Carboxylic acids Dr AKM Shafiqul Islam School of Bioprocess Engineering Carboxylic Acids The functional group of a carboxylic acid is a carboxyl group, Carbonyl with hydroxy represented in one of three ways O C-OH COOH CO2 H Fill in the table below. Compound Common Name IUPAC Name HCOOH formic acid methanoic acid CH3COOH acetic acid ethanoic acid CH3CH2COOH propionic acid propanoic acid CH3(CH2)2COOH butyric acid butanoic acid CH3(CH2)3COOH pentanoic acid CH3(CH2)4COOH valeric acid caproic acid C6H5COOH benzoic acid benzoic acid hexanoic acid 2. Carboxylic acids that have branches are often given IUPAC names, where the numbering of the longest carbon chain begins at the Carbonyl carbon. Name the following compounds. Compound Br O CH3 CH C Cl CH3 CH 2-bromopropanoic acid OH O CH2 C NH2 CH3 CH3 CH2 CH IUPAC Name CH 3-chlorobutanoic acid OH O CH2 C OH 4-amino-3-methylhexanoic acid Carboxylic Aromatic Acids HO HO O C C HO O O C OH OH C O salicylic acid o-phthalic acid (2-hydroxybenzenecarboxylic) (benzene-1,2-dicarboxylic) benzoic acid HO O HO C O C CH CH HO gallic acid (3,4,5-trihydroxy -benzenecarboxylic) C CH3 OH OH O O OH vanillic acid (4-hydroxy-3-methoxy -benzenecarboxylic) OH cinnamic acid {(2E)-3-phenylprop -2-enoic} Where are they to be found? Carboxylic acids and their derivatives: Tart taste of citrus fruits, vinegar, and rhubarb Sharp sting of red ants Unsavory smell and taste of rancid butter Vitamin C is a carboxylic acid Pleasant taste and odor of fruits are due to carboxylic acid derivatives: esters Physical Properties The carboxyl group contains three polar covalent bonds; C=O, C-O, and O-H the polarity of these bonds determines the major physical properties of carboxylic acids δ-O – Hδ+ δ+C=Oδ- R Electron Delocalization R C •• • O• •• O •• R + C •• •– O• •• •• O •• H H Electron Delocalization R C •• • O• R •• O •• + C •• •– O• •• •• O •• H C + O •• H R •• •– O• •• stabilizes carbonyl group H Physical Properties The carbonyl group has a large dipole The hydroxy group is capable of hydrogen bonding. The molecules can H-bond to each other How does this affect boiling point? Higher than aldehydes and ketone – no H-bonds Higher than alcohols – H-bonds, not strong dipole hydrogen bondin g betw een tw o molecules H3 C O + H O C C O H + O - CH3 Solubility in Water Carboxylic acids are similar to alcohols in respect to their solubility in water Form hydrogen bonds to water H O H O H3CC H O H O H Physical Properties carboxylic acids are more soluble in water than are alcohols, ethers, aldehydes, and ketones of comparable molecular weight Structu re N ame CH3 COOH CH3 CH2 CH2 OH CH3 CH2 CHO acetic acid 1-prop anol prop anal Boilin g Solubility Molecular Poin t Weigh t (°C) (g/100 mL H 2O) 60.5 118 infinite 97 60.1 infinite 58.1 48 16 CH3 (CH2 ) 2 COOH butan oic acid CH3 (CH2 ) 3 CH2 OH 1-pentan ol pentan al CH3 (CH2 ) 3 CHO 88.1 88.1 86.1 163 137 103 infinite 2.3 slight R OH acid + H2O base R O + + H3O alkoxide anion Alkoxide anions don’t have resonance-stabilization. Alcohols are weaker acids than carboxylic acids. Copyright© 2005, Michael J. Wovkulich. All rights reserved. Physical Properties Sharp and or sour odor/taste Vinegar, rancid butter, sweat, sauerkraut Carboxylic acids derivatives When the carbonyl group is substituted by atoms other than carbon or hydrogen, as in O R C Z where Z is oxygen, nitrogen, or halogen, the compound becomes a carboxylic acid or a carboxylic acid derivative. Carboxylic acid and derivatives Ester Acid anhydride Amide R R O R R'O O R O O Acid halide R O H2 N O X Carboxylic Acid Derivatives Hydroxy Acids Beta-hydroxy acid Salicylic Acid exfoliant oil soluble – penetrates oil containing pores to remove dead skin cells less irritating than alpha acids O OH OH Reactions carboxylic acids 1. 