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Carboxylic Acids: Structure, Properties, and Reactions

Explore carboxylic acids, including common names, nomenclature, reactions, and why they are more acidic than alcohols. Learn about their synthesis, physical properties, and how substituent groups affect their acid strength.

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Carboxylic Acids: Structure, Properties, and Reactions

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  1. Carboxylic acids: R-COOH, R-CO2H, Common names: HCO2H formic acid L. formica ant CH3CO2H acetic acid L. acetum vinegar CH3CH2CO2H propionic acid G. “first salt” CH3CH2CH2CO2H butyric acid L. butyrum butter CH3CH2CH2CH2CO2H valeric acid L. valerans

  2. Carboxylic acids, common names: … CH3(CH2)4CO2H caproic acid L. caper goat CH3(CH2)5CO2H --- CH3(CH2)6CO2H caprylic acid CH3(CH2)7CO2H --- CH3(CH2)8CO2H capric acid CH3(CH2)9CO2H --- CH3(CH2)10CO2H lauric acid oil of lauryl

  3. 5 4 3 2 1 C—C—C—C—C=O δ γ β α used in common names

  4. special names

  5. IUPAC nomenclature for carboxylic acids: parent chain = longest, continuous carbon chain that contains the carboxyl group  alkane, drop –e, add –oic acid HCOOH methanoic acid CH3CO2H ethanoic acid CH3CH2CO2H propanoic acid CH3 CH3CHCOOH 2-methylpropanoic acid Br CH3CH2CHCO2H 2-bromobutanoic acid

  6. dicarboxylic acids: HOOC-COOH oxalic acid HO2C-CH2-CO2H malonic acid HO2C-CH2CH2-CO2H succinic acid HO2C-CH2CH2CH2-CO2H glutaric acid HOOC-(CH2)4-COOH adipic acid HOOC-(CH2)5-COOH pimelic acid Oh, my! Such good apple pie!

  7. salts of carboxylic acids: name of cation + name of acid: drop –ic acid, add –ate CH3CO2Na sodium acetate or sodium ethanoate CH3CH2CH2CO2NH4 ammonium butyrate ammonium butanoate (CH3CH2COO)2Mg magnesium propionate magnesium propanoate

  8. physical properties: polar + hydrogen bond  relatively high mp/bp water insoluble exceptions: four carbons or less acidic turn blue litmus  red soluble in 5% NaOH RCO2H + NaOH  RCO2-Na+ + H2O stronger stronger weaker weaker acid base base acid

  9. RCO2H RCO2- • covalent ionic • water insoluble water soluble • Carboxylic acids are insoluble in water, but soluble in 5% NaOH. • Identification. • Separation of carboxylic acids from basic/neutral organic compounds. • The carboxylic acid can be extracted with aq. NaOH and then regenerated by the addition of strong acid.

  10. Carboxylic acids, syntheses: • oxidation of primary alcohols • RCH2OH + K2Cr2O7 RCOOH • 2. oxidation of arenes • ArR + KMnO4, heat  ArCOOH • 3. carbonation of Grignard reagents • RMgX + CO2 RCO2MgX + H+  RCOOH • 4. hydrolysis of nitriles • RCN + H2O, H+, heat  RCOOH

  11. oxidation of 1o alcohols: • CH3CH2CH2CH2-OH + CrO3 CH3CH2CH2CO2H • n-butyl alcohol butyric acid • 1-butanol butanoic acid • CH3 CH3 • CH3CHCH2-OH + KMnO4  CH3CHCOOH • isobutyl alcohol isobutyric acid • 2-methyl-1-propanol` 2-methylpropanoic acid

  12. oxidation of arenes: note: aromatic acids only!

  13. carbonation of Grignard reagent: • R-X RMgX RCO2MgX RCOOH • Increases the carbon chain by one carbon. • Mg CO2 H+ • CH3CH2CH2-Br CH3CH2CH2MgBr CH3CH2CH2COOH • n-propyl bromide butyric acid Mg CO2 H+

  14. Hydrolysis of a nitrile: • H2O, H+ • R-CN R-CO2H • heat • H2O, OH- • R-CN R-CO2- + H+ R-CO2H • heat • R-X + NaCN  R-CN + H+, H2O, heat  RCOOH • 1o alkyl halide • Adds one more carbon to the chain. • R-X must be 1o or CH3!

