ORGANIC ACID
MF: C_nH_2nO₂ (alkanoic acid)
functional group: carboxylic group: -C(=O)OH (H attached to O can be ionised > H⁺/H₃O⁺ > acidic properties)

Organic acids in nature
methanoic/ formic acid: in ants
ethanoic acid: in vinegar (vinegar: 7% ethanoic acid)
citric acid (white crystalline solid at rtp): fruits
lactic acid: in sour milk, produced in muscles during exercise when O₂ lacking> cramps
benzoic acid (white solid at rtp): in some plants (sodium benzoate: C₆H₅C(=O)O^-Na⁺ used as preservative)
steoric acid: CH₃(CH₂)₁₆C(=O)OH

Formation of alkanoic acids
Lab preparation
Oxidation of 1° alcohols or aldehydes
reagent: acidified oxidising agent
condition: heat under reflux
  R-CH₂OH + [O] >(oxidising agent, heat, reflux)> R-C(=O)OH + H₂O
  R-C(=O)H + [O] >(oxidising agent, heat, reflux)> R-C(=O)OH observation: excess alcohol / aldehyde: KMnO₄: purple > colourless, K₂Cr₂O₇: orange > green

Hydrolysis of alkanonitrile
reagent: dil HCl
condition: heat under reflux
R-CN + HCl + 2H₂O >(heat, reflux)> R-C(=O)OH + NH₄Cl [-CN hydrolysed to -COOH]

Hydrolysis of amide or acid amide
reagent: dil HCl
condition: heat under reflux
R-C(=O)-NH₂ + HCl + 2H₂O >(heat, reflux)> R-C(=O)OH + NH₄Cl [-C(=O)NH₂ hydrolysed to -COOH]

Hydrolysis of ester
reagent: dil HCl / dil H₂SO₄
condition: heat under reflux


Industrial preparation of ethanoic acid
reagent: bacteria
condition: O₂ available
CH₃CH₂OH + O₂ >(bacteria)> CH₃COOH + H₂O

Physical properties
-lower members: liquids w/ sharp, characteristic smell of vinegar, higher members: white, waxy solids
-higher mp,bp than expected due to H-bond
-lower members: soluble in water, higher members: insoluble in water (H-bonds bet acid molecules too strong)

Acidic properties
-Neutralised by alkalis/ bases
reagent: NaOH(aq)
condition: room temp
R-C(=O)OH + NaOH > R-C(=O)O^-Na⁺ + H₂O
-Reaction w/ Na₂CO₃
reagent: Na₂CO₃ (aq)
condition: room temp
2R-C(=O)OH + Na₂CO₃ > 2R-C(=O)O^-Na⁺ + CO₂ + H₂O
observation: -white Na₂CO₃ dissolves, -bubbles of CO₂ evovled
Formation of acid derivatives
alkanoic acid; -OH group replaced to form acid derivatives
-OH replaced by:
-OR > R-C(=O)OR: ester
-Cl > R-C(=O)Cl: acid chloride
-NH₂ > R-C(=O)NH₂: amide
-OC(=O)R > R-C(=O)OC(=O)R: acid anhydride

Formation of esters
reagent: alcohol, conc H₂SO₄
condition: heat under reflux


Formation of acid chloride
regaent: PCl₅
condition: room temp
R-C(=O)OH + PCl₅ > R-C(=O)Cl + POCl₃ + HCl
observation: steamy white fumes of HCl (shows presence of -OH group)

Formation of amides
reagent: NH₃ (aq)
condition: neutralise acid at room temp > ammonium alkanoate, NH₄⁺ alkanoate: heat & dehydrate > amide
R-C(+O)OH(l) + NH₃(aq) > R-C(=O)O^-NH₄⁺ >(heat-dehydrate)> R-C(=O)NH₂(alkanoamide) + H₂O

Reduction to 1° alcohol
reagent: LiAlH₄ (tetrahydriodoaluminate(III)/ Lithium Aluminium Hydride) in dry ether
condition: ice cold (<10°C)
R-C(=O)OH + 4[H] >(LiAlH₄, <10°C)> R-CH₂OH + H₂O

Acidity of organic acids
alkanoic acid molecule alkanoate ion + hydrogen ion


Structural factors influencing acidity
-Electron-withdrawing group attached to C chain

electron-withdrawing groups draw e-s from carboxyl group > reducing -ve charge density on alkanoate anion > stabilises anion & weakens O-H bond > more alkanoate ions and H⁺ formed > acidity increases
electron-withdrawing groups: halogen group, phenyl group; C₆H₅-, groups with unsaturated bonds (-C=O, -NO₂)
-greater # of electron-withdrawing groups > alkanoate ion stablised more + O-H weakened more > stronger acid
-greater dist of electron-withdrawing groups from -C(=O)OH group > less e^- withdrawing effect > weaker acid
-more electronegative electron-withdrawing group > stronger e^- withdrawing effect > stronger acid
-Electron-donating group attached to C chain

electron-donating group pushes e^-s towards carboxyl group > increase +ve charge density of alkanoate ion > O-H bond stronger > less alkanoate & H⁺ ions > weaker acid
electron-donating groups: alkyl group; C_nH_2n+1-, hydroxy group; -OH , amino group; -NH₂
-more electron-donating groups > more electron-donating effect > weaker acid
-greater dist from -C(=O)OH group > less electron-donating effect > stronger acid


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