Complete interactive resource covering preparation, reactions, properties, and derivatives
Carboxylic acids are organic compounds containing the carboxyl functional group (-COOH). They are important in both industrial and biological contexts, serving as precursors to many other compounds.
Structural representation of carboxylic acid functional group
Image: Carboxylic acid structure showing -COOH groupCarboxylic acids are stronger acids than alcohols due to the stability of the carboxylate anion formed after deprotonation. The resonance stabilization of the carboxylate ion distributes the negative charge over two oxygen atoms, making it more stable than the alkoxide ion from alcohols.
pKa values comparison:
The lower pKa value indicates stronger acidity. Carboxylic acids are about 1010 times more acidic than alcohols.
RESONANCE is the key! Remember: Carboxylic acids have resonance-stabilized conjugate bases (carboxylate ions), while alcohols don’t. The negative charge in carboxylate is delocalized over two oxygen atoms, making it more stable.
Mnemonic: “CARBoxylic acids have COOL Resonance to Be acidic”
When a Grignard reagent (R-MgX) reacts with carbon dioxide (dry ice or CO2 gas), it forms an addition product which upon acidic hydrolysis yields a carboxylic acid.
Example: Methylmagnesium bromide with CO2 gives acetic acid after hydrolysis.
Key points:
Remember: Grignard + CO2 = Carboxylic Acid. Visualize it as “adding a carbon” to the alkyl chain. The reaction increases the carbon chain by one atom.
Common mistake: Forgetting the acidic hydrolysis step. The initial product is a magnesium salt that needs acid to release the carboxylic acid.
Alkyl cyanides (nitriles) undergo hydrolysis in acidic or basic conditions to form carboxylic acids with the liberation of ammonia.
Example: Acetonitrile (CH3CN) hydrolyzes to acetic acid (CH3COOH).
Mechanism: The nitrile group is first protonated, then water attacks the electrophilic carbon, followed by tautomerization to yield the amide intermediate, which further hydrolyzes to the acid.
Note: Basic hydrolysis yields carboxylate salt, which needs acidification to get the free acid.
Think: Nitrile → Amide → Acid. The hydrolysis occurs in two stages. Remember that nitriles can be prepared from alkyl halides via SN2 with cyanide ion, providing a two-step route from RX to RCOOH.
Visual cue: C≡N becomes C=O(OH) by adding H2O and losing NH3.
Primary alcohols can be oxidized to carboxylic acids using strong oxidizing agents like potassium dichromate (K2Cr2O7) or potassium permanganate (KMnO4) in acidic medium.
Example: Ethanol oxidizes to acetaldehyde then to acetic acid.
Key oxidizing agents:
Note: The reaction proceeds through an aldehyde intermediate. To stop at aldehyde, use milder conditions (PCC for Cr(VI) based oxidation).
Remember the oxidation states: Alcohol (-OH) → Aldehyde (CHO) → Acid (COOH). Primary alcohols give acids, secondary give ketones, tertiary don’t oxidize easily.
Color change: K2Cr2O7 changes from orange to green (Cr6+ to Cr3+). KMnO4 changes from purple to colorless (Mn7+ to Mn2+).
Aldehydes are readily oxidized to carboxylic acids even with mild oxidizing agents due to the presence of the reactive carbonyl group.
Example: Acetaldehyde (CH3CHO) oxidizes to acetic acid (CH3COOH).
Common reagents:
Note: This oxidation is easier than alcohol oxidation because aldehydes are more susceptible to oxidation.
Think: Aldehydes are “half-way” oxidized. They have a hydrogen on the carbonyl carbon that can be replaced by OH to give the acid.
Test for aldehydes: Tollens’ test gives silver mirror; Fehling’s test gives brick red precipitate. Both confirm presence of aldehyde group which can be oxidized to acid.
Alkyl benzenes with at least one benzylic hydrogen can be oxidized to benzoic acid derivatives using strong oxidizing agents like KMnO4.
Example: Ethylbenzene oxidizes to benzoic acid with KMnO4/H2SO4.
Mechanism: The benzylic position is particularly susceptible to oxidation. The alkyl side chain is completely oxidized to carboxyl group regardless of its length (except tert-butyl which resists oxidation).
Limitations: Tertiary alkyl groups (no α-hydrogen) are resistant to oxidation under these conditions.
Remember: Any alkyl benzene with α-H oxidizes to benzoic acid. The entire side chain (except the carbon directly attached to benzene) gets oxidized away to CO2 and H2O.
Exception: tert-butylbenzene doesn’t oxidize because it has no benzylic hydrogens. This is a test to distinguish between alkyl benzenes.
R-COOR’
Formed by esterification with alcohols
Fruity smells
R-COCl
Formed with SOCl2, PCl3, PCl5
Very reactive
R-CONH2
Formed with NH3
Important in proteins
(RCO)2O
Formed with P2O5
Good acylating agents
Carboxylic acids react with phosphorus halides (PCl5, PCl3) or thionyl chloride (SOCl2) to form acyl halides.
Why SOCl2 is preferred: By-products (SO2 and HCl) are gases that escape, leaving pure acyl chloride.
Reactivity order: Acid chlorides > Acid anhydrides > Esters > Amides
Remember: SOCl2 is the best because gaseous by-products mean easy purification. Think “SOCl2 = Simple Clean Chlorination”.
Reactivity: Acid chlorides are the most reactive derivatives – they react with almost any nucleophile.
Acid anhydrides are formed by the condensation of two carboxylic acid molecules with a dehydrating agent like P2O5.
Example: Two acetic acid molecules form acetic anhydride.
Uses: Acetic anhydride is used in aspirin synthesis, dye manufacturing, and cellulose acetate production.
Think: Anhydride = “without water”. Two acid molecules lose a water molecule between them.
