Carboxylic acids and derivatives (AS)

👋 Welcome to Carboxylic Acids and Derivatives!

This chapter dives into one of the most important classes of organic compounds: Carboxylic acids, and their closely related family, the Carboxylic Acid Derivatives.
These molecules are everywhere, from the vinegar on your chips (ethanoic acid) to the fats that make up cell membranes (esters). Understanding their structure and highly versatile reactions is crucial for tackling advanced organic synthesis.

Key takeaway from this introduction:

Carboxylic acids and their derivatives (\(\text{R-CO-X}\)) act as central building blocks in organic chemistry, particularly due to their characteristic carbonyl group (\(\text{C=O}\)) combined with a strong electron-withdrawing group.

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1. Carboxylic Acids (R-COOH)

1.1 Structure, Nomenclature, and Physical Properties

The Carboxyl Functional Group

The defining feature of a carboxylic acid is the carboxyl group, which combines a carbonyl group (\(\text{C=O}\)) and a hydroxyl group (\(\text{-OH}\)).

\(\text{R-C}(=\text{O})-\text{OH}\)

The general formula is \(\text{RCOOH}\).

Nomenclature (Naming)

Carboxylic acids are named by replacing the 'e' of the corresponding alkane with 'oic acid'. The carbon in the carboxyl group is always assigned position 1.

• \(\text{C}_1\): Methanoic acid (\(\text{HCOOH}\))
• \(\text{C}_2\): Ethanoic acid (\(\text{CH}_{3}\text{COOH}\)) - the acid found in vinegar.
• \(\text{C}_3\): Propanoic acid (\(\text{CH}_{3}\text{CH}_{2}\text{COOH}\))

High Boiling Points (The Dimer Trick)

Carboxylic acids have surprisingly high melting and boiling points compared to alcohols of similar molecular mass. Why?

They form dimers in both the liquid and sometimes the gaseous phase. A dimer is a pair of identical molecules linked together.

Key concept: Two carboxylic acid molecules are held together by two strong hydrogen bonds, forming a stable ring structure.
\(2\text{ RCOOH} \rightleftharpoons (\text{RCOOH})_2\)

Analogy: Imagine an alcohol molecule has one strong hand (one -OH group) to hydrogen bond. A carboxylic acid molecule has two strong hands (one C=O acceptor site and one O-H donor site), allowing two molecules to lock together tightly. This takes much more energy to break apart, leading to a high boiling point!

1.2 Acidity of Carboxylic Acids

Carboxylic acids are weak acids, meaning they partially dissociate in water to release a hydrogen ion (\(\text{H}^+\)).

\(\text{RCOOH} + \text{H}_2\text{O} \rightleftharpoons \text{RCOO}^- + \text{H}_3\text{O}^+\)

Why are they acidic?

The acidity comes from the stability of the resultant ion: the carboxylate ion (\(\text{RCOO}^-\)).

The negative charge on the carboxylate ion is delocalised over the two oxygen atoms due to resonance (movement of \(\pi\) electrons). This delocalisation spreads the charge and makes the ion much more stable than, for example, the alkoxide ion (\(\text{RO}^-\)) formed when an alcohol loses \(\text{H}^+\).

Did you know? The more stable the conjugate base (\(\text{RCOO}^-\)), the stronger the acid (\(\text{RCOOH}\)).

Comparing Relative Acidities (Syllabus 33.1(4))

We compare the ability of molecules to donate an \(\text{H}^+\) ion.

Acidity Order:
\(\text{Carboxylic acid} > \text{Phenol} > \text{Water} > \text{Alcohol}\)

• Carboxylic acids are the strongest because the carboxylate ion (\(\text{RCOO}^-\)) is highly resonance-stabilised.
• Phenol (\(\text{C}_6\text{H}_5\text{OH}\)) is weakly acidic because the phenoxide ion is resonance-stabilised by the benzene ring (less stable than carboxylate).
• Water is very weak, but stronger than alcohols.
• Alcohols (\(\text{ROH}\)) are the weakest acids because the negative charge on the alkoxide ion (\(\text{RO}^-\)) is fixed on one oxygen atom and destabilised by the electron-donating alkyl group (\(\text{R}\)).

Effect of Substituents (Syllabus 33.1(5))

Adding electron-withdrawing groups (like chlorine) near the carboxyl group makes the acid stronger.

Example: Chloroethanoic acid (\(\text{ClCH}_2\text{COOH}\)) is stronger than ethanoic acid (\(\text{CH}_3\text{COOH}\)).

Explanation: The highly electronegative chlorine atom pulls electron density away from the carboxylate ion (\(\text{RCOO}^-\)) via the sigma bond (this is called the negative inductive effect). This withdrawal helps disperse the negative charge, further stabilising the conjugate base, and thus increasing the acidity.

