Welcome to the World of Carboxylic Acids!
Hello future Chemists! This chapter takes us into one of the most important functional groups in organic chemistry: the Carboxylic Acid group. You encounter these every day—from the tangy bite of vinegar (ethanoic acid) to the structure of fatty acids in your diet.
Carboxylic acids ($\text{RCOOH}$) bridge the gap between simple organic molecules and complex biological compounds. Understanding their structure, their surprising acidity, and their derivatives (like esters and acyl chlorides) is crucial for success in A Level Chemistry. Don't worry if the reactions look intimidating at first; we will break them down step-by-step!
1. Structure and Physical Properties (The $\text{-COOH}$ Group)
1.1 The Carboxyl Functional Group
The defining feature of a carboxylic acid is the Carboxyl Group ($\text{-COOH}$).
It is a hybrid structure, containing two other functional groups:
- A Carbonyl group ($\text{C=O}$)
- A Hydroxyl group ($\text{-OH}$)
The general formula for an aliphatic carboxylic acid is $\text{C}_{n-1}\text{H}_{2n-1}\text{COOH}$.
1.2 Naming (Nomenclature)
Carboxylic acids are named by replacing the 'e' of the corresponding alkane with 'oic acid'.
Examples:
- $\text{CH}_3\text{COOH}$ (2 carbons) $\rightarrow$ Ethane $\rightarrow$ Ethanoic acid (Commonly known as Acetic Acid/Vinegar)
- $\text{CH}_3\text{CH}_2\text{COOH}$ (3 carbons) $\rightarrow$ Propane $\rightarrow$ Propanoic acid
- $\text{C}_6\text{H}_5\text{COOH}$ (Benzene ring + $\text{COOH}$) $\rightarrow$ Benzoic acid
1.3 Physical Properties: Hydrogen Bonding and Dimerisation
Carboxylic acids have significantly higher boiling points than alcohols, aldehydes, or ketones of comparable $M_r$. Why?
They can form strong intermolecular hydrogen bonds via both the $\text{C=O}$ (as an acceptor) and the $\text{O-H}$ (as a donor) parts of the carboxyl group.
In the liquid state and often in the gaseous phase (or in non-polar solvents), two carboxylic acid molecules bond together very strongly to form a stable structure called a dimer.
Analogy: Imagine two magnets sticking together perfectly (head-to-tail). This strong pairing means it takes much more energy (a higher temperature) to separate them and turn them into a gas.
Quick Review: Key Takeaways
The $\text{-COOH}$ group dictates the properties. Strong hydrogen bonding leads to stable dimers and high boiling points.
2. Preparation of Carboxylic Acids (Synthesis Routes)
There are three main ways you need to know to synthesize carboxylic acids. These are great examples of how functional groups can be interconverted.
2.1 Oxidation of Primary Alcohols and Aldehydes (Syllabus 18.1a)
This is the most common method, requiring strong oxidizing agents and heating under reflux.
Reagents: Acidified $\text{Potassium(VI) Dichromate}$ ($\text{K}_2\text{Cr}_2\text{O}_7$) or $\text{Acidified Potassium Manganate(VII)}$ ($\text{KMnO}_4$).
Conditions: Reflux (This ensures all the aldehyde intermediate is fully oxidized to the carboxylic acid).
$$ \text{Primary Alcohol } \xrightarrow{[\text{O}]} \text{ Aldehyde } \xrightarrow{[\text{O}]} \text{ Carboxylic Acid} $$
Remember: Oxidation must be carried out under reflux. If you distil the product immediately, you isolate the aldehyde instead.
2.2 Hydrolysis of Nitriles (Syllabus 18.1b)
Nitriles contain the $\text{R-C}\equiv\text{N}$ group. Hydrolyzing (breaking with water) this group adds one carbon atom to the chain of the starting material, which is useful for chain extension.
Reagents: Dilute acid ($\text{H}^+$) or Dilute alkali ($\text{OH}^-$)
Conditions: Heat (and subsequent acidification if using alkali).
$$ \text{R-C}\equiv\text{N} + 2\text{H}_2\text{O} + \text{H}^+ \xrightarrow{\text{Heat}} \text{R-COOH} + \text{NH}_4^+ $$
Note: If you use alkali ($\text{OH}^-$), you first get the carboxylate salt ($\text{RCOO}^-$). You must then add acid ($\text{H}^+$) to isolate the carboxylic acid ($\text{RCOOH}$).
2.3 Hydrolysis of Esters (Syllabus 18.1c)
Esters can be broken back down into the carboxylic acid and alcohol from which they were formed.
Reagents & Conditions:
- Acid Hydrolysis: Dilute acid ($\text{H}^+$) and heat (Reversible reaction).
