Carboxylic Acids: A2 Chemistry Study Notes
Welcome to your comprehensive guide to Carboxylic Acids! Whether you love organic chemistry or find it a bit daunting, this guide breaks down everything step-by-step. Carboxylic acids are everywhere in daily life: ethanoic acid gives vinegar its sharp taste, methanoic acid is found in ant stings, and citric acid gives lemons their zesty kick. In this topic, we will explore their structure, physical properties, why they behave as weak acids, how to make them, and how they react.
1. Structure and Nomenclature
A carboxylic acid is an organic compound that contains the carboxyl functional group: \(-\text{COOH}\).
This group is made of two parts attached to the same carbon atom:
1. A carbonyl group (\(\text{C}=\text{O}\))
2. A hydroxyl group (\(-\text{O}-\text{H}\))
The general formula for an aliphatic carboxylic acid is \(\text{R}-\text{COOH}\) (or \(\text{C}_n\text{H}_{2n+1}\text{COOH}\)).
How to Name Carboxylic Acids (IUPAC Rules)
1. Find the longest continuous carbon chain that contains the \(-\text{COOH}\) group.
2. The carbon in the \(-\text{COOH}\) group is always carbon-1 (you do not need to write the number "1" in the name).
3. Replace the "-e" ending of the alkane name with "-oic acid".
Examples:
• \(\text{HCOOH}\) : Methanoic acid (1 carbon)
• \(\text{CH}_3\text{COOH}\) : Ethanoic acid (2 carbons)
• \(\text{CH}_3\text{CH}_2\text{COOH}\) : Propanoic acid (3 carbons)
• \(\text{CH}_3\text{CH(CH}_3\text{)COOH}\) : 2-methylpropanoic acid (longest chain is 3 carbons, methyl on carbon-2)
Key Takeaway: The carboxyl carbon is always carbon-1 in numbering the main carbon chain.
2. Physical Properties
Boiling Points
Carboxylic acids have significantly higher boiling points than alkanes, aldehydes, ketones, and even alcohols of similar molecular mass. Why is this?
• Carboxylic acids can form strong hydrogen bonds with each other due to the highly polar \(-\text{O}-\text{H}\) and \(\text{C}=\text{O}\) groups.
• In pure liquid state or non-polar solvents, two carboxylic acid molecules often pair up to form a stable dimer held together by two hydrogen bonds:
\(\text{R}-\text{C}(=\text{O}\cdots\text{H}-\text{O})-\text{O}-\text{H}\cdots\text{O}=\text{C}-\text{R}\)
This dimerization effectively doubles the size of the particle, which dramatically increases both London dispersion (van der Waals) forces and hydrogen bonding, requiring much more thermal energy to separate the molecules.
Solubility in Water
• Lower members (1 to 4 carbons): Completely miscible (mixes fully) with water because they can readily form hydrogen bonds with water molecules.
• Higher members (5+ carbons): Solubility decreases rapidly as chain length increases. The non-polar, hydrophobic hydrocarbon chain becomes larger and disrupts the hydrogen-bonded network of water without providing strong enough interactions to compensate.
Common Mistake to Avoid: Do not say that long-chain acids cannot form hydrogen bonds. The \(-\text{COOH}\) group can still form hydrogen bonds, but the large non-polar chain dominates the overall physical behavior!
Key Takeaway: Dimer formation leads to high boiling points; short-chain carboxylic acids dissolve well in water because of hydrogen bonding.
3. Acidity of Carboxylic Acids
Carboxylic acids are weak acids. In aqueous solution, they partially dissociate to release hydrogen ions (\(\text{H}^+\)):
\(\text{RCOOH(aq)} \rightleftharpoons \text{RCOO}^-\text{(aq)} + \text{H}^+\text{(aq)}\)
Why are Carboxylic Acids more acidic than Alcohols?
