CHAPTER 20 CARBOXYLIC ACIDS Based on McMurry’s Organic Chemistry, 6th edition Dr.Mohanad Al-Hachamii The Importance of Carboxylic Acids (RCO2H) Starting materials for acyl derivatives (esters, amides, and acid chlorides) Abundant in nature from oxidation of aldehydes and alcohols in metabolism Acetic acid, CH3CO2H, - vinegar Butanoic acid, CH3CH2CH2CO2H (rancid butter) Long-chain aliphatic acids from the breakdown of fats Based on McMurry, Organic Chemistry, Chapter 20, 6th edition, (c) 2003 Dr.Mohanad Al-Hachamii 2 20.1 Naming Carboxylic Acids •Carboxylic Acids, RCO2H •If derived from open-chain alkanes, replace the terminal -e of the alkane name with -oic acid •The carboxyl carbon atom is C1 Based on McMurry, Organic Chemistry, Chapter 20, 6th Dr.Mohanad Al-Hachamii edition, (c) 2003 3 Alternative Names Compounds with CO2H bonded to a ring are named using the suