to form an aldehyde (by distilling off the product) + solution turns green
\[
\text{CH}_3\text{CH}_2\text{OH} + [O] \rightarrow \text{CH}_3\text{CHO} + \text{H}_2\text{O}
\]
- Aldehydes can be further oxidized to form carboxylic acids (by refluxing with excess acidified K2Cr2O7)
\[
\text{CH}_3\text{CHO} + [O] \rightarrow \text{CH}_3\text{COOH}
\]
🔥 Using acidified KMnO4
- Stronger oxidizing agent
- It can directly oxidize a primary alcohol to a carboxylic acid
- It does not require heat in most cases
