Chemistry (9701) | Carboxylic acids and derivatives

Welcome to Chapter 33: Carboxylic Acids and Derivatives!

Hello future chemists! This chapter dives deep into a fundamental area of A-Level Organic Chemistry: compounds containing the carboxyl group ($\text{COOH}$). These molecules are everywhere, from the vinegar in your kitchen to the painful sting of an ant bite.

We will explore how these compounds are made, how incredibly acidic they are compared to other organic molecules, and look at their highly reactive relatives: esters and acyl chlorides. Don't worry if mechanisms seem tough—we'll break down the reactions step-by-step!

18.1 Carboxylic Acids (RCOOH)

Structure and Properties

Carboxylic acids are defined by the carboxyl functional group ($\text{–COOH}$). This group is a hybrid of a carbonyl ($\text{C=O}$) and a hydroxyl ($\text{–OH}$) group.

• Nomenclature: Named by replacing the ‘-e’ of the corresponding alkane with ‘-oic acid’ (e.g., Ethane $\to$ Ethanoic acid).
• Intermolecular Forces: Carboxylic acids can form two hydrogen bonds between molecules, creating stable dimers (pairs) in the liquid and solid state.
• Key Takeaway: Due to dimerization via two hydrogen bonds, carboxylic acids have significantly higher melting and boiling points than similarly sized alcohols or aldehydes.

A. Preparation of Carboxylic Acids

1. Oxidation of Primary Alcohols or Aldehydes (Syllabus 33.1.1(a))

To make a carboxylic acid, you must fully oxidise the primary alcohol or aldehyde.

Reagents and Conditions:

• Reagents: Acidified potassium dichromate(VI) ($\text{K}_2\text{Cr}_2\text{O}_7$/$\text{H}^+$) or acidified potassium manganate(VII) ($\text{KMnO}_4$/$\text{H}^+$).
• Conditions: Reflux (heating the mixture strongly with a condenser) is essential to ensure the intermediate aldehyde is not distilled off and is fully oxidized to the acid.

Example: Oxidation of Propan-1-ol to Propanoic acid.
$$\text{CH}_3\text{CH}_2\text{CH}_2\text{OH} + 2[\text{O}] \xrightarrow{\text{acidified } \text{K}_2\text{Cr}_2\text{O}_7, \text{ reflux}} \text{CH}_3\text{CH}_2\text{COOH} + \text{H}_2\text{O}$$

2. Hydrolysis of Nitriles (Syllabus 33.1.1(b))

Nitriles ($\text{RCN}$) are hydrolysed (broken down by water) under acidic or alkaline conditions to produce a carboxylic acid (via the salt).

Reagents and Conditions:

• Acid Hydrolysis: Dilute acid (e.g., $\text{H}_2\text{SO}_4$ or $\text{HCl}$) and reflux. $$\text{RCN} + 2\text{H}_2\text{O} + \text{H}^+ \xrightarrow{\text{heat}} \text{RCOOH} + \text{NH}_4^+$$
• Alkaline Hydrolysis: Dilute alkali (e.g., $\text{NaOH}$) and reflux, followed by acidification ($\text{H}^+$). This gives the sodium salt first, which is then converted to the free acid. $$\text{RCN} + \text{OH}^- + \text{H}_2\text{O} \xrightarrow{\text{heat}} \text{RCOO}^- \text{Na}^+ + \text{NH}_3$$ $$\text{RCOO}^- + \text{H}^+ \to \text{RCOOH}$$

3. Hydrolysis of Esters (Syllabus 33.1.1(c))

Esters are derivatives of carboxylic acids. Breaking them down (hydrolysis) regenerates the original acid and alcohol.

Reagents and Conditions: Dilute acid or dilute alkali, and heat. (Same as the Esters section 18.2.2).

B. Reactions of Carboxylic Acids (33.1.2 & 33.1.5)

1. Acidic Reactions (33.1.2(a)-(c))

Carboxylic acids are weak acids, but they display all the classic reactions of acids:

• With Reactive Metals ($\text{Na}$): Produces a salt and hydrogen gas ($\text{H}_2$).
  $2\text{RCOOH} + 2\text{Na} \to 2\text{RCOO}^-\text{Na}^+ + \text{H}_2$
• With Alkalis ($\text{NaOH}$): Neutralisation reaction, producing a salt and water.
  $\text{RCOOH} + \text{NaOH} \to \text{RCOO}^-\text{Na}^+ + \text{H}_2\text{O}$
• With Carbonates/Hydrogen Carbonates ($\text{Na}_2\text{CO}_3$): Produces salt, water, and carbon dioxide ($\text{CO}_2$). This is a key test! Carboxylic acids are strong enough to react with carbonates, unlike phenols or alcohols.
  $2\text{RCOOH} + \text{Na}_2\text{CO}_3 \to 2\text{RCOO}^-\text{Na}^+ + \text{H}_2\text{O} + \text{CO}_2$

2. Conversion to Esters (Esterification) (33.1.2(d))

Reaction with an alcohol. This is a reversible condensation reaction.

