Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 15.12 Reactions-Study Notes - New Syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 15.12 Reactions- Study Notes- New syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 15.12 Reactions- Study Notes -International A Level (IAL) Chemistry (YCH11) – per latest Syllabus.
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Edexcel International A Level (IAL) Chemistry (YCH11) -Concise Summary Notes- All Topics
15.12 Reactions of Carboxylic Acids
(i) Reaction with Lithium Tetrahydridoaluminate(III)
Carboxylic acids can be reduced by:
\( \mathrm{LiAlH_4} \)
(lithium tetrahydridoaluminate(III), also called lithium aluminium hydride).
Reagent and Conditions
- \( \mathrm{LiAlH_4} \) in dry ether (ethoxyethane)
- Anhydrous conditions required
Reason for Dry Conditions
- \( \mathrm{LiAlH_4} \) reacts violently with water.
- Water would destroy the reducing agent.
Reaction Type
Reduction
Product Formed
- Carboxylic acids are reduced to:
Primary alcohols
General Equation
\( \mathrm{RCOOH + 4[H] \rightarrow RCH_2OH + H_2O} \)
Example
\( \mathrm{CH_3COOH + 4[H] \rightarrow CH_3CH_2OH + H_2O} \)
Key Point
- Strong reducing agent required because carboxylic acids are relatively resistant to reduction.
(ii) Reaction with Bases
Carboxylic acids behave as weak acids and react with bases in neutralisation reactions to form:
- Salts
- Water
Reaction Type
Neutralisation
Products
- Carboxylate salts formed.
General Equation
\( \mathrm{RCOOH + NaOH \rightarrow RCOONa + H_2O} \)
Example
\( \mathrm{CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O} \)
Reaction with Carbonates
Carboxylic acids also react with carbonates and hydrogencarbonates.
Products
- Salt
- Water
- Carbon dioxide
General Equation
\( \mathrm{2RCOOH + Na_2CO_3 \rightarrow 2RCOONa + H_2O + CO_2} \)
Observation
- Effervescence due to \( \mathrm{CO_2} \) gas.
Why Carboxylic Acids Are Acidic
- They can donate a proton from the \( \mathrm{O-H} \) bond.
- Carboxylate ion formed is stabilised by resonance.
Summary Table
| Reaction | Type | Main Product |
|---|---|---|
| \( \mathrm{LiAlH_4} \) | Reduction | Primary alcohol |
| Reaction with base | Neutralisation | Carboxylate salt |
Key Features
- \( \mathrm{LiAlH_4} \) reduces carboxylic acids to primary alcohols.
- Dry ether required for reduction reactions.
- Carboxylic acids react with bases to form salts.
- Reaction with carbonates produces \( \mathrm{CO_2} \).
Example 1:
State the product formed when propanoic acid is reduced using \( \mathrm{LiAlH_4} \).
▶️ Answer/Explanation
Reduction converts the carboxylic acid group into a primary alcohol.
Product:
\( \mathrm{propan\text{-}1\text{-}ol} \)
Example 2:
Describe the observations when ethanoic acid reacts with sodium carbonate.
▶️ Answer/Explanation
Effervescence is observed due to formation of carbon dioxide gas.
Sodium ethanoate and water are also formed.
(iii) Reaction with Phosphorus(V) Chloride
Carboxylic acids react with phosphorus(V) chloride:
\( \mathrm{PCl_5} \)
to produce acyl chlorides.
Reaction Type
Substitution reaction
The \( \mathrm{-OH} \) group in the carboxylic acid is replaced by chlorine.
General Equation
\( \mathrm{RCOOH + PCl_5 \rightarrow RCOCl + POCl_3 + HCl} \)
Example
\( \mathrm{CH_3COOH + PCl_5 \rightarrow CH_3COCl + POCl_3 + HCl} \)
Observations
- Steamy white fumes of:
\( \mathrm{HCl} \)
- Solid \( \mathrm{PCl_5} \) disappears.
Importance of Acyl Chlorides
- Acyl chlorides are highly reactive organic intermediates.
- Used to prepare esters and amides.
(iv) Reaction with Alcohols in the Presence of an Acid Catalyst
Carboxylic acids react with alcohols to form:
- Esters
- Water
Reaction Type
Esterification
Catalyst
Concentrated \( \mathrm{H_2SO_4} \)
Conditions
- Heat under reflux.
General Equation
\( \mathrm{RCOOH + R’OH \rightleftharpoons RCOOR’ + H_2O} \)
Example
\( \mathrm{CH_3COOH + CH_3CH_2OH \rightleftharpoons CH_3COOCH_2CH_3 + H_2O} \)
Ethanoic acid reacts with ethanol to form:
\( \mathrm{ethyl\ ethanoate} \)
Properties of Esters
- Usually have pleasant fruity smells.
- Used in flavourings and perfumes.
Role of Concentrated \( \mathrm{H_2SO_4} \)
- Acts as acid catalyst.
- Removes water, helping equilibrium move towards ester formation.
Summary Table
| Reaction | Main Product | Observation/Feature |
|---|---|---|
| \( \mathrm{PCl_5} \) | Acyl chloride | Steamy fumes of \( \mathrm{HCl} \) |
| Alcohol + acid catalyst | Ester | Fruity smell |
Key Features
- \( \mathrm{PCl_5} \) converts carboxylic acids into acyl chlorides.
- Steamy \( \mathrm{HCl} \) fumes are produced.
- Carboxylic acids react with alcohols to form esters.
- Esterification requires concentrated \( \mathrm{H_2SO_4} \) catalyst.
- Esters often have fruity odours.
Example 1:
State the observations when ethanoic acid reacts with phosphorus(V) chloride.
▶️ Answer/Explanation
Steamy white fumes of hydrogen chloride are produced.
The solid phosphorus(V) chloride disappears during the reaction.
Example 2:
Write the equation for the formation of methyl ethanoate from ethanoic acid.
▶️ Answer/Explanation
\( \mathrm{CH_3COOH + CH_3OH \rightleftharpoons CH_3COOCH_3 + H_2O} \)
Concentrated \( \mathrm{H_2SO_4} \) is used as the catalyst.