- Alkaline Hydrolysis (Saponification): Dilute alkali ($\text{OH}^-$) and heat, followed by acidification to get the final $\text{RCOOH}$.
2.4 Preparation of Benzoic Acid (A-Level Synthesis, Syllabus 33.1a)
Benzoic acid (a carboxylic acid attached to a benzene ring) is prepared by oxidizing an alkyl side-chain attached to the benzene ring, such as methylbenzene.
Reagents: Hot alkaline $\text{Potassium Manganate(VII)}$ ($\text{KMnO}_4$)
Conditions: Heat, followed by addition of dilute acid.
Did you know? Even if the side chain is longer (like ethylbenzene), the strong oxidation chops off the entire chain, leaving only the single carbon atom that forms the $\text{-COOH}$ group attached to the ring.
Quick Review: Key Takeaways
Use refluxing oxidizer for $\text{ROH} \rightarrow \text{RCOOH}$. Use hydrolysis ($\text{H}^+$ or $\text{OH}^-$) for nitriles and esters.
3. Reactions of Carboxylic Acids
Carboxylic acids react in two primary ways: they act as acids (donating a proton) or they undergo reactions involving the whole functional group ($\text{-OH}$ and $\text{C=O}$).
3.1 Acidic Reactions (Syllabus 18.1b, 18.1c)
Carboxylic acids are weak acids, meaning they partially dissociate in water: $$ \text{RCOOH}(aq) \rightleftharpoons \text{RCOO}^-(aq) + \text{H}^+(aq) $$
They react with three main types of base/metal to form salts:
-
With Reactive Metals ($\text{Na}, \text{Mg}, \text{Zn}$): Produces a salt and hydrogen gas ($\text{H}_2$).
$$ 2\text{RCOOH}(aq) + \text{Mg}(s) \rightarrow (\text{RCOO})_2\text{Mg}(aq) + \text{H}_2(g) $$ -
With Alkalis ($\text{NaOH}, \text{KOH}$): Neutralisation reaction, producing a salt and water ($\text{H}_2\text{O}$).
$$ \text{RCOOH}(aq) + \text{NaOH}(aq) \rightarrow \text{RCOONa}(aq) + \text{H}_2\text{O}(l) $$ -
With Carbonates ($\text{Na}_2\text{CO}_3, \text{CaCO}_3$): Produces a salt, water, and carbon dioxide ($\text{CO}_2$). This is a characteristic test for acids (effervescence of $\text{CO}_2$).
$$ 2\text{RCOOH}(aq) + \text{Na}_2\text{CO}_3(s) \rightarrow 2\text{RCOONa}(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g) $$
3.2 Reactions involving the $\text{C-OH}$ bond
A. Esterification (Syllabus 18.1d)
Carboxylic acids react with alcohols ($\text{ROH}$) to form esters. This is a condensation reaction (water is eliminated).
Reagents: Alcohol (e.g., Ethanol)
Conditions: Concentrated $\text{H}_2\text{SO}_4$ catalyst and heat.
$$
\text{RCOOH} + \text{R}'\text{OH} \rightleftharpoons \text{RCOOR}' + \text{H}_2\text{O}
$$
Example: Ethanoic acid + Ethanol $\rightleftharpoons$ Ethyl ethanoate + Water.
B. Reduction (Syllabus 18.1e)
Carboxylic acids can be reduced back to primary alcohols.
Reagents: $\text{Lithium Aluminium Hydride}$ ($\text{LiAlH}_4$) in dry ether.
Conditions: Room temperature, followed by acid workup.
$$
\text{RCOOH} \xrightarrow{\text{LiAlH}_4} \text{RCH}_2\text{OH}
$$
Warning: $\text{LiAlH}_4$ is an extremely powerful and expensive reducing agent. We use the symbol $\text{[H]}$ for simplification in equations.
3.3 Converting to Acyl Chlorides (Syllabus 33.1b)
This reaction replaces the $\text{-OH}$ group with a $\text{-Cl}$ atom, forming a highly reactive carboxylic acid derivative.
Reagents: $\text{PCl}_5$ (Phosphorus pentachloride), $\text{PCl}_3$ (Phosphorus trichloride) + heat, or $\text{SOCl}_2$ (Thionyl chloride).