Naming: Named by replacing “acid” with “anhydride” (e.g., acetic acid → acetic anhydride).
Carboxylic acids react with alcohols in the presence of acid catalyst (usually conc. H2SO4) to form esters and water.
Example: Acetic acid + ethanol → ethyl acetate (fruity smell).
Key points:
Esterification = Acid + Alcohol → Ester + Water. Remember: It’s a reversible reaction. To get good yield, use excess alcohol or remove water.
Fruity smells: Esters are responsible for fruit flavors (e.g., isoamyl acetate = banana, ethyl butanoate = pineapple).
Carboxylic acids react with ammonia to form ammonium salts, which upon heating lose water to yield amides.
Example: Acetic acid + ammonia → acetamide.
Alternative route: More commonly, amides are prepared from acid chlorides or anhydrides which react readily with ammonia.
Importance: Amide bond is the key linkage in proteins (peptide bond).
Remember: Acid → Ammonium salt → Amide. Heating drives off water. Amides are the least reactive derivatives.
Biological connection: The amide bond (peptide bond) links amino acids in proteins. This is one of the most important bonds in biochemistry.
Carboxylic acids can be reduced to primary alcohols using strong reducing agents like lithium aluminum hydride (LiAlH4).
Example: Acetic acid reduces to ethanol.
Important: LiAlH4 is a powerful reducing agent that reduces many functional groups. NaBH4 is milder and doesn’t reduce carboxylic acids.
Mechanism: The carbonyl carbon is attacked by hydride, eventually leading to alcohol after workup.
Remember: LiAlH4 reduces acids to alcohols (adds 2 H2 molecules). NaBH4 doesn’t work for acids – it only reduces aldehydes and ketones.
Reduction equivalents: Carboxylic acid needs 4 H atoms (2 H2 molecules) to become primary alcohol.
When carboxylic acid salts are heated with soda lime (NaOH + CaO), they undergo decarboxylation to yield alkanes with one less carbon atom.
Example: Sodium acetate decarboxylates to methane.
Mechanism: Free radical mechanism involving formation of carbonate intermediate.
Limitation: Only works well for sodium salts of simple carboxylic acids. Aromatic acids give poor yields.
Think: Decarboxylation = removing CO2. The acid loses one carbon atom (as CO2) to become an alkane.
Remember: Soda lime = NaOH + CaO. The CaO keeps the mixture dry and helps absorb CO2.
CH3COOH
Vinegar, preservative
Chemical synthesis
C6H8O7
Citrus fruits
Preservative, flavor
C6H5COOH
Food preservative
Dye intermediate
o-HOC6H4COOH
Aspirin precursor
Skin care products
Acetic Acid (CH3COOH): Main component of vinegar (5-8%), used as preservative, solvent, and chemical feedstock for vinyl acetate, acetic anhydride, etc.
Citric Acid (C6H8O7): Found in citrus fruits, used as preservative, flavor enhancer, acidity regulator in food and beverages.
Malic Acid (C4H6O5): Found in apples, used as food additive and pH control agent.
Tartaric Acid (C4H6O6): Found in grapes, used in baking powder and as antioxidant in food.
Salicylic Acid (o-HOC6H4COOH): Used in acne treatments, wart removal, and as precursor to aspirin (acetylsalicylic acid).
Benzoic Acid (C6H5COOH): Used as food preservative (E210), and in production of dyes, perfumes, and plastics.
Butyric Acid (C3H7COOH): Found in butter, cheese, and milk; responsible for their characteristic smell.
Acetic = Vinegar (think cooking)
Citric = Citrus fruits (lemons, oranges)
Malic = Apples (malus = apple in Latin)
Tartaric = Grapes (wine tartar)
Salicylic = Willow bark (salix = willow)
Benzoic = Benzoin resin (traditional source)
Interactive concept map showing interconversions between carboxylic acids and their derivatives
Acid → (SOCl2) → Acid Chloride → (NH3) → Amide → (H2O/H+) → Acid Acid → (Alcohol/H+) → Ester → (H2O/H+) → Acid Acid → (LiAlH4) → Alcohol → (Oxidation) → Aldehyde → (Oxidation) → Acid1. Understand Mechanisms: Don’t just memorize reactions – understand the electron movement. Carboxylic acid chemistry is largely about nucleophilic acyl substitution.
2. Create Reaction Maps: Draw interconnected pathways showing how carboxylic acids can be converted to various derivatives and vice versa.
3. Compare pKa Values: Make a table comparing pKa of carboxylic acids, alcohols, phenols, and mineral acids to understand relative acidities.
4. Practice Naming: Learn IUPAC naming of carboxylic acids and their derivatives systematically.
5. Relate to Real World: Connect each carboxylic acid to its natural source and industrial use (e.g., acetic acid in vinegar, citric acid in lemons).
6. Focus on Key Conversions: Master these: acid → acid chloride (SOCl2), esterification, hydrolysis of nitriles, oxidation of 1° alcohols/aldehydes.
7. Solve Problems Regularly: Practice conversion problems and mechanism questions daily.
8. Use Mnemonics: Create memory aids for reaction conditions and reagent selectivity.
9. Review Physical Properties: Understand why carboxylic acids have high boiling points (hydrogen bonding) and solubility trends.
10. Test Yourself: Use the quiz section regularly to identify weak areas.
Time Management: Allocate time based on marks – spend more on mechanism questions.
Read Questions Carefully: Note if they ask for reagents, conditions, or mechanisms.
Show Your Work: Even if final answer is wrong, partial credit for correct steps.
Common Mistakes to Avoid:
© 2023 EverExams.com | Comprehensive Chemistry Learning Resources
This interactive module is part of the EverExams Organic Chemistry Series