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Chapter 14: Carboxylic Acids Sections 14.1-14.7 Chemistry 2060, Spring 2060, LSU 14-1 Sections Chapter 14: Carboxylic Acids 1. 2. 3. 4. 5. 6. 7. 8. 9. Introduction Structure Nomenclature Physical properties Acidity Reduction Fisher esterification Conversion to acid halides Decaroxylation Chemistry 2060, Spring 2060, LSU 14-2 Structure The functional group of a carboxylic acid is a carboxyl group : O: : C :O COOH CO 2 H H • the general formula of an aliphatic carboxylic acid is RCOOH • that of an aromatic carboxylic acid is ArCOOH Chemistry 2060, Spring 2060, LSU 14-3 Nomenclature IUPAC names: drop the -e from the parent alkane and add the suffix -oic acid • if the compound contains a carbon-carbon double bond, change the infix -an- to -enCOOH 3-Methylbutanoic acid (Isovaleric acid) Chemistry 2060, Spring 2060, LSU C6 H5 COOH t rans -3-Phenylpropenoic acid (Cinnamic acid) 14-4 Nomenclature The carboxyl group takes precedence over most other functional groups OH COOH (R)-5-Hydroxyhexanoic acid O COOH 5-Oxohexanoic acid Chemistry 2060, Spring 2060, LSU H2N COOH 4-Aminobutanoic acid 14-5 Nomenclature • dicarboxylic acids: add -dioic acid to the name of the parent alkane containing both carboxyl groups • there is no need to use numbers to locate the carboxyl groups; they can only be on the ends of the chain O HO O OH O Ethanedioic acid (Oxalic acid) O HO O OH O Butanedioic acid (Succinic acid) Chemistry 2060, Spring 2060, LSU HO O HO OH Propanedioic acid (Malonic acid) O O OH Pentanedioic acid (Glutaric acid) HO OH O Hexanedioic acid (Adipic acid) 14-6 Nomenclature • if the carboxyl group is bonded to a ring, name the ring compound and add the suffix -carboxylic acid 2 3 1 COOH 2-Cyclohexenecarboxylic acid Chemistry 2060, Spring 2060, LSU H OOC COOH trans- 1,3-Cyclopentanedicarboxylic acid 14-7 Nomenclature • benzoic acid is the simplest aromatic carboxylic acid • use numbers to show the location of substituents COOH COOH COOH OH COOH COOH COOH Benzoic acid 2-Hydroxybenzoic acid (Salicylic acid) Chemistry 2060, Spring 2060, LSU 1,2-Benzenedicarboxylic acid (Phthalic acid) 1,4-Benzenedicarboxylic acid (Terephthalic acid) 14-8 Nomenclature • when common names are used, the letters etc. are often used to locate substituents O O 5 1 OH 4 3 2 H2 N O OH 4-A min ob utanoic acid (-Aminobutyric acid; GABA ) OH NH2 2-Aminopropanoic acid (-Aminop ropionic acid; alanin e) • in common nomenclature, keto indicates the presence of a ketone, and CH3CO- is named an aceto group O O OH 3-oxobutanoic acid -Ketobutyric acid ; acetoacetic acid) Chemistry 2060, Spring 2060, LSU O CH3 CAcetyl group (aceto group) 14-9 Physical Properties In the liquid and solid states, carboxylic acids are associated by hydrogen bonding into dimeric structures Chemistry 2060, Spring 2060, LSU 14-10 Physical Properties Carboxylic acids have significantly higher boiling points than other types of organic compounds of comparable molecular weight • they are polar compounds and form very strong intermolecular hydrogen bonds Carboxylic acids are more soluble in water than alcohols, ethers, aldehydes, and ketones of comparable molecular weight • they form hydrogen bonds with water molecules through their C=O and OH groups Chemistry 2060, Spring 2060, LSU 14-11 Physical Properties • water solubility decreases as the relative size of the hydrophobic portion of the molecule increases Chemistry 2060, Spring 2060, LSU 14-12 Acidity Carboxylic acids are weak acids • values of pKa for most aliphatic and aromatic carboxylic acids fall within the range 4 to 5 CH3 COO- + H3 O+ CH3 COOH + H2 O - + K a = [CH3 COO ] [H3 O ] [CH3 COOH] = 1.74 x 10-5 pK a = 4.76 • the greater acidity of carboxylic acids relative to alcohols, both of which contain an OH group, is due to resonance stabilization of the carboxylate anion Chemistry 2060, Spring 2060, LSU 14-13 Acidity electron-withdrawing substituents near the carboxyl group increase acidity through their inductive effect CH3 COOH Acetic acid pKa : 4.76 Chemistry 2060, Spring 2060, LSU ClCH2 COOH Cl2 CHCOOH Chloroacetic Dichloroacetic acid acid 2.86 1.48 Increasing acid strength Cl3 CCOOH Trich loroacetic acid 0.70 14-14 Acidity • the acid-strengthening effect of a halogen substituent falls off rapidly with increasing distance from the carboxyl group Cl COOH COOH Cl 2-Chlorob utanoic 3-Chlorobutan oic acid acid (pK a 2.83) (p Ka 3.98) Cl COOH 4-Ch lorobutanoic acid (pK a 4.52) COOH Butan oic acid (pK a 4.82) Decreas ing acid strength Chemistry 2060, Spring 2060, LSU 14-15 Reaction with Bases Carboxylic acids, whether soluble or insoluble