2. as acids conversion into functional derivatives a) b) c) 3. 4. 5. acid chlorides esters amides reduction alpha-halogenation EAS Reactions : Acid/Base Carboxylic acids are weak acids Give up the H bonded to O to water or base Tendency for acid to give up proton (H+) is indicated by pKa; lower pKa indicates stronger acid Carboxylic acids: pKa 4.0 - 5.0 O CH3COH O H2O In comparison: Hydrochloric acid Sulfuric acid Alcohol CH3CO H3O+ pKa = -7 pKa = -3 pKa = 15-16 Comparison Of pKa Values Hydrochloric acid Sulfuric acid Alcohol Carboxylic acid pKa = -7 pKa = -3 pKa = 15-16 pKa = 4 Acidity of Carboxylic Acids 1. Both acetic acid and ethanol have an acidic hydrogen on the OH group. Why is the pKa value for acetic acid (pKa = 4.74) lower than the pKa for ethanol (pKa = 15.9)? In other words, why are carboxylic acids more acidic than alcohols. Consider resonance-stabilized anions. O O R C acid OH + H2O base R C O carboxylate anion + + H3O O O R R C = C O O O R C O ½½- resonance-stabilized carboxylate anion O O R C acid OH + H2O base R C O + + H3O carboxylate anion The greater the stability of the carboxylate anion, the farther the shift to the right. The farther the shift to the right, the greater the acidity. As acids: a) with active metals RCO2H + Na RCO2-Na+ + H2(g) b) with bases RCO2H + NaOH RCO2-Na+ + H2O c) relative acid strength? CH4 < NH3 < HCCH < ROH < HOH < H2CO3 < RCO2H < HF d) quantitative HA + H2O H3O+ + A- onization in water Ka = [H3O+] [A-] / [HA] Dehydration of Carboxylic Acid 2 H3C COOH O 800o O + H2O(g) H3C O CH3 O COOH 200o O + H2O(g) COOH O * water must be removed (to avoid hydrolysis) * limited use Reaction With Bases All carboxylic acids react with strong bases to form water-soluble salts COOH + NaOH H2 O Ben zoic acid (slightly soluble in w ater) COOH + + COO Na + H2 O Sodiu m b enzoate (60 g/100 mL w ater) NH3 Benzoic acid (s ligh tly solub le in w ater) H2 O - COO NH4 + Ammoniu m b enzoate (20 g/100 mL water) Reactions with bases Salts of carboxylic acids Drop the –ic acid Change to –ate Sodium benzoate & monosodium glutamate Sodium benzoate – inhibit mold MSG – flavor enhancer O O O - C + O Na HO + CH2 C - Na CH2 O CH NH2 sodium benzoate MSG Conversion into functional derivatives a) acid chlorides O R C OH SOCl2 O R C Cl or PCl3 orPCl5 CO2H + SOCl2 O CH3CH2CH2 C OH COCl PCl3 O CH3CH2CH2 C Cl b) 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 O C OH H+ + CH3OH SOCl2 O C Cl CH3OH O + H2O C O CH3 c) amides “indirect” only! RCOOH + SOCl2 RCOCl + NH3 RCONH2 amide O OH 3-Methylbutanoic acid PCl3 O NH3 Cl O NH2 Directly reacting ammonia with a carboxylic acid results in an ammonium salt: RCOOH + NH3 RCOO-NH4+ acid base O C OH PCl3 O C Cl NH3 O C NH2 amide NH3 O C O NH4 ammonium salt 3. Reduction: RCO2H + LiAlH4; then H+ RCH2OH 1o alcohol LiAlH4 H+ CH3CH2CH2CH2CH2CH2CH2COOH Octanoic acid (Caprylic acid) CH3CH2CH2CH2CH2CH2CH2CH2OH 1-Octanol Carboxylic acids resist catalytic reduction under normal conditions. RCOOH + H2, Ni NR O CH2 C OH H2, Pt LiAlH4 H+ CH2CH2OH NR 4. Alpha-halogenation: (Hell-Volhard-Zelinsky reaction) RCH2COOH + X2, P RCHCOOH + HX X α-haloacid X2 = Cl2, Br2 CH3CH2CH2CH2COOH + Br2,P pentanoic acid COOH Br2,P NR (no alpha H) CH3CH2CH2CHCOOH Br 2-bromopentanoic acid RCH2COOH + Br2,P RCHCOOH + HBr + H; O Na H n the Br NH3 RCHCOOH RCHCOOH NH2 OH KOH(alc) RCH2CHCOOH Br then H+ RCH=CHCOOH aminoacid 5. EAS: (-COOH is deactivating and meta- directing) CO2H HNO3,H2SO4 NO2 CO2H CO2H H2SO4,SO3 SO3H CO2H benzoic acid Br2,Fe Br CH3Cl,AlCl3 NR