Dr AKM Shafiqul Islam School of Bioprocess Engineering 06.10.08 Carboxylic acids Dr AKM Shafiqul Islam School of Bioprocess Engineering 06.10.08

Carboxylic Acids The functional group of a carboxylic acid is a carboxyl group, Carbonyl with hydroxy represented in one of three ways

Fill in the table below. formic acid methanoic acid acetic acid ethanoic acid propionic acid propanoic acid butyric acid butanoic acid valeric acid pentanoic acid caproic acid hexanoic acid benzoic acid benzoic acid

4-amino-3-methylhexanoic 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. 2-bromopropanoic acid 3-chlorobutanoic acid 4-amino-3-methylhexanoic acid

Carboxylic Aromatic Acids salicylic acid (2-hydroxybenzenecarboxylic) benzoic acid o-phthalic acid (benzene-1,2-dicarboxylic) gallic acid (3,4,5-trihydroxy -benzenecarboxylic) vanillic acid (4-hydroxy-3-methoxy -benzenecarboxylic) cinnamic acid (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 H •• • • + – R C O H •• • • 6

Electron Delocalization H •• • • + – R C O H •• • • R C O H •• • • + – stabilizes carbonyl group 6

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

Solubility in Water Carboxylic acids are similar to alcohols in respect to their solubility in water Form hydrogen bonds to water H3CC O H 10

Physical Properties carboxylic acids are more soluble in water than are alcohols, ethers, aldehydes, and ketones of comparable molecular weight

+ + acid base alkoxide anion Alkoxide anions don’t have resonance-stabilization. Alcohols are weaker acids than carboxylic acids.

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 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 Acid halide

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

Reactions carboxylic acids as acids conversion into functional derivatives  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 In comparison: Hydrochloric acid pKa = -7 Sulfuric acid pKa = -3 Alcohol pKa = 15-16

Comparison Of pKa Values Hydrochloric acid pKa = -7 Sulfuric acid pKa = -3 Alcohol pKa = 15-16 Carboxylic acid 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. + acid base carboxylate anion

resonance-stabilized carboxylate anion ½- = ½- resonance-stabilized carboxylate anion + acid base 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 < HCCH < ROH < HOH < H2CO3 < RCO2H < HF d) quantitative HA + H2O  H3O+ + A- onization in water Ka = [H3O+] [A-] / [HA]

Dehydration of Carboxylic Acid * water must be removed (to avoid hydrolysis) * limited use

Reaction With Bases All carboxylic acids react with strong bases to form water-soluble salts

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

Conversion into functional derivatives a) 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)