Using $\text{SOCl}_2$ is often preferred because the by-products ($\text{SO}_2$ and $\text{HCl}$) are gases, making purification easy.
$$
\text{RCOOH} + \text{SOCl}_2 \rightarrow \text{RCOCl} + \text{SO}_2 + \text{HCl}
$$
3.4 Further Oxidation (A-Level Redox, Syllabus 33.1c)
Most carboxylic acids are resistant to further oxidation. However, two specific small acids can be oxidized:
1. Methanoic Acid ($\text{HCOOH}$): Can be oxidised to $\text{CO}_2$ and $\text{H}_2\text{O}$ because it contains a reactive $\text{H}$ atom directly bonded to the carboxyl carbon. It reacts positively with mild oxidizing agents like:
- $\text{Fehling's reagent}$ (Copper(II) ions reduced to red-brown $\text{Cu}_2\text{O}$)
- $\text{Tollens' reagent}$ (Silver ions reduced to silver mirror)
2. Ethanedioic Acid ($\text{HOOCCOOH}$): Can be oxidized by warm acidified $\text{KMnO}_4$ to form $\text{CO}_2$ and $\text{H}_2\text{O}$. $$ \text{HOOCCOOH} + 2\text{MnO}_4^- + 6\text{H}^+ \rightarrow 2\text{CO}_2 + 2\text{H}_2\text{O} + 2\text{Mn}^{2+} $$
Quick Review: Key Takeaways
Carboxylic acids are weak acids (react with metals, alkalis, carbonates). They form esters with alcohols and highly reactive acyl chlorides with agents like $\text{SOCl}_2$. Small acids like methanoic acid can undergo redox tests.
4. Acidity Comparison (A-Level Depth)
4.1 Explaining Relative Acidity (Syllabus 33.1d)
We compare acidity based on the stability of the conjugate base formed after the proton ($\text{H}^+$) is released. A more stable conjugate base means the acid is stronger and dissociates more readily.
Ranking Acidity (Strongest to Weakest):
Carboxylic Acid $\text{RCOOH}$ > Phenol $\text{ArOH}$ > Alcohol $\text{ROH}$ > Water $\text{H}_2\text{O}$
Why are Carboxylic Acids Stronger than Alcohols and Phenols?
- Alcohols ($\text{ROH}$): The conjugate base is the alkoxide ion ($\text{RO}^-$). The attached alkyl group ($\text{R}$) is electron-donating, concentrating negative charge on the oxygen. This makes the $\text{RO}^-$ ion highly unstable, thus alcohols are very weak acids.
- Carboxylic Acids ($\text{RCOOH}$): The conjugate base is the Carboxylate ion ($\text{RCOO}^-$). The negative charge is spread out (delocalised) over both oxygen atoms due to resonance with the nearby $\text{C=O}$ group. This resonance stabilisation makes the carboxylate ion very stable, making $\text{RCOOH}$ a much stronger acid than an alcohol.
- Phenols ($\text{ArOH}$): The conjugate base is the phenoxide ion. The negative charge is delocalised into the benzene ring. While this stabilises the ion (making phenol stronger than alcohol), the delocalisation involves less electronegative carbon atoms and is therefore less effective than the resonance stabilization in the carboxylate ion.
Memory Aid: Stability of conjugate base (Carboxylate) $>$ (Phenoxide) $>$ (Alkoxide).
4.2 Effect of Substituents on Acidity (Syllabus 33.1e)
If we substitute a hydrogen atom on the $\alpha$-carbon (the carbon next to the $\text{-COOH}$ group) with an electron-withdrawing atom (like Chlorine, $\text{Cl}$), the acid becomes stronger.
This is explained by the Negative Inductive Effect:
- Chlorine is highly electronegative, pulling electron density away from the carboxylate ion.
- This withdrawal effectively reduces the negative charge density on the carboxylate ion, stabilizing it further.
- A more stable conjugate base means a stronger acid.
Example Ranking (Strongest to Weakest):
$\text{ClCH}_2\text{COOH}$ (Chloroethanoic acid) > $\text{CH}_3\text{COOH}$ (Ethanoic acid)
The more electron-withdrawing groups you add, the stronger the acid becomes (e.g., $\text{Cl}_3\text{CCOOH}$ is much stronger than $\text{CH}_3\text{COOH}$).
5. Carboxylic Acid Derivatives: Esters and Acyl Chlorides
Carboxylic acids are often converted into derivatives to change their reactivity. The two key derivatives you must know are esters and acyl chlorides.
5.1 Esters (Syllabus 18.2, 33.2)
Esters are responsible for many sweet, fruity smells. They are formed when the $\text{OH}$ group of a carboxylic acid is replaced by an $\text{OR}$ group ($\text{RCOOR}'$).
A. Preparation of Esters (Revisited and Expanded)
- Carboxylic Acid + Alcohol: (Fischer Esterification - slow and reversible, requires $\text{H}_2\text{SO}_4$ catalyst and heat).
- Acyl Chloride + Alcohol: (Fast, non-reversible, room temperature) $$ \text{RCOCl} + \text{R}'\text{OH} \rightarrow \text{RCOOR}' + \text{HCl} $$
B. Hydrolysis of Esters
This reverses ester formation to yield the parent acid and alcohol (covered in Section 2.3).