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2. Synthesis of Carboxylic Acids (Preparation)

There are three main routes to synthesise carboxylic acids that you need to know (Syllabus 18.1(1)):

2.1 Oxidation of Primary Alcohols and Aldehydes

To get a carboxylic acid, you must fully oxidise the starting material. This requires strong oxidising agents and refluxing (heating in a closed system to prevent volatile compounds from escaping).

\(\text{Primary Alcohol} \xrightarrow{\text{Reflux, Acidified } \text{K}_2\text{Cr}_2\text{O}_7 \text{ or } \text{KMnO}_4} \text{Carboxylic Acid}\)

\(\text{Aldehyde} \xrightarrow{\text{Reflux, Acidified } \text{K}_2\text{Cr}_2\text{O}_7 \text{ or } \text{KMnO}_4} \text{Carboxylic Acid}\)

(Remember: If you oxidise a primary alcohol using distillation, you stop at the aldehyde stage.)

2.2 Hydrolysis of Nitriles

Nitriles (\(\text{R-C}\equiv\text{N}\)) can be hydrolysed (broken down by water, usually with acid or base catalysis) to form carboxylic acids.

Reagents & Conditions: Dilute acid or dilute alkali, followed by acidification, and heat.

\(\text{R-C}\equiv\text{N} + 2\text{H}_2\text{O} + \text{H}^+ \xrightarrow{\text{Heat}} \text{RCOOH} + \text{NH}_4^+\)

Note: If you use alkaline hydrolysis (\(\text{OH}^-\)), the final product will be the carboxylate salt (\(\text{RCOO}^-\)). You must add dilute acid (\(\text{H}^+\)) at the end to convert the salt back into the free carboxylic acid (\(\text{RCOOH}\)).

2.3 Hydrolysis of Esters

Esters can be split back into the acid and alcohol components using hydrolysis, requiring heat and either acid or alkali (Syllabus 18.1(1)(c)).

Reagents & Conditions: Dilute acid or dilute alkali, and heat, followed by acidification (if alkali used).

2.4 Synthesis of Benzoic Acid (Aromatic Example)

Benzoic acid (\(\text{C}_6\text{H}_5\text{COOH}\)) can be made by oxidising an alkylbenzene (like methylbenzene). (Syllabus 33.1(1)(a))

Reagents & Conditions: Hot alkaline \(\text{KMnO}_4\), followed by dilute acid (\(\text{H}^+\)).

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3. Reactions of Carboxylic Acids (18.1(2))

The reactions of carboxylic acids are dominated by the acidity of the \(\text{O-H}\) bond and the reactivity of the \(\text{C=O}\) group.

3.1 Reactions Showing Acidic Character

(a) Reaction with Reactive Metals

Carboxylic acids react with highly reactive metals (like \(\text{Na}\) or \(\text{Mg}\)) to produce a salt and hydrogen gas (\(\text{H}_2\)).

\(2\text{RCOOH} + \text{Mg} \rightarrow (\text{RCOO})_2\text{Mg} + \text{H}_2\text{(g)}\)

(b) Neutralisation with Alkalis

They undergo standard neutralisation reactions with bases (alkalis) to produce a salt and water.

\(\text{RCOOH} + \text{NaOH} \rightarrow \text{RCOONa} + \text{H}_2\text{O}\)

(c) Acid-Base Reaction with Carbonates (The Key Test)

This is the characteristic reaction used to distinguish carboxylic acids from less acidic compounds like phenol or alcohol.

Carboxylic acids are strong enough acids to react with carbonates (\(\text{CO}_{3}^{2-}\)) or hydrogencarbonates (\(\text{HCO}_{3}^{-}\)) to produce carbon dioxide gas (\(\text{CO}_2\)).

\(2\text{RCOOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{RCOONa} + \text{H}_2\text{O} + \text{CO}_2\text{(g)}\)

Observation: You will see effervescence (bubbling) of \(\text{CO}_2\), which can be tested using limewater.

3.2 Reactions Involving the \(\text{C=O}\) Group

(a) Esterification (Syllabus 18.1(2)(d))

Carboxylic acids react with alcohols (\(\text{R'OH}\)) to form an ester (\(\text{RCOOR'}\)) and water. This is a condensation reaction (elimination of water).

Reagents & Conditions: Alcohol + Carboxylic Acid, with concentrated \(\text{H}_2\text{SO}_4\) as a catalyst, and heat (often gentle warming).

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

This reaction is reversible, hence the equilibrium sign. Concentrated \(\text{H}_2\text{SO}_4\) acts both as a catalyst and a dehydrating agent, helping to shift the equilibrium to the right.

