Carboxylic Acids (AQA A-Level Chemistry): Revision Notes

7.3.1 Carboxylic Acids

Carboxylic Acids

Carboxylic acids are a family of organic compounds characterised by the presence of the carboxyl functional group ($-COOH$).

Structure and Nomenclature

General Formula: The general formula for a carboxylic acid is $RCOOH$ (where R is an alkyl or aryl group).

Carboxyl Group ($-COOH$): The functional group consists of a carbonyl group ($C=O$) and a hydroxyl group ($OH$) bonded to the same carbon atom, giving carboxylic acids their distinct properties.

Naming Carboxylic Acids: The naming follows the structure alkan + oic acid (e.g., ethanoic acid for $CH₃COOH$).

• There is no need to number the position of the carboxyl group, as it is always at the end of the carbon chain (carbon 1). Carboxylic Acids Examples:

Structural Formula	Name	Molecular Formula
$[H-C(=O)-O-H]$	Methanoic acid (formic acid)	$HCOOH$
$[CH3-C(=O)-O-H]$	Ethanoic acid (acetic acid)	$CH₃COOH$
$[CH3-CH2-C(=O)-O-H]$	Propanoic acid	$CH₃CH₂COOH$

Weak Acid Properties of Carboxylic Acids

• Weak Acid Nature:
  • Carboxylic acids are weak acids, meaning they only partially ionise in aqueous solution.
  • In water, they establish an equilibrium where only a small fraction of molecules donate $H⁺$ions:

$$RCOOH⇌RCOO−+H+$$

The position of this equilibrium lies significantly to the left, indicating low levels of $H⁺$ions, which results in a weakly acidic solution.

• Solubility:
  • Carboxylic acids with up to five or six carbons are soluble in water due to hydrogen bonding between the $-COOH$ group and water molecules.
  • Larger carboxylic acids have limited solubility in water due to the increased hydrophobic nature of the alkyl chain.
• Reactions with Carbonates:
  • Despite being weak acids, carboxylic acids react with carbonates ($CO₃²⁻$) and hydrogen carbonates ($HCO₃⁻$)to release carbon dioxide ($CO₂$).
  • Reaction with Sodium Carbonate ($Na₂CO₃$):

$$2\text{RCOOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{RCOO}^-\text{Na}^+ + \text{CO}_2 + \text{H}_2\text{O}$$

• Reaction with Sodium Hydrogen Carbonate (NaHCO₃):

$${RCOOH} + {NaHCO}_3 {RCOO}^-\text{Na}^+ + \text{CO}_2 + \text{H}_2\text{O}$$

• These reactions are useful as qualitative tests for carboxylic acids, as the production of $CO₂$ gas (observable by effervescence) confirms the acidic functional group.

Esterification: Formation of Esters

Carboxylic acids react with alcohols in the presence of a strong acid catalyst (such as concentrated sulfuric acid) to form esters. This process, known as esterification, is a type of condensation reaction where a small molecule (water) is eliminated.

Reaction Mechanism:

• Carboxylic acid + alcohol → ester + water.
• For example, ethanoic acid reacts with ethanol to form ethyl ethanoate:

$${CH}_3\text{COOH} + \text{CH}_3\text{CH}_2\text{OH} \rightarrow \text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O}$$

• Naming Esters:
  • The ester's name comes from both the alcohol (prefix for the alkyl part) and the carboxylic acid (the acid part gives the suffix):
  • For example, ethyl ethanoate is derived from ethanol (ethyl group) and ethanoic acid.
  • This reaction is often reversible, requiring reflux to ensure maximum yield of ester.
• Key Points about Esterification:
  • Catalyst: Concentrated sulfuric acid or hydrochloric acid speeds up the reaction.
  • Conditions: Heating under reflux helps achieve a high yield.
  • Condensation Reaction: Esterification is classified as a condensation reaction because it eliminates water in the process.

Physical Properties

Solubility in water

• Short chain carboxylic acids are very soluble in water.
• This is because the polar $C=O$ and $-OH$ can form H-bonds with water.
• However, as the number of C atoms in the chain increases, the solubility decreases
• This is due to the longer non-polar hydrocarbon chain.

Boiling point

The boiling point of carboxylic acids is higher than corresponding alcohols.

• For propan-1-ol and ethanoic acid: - They have the same relative $M_r$ so the same number of electrons.
• This means they have similar Van der Waals forces between the molecules, both of them have H-bonds between their molecules.
• The higher boiling point of the carboxylic acid is because the H-bonding occurs between 2 molecules of the acid (to form a DIMER).
• This doubles the size of the molecule and increases the van der Waals' forces between the dimers resulting in a higher boiling point.

Acid Reactions of Carboxylic Acids

Carboxylic acids react with bases, alkalis and reactive metals to form salts (just as other acids do)

Acid + Carbonate → Salt + Water + Carbon Dioxide

• Ethanoic acid with sodium carbonate:

$$2\text{CH}_3\text{COOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2$$

• Ethanoic acid with sodium hydrogen carbonate:

$${CH}_3\text{COOH} + \text{NaHCO}_3 \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2$$

With Metals

Observations: Effervescence occurs, and the solid metal is used up, producing a colourless solution.

Acid + Metal → Salt + Hydrogen

• Ethanoic acid with magnesium:

$$2\text{CH}_3\text{COOH} + \text{Mg} \rightarrow (\text{CH}_3\text{COO})_2\text{Mg} + \text{H}_2$$

With Bases

Observations: Release of heat, and a colourless solution remains.

Acid + Base → Salt + Water

• Ethanoic acid with sodium hydroxide:

$$\text{CH}_3\text{COOH} + \text{NaOH} \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O}$$

With Ammonia

Observations: There is a release of heat, and a colourless solution remains.

Acid + Ammonia → Ammonium Salt

• Ethanoic acid with ammonia:

$$\text{CH}_3\text{COOH} + \text{NH}_3 \rightarrow \text{CH}_3\text{COONH}_4$$

(Ammonium ethanoate formed)