• Reagents: Alcohol ($\text{ROH}$)
• Catalyst/Conditions: Concentrated $\text{H}_2\text{SO}_4$ as a catalyst, and heat.

$$\text{RCOOH} + \text{R}'\text{OH} \rightleftharpoons \text{RCOOR}' + \text{H}_2\text{O}$$

3. Reduction (33.1.2(e))

Carboxylic acids can be reduced back to primary alcohols.

• Reagent: The very powerful reducing agent, Lithium Aluminium Hydride ($\text{LiAlH}_4$).
• Conditions: In dry ether, followed by reaction with dilute acid.
• Note: $\text{LiAlH}_4$ is hazardous and reacts violently with water/alcohol. Simpler reducing agents like $\text{NaBH}_4$ are not strong enough to reduce carboxylic acids.

4. Conversion to Acyl Chlorides (33.1.3)

Carboxylic acids can be converted into the highly reactive acyl chlorides (covered in detail in Section 18.3).

• Reagents: Phosphorus(V) chloride ($\text{PCl}_5$), Phosphorus(III) chloride ($\text{PCl}_3$) and heat, or Thionyl chloride ($\text{SOCl}_2$). $$\text{RCOOH} + \text{SOCl}_2 \to \text{RCOCl} + \text{SO}_2 + \text{HCl}$$

C. Acidity Comparison (Syllabus 33.1.4 & 33.1.5)

We must compare the acidity of carboxylic acids, phenols, and alcohols. Acidity is measured by the extent to which a molecule dissociates ($\text{R-H} \rightleftharpoons \text{R}^- + \text{H}^+$). The more stable the anion ($\text{R}^-$), the stronger the acid.

Explanation: Stability of the Anion

1. Carboxylate Ion ($\text{RCOO}^-$): The negative charge is spread (delocalised) over both oxygen atoms due to the adjacent $\text{C=O}$ group. This results in two equivalent resonance structures, making the ion highly stable and thus, the parent acid strong.
2. Phenoxide Ion ($\text{C}_6\text{H}_5\text{O}^-$): The negative charge on the oxygen can be partially delocalised into the benzene ring. This stabilization is less effective than in the carboxylate ion, making phenol weaker than a carboxylic acid.
3. Alkoxide Ion ($\text{RO}^-$): The alkyl group ($\text{R}$) is electron-releasing. It pushes electron density onto the already negative oxygen atom, concentrating the charge and destabilising the ion. This makes alcohols the weakest acids.

Effect of Substituents (Chlorine-Substituted Acids) (33.1.5)

Substituents affect the stability of the carboxylate ion via the inductive effect.

• Electron-Withdrawing Groups (EWGs) (e.g., Cl, F, $\text{NO}_2$): These groups pull electron density away from the carboxylate group, helping to spread the negative charge and increase stability.

  $\implies$ EWGs increase acidity.

  Example: Chloroethanoic acid ($\text{ClCH}_2\text{COOH}$) is stronger than ethanoic acid ($\text{CH}_3\text{COOH}$).
• Electron-Donating Groups (EDGs) (e.g., alkyl groups R): These groups push electron density towards the carboxylate group, decreasing stability.

  $\implies$ EDGs decrease acidity.

18.2 Esters (RCOOR')

Esters are famous for their sweet, fruity smells (Did you know? The smell of bananas, pineapples, and apples often comes from different esters!).

A. Production of Esters (Esterification) (Syllabus 33.2.1)

1. Carboxylic Acid + Alcohol (AS Recap, 33.1.2(d))

As reviewed above, heating a carboxylic acid and an alcohol with concentrated $\text{H}_2\text{SO}_4$ catalyst produces an ester.

2. Acyl Chloride + Alcohol (33.2.1(a))

This is the preferred A-Level method because it is much faster and is irreversible (no equilibrium).

