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Carboxylic acids

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

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

special names

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

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!

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

physical properties: polar + hydrogen bond 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

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.

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

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

oxidation of arenes: note: aromatic acids only!

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+

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!

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

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]

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.

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

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.

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.

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.

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

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

Conversion into functional derivatives: acid chlorides

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

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

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

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

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

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

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