27 – Synthesising & Investigating Benzoic Acid (LC 2027) Revision Notes for Leaving Cert Chemistry

27 – Synthesising & Investigating Benzoic Acid

Introduction

Benzoic acid is an important organic compound with the molecular formula $\mathrm{C_6H_5COOH}$ . It appears as white crystalline solid and is commonly found in our everyday lives as its sodium salt, sodium benzoate (E211), which serves as a food preservative to prevent bacterial, yeast, and fungal growth.

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Benzoic acid and its derivatives are among the most widely used preservatives in the food industry due to their effectiveness against microorganisms and their safety profile when used in appropriate concentrations.

This experiment involves three main stages: synthesising benzoic acid through oxidation, purifying it by recrystallisation, and testing its purity by measuring its melting point.

Part 1: Synthesis of benzoic acid by oxidation

Theory and chemical reactions

The synthesis involves oxidising phenylmethanol (benzyl alcohol) using potassium permanganate under alkaline conditions. This is a redox reaction where the alcohol is oxidised to a carboxylic acid.

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Main Oxidation Reaction

The balanced chemical equation is:

$3\mathrm{C_6H_5CH_2OH} + 4\mathrm{KMnO_4} \rightarrow 3\mathrm{C_6H_5COOH} + 4\mathrm{MnO_2} + \mathrm{H_2O} + 4\mathrm{KOH}$

During this reaction, several important changes occur:

The purple colour of permanganate changes to brown as manganese dioxide precipitate forms
Sodium benzoate forms as an intermediate because the reaction occurs under alkaline conditions
The final conversion to benzoic acid requires acidification with hydrochloric acid

The conversion equation is: $\mathrm{C_6H_5COONa} + \mathrm{HCl} \rightarrow \mathrm{C_6H_5COOH} + \mathrm{NaCl}$

Laboratory procedure

The synthesis follows these key steps:

Mixing reagents: Combine phenylmethanol, potassium permanganate solution, and sodium carbonate in a conical flask
Heating: Heat the mixture using boiling water for approximately one hour
Cooling: Cool the solution under running cold water
Acidification: Add concentrated hydrochloric acid in a fume cupboard to convert sodium benzoate to benzoic acid
Addition of sodium sulfite: Remove excess permanganate ions
Filtration: Philtre the benzoic acid crystals using a Buchner funnel

Observations and explanations

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Key Observable Changes During Synthesis

Colour change: The solution changes from purple to brown due to the formation of manganese dioxide ( $\mathrm{MnO_2}$ )
Precipitate formation: A brown precipitate of manganese dioxide appears, which is insoluble in water
Crystal formation: White crystals of benzoic acid become visible after acidification and cooling

The potassium permanganate acts as the oxidising agent, while the sodium carbonate maintains alkaline conditions necessary for the reaction to proceed efficiently.

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Safety Considerations

Wear gloves when handling concentrated potassium permanganate and hydrochloric acid
Use a fume cupboard during acidification to avoid inhaling acid vapours
Handle hot solutions with care to prevent burns

Part 2: Purification by recrystallisation

Theory of recrystallisation

Recrystallisation is a purification technique that takes advantage of different solubilities at different temperatures. Most solids are more soluble in hot solvents than in cold solvents. Benzoic acid is very soluble in hot water but only slightly soluble in cold water.

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The solubility curve shows how benzoic acid's solubility increases dramatically with temperature, reaching about 5.7 g per 100g of water at 100°C.

Recrystallisation procedure

The purification process involves five stages:

Dissolving: Dissolve the impure benzoic acid in the minimum amount of hot water
Hot filtration: Philtre the hot solution using a heated funnel and fluted philtre paper to remove insoluble impurities
Crystallisation: Cool the solution in an ice bath to allow pure benzoic acid crystals to form
Cold filtration: Philtre the crystals using a Buchner funnel to separate them from soluble impurities
Drying: Dry the purified crystals under a watch glass

Key principles

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Understanding Recrystallisation Principles

Minimum solvent: Using the minimum amount of hot water ensures maximum crystal yield upon cooling
Hot filtration: Removes insoluble impurities while keeping the product dissolved
Cold filtration: Separates pure crystals from soluble impurities that remain dissolved in cold water
Vacuum filtration: Speeds up the filtration process and helps dry the crystals

Part 3: Measuring melting point

Purpose and theory

The melting point is a key indicator of purity. Pure benzoic acid has a sharp melting point of 121-122°C. Impure samples typically:

Melt over a wider temperature range
Have lower melting points
Show gradual softening rather than sharp melting

Procedure for melting point determination

Sample preparation: Ensure crystals are completely dry and finely ground
Capillary tube filling: Seal one end of a melting point tube and fill with sample to about 1 cm depth
Apparatus setup: Place the tube in an aluminium heating block connected to a digital thermometer
Heating: Heat slowly and observe when crystals begin to melt
Recording: Note both the temperature when melting starts and when it's complete

Interpreting results

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Worked Example: Interpreting Melting Point Data

Pure benzoic acid: Sharp melting point at 121-122°C
Impure benzoic acid: Lower melting point and wider range (e.g., 116-119°C)
Very impure samples: May show gradual softening over an even wider range

What this tells us: A sharp, high melting point indicates high purity, while a broad, lower melting point suggests the presence of impurities.

Quality control and calculations

Percentage yield calculation

The efficiency of the synthesis can be calculated using:

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Worked Example: Calculating Percentage Yield

Formula: $\text{Percentage yield} = \frac{\text{Final mass}}{\text{Initial mass}} \times 100\%$

Example calculation: If 2.5g of impure benzoic acid yields 2.25g of pure crystals:

$\text{Percentage yield} = \frac{2.25}{2.5} \times 100\% = 90\%$

Factors affecting yield

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Factors That Influence Crystal Yield

Temperature control: Proper cooling maximises crystallisation
Solvent amount: Using minimum hot water increases yield
Filtration technique: Efficient filtration prevents product loss
Drying method: Complete drying ensures accurate mass measurements

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Key Points to Remember:

Benzoic acid synthesis uses oxidation of phenylmethanol with potassium permanganate under alkaline conditions, producing sodium benzoate intermediate that must be acidified to form the final product
Recrystallisation purifies the crude product by dissolving in minimum hot water, filtering hot to remove insoluble impurities, cooling to crystallise, then filtering cold to separate pure crystals
Melting point determination tests purity - pure benzoic acid melts sharply at 121-122°C, while impure samples melt over wider, lower temperature ranges
Safety is crucial - always use fume cupboards for acidification steps and wear appropriate protective equipment when handling strong oxidising agents and acids
The colour change from purple permanganate to brown manganese dioxide precipitate indicates the oxidation reaction is proceeding correctly

27 – Synthesising & Investigating Benzoic Acid (LC 2027) (Leaving Cert Chemistry): Revision Notes