Phenols and carboxylic acids show characteristic test-tube behaviours that allow their differentiation through simple reactions, especially acid–base interactions and observable gas evolution.

This diagram illustrates the carbonate test for a carboxylic acid, where reaction with aqueous sodium carbonate produces carbon dioxide gas, observed as effervescence. The release of CO₂ is the key qualitative observation used to identify a –COOH group. Source

Test-Tube Identification of Phenols and Carboxylic Acids

Phenols and carboxylic acids are both acidic organic compounds, yet their reactivity in qualitative tests differs significantly. Understanding these differences is essential for recognising functional groups in laboratory analysis and for interpreting organic reaction behaviour.

Comparing Acidity and Ion Formation

Phenols contain an –OH group directly attached to an aromatic ring, giving them distinctive reactivity.

This structural formula shows phenol, with an –OH group directly bonded to an aromatic benzene ring. This bonding arrangement distinguishes phenols from alcohols and influences their acidity and test-tube reactions. Source

Carboxylic acids contain the –COOH functional group. Their differing abilities to donate protons influence their reactions with bases and carbonates.

Because phenols are weak acids, they only partially form phenoxide ions in aqueous solution and cannot neutralise weak bases such as carbonates. This behaviour directly links to their laboratory test outcomes.

A brief examination of structural differences highlights why carboxylic acids are sufficiently acidic to react with carbonates, generating carbon dioxide gas, whereas phenols are not.

Reaction of Phenols with Carbonates

A key diagnostic test is the addition of solid sodium carbonate or sodium hydrogen carbonate to the organic compound.

• Phenols do not react with carbonates.

• No effervescence (bubbling) is observed.

• The absence of carbon dioxide release is a primary indicator that the unknown compound may be a phenol rather than a carboxylic acid.

This lack of reactivity results from phenols being too weakly acidic to donate protons to carbonate ions.

Reaction of Carboxylic Acids with Carbonates

Carboxylic acids readily undergo acid–base reactions due to their greater acidity.

• Carboxylic acids react vigorously with carbonates.

• Effervescence occurs due to the release of carbon dioxide gas.

• The reaction proceeds even in dilute solutions.

In test-tube analysis, the appearance of bubbles is the crucial qualitative observation, signalling the presence of a carboxylic acid functional group.

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Formation of Salts in Acid–Base Reactions

Carboxylic acids form carboxylate salts (e.g., ethanoates, propanoates) when reacting with metal carbonates or hydroxides. Phenols form phenoxide ions only when treated with strong bases such as sodium hydroxide, not with carbonates.

Unlike carboxylates, phenoxide salts are only produced in the presence of hydroxides due to phenols’ lower acidity.

Testing with Strong Bases

The different behaviours of phenols and carboxylic acids become clearer when strong bases are used:

• Phenols react with sodium hydroxide, forming soluble sodium phenoxide.

• Carboxylic acids also react with sodium hydroxide, producing carboxylate salts and water.

• Both dissolve on addition of strong base, meaning this test cannot distinguish them reliably.

The carbonate test remains the key differentiation method.

Role of Delocalisation and Acidity Strength

The acidity of carboxylic acids arises from stabilisation of the carboxylate ion by resonance delocalisation between both oxygens. Phenols also exhibit some delocalisation when forming phenoxide ions, but the stabilisation is much weaker, reducing their acidic strength.

This difference in stabilisation energy explains why only carboxylic acids are strong enough to release carbon dioxide from carbonates.

This figure compares a localised negative charge in an alkoxide ion with the delocalised charge in a carboxylate ion. It also includes alcohol–acid comparisons, which go beyond the OCR test-tube requirement but help explain why carboxylic acids are stronger acids than phenols. Source

Practical Considerations in Test-Tube Procedures

When carrying out these tests in a laboratory setting:

• Use small quantities of reagents to observe changes clearly.

• Add carbonate powders slowly to prevent vigorous reactions with strong acids.

• Warmth is not required; reactions occur at room temperature.

• Always compare the compound’s behaviour with a known reference sample.

Visual cues are central to these qualitative tests.

Additional Observations with Phenols

Although they do not react with carbonates, phenols exhibit other distinctive behaviours that help support their identification:

• No effervescence on contact with carbonate solutions.

• Slight acidity, sufficient to react with strong bases but not weak ones.

• Insolubility in water unless converted into phenoxide ions.

These properties help differentiate phenols from alcohols and carboxylic acids in broader qualitative schemes.

Additional Observations with Carboxylic Acids

Carboxylic acids show predictable acid reactions:

• Effervescence with carbonates due to CO₂ release.

• Characteristic vinegar-like odours for many lower carboxylic acids.

• Formation of salts with both strong and weak bases.

Their behaviour is consistent and easily recognisable in a laboratory context.

Summary of Key Distinguishing Features

For quick reference in test-tube identification:

• Phenols:

  • Weak acids

  • Do not react with carbonates

  • No effervescence

  • React only with strong bases

• Carboxylic acids:

  • Stronger acids

  • React with carbonates

  • Effervescence due to CO₂

  • React with both weak and strong bases

The contrast in carbonate reactivity forms the basis of the OCR specification requirement for distinguishing phenols from carboxylic acids during qualitative analysis.