  15. carboxylic acids, reactions: • as acids • conversion into functional derivatives • a)  acid chlorides • b)  esters • c)  amides • reduction • alpha-halogenation • EAS

  16. as acids: • with active metals • RCO2H + Na  RCO2-Na+ + H2(g) • with bases • RCO2H + NaOH  RCO2-Na+ + H2O • relative acid strength? • CH4 < NH3 < HCCH < ROH < HOH < H2CO3 < RCO2H < HF • quantitative • HA + H2O  H3O+ + A- ionization in water • Ka = [H3O+] [A-] / [HA]

  17. Ka for carboxylic acids  10-5 Why are carboxylic acids more acidic than alcohols? ROH + H2O  H3O+ + RO- RCOOH + H2O  H3O+ + RCOO- ΔGo = -2.303 R T log Keq The position of the equilibrium is determined by the free energy change, ΔGo. ΔGo = ΔH - TΔS ΔGo  ΔH Ka is inversely related to ΔH, the potential energy difference between the acid and its conjugate base. The smaller the ΔH, the larger the Ka and the stronger the acid.

  18. ΔH The smaller the ΔH, the more the equilibrium lies to the right, giving a larger Ka ( a stronger acid ).

  19. Resonance stabilization of the carboxylate ion decreases the ΔH, shifts the ionization in water to the right, increases the Ka, and results in carboxylic acids being stronger acids.

  20. Effect of substituent groups on acid strength? CH3COOH 1.75 x 10-5 ClCH2COOH 136 x 10-5 Cl2CHCOOH 5,530 x 10-5 Cl3CCOOH 23,200 x 10-5 -Cl is electron withdrawing and delocalizes the negative charge on the carboxylate ion, lowering the PE, decreasing the ΔH, shifting the ionization to the right and increasing acid strength.

  21. Effect of substituent groups on acid strength of benzoic acids? Electron withdrawing groups will stabilize the anion, decrease the ΔH, shift the ionization to the right, increasing the Ka, increasing acid strength. Electron donating groups will destabilize the anion, increase the ΔH, shift the ionization in water to the left, decreasing the Ka, decreasing acid strength.

  22. -NH2, -NHR, -NR2 -OH -OR electron donating -NHCOCH3 -C6H5 -R -H -X -CHO, -COR -SO3H -COOH, -COOR electron withdrawing -CN -NR3+ -NO2

  23. Relative acid strength? Ka p-aminobenzoic acid 1.4 x 10-5 p-hydroxybenzoic acid 2.6 x 10-5 p-methoxybenzoic acid 3.3 x 10-5 p-toluic acid 4.2 x 10-5 benzoic acid 6.3 x 10-5 p-chlorobenzoic acid 10.3 x 10-5 p-nitrobenzoic acid 36 x 10-5

  24. Conversion into functional derivatives: •  acid chlorides

  25.  esters • “direct” esterification: H+ • RCOOH + R´OH  RCO2R´ + H2O • -reversible and often does not favor the ester • -use an excess of the alcohol or acid to shift equilibrium • -or remove the products to shift equilibrium to completion • “indirect” esterification: • RCOOH + PCl3 RCOCl + R´OH  RCO2R´ • -convert the acid into the acid chloride first; not reversible

  26.  amides • “indirect” only! • RCOOH + SOCl2 RCOCl + NH3  RCONH2 • amide • Directly reacting ammonia with a carboxylic acid results in an ammonium salt: • RCOOH + NH3 RCOO-NH4+ • acid base

  27. Reduction: • RCO2H + LiAlH4; then H+ RCH2OH • 1o alcohol • Carboxylic acids resist catalytic reduction under normal conditions. • RCOOH + H2, Ni  NR

  28. Alpha-halogenation: (Hell-Volhard-Zelinsky reaction) • RCH2COOH + X2, P  RCHCOOH + HX • X • α-haloacid • X2 = Cl2, Br2

  29. 5.EAS: (-COOH is deactivating and meta- directing)

  30. spectroscopy: IR: -COOH O—H stretch 2500 – 3000 cm-1 (b) C=O stretch 1680 – 1725 (s) nmr: -COOH 10.5 – 12 ppm

  31. p-toluic acid -COO—H stretch C=O

  32. c b a

  33. Carboxylic acids, syntheses: • oxidation of primary alcohols • RCH2OH + K2Cr2O7 RCOOH • 2. oxidation of arenes • ArR + KMnO4, heat  ArCOOH • 3. carbonation of Grignard reagents • RMgX + CO2 RCO2MgX + H+  RCOOH • 4. hydrolysis of nitriles • RCN + H2O, H+, heat  RCOOH

  34. carboxylic acids, reactions: • as acids • conversion into functional derivatives • a)  acid chlorides • b)  esters • c)  amides • reduction • alpha-halogenation • EAS

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