Welcome to Topic 17C: Carboxylic Acids, Acyl Chlorides, and Esters

In this chapter, we explore three functional groups that are very closely related. You can think of them as a family: carboxylic acids are the "parents," while acyl chlorides and esters are their more reactive or sweet-smelling "offspring." This topic is a core part of your Paper 2 and Paper 3 exams, building on the organic chemistry foundations you laid in Year 12.

We will look at how to make these compounds, how they react with each other, and how they contribute to the world around us—from the vinegar in your kitchen to the polyester in your clothes.

1. Carboxylic Acids

Carboxylic acids contain the carboxyl group \( -COOH \). They are weak acids, meaning they only partially dissociate in water to release \( H^+ \) ions.

Preparation of Carboxylic Acids

There are two main ways you need to know to prepare carboxylic acids:

  1. Oxidation: You can oxidize primary alcohols or aldehydes.
    • Reagent: Acidified potassium dichromate(VI) \( (K_2Cr_2O_7 / H_2SO_4) \).
    • Conditions: Reflux (to ensure full oxidation).
    • Equation: \( RCH_2OH + 2[O] \rightarrow RCOOH + H_2O \)
  2. Hydrolysis of Nitriles: Nitriles \( (RCN) \) can be broken down by water.
    • Conditions: Reflux with either a dilute acid (like \( HCl \)) or a dilute alkali (like \( NaOH \)) followed by acidification.
    • Acidic Hydrolysis Equation: \( RCN + 2H_2O + HCl \rightarrow RCOOH + NH_4Cl \)

Reactions of Carboxylic Acids

Carboxylic acids aren't just "end products"; they are very useful starting materials for other reactions:

  • Reduction: Using the powerful reducing agent lithium tetrahydridoaluminate \( (LiAlH_4) \) in dry ether. This reduces the acid all the way back to a primary alcohol.
    Note: You cannot stop at the aldehyde stage with \( LiAlH_4 \).
  • Neutralization (as Bases): Carboxylic acids react with bases (like \( NaOH \)), carbonates (like \( Na_2CO_3 \)), or metals to form salts.
    Example: \( CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O \)
  • Reaction with \( PCl_5 \): Phosphorus(V) chloride reacts with carboxylic acids to produce acyl chlorides. This is a key way to increase the reactivity of the molecule.
    Example: \( CH_3COOH + PCl_5 \rightarrow CH_3COCl + POCl_3 + HCl \)
  • Esterification: Reacting a carboxylic acid with an alcohol in the presence of an acid catalyst (usually concentrated \( H_2SO_4 \)) to form an ester and water. This is a reversible reaction.

Quick Review: Carboxylic acids can be identified in Infrared (IR) Spectroscopy by two distinct peaks: a C=O peak (around \( 1700\text{ cm}^{-1} \)) and a very broad O-H peak (around \( 2500–3300\text{ cm}^{-1} \)).

2. Acyl Chlorides

Acyl chlorides have the functional group \( -COCl \). They are much more reactive than carboxylic acids because the \( Cl \) atom is a very good "leaving group."

Important Observation: Reactions with acyl chlorides almost always produce steamy white fumes of Hydrogen Chloride \( (HCl) \) gas. This makes them easy to spot in a lab!

Key Reactions of Acyl Chlorides

Acyl chlorides undergo nucleophilic addition-elimination reactions (though you just need to know the reagents and products for this section):

  • With Water: Forms a carboxylic acid and \( HCl \). This reaction is very vigorous.
    \( RCOCl + H_2O \rightarrow RCOOH + HCl \)
  • With Alcohols: Forms an ester and \( HCl \). This is a faster, non-reversible way to make esters compared to using carboxylic acids.
    \( RCOCl + R'OH \rightarrow RCOOR' + HCl \)
  • With Ammonia: Forms a primary amide and \( HCl \).
    \( RCOCl + NH_3 \rightarrow RCONH_2 + HCl \)
  • With Amines: Forms an N-substituted amide and \( HCl \).
    \( RCOCl + R'NH_2 \rightarrow RCONHR' + HCl \)

Don't worry if these seem tricky! Just remember: the \( Cl \) is replaced by whatever nucleophile is attacking, and the "leftover" \( H \) from the nucleophile joins with the \( Cl \) to make \( HCl \).

3. Esters

Esters have the functional group \( -COOR \). They are famous for their sweet, fruity smells and are used in perfumes and food flavorings.

Hydrolysis of Esters

Hydrolysis is the splitting of a molecule using water. Esters can be hydrolyzed in two ways:

  1. Acid Hydrolysis:
    • Reagents: Dilute acid (e.g., \( HCl \)) and reflux.
    • Result: Reversible reaction forming the carboxylic acid and alcohol.
    • \( RCOOR' + H_2O \rightleftharpoons RCOOH + R'OH \)
  2. Alkaline Hydrolysis (Saponification):
    • Reagents: Dilute alkali (e.g., \( NaOH \)) and reflux.
    • Result: Irreversible reaction forming a carboxylate salt and the alcohol.
    • \( RCOOR' + NaOH \rightarrow RCOONa + R'OH \)
    • Key Tip: To get the carboxylic acid from the salt, you must add a strong acid at the end.

Did you know? Alkaline hydrolysis of fats (which are naturally occurring esters) is how soap is made! That is why it is called saponification.

4. Polyesters

Polyesters are condensation polymers. Unlike addition polymers (like polyethene), condensation polymers form when two monomers join and lose a small molecule (usually water or \( HCl \)) in the process.

To make a polyester, you need either:

  • A dicarboxylic acid and a diol.
  • A single monomer that has both an alcohol group and a carboxylic acid group (a hydroxycarboxylic acid).

Example: Terylene (PET)
This is formed from benzene-1,4-dicarboxylic acid and ethane-1,2-diol. The \( -OH \) from the acid and the \( -H \) from the alcohol's hydroxyl group leave to form water, creating an ester link \( (-COO-) \) between the monomers.

Common Mistakes to Avoid

  • Mixing up reagents: Remember that \( LiAlH_4 \) is for reduction (adding hydrogen), while acidified dichromate is for oxidation (adding oxygen).
  • Missing the \( HCl \) fumes: In exam questions, "misty white fumes" is a massive hint that an acyl chloride is reacting.
  • Forgetting the "reversible" sign: Acid hydrolysis of esters and esterification are equilibrium reactions. Alkaline hydrolysis is not.
  • Solvent choice: Remember that \( LiAlH_4 \) reacts violently with water, so it must be used in dry ether.

Quick Takeaway Table

Functional Group Suffix Key Reaction Partner Main Product
Carboxylic Acid -oic acid Alcohol (+ acid cat.) Ester + \( H_2O \)
Acyl Chloride -oyl chloride Alcohol Ester + \( HCl \)
Ester -yl -oate Dilute \( NaOH \) (reflux) Salt + Alcohol

For more on how these molecules are used in complex reaction schemes, see Topic 18C: Organic Synthesis. For more on the amides formed from acyl chlorides, see Topic 18B.