• Reagents: Acyl chloride ($\text{RCOCl}$) and Alcohol ($\text{R}'\text{OH}$).
• Conditions: Room temperature. No catalyst needed. $$\text{RCOCl} + \text{R}'\text{OH} \to \text{RCOOR}' + \text{HCl}$$

Example: Ethanoyl chloride + Ethanol $\to$ Ethyl ethanoate + $\text{HCl}$

The syllabus requires examples using ethyl ethanoate and phenyl benzoate. Phenyl benzoate is formed by reacting benzoyl chloride ($\text{C}_6\text{H}_5\text{COCl}$) with phenol ($\text{C}_6\text{H}_5\text{OH}$).

B. Hydrolysis of Esters (33.2.2)

Hydrolysis is the reverse of esterification, breaking the ester link to reform the acid and alcohol.

1. Acid Hydrolysis (Dilute acid and heat)

The reaction is reversible. The acid catalyst ($\text{H}^+$) attacks the oxygen atom of the carbonyl group.

$$\text{RCOOR}' + \text{H}_2\text{O} \xrightarrow{\text{dilute acid, heat}} \text{RCOOH} + \text{R}'\text{OH}$$

2. Alkaline Hydrolysis (Saponification) (Dilute alkali and heat)

This reaction uses an alkali (like $\text{NaOH}$) and is irreversible.

• The products are the salt of the carboxylic acid and the alcohol.
• Since the acid is formed as its salt ($\text{RCOO}^-$), it cannot react with the alcohol to reform the ester, making the reaction complete (irreversible).

$$\text{RCOOR}' + \text{NaOH} \xrightarrow{\text{heat}} \text{RCOO}^-\text{Na}^+ + \text{R}'\text{OH}$$

Analogy: Saponification is how soap is made! Fats (which are esters of glycerol) are boiled with alkali to produce the sodium salt (soap) and glycerol (an alcohol).

18.3 Acyl Chlorides (RCOCl)

Acyl chlorides (also known as acid chlorides) are the most reactive derivatives of carboxylic acids. They are extremely useful in synthesis because they react quickly under mild conditions.

A. Production of Acyl Chlorides (33.3.1)

Acyl chlorides are made by replacing the ($\text{–OH}$) group of a carboxylic acid with a ($\text{–Cl}$) atom.

Reagents:

• Thionyl chloride ($\text{SOCl}_2$): Best choice, as gaseous byproducts ($\text{SO}_2$ and $\text{HCl}$) are easily removed.
• Phosphorus(V) chloride ($\text{PCl}_5$) or Phosphorus(III) chloride ($\text{PCl}_3$ and heat).

B. The Characteristic Reaction: Nucleophilic Addition-Elimination (33.3.2 & 33.3.3)

Acyl chlorides are highly reactive because the carbonyl carbon atom is strongly electrophilic ($\delta+$). This is due to electron withdrawal by the oxygen and the chlorine atom.

Acyl chlorides react readily with nucleophiles ($\text{Nu}^-$) through a two-step process: nucleophilic addition-elimination.

Step-by-step Mechanism (Addition-Elimination):

1. Addition: The nucleophile ($\text{Nu}^-$, e.g., water, alcohol, ammonia) attacks the electrophilic carbonyl carbon. The $\pi$ bond breaks, forming a negatively charged tetrahedral intermediate.
2. Elimination: The $\text{C=O}$ double bond reforms, and the chlorine atom (a good leaving group) is eliminated as $\text{Cl}^-$.
3. In the final step, a proton ($\text{H}^+$) is usually lost from the nucleophile to form the final stable product and $\text{HCl}$.

C. Reactions of Ethanoyl Chloride ($\text{CH}_3\text{COCl}$) (33.3.2)

All reactions occur rapidly at room temperature and release $\text{HCl}$ fumes.

1. Hydrolysis (Reaction with Water) (33.3.2(a))

• Nucleophile: $\text{H}_2\text{O}$
• Product: Carboxylic Acid ($\text{RCOOH}$)
• $$\text{RCOCl} + \text{H}_2\text{O} \to \text{RCOOH} + \text{HCl}$$

2. Reaction with Alcohols (33.3.2(b))

• Nucleophile: Alcohol ($\text{R}'\text{OH}$)
• Product: Ester ($\text{RCOOR}'$)
• $$\text{RCOCl} + \text{R}'\text{OH} \to \text{RCOOR}' + \text{HCl}$$