(b) Reduction to Primary Alcohols (Syllabus 18.1(2)(e))

Carboxylic acids are tough to reduce. They require a very powerful reducing agent:

Reagents & Conditions: \(\text{LiAlH}_4\) (Lithium Aluminium Hydride) in dry ether, followed by dilute acid (\(\text{H}^+\)).

\(\text{RCOOH} \xrightarrow{\text{1. LiAlH}_4 \text{ 2. H}^+} \text{RCH}_2\text{OH}\)

Note: \(\text{NaBH}_4\) (Sodium Borohydride) is not powerful enough to reduce carboxylic acids, though it is used for aldehydes and ketones.

3.3 Converting Carboxylic Acids to Acyl Chlorides (Syllabus 33.1(2))

This is a critical step because acyl chlorides are much more reactive intermediates than carboxylic acids.

Reagents: \(\text{PCl}_3\) and heat, \(\text{PCl}_5\), or \(\text{SOCl}_2\) (Thionyl chloride).

\(\text{RCOOH} + \text{PCl}_5 \rightarrow \text{RCOCl} + \text{POCl}_3 + \text{HCl}\)

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4. Carboxylic Acid Derivatives: Esters (\(\text{RCOOR'}\))

4.1 Structure and Preparation (Syllabus 18.2(1))

Esters are derived from carboxylic acids by replacing the \(\text{-OH}\) group with an \(\text{-OR'}\) group. They often smell pleasant (fruits and flowers).

Preparation Methods

1. Esterification: Carboxylic acid + Alcohol (\(\text{H}_2\text{SO}_4\) catalyst, heat). (Covered in 3.2(a))
2. Acyl Chloride Reaction: Alcohol + Acyl Chloride (room temperature, highly efficient). (Syllabus 33.2(1))
   \(\text{RCOCl} + \text{R'OH} \rightarrow \text{RCOOR'} + \text{HCl}\)

4.2 Hydrolysis of Esters (Reversal of Esterification) (Syllabus 18.2(2))

Hydrolysis is the reaction of an ester with water (often under acid or alkali conditions) to return to the parent carboxylic acid (or salt) and alcohol.

(a) Acid Hydrolysis

Reagents & Conditions: Dilute acid (\(\text{H}^+\)), heat.
\(\text{RCOOR'} + \text{H}_2\text{O} \rightleftharpoons \text{RCOOH} + \text{R'OH}\)

Note: This is the reverse of esterification and is an equilibrium reaction.

(b) Alkaline Hydrolysis (Saponification)

Reagents & Conditions: Dilute alkali (\(\text{OH}^-\)), heat.
\(\text{RCOOR'} + \text{NaOH} \rightarrow \text{RCOONa} + \text{R'OH}\)

Note: This reaction is irreversible because the carboxylic acid is instantly converted into the stable carboxylate salt (\(\text{RCOONa}\)). This makes alkaline hydrolysis better for quantitative yield. This process is historically important for making soap (saponification).

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5. Carboxylic Acid Derivatives: Acyl Chlorides (\(\text{RCOCl}\))

Acyl chlorides (also called alkanoyl chlorides) are the most reactive carboxylic acid derivatives and are essential synthetic intermediates.

5.1 Structure and Preparation (Syllabus 33.3(1))

They have the functional group \(\text{-COCl}\). They are highly reactive due to the highly polar \(\text{C=O}\) bond and the presence of the easily displaced \(\text{Cl}\) atom.

Preparation: From a carboxylic acid using \(\text{PCl}_3\) and heat, \(\text{PCl}_5\), or \(\text{SOCl}_2\). (Covered in 3.3)

5.2 Nucleophilic Addition-Elimination Reactions (Syllabus 33.3(2), 33.3(3))

Acyl chlorides react vigorously with many nucleophiles, replacing the chlorine atom with another group. The mechanism is nucleophilic addition-elimination (or substitution).

Don't worry if the name is long! It just means a nucleophile adds to the carbonyl carbon, and then the \(\text{Cl}\) (the leaving group) is eliminated.

(a) Reaction with Water (Hydrolysis)

Reagents & Conditions: Water (\(\text{H}_2\text{O}\)), room temperature.
\(\text{RCOCl} + \text{H}_2\text{O} \rightarrow \text{RCOOH} + \text{HCl}\)

Observation: This is a very vigorous reaction producing white fumes of \(\text{HCl}\) gas, much faster than the hydrolysis of a halogenoalkane.

(b) Reaction with Alcohols/Phenols (Ester Formation)

Reagents & Conditions: Alcohol (\(\text{R'OH}\)) or Phenol (\(\text{C}_6\text{H}_5\text{OH}\) with aqueous \(\text{NaOH}\)), room temperature.
\(\text{RCOCl} + \text{R'OH} \rightarrow \text{RCOOR'} + \text{HCl}\)

This is a cleaner and faster way to make esters compared to the reversible reaction using a carboxylic acid.

