Carboxylic Acids O R C + H + O - Chemistry 618B

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1 arboxylic Acids R H R + H + - R -

2 Nomenclature - IUPA IUPA 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-en- H 2 =H 2 H Propenoic acid (Acrylic acid) 6 H 5 H H 2 H trans-3-phenylpropenoic acid (innamic acid) H 3 H H 2 H trans-2-butenoic acid (rotonic acid)

3 Nomenclature-ommon When common names are used, the letters α, β, γ, δ, etc. are often used to locate substituents HH 2 H 2 H 2 2 H 4-Hydroxybutanoic acid (γ-hydroxybutyric acid) δ γ β α -----H NH 2 H 3 H 2 H 2-Aminopropanoic acid (α-aminopropionic acid; Alanine)

4 Naming the Salts To name the salt of the carboxylic acid, name the cation followed by the name of the anion (two words). The anion is named by removing -oic acid and adding ate Benzoic acid Butyric acid Sodium benzoate Ammonium butyrate

5 Boiling Points H bp (1 atm) H Intermolecular forces, especially hydrogen bonding, are stronger in carboxylic acids than in other compounds of similar shape and molecular weight

6 Physical Properties In the liquid and solid states, carboxylic acids are associated by hydrogen bonding into dimeric structures δ- δ+ δ+ δ- H 3 H H H 3

7 Physical Properties arboxylic 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 arboxylic acids are more soluble in water than alcohols, ethers, aldehydes, and ketones of comparable molecular weight they form hydrogen bonds with water molecules through both the = and H groups

8 Physical Properties Water solubility decreases as the relative size of the hydrophobic portion of the molecule increases hydrophilic region; increases water solubility hydrophobic region; decreases water solubility

9 Saponification of Fat R H 2 H H 2 R R H 2 H NaH H H H 2 H R - Na + Fat glycerol Fatty Acid Salt "Soap" R = H 3 (H 2 ) 16 H Stearic Acid H 3 (H 2 ) 7 H H(H 2 ) 7 H leic acid etc.

10 Soaps and Detergents Surface Active Agents = surfactants H 3 (H 2 ) 16 - Na + Soap H 3 (H 2 ) 11 S 'Na + Anionic detergent H 3 l H 2 N (H 2 ) 15 H 3 H 3 ationic Detergent

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13 Infrared Spectroscopy A carboxylic acid is characterized by peaks due to H and = groups in its infrared spectrum. = stretching gives an intense absorption near 1700 cm -1. H peak is broad and overlaps with H absorptions.

14 Infrared Spectrum of 4-Phenylbutanoic4 acid 6 H 5 H 2 H 2 H 2 2 H H and H stretch = monosubstituted benzene Wave number, cm -1

15 1 H NMR of arboxylic acids The acidic proton in the H- croup of a carboxylic acid is normally the least shielded of all protons in a 1 H nmr spectrum: (d ppm; ; broad)

16 H 2 H 2 H 2 H hemical shift (δ,( ppm)

17 13 NMR of arboxylic acids The arbonyl carbon on the carboxylic acid group is at low field (δ( ppm), but not quite as deshielded as the carbonyl carbon of an aldehyde or ketone (δ( ppm).

18 Mass Spectrometry The McLafferty rearrangement gives a characteristic peak at m/z = 60 H 2 H + McLafferty rearrangement H 2 H 2 H H 2 H 2 + H H 2 H + m/z 60

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20 Acidity arboxylic acids are weak acids The pk a of typical aliphatic and aromatic carboxylic acids falls within the range 4 to 5 The greater acidity of carboxylic acids relative to alcohols, both of which have oxyanions conjugate bases is because: R H R + H + R - - the carboxylate anion is stabilized by resonance

21 Review definitions of: Ka pka [H + ] pka Assuming equal molar concentations, which produces the solution with the lower ph, an acid with high or low pka??