Don't worry if this concept feels tricky at first! Let's break down why the \(-\text{O}-\text{H}\) bond in a carboxylic acid breaks much more easily than in an alcohol:
1. Delocalisation of Charge in the Carboxylate Ion: When a carboxylic acid loses a proton, it forms a carboxylate ion (\(\text{RCOO}^-\)). In this ion, the negative charge is not stuck on one oxygen; instead, the \(\pi\)-electrons are delocalised across both oxygen atoms and the central carbon (\(\text{O}-\text{C}-\text{O}\)). This delocalisation spreads the negative charge out, making the conjugate base stable and lowering the energy needed to form it.
2. Alcohol Comparison: When an alcohol (\(\text{ROH}\)) loses a proton, it forms an alkoxide ion (\(\text{RO}^-\)). The negative charge is localized entirely on a single oxygen atom, making it unstable and eager to grab the proton back.
Key Takeaway: Carboxylic acids are weak acids because they partially dissociate, but they are much more acidic than alcohols due to the resonance stabilisation / delocalisation of negative charge in the carboxylate anion.
4. Synthesis / Preparation of Carboxylic Acids
There are two primary methods to make carboxylic acids in the laboratory:
Method 1: Oxidation of Primary Alcohols and Aldehydes
• Reagents: Acidified potassium dichromate(\(\text{VI}\)), \(\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}_2\text{SO}_4\)
• Conditions: Heat under reflux (excess oxidising agent ensures complete oxidation to the acid)
• Observation: Color change from orange (\(\text{Cr}_2\text{O}_7^{2-}\)) to green (\(\text{Cr}^{3+}\))
Equation:
\(\text{RCH}_2\text{OH} + 2[\text{O}] \rightarrow \text{RCOOH} + \text{H}_2\text{O}\)
Method 2: Hydrolysis of Nitriles
Nitriles (\(\text{R}-\text{C}\equiv\text{N}\)) can be hydrolyzed by heating with either an aqueous acid or an aqueous alkali:
a) Acid Hydrolysis:
• Reagents & Conditions: Heat under reflux with dilute hydrochloric acid (\(\text{HCl}\)) or sulfuric acid (\(\text{H}_2\text{SO}_4\)).
• Products: Carboxylic acid and ammonium salt.
• Equation:
\(\text{RCN} + 2\text{H}_2\text{O} + \text{H}^+ \rightarrow \text{RCOOH} + \text{NH}_4^+\)
Example: \(\text{CH}_3\text{CN} + 2\text{H}_2\text{O} + \text{HCl} \rightarrow \text{CH}_3\text{COOH} + \text{NH}_4\text{Cl}\)
b) Alkaline Hydrolysis:
• Reagents & Conditions: Heat under reflux with aqueous sodium hydroxide (\(\text{NaOH}\)), followed by acidification with dilute strong acid (\(\text{HCl}\)).
• Step 1 (Alkaline hydrolysis produces carboxylate salt and ammonia gas):
\(\text{RCN} + \text{H}_2\text{O} + \text{OH}^- \rightarrow \text{RCOO}^- + \text{NH}_3\)
• Step 2 (Acidification releases the free carboxylic acid):
\(\text{RCOO}^- + \text{H}^+ \rightarrow \text{RCOOH}\)
Key Takeaway: Oxidation of primary alcohols uses reflux with acidified \(\text{K}_2\text{Cr}_2\text{O}_7\). Hydrolysis of nitriles adds one carbon to an original haloalkane chain and converts \(-\text{C}\equiv\text{N}\) to \(-\text{COOH}\).
5. Chemical Reactions of Carboxylic Acids
1. Reactions as Acids (Salt Formation)
Carboxylic acids undergo typical acid reactions to form salts known as carboxylates:
• With Metals:
\(\text{Acid} + \text{Metal} \rightarrow \text{Salt} + \text{Hydrogen}\)
\(2\text{CH}_3\text{COOH} + \text{Mg} \rightarrow (\text{CH}_3\text{COO})_2\text{Mg} + \text{H}_2\)
Observation: Effervescence (bubbles of \(\text{H}_2\) gas), metal dissolves.