3. Reaction with Phenol (33.3.2(c))

• Nucleophile: Phenol ($\text{C}_6\text{H}_5\text{OH}$)
• Product: Phenyl Ester (e.g., Phenyl ethanoate)
• $$\text{RCOCl} + \text{C}_6\text{H}_5\text{OH} \to \text{RCOOC}_6\text{H}_5 + \text{HCl}$$

4. Reaction with Ammonia (33.3.2(d))

• Nucleophile: Ammonia ($\text{NH}_3$)
• Product: Amide ($\text{RCONH}_2$). Two moles of $\text{NH}_3$ are usually needed (one for the nucleophilic reaction, one to neutralise the $\text{HCl}$ produced).
• $$\text{RCOCl} + 2\text{NH}_3 \to \text{RCONH}_2 + \text{NH}_4\text{Cl}$$

5. Reaction with Primary or Secondary Amines (33.3.2(e))

• Nucleophile: Primary amine ($\text{R}'\text{NH}_2$) or Secondary amine ($\text{R}'_2\text{NH}$).
• Product: Substituted Amide.
• $$\text{RCOCl} + 2\text{R}'\text{NH}_2 \to \text{RCONHR}' + \text{R}'\text{NH}_3^+\text{Cl}^-$$

D. Relative Reactivity of Chlorides (33.3.4)

We need to compare the reactivity towards hydrolysis (nucleophilic attack) for three types of organic chlorides:

1. Acyl Chloride ($\text{RCOCl}$)
2. Halogenoalkane ($\text{RCl}$, or Alkyl Chloride)
3. Halogenoarene ($\text{ArCl}$, or Aryl Chloride)

1. Acyl Chlorides are Most Reactive

• The carbonyl carbon ($\text{C=O}$) and the chlorine atom both withdraw electrons powerfully.
• This creates a large partial positive charge ($\delta+$) on the carbon, making it a powerful electrophilic centre.
• The $\text{C-Cl}$ bond is easily broken because the nucleophile attacks easily, leading to the stable tetrahedral intermediate (addition-elimination).

2. Halogenoalkanes (Alkyl Chlorides)

• The C-Cl bond is polar ($\text{C}^{\delta+}-\text{Cl}^{\delta-}$), allowing nucleophilic substitution ($\text{S}_{\text{N}}1$ or $\text{S}_{\text{N}}2$).
• However, there is only one electron-withdrawing atom (Cl), so the $\delta+$ charge on the carbon is much smaller than in acyl chlorides. They require heating with aqueous $\text{NaOH}$ for hydrolysis.

3. Halogenoarenes (Aryl Chlorides, e.g., Chlorobenzene) are Least Reactive

The chlorine atom's lone pair of electrons is partially delocalised into the $\pi$ system of the benzene ring.

• This delocalisation gives the $\text{C-Cl}$ bond partial double bond character.
• The bond is therefore stronger and shorter than a single $\text{C-Cl}$ bond in a halogenoalkane.
• It is very difficult for a nucleophile to attack and break this stronger bond, so halogenoarenes resist nucleophilic substitution/hydrolysis entirely, even under heating.

D. Oxidation of Methanoic and Ethanedioic Acid (33.1.3)

While most carboxylic acids are resistant to further oxidation, two specific acids can be oxidised because they contain a structural feature that makes them susceptible.

1. Methanoic Acid ($\text{HCOOH}$)

Methanoic acid is unique because it contains both the carboxylic acid group and an aldehyde group ($\text{H-C=O}$) character.

• It can be oxidized to $\text{CO}_2$ and $\text{H}_2\text{O}$.
• Reagents: It reacts positively with mild oxidising agents, just like an aldehyde:
  • Fehling's reagent (changes from blue to brick-red precipitate).
  • Tollens' reagent (silver mirror forms).
  • Acidified $\text{KMnO}_4$ or $\text{K}_2\text{Cr}_2\text{O}_7$.
• Reaction: $\text{HCOOH} + [\text{O}] \to \text{CO}_2 + \text{H}_2\text{O}$

2. Ethanedioic Acid ($\text{HOOCCOOH}$)

Ethanedioic acid (oxalic acid) is a dicarboxylic acid that can be oxidised by warm acidified $\text{KMnO}_4$ to produce carbon dioxide and water.

• Ethanedioate ion ($\text{C}_2\text{O}_4^{2-}$) is often used in redox titrations against permanganate ions.

$5\text{C}_2\text{O}_4^{2-} + 2\text{MnO}_4^- + 16\text{H}^+ \to 10\text{CO}_2 + 2\text{Mn}^{2+} + 8\text{H}_2\text{O}$