(c) Reaction with Ammonia or Amines (Amide Formation)

Acyl chlorides react with ammonia (\(\text{NH}_3\)) or primary/secondary amines (\(\text{RNH}_2\)) to form amides.

Reagents & Conditions: Ammonia or Amine, room temperature.

Ammonia forms a primary amide:
\(\text{RCOCl} + 2\text{NH}_3 \rightarrow \text{RCONH}_2 + \text{NH}_4\text{Cl}\)

A primary amine forms a secondary amide:
\(\text{RCOCl} + 2\text{R'NH}_2 \rightarrow \text{RCONHR'} + \text{R'NH}_3\text{Cl}\)

5.3 Relative Ease of Hydrolysis (Reactivity) (Syllabus 33.3(4))

The speed of hydrolysis determines the overall reactivity of the C-X bond:

\(\text{Acyl Chloride} \gg \text{Halogenoalkane} \gg \text{Halogenoarene}\)

• Acyl Chloride (\(\text{RCOCl}\)): Hydrolyses instantly with cold water. The carbon atom is strongly \(\text{C}^{\delta+}\) due to the two adjacent electronegative oxygen and chlorine atoms. The addition of the nucleophile (\(\text{H}_2\text{O}\)) is easy.
• Halogenoalkane (\(\text{RCl}\)): Hydrolyses slowly with hot aqueous \(\text{NaOH}\). The \(\text{C}^{\delta+}\) is less polar than in acyl chloride.
• Halogenoarene (\(\text{C}_6\text{H}_5\text{Cl}\)): Does not hydrolyse easily. The lone pair on the chlorine atom is delocalised into the benzene ring, giving the \(\text{C-Cl}\) bond partial double bond character. This makes the \(\text{C-Cl}\) bond stronger and much harder to break.

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6. Oxidation of Carboxylic Acids (Special Cases) (Syllabus 33.1(3))

Generally, carboxylic acids are resistant to further oxidation. However, two specific small acids can be oxidized because they contain structural elements that mimic an aldehyde (\(\text{-CHO}\)) or an alcohol (\(\text{-CH(OH)-}\)).

6.1 Methanoic Acid (\(\text{HCOOH}\))

Methanoic acid can be viewed as having a \(\text{-CHO}\) group (aldehyde group) attached to an \(\text{-OH}\) group. Because of this, it can be oxidized further to \(\text{CO}_2\) and \(\text{H}_2\text{O}\).

Reagents: Fehling's reagent, Tollens' reagent, acidified \(\text{KMnO}_4\) or \(\text{K}_2\text{Cr}_2\text{O}_7\).
\(\text{HCOOH} + [\text{O}] \rightarrow \text{CO}_2 + \text{H}_2\text{O}\)

This allows methanoic acid to give positive results for the tests used to detect aldehydes (Tollens' silver mirror, Fehling's red precipitate).

6.2 Ethanedioic Acid (\(\text{HOOC-COOH}\))

Ethanedioic acid (oxalic acid) is a dicarboxylic acid that is easily oxidized by acidified potassium manganate(VII) (\(\text{KMnO}_4\)).

Reagents & Conditions: Warm acidified \(\text{KMnO}_4\).
\(\text{HOOC-COOH} + [\text{O}] \xrightarrow{\text{Warm Acidified } \text{KMnO}_4} 2\text{CO}_2 + \text{H}_2\text{O}\)

Observation: The purple \(\text{KMnO}_4\) solution is decolourised.

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Summary of Reagents for Carboxylic Acids and Derivatives

Mastering this topic means knowing the reagents, conditions, and products for interconverting these functional groups.

Key Transformations Involving RCOOH:

• \(\text{RCOOH} \rightarrow \text{RCH}_2\text{OH}\) (Reduction): \(\text{LiAlH}_4\) in dry ether followed by \(\text{H}^+\).
• \(\text{RCOOH} \rightarrow \text{RCOCl}\) (Activation): \(\text{PCl}_3\) and heat, \(\text{PCl}_5\), or \(\text{SOCl}_2\).
• \(\text{RCOOH} + \text{R'OH} \rightarrow \text{RCOOR'}\) (Esterification): Conc. \(\text{H}_2\text{SO}_4\), heat.

Key Transformations Involving RCOCl:

• \(\text{RCOCl} \rightarrow \text{RCOOH}\) (Hydrolysis): \(\text{H}_2\text{O}\), room temp. (Vigorous).
• \(\text{RCOCl} + \text{R'OH} \rightarrow \text{RCOOR'}\) (Ester formation): \(\text{R'OH}\), room temp.
• \(\text{RCOCl} + \text{NH}_3 \rightarrow \text{RCONH}_2\) (Amide formation): \(\text{NH}_3\), room temp.