1. Acid Hydrolysis: (Dilute $\text{H}^+$ / heat). Gives $\text{RCOOH}$ and $\text{R}'\text{OH}$. $$ \text{RCOOR}' + \text{H}_2\text{O} \rightleftharpoons \text{RCOOH} + \text{R}'\text{OH} $$
2. Alkaline Hydrolysis (Saponification): (Dilute $\text{OH}^-$ / heat). Gives the carboxylate salt ($\text{RCOO}^-$) and $\text{R}'\text{OH}$. The acid ($\text{RCOOH}$) is released upon subsequent acidification.
This reaction is used to make soap! Soap is the sodium salt of a long-chain fatty acid.
5.2 Acyl Chlorides (Acid Chlorides) (A-Level Focus, Syllabus 33.3)
Acyl chlorides ($\text{RCOCl}$) are characterized by their extreme reactivity due to the highly polarized $\text{C=O}$ bond and the good leaving group ($\text{Cl}^-$).
A. Reactions and Addition-Elimination Mechanism
Acyl chlorides react rapidly at room temperature with compounds containing a lone pair of electrons (nucleophiles). These reactions follow a mechanism called addition-elimination.
The Mechanism Step-by-Step (Simplified):
- The nucleophile (Nuc) attacks the partially positive ($\delta+$) carbon in the carbonyl group ($\text{C=O}$).
- The $\pi$ electrons of the $\text{C=O}$ bond shift up onto the oxygen, forming a tetrahedral intermediate. (Addition step)
- The electrons shift back down from the oxygen, reforming the $\text{C=O}$ bond, and the chloride ion ($\text{Cl}^-$) is kicked out as the leaving group. (Elimination step)
B. Key Reactions of Acyl Chlorides ($\text{RCOCl}$)
All these reactions happen easily at room temperature:
-
Hydrolysis (Reaction with Water): Forms the parent carboxylic acid, often vigorously.
$$ \text{RCOCl} + \text{H}_2\text{O} \rightarrow \text{RCOOH} + \text{HCl} $$ -
Reaction with Alcohols ($\text{R}'\text{OH}$): Forms an ester.
$$ \text{RCOCl} + \text{R}'\text{OH} \rightarrow \text{RCOOR}' + \text{HCl} $$ -
Reaction with Ammonia ($\text{NH}_3$): Forms a primary amide ($\text{RCONH}_2$). (Need two equivalents of $\text{NH}_3$: one for reaction, one to mop up $\text{HCl}$).
$$ \text{RCOCl} + 2\text{NH}_3 \rightarrow \text{RCONH}_2 + \text{NH}_4\text{Cl} $$ -
Reaction with Primary/Secondary Amines: Forms secondary/tertiary amides.
$$ \text{RCOCl} + 2\text{R}'\text{NH}_2 \rightarrow \text{RCONHR}' + \text{R}'\text{NH}_3^+\text{Cl}^- $$ -
Reaction with Phenols: Forms phenyl esters (similar to alcohol reaction).
$$ \text{RCOCl} + \text{ArOH} \rightarrow \text{RCOOAr} + \text{HCl} $$
C. Relative Ease of Hydrolysis (Reactivity) (Syllabus 33.3d)
Acyl chlorides are far more reactive towards hydrolysis (reaction with water/nucleophiles) than halogenoalkanes or halogenoarenes. This is about the strength of the $\text{C-X}$ bond and the mechanism.
Ranking Reactivity (Most Reactive First):
Acyl Chloride ($\text{RCOCl}$) >> Halogenoalkane ($\text{RCl}$) > Halogenoarene ($\text{ArCl}$)
1. Acyl Chlorides: High reactivity because the addition-elimination mechanism (Section 5.2A) is easy. The $\text{C}$ atom is highly $\delta+$ and the $\text{Cl}^-$ is a great leaving group.
2. Halogenoalkanes: Moderate reactivity (requires heating with $\text{OH}^-$ or $\text{H}_2\text{O}$). React via nucleophilic substitution ($\text{S}_N1$/$\text{S}_N2$).
3. Halogenoarenes (Aryl Chlorides): Very low reactivity. The lone pair on the $\text{Cl}$ atom is delocalized into the benzene ring, giving the $\text{C-Cl}$ bond partial double bond character. This makes the bond much stronger and harder to break via substitution or hydrolysis.
Quick Review: Key Takeaways
Acyl chlorides are the most reactive derivative, undergoing addition-elimination with water, alcohols, ammonia, and amines, typically at room temperature. Their reactivity is much higher than that of alkyl or aryl halides.