• With Alkalis / Bases:
\(\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}\)
\(\text{CH}_3\text{COOH} + \text{NaOH} \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O}\)
• With Carbonates and Hydrogencarbonates:
\(\text{Acid} + \text{Carbonate} \rightarrow \text{Salt} + \text{Water} + \text{Carbon Dioxide}\)
\(2\text{CH}_3\text{COOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2\)
\(\text{CH}_3\text{COOH} + \text{NaHCO}_3 \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2\)
Observation: Vigorous effervescence; the gas turns limewater milky (\(\text{CO}_2\)). This reaction is a useful diagnostic test to distinguish carboxylic acids from phenols and alcohols!
2. Reduction to Primary Alcohols
• Reducing Agent: Lithium tetrahydridoaluminate(\(\text{III}\)) / Lithium aluminium hydride, \(\text{LiAlH}_4\)
• Condition: In dry ether solvent at room temperature (followed by careful addition of dilute acid to hydrolyse the complex).
• Note: Sodium borohydride (\(\text{NaBH}_4\)) is not strong enough to reduce carboxylic acids; you must use \(\text{LiAlH}_4\).
Equation:
\(\text{RCOOH} + 4[\text{H}] \rightarrow \text{RCH}_2\text{OH} + \text{H}_2\text{O}\)
3. Esterification (Reaction with Alcohols)
• Reagents: Carboxylic acid + Alcohol
• Catalyst: Concentrated sulfuric acid (\(\text{H}_2\text{SO}_4\))
• Conditions: Heat under reflux
• Reaction type: Condensation / Esterification (reversible reaction)
General Equation:
\(\text{RCOOH} + \text{R}'\text{OH} \rightleftharpoons \text{RCOOR}' + \text{H}_2\text{O}\)
Example:
\(\text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH} \rightleftharpoons \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}\)
(Ethanoic acid + Ethanol \(\rightleftharpoons\) Ethyl ethanoate + Water)
Observation: Formation of a sweet, fruity smell.
4. Reaction with Phosphorus Pentachloride (\(\text{PCl}_5\))
Carboxylic acids react vigorously with \(\text{PCl}_5\) at room temperature to form acyl chlorides (acid chlorides). This replaces the \(-\text{OH}\) group with a \(-\text{Cl}\) atom.
• Reagents & Conditions: Solid \(\text{PCl}_5\), dry conditions, room temperature.
• Equation:
\(\text{RCOOH} + \text{PCl}_5 \rightarrow \text{RCOCl} + \text{POCl}_3 + \text{HCl}\)
• Observations: Steamy, white fumes of hydrogen chloride (\(\text{HCl}\)) gas that turn damp blue litmus paper red.
Key Takeaway:
• Carbonate test (\(\text{Na}_2\text{CO}_3\)) \(\rightarrow\) \(\text{CO}_2\) gas produced.
• Reduction with \(\text{LiAlH}_4\) \(\rightarrow\) primary alcohol.
• Reaction with alcohol + conc. \(\text{H}_2\text{SO}_4\) \(\rightarrow\) ester (fruity smell).
• Reaction with \(\text{PCl}_5\) \(\rightarrow\) acyl chloride + steamy fumes of \(\text{HCl}\).
Quick Summary & Exam Checklist
• Functional Group: \(-\text{COOH}\) (contains \(\text{C}=\text{O}\) and \(-\text{OH}\)).
• Boiling Point: High due to hydrogen bonding and dimerisation.
• Solubility: First four carbons dissolve readily in water; decreases with longer hydrocarbon chains.
• Acidity: Weak acids; carboxylate anion is stabilized by delocalisation of negative charge across \(\text{O}-\text{C}-\text{O}\).
• Preparation: Reflux \(1^\circ\) alcohol with \(\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}_2\text{SO}_4\) OR hydrolyse nitriles with dilute acid/alkali.
• Key Reactions to Memorise: Neutralization (\(\text{NaHCO}_3 \rightarrow \text{CO}_2\)), Reduction (\(\text{LiAlH}_4 \rightarrow 1^\circ\) alcohol), Esterification (\(\text{ROH} / \text{H}_2\text{SO}_4 \rightarrow\) ester), Chlorination (\(\text{PCl}_5 \rightarrow\) acyl chloride + \(\text{POCl}_3\) + \(\text{HCl}\)).