Carboxylic Acids
Section: 11. Organic Chemistry | Syllabus: Cambridge AS Level Physics 9702
Carboxylic Acids Carboxylic acids are organic compounds that contain the carboxyl functional group (-COOH). This group consists of a carbonyl group (C=O) and a hydroxyl group (-OH) attached to the same carbon atom.
Carboxylic Acid An organic compound containing the carboxyl functional group (-COOH), which consists of a carbonyl group (C=O) and a hydroxyl group (-OH) attached to the same carbon atom. The Carboxyl Group The functional group is written as -COOH or -CO₂H Structure: O ‖ -C-OH The carbon atom is double-bonded to an oxygen atom (C=O) The same carbon is also bonded to an -OH group This functional group gives carboxylic acids their acidic properties General Formula The general formula for carboxylic acids is: C n H 2n+1 COOH or R-COOH where: n = number of carbon atoms in the alkyl chain (can be 0) R = alkyl group or hydrogen -COOH = carboxyl group (functional group) Homologous Series Carboxylic acids form a homologous series with: The same functional group (-COOH) Similar chemical properties Gradual change in physical properties Each member differs from the next by CH₂ The First Six Carboxylic Acids Name Molecular Formula Structural Formula State at Room Temp Methanoic acid (Formic acid) HCOOH H-COOH Liquid Ethanoic acid (Acetic acid) CH₃COOH CH₃-COOH Liquid Propanoic acid C₂H₅COOH CH₃-CH₂-COOH Liquid Butanoic acid C₃H₇COOH CH₃-CH₂-CH₂-COOH Liquid Pentanoic acid C₄H₉COOH CH₃-(CH₂)₂-CH₂-COOH Liquid Hexanoic acid C₅H₁₁COOH CH₃-(CH₂)₃-CH₂-COOH Liquid Naming Carboxylic Acids Carboxylic acids are named by: Finding the longest carbon chain including the -COOH carbon Using the alkane name but changing the ending from -ane to -oic acid The carbon in -COOH is always carbon number 1 Examples: Meth-anoic acid = 1 carbon total (including COOH) Eth-anoic acid = 2 carbons total (CH₃-COOH) Prop-anoic acid = 3 carbons total (CH₃-CH₂-COOH) Common Names Many carboxylic acids also have common (trivial) names: Methanoic acid = Formic acid (from Latin "formica" = ant) Ethanoic acid = Acetic acid (from Latin "acetum" = vinegar) Physical Properties of Carboxylic Acids Acidity Carboxylic acids are weak acids They partially dissociate in water to release H⁺ ions pH typically around 3-4 for dilute solutions Stronger acids than phenols but weaker than mineral acids (HCl, H₂SO₄) Ionization in water: CH₃COOH ⇌ CH₃COO⁻ + H⁺ (ethanoic acid ⇌ ethanoate ion + hydrogen ion) Boiling Points Higher boiling points than alcohols and alkanes of similar molecular mass Reason: Can form strong hydrogen bonds between molecules Can form dimers (pairs) through double hydrogen bonding Boiling points increase as chain length increases Solubility in Water Short-chain carboxylic acids are very soluble in water Reason: -COOH group can form hydrogen bonds with water Also ionize slightly to form ions which dissolve well Solubility decreases as chain length increases Longer hydrocarbon chains are non-polar Smell Short-chain acids have sharp, pungent, vinegary smells Medium-chain acids smell unpleasant (like rancid butter, sweaty feet) Example: Butanoic acid smells like vomit/rancid butter Production of Carboxylic Acids 1.
Oxidation of Primary Alcohols Primary alcohols can be oxidized to carboxylic acids using oxidizing agents. Oxidizing Agents Potassium dichromate(VI) in dilute sulfuric acid Potassium manganate(VII) Heated with excess oxidizing agent under reflux Example - Ethanol to Ethanoic Acid: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O (ethanol + oxygen → ethanoic acid + water) Observations Orange dichromate(VI) turns green Purple manganate(VII) turns colorless 2.
Oxidation of Aldehydes Aldehydes are easily oxidized to carboxylic acids. Example: CH₃CHO + [O] → CH₃COOH (ethanal + oxygen → ethanoic acid) 3. Bacterial Oxidation Ethanol in wine/beer can be oxidized to ethanoic acid by bacteria in air.
Acetobacter bacteria use oxygen to oxidize ethanol This is how wine turns to vinegar if left open to air Chemical Properties of Carboxylic Acids 1. Typical Acid Reactions As acids, carboxylic acids show all the typical reactions of acids: a) Reaction with Metals React with reactive metals to produce salt + hydrogen gas 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂ (ethanoic acid + magnesium → magnesium ethanoate + hydrogen) Metal fizzes/dissolves Hydrogen gas produced (pops with lit splint) b) Reaction with Metal Oxides React with metal oxides to produce salt + water 2CH₃COOH + CuO → (CH₃COO)₂Cu + H₂O (ethanoic acid + copper oxide → copper ethanoate + water) Black copper oxide dissolves Blue solution of copper ethanoate forms c) Reaction with Metal Hydroxides (Bases) React with bases to produce salt + water (neutralization) CH₃COOH + NaOH → CH₃COONa + H₂O (ethanoic acid + sodium hydroxide → sodium ethanoate + water) d) Reaction with Metal Carbonates React with carbonates to produce salt + water + carbon dioxide 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂ (ethanoic acid + sodium carbonate → sodium ethanoate + water + carbon dioxide) Observations Fizzing/effervescence (CO₂ bubbl…
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