in water, react with NaOH, KOH, and other strong bases to give water-soluble salts COOH + NaOH - H2 O Benzoic acid (slightly soluble in water) COO Na + + H2 O Sodium benzoate (60 g/100 mL water) They also form water-soluble salts with ammonia and amines COOH + NH3 Benzoic acid (slightly soluble in water) Chemistry 2060, Spring 2060, LSU - H2 O COO NH4 + Ammonium benzoate (20 g/100 mL water) 14-16 Reaction with Bases Carboxylic acids react with sodium bicarbonate and sodium carbonate to form water-soluble salts and carbonic acid • carbonic acid, in turn, breaks down to carbon dioxide and water + CH3 COOH + Na HCO3 - H2 CO3 + CH3 COOH + Na HCO3 Chemistry 2060, Spring 2060, LSU - H2 O + CH3 COO Na + H2 CO3 CO2 + H2 O + CH3 COO Na + CO2 + H2 O 14-17 Reaction with Bases Chemistry 2060, Spring 2060, LSU 14-18 Reduction The carboxyl group is very resistant to reduction • it is not affected by catalytic hydrogenation under conditions that easily reduce aldehydes and ketones to alcohols, and reduce alkenes and alkynes to alkanes • it is not reduced by NaBH4 Chemistry 2060, Spring 2060, LSU 14-19 Reduction Lithium aluminum hydride reduces a carboxyl group to a 1° alcohol • reduction is carried out in diethyl ether, THF, or other nonreactive, aprotic solvents O H 1 . LiAlH4 , eth er 2 . H2 O Chemistry 2060, Spring 2060, LSU OH + LiOH + Al(OH) 3 14-20 Selective Reduction Catalytic hydrogenation does not reduce a COOH group • we can use H2/M to reduce an alkene in the presence of a COOH group O O OH + H2 5-Hexen oic acid Pt 25°C, 2 atm OH Hexanoic acid • we can use NaBH4 to reduce an aldehyde or ketone in the presence of a COOH group O C6 H5 OH O OH 5-Oxo-5-p henylpentan oic acid Chemistry 2060, Spring 2060, LSU 1 . NaBH4 2 . H2 O C6 H5 O OH 5-Hyd roxy-5-phen ylpen tanoic acid 14-21 Fischer Esterification Esters can be prepared by treating a carboxylic acid with an alcohol in the presence of an acid catalyst, commonly H2SO4 or gaseous HCl O H2 SO4 CH3 COH + CH3 CH2 OH Ethanoic acid Eth anol (A cetic acid ) (Ethyl alcoh ol) Chemistry 2060, Spring 2060, LSU O CH3 COCH2 CH3 + H2 O Ethyl ethanoate (Ethyl acetate) 14-22 Fischer Esterification Fischer esterification is an equilibrium reaction • by careful control of experimental conditions, it is possible to prepare esters in high yield • if the alcohol is inexpensive relative to the carboxylic acid, it can be used in excess to drive the equilibrium to the right • a key intermediate in Fischer esterification is the tetrahedral carbonyl addition intermediate formed by addition of ROH to the C=O group H O R C OH + HOCH3 Chemistry 2060, Spring 2060, LSU H+ O R C OCH3 O H H+ O R C OCH3 + HOH 14-23 Acid Chlorides The functional group of an acid halide is a carbonyl group bonded to a halogen atom • among the acid halides, acid chlorides are by far the most common and the most widely used O - C- X Functional group of an acid halide Chemistry 2060, Spring 2060, LSU O CH3 CCl A cetyl chloride O C-Cl Benzoyl chloride 14-24 Acid Chlorides Acid chlorides are most often prepared by treating a carboxylic acid with thionyl chloride O OH + SOCl 2 Butanoic Thionyl acid chloride Chemistry 2060, Spring 2060, LSU O Cl + SO 2 + HCl Butanoyl chloride 14-25 Decarboxylation Decarboxylation: loss of CO2 from a carboxyl group • most carboxylic acids, if heated to a very high temperature, undergo thermal decarboxylation O RCOH decarb oxylation heat RH + CO2 • most carboxylic acids, however, are quite resistant to • moderate heat and melt or even boil without decarboxylation Chemistry 2060, Spring 2060, LSU 14-26 Decarboxylation • exceptions are carboxylic acids that have a carbonyl group beta to the carboxyl group • this type of carboxylic acid undergoes decarboxylation on mild heating O O warm OH 3-Oxobu tan oic acid (Acetoacetic acid) Chemistry 2060, Spring 2060, LSU O + CO2 Acetone 14-27 Decarboxylation • thermal decarboxylation of a -ketoacid involves rearrangement of six electrons in a cyclic sixmembered transition state enol of a ketone O H O (1) O O H O C (2) O O + CO 2 (A cyclic six-membered trans ition s tate) Chemistry 2060, Spring 2060, LSU 14-28 Decarboxylation • thermal decarboxylation of malonic acids also involves rearrangement of six electrons in a cyclic sixmembered transition state O H HO O O A cyclic six-membered trans ition s tate Chemistry 2060, Spring 2060, LSU O (1) HO H O C O (2) O + HO CO 2 Enol of a carboxylic acid 14-29 Decarboxylation • Problem: draw the product of decarboxylation O COOH heat • Problem: draw the -ketoacid that undergoes decarboxylation to give this ketone O -ketoacid Chemistry 2060, Spring 2060, LSU heat + CO2 14-30