22 Acidity Electron-withdrawing substituents near the carboxyl group increase acidity through their inductive effect H I Br l F H 2 2 H H 2 2 H H 2 2 H H 2 2 H H 2 2 H Acid Strength

23 Acidity Substitution by multiple electronwithdrawing groups further increases acidity H 3 2 H H 2 l 2 H Hl 2 2 H l 3 2 H Acetic hloroacetic Dichloroacetic Trichloroacetic pk : a Acid Strength

24 Acidity The inductive effect of an electronwithdrawing substituent falls off rapidly with its distance from the carboxyl group l l l H 2 H 2 H 2 2 H H 3 HH 2 2 H H 3 H 2 H 2 H 4-hlorobutanoic 3-hlorobutanoic 2-hlorobutanoic pk a : Acid Strength

25 The key to understanding acid strength is in being able to understand the stability of the conjugate base, the anion R H R + H + - R -

26 Reactions of Acids Reduction Decarboxylation Esterification Formation of Acid Halides

27 Reduction The carboxyl groups is one of the organic functional groups most 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 NaBH 4

28 Reduction by LiAlH 4 Lithium aluminum hydride reduces a carboxyl group to a 1 alcohol reduction is carried out in diethyl ether, THF, or other nonreactive, aprotic solvent H 1. LiAlH 4, ether 2. H 2 H 2 H + LiH + Al(H) 3

29 Selective Reduction Using the less reactive NaBH 4, it is possible to reduce the carbonyl group of an aldehyde or ketone without affecting a carboxyl group H 2 H 2 H 2 H 1.NaBH 4 2. H 2 H HH 2 H 2 H 2 H

30 Decarboxylation Decarboxylation: loss of 2 from a carboxyl group Most carboxylic acids, if heated to a very high temperature (fried), undergo thermal decarboxylation Most carboxylic acids, however, are quite resistant to reasonable heat and melt or even boil without decarboxylation

31 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 β α warm H 3 --H 2 --H H 3 --H xobutanoic acid Acetone (Acetoacetic acid)

32 Decarboxylation Decarboxylation occurs readily if there is any carbonyl group beta to the carboxyl Malonic acid and substituted malonic acids, for example, also undergo thermal decarboxylation HH + 2 H H 3 H 2 Propanedioic acid (Malonic acid)

33 Decarboxylation Thermal decarboxylation of a β-ketoacid involves rearrangement of six electrons in a cyclic six-membered transition state H H 3 H H (A cyclic six-membered transition state) H 3 enol of a ketone H H H H 3 --H 3 + 2

34 Decarboxylation Thermal decarboxylation of malonic acids also involves rearrangement of six electrons in a cyclic six-membered transition state H H H H H H H 3 --H + 2 H H enol of a carboxylic acid

35 Fischer Esterification Esters can be prepared by treatment of a carboxylic acid with an alcohol in the presence of an acid catalyst, commonly H 2 S 4 or gaseous Hl H 3 H Ethanoic acid (Acetic acid) + H 3 H 2 H Ethanol (Ethyl alcohol) H 2 S 4 H 3 H 2 H 3 Ethyl ethanoate (Ethyl acetate) + H 2

36 Fischer Esterification The key intermediate in Fischer esterification is the tetrahedral addition intermediate formed by addition of RH to the = group R H H 3 tetrahedral carbonyl addition intermediate R H + HH 3 H H + H + R H 3 + HH

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38 Reaction of Esters with H - Hydrolysis of an esters is aqueous base is called saponification Each mol of ester hydrolyzed requires 1 mol of base; for this reason, ester hydrolysis in aqueous base is said to be base-promoted (not catalyzed) H 2 RH + 3 NaH R - Na + + H 3 H Hydrolysis of an ester in aqueous base involves formation of a tetrahedral carbonyl addition intermediate followed by collapse and proton transfer (Nucleophilic acyl substitution)

39 Saponification Base-promoted ester hydrolysis Irreversible due to deprotonation of the acid R---H 3 + H R H H 3 R- + -H + 3 R- -H 3 H H -

40 Hydrolysis of Amides Hydrolysis of amides is also irreversible. In acid solution the amine product is protonated to give an ammonium salt. RNHR' + + H 2 + H RH + + R'NH 3

41 Grignard Reagents Addition to 2 gives a carboxylic acid - + H 3 + H 3 H 2 MgBr H 3 H 2 - MgBr + THF dil. H 3 H 2 H + Mg +2 This is a great way to add a carbon and make an acid

42 Diazomethane Diazomethane, H 2 N 2, a potentially explosive, toxic yellow gas, is best drawn as a hybrid of two contributing structures Treatment of a carboxylic acid with diazomethane gives a methyl ester H + + N N H ether RH + H 2 N 2 RH 3 + N 2 Diazomethane H H N N A methyl ester

43 Diazomethane Esterification occurs in two steps Step 1: proton transfer from the carboxyl group to diazomethane R H + H 2 + N N R A carboxylate anion + H 3 + N N

44 Diazomethane Step 2: nucleophilic displacement (S N 2) of N 2, an extraordinarily good leaving group, by the carboxylate anion R + + S N 2 H 3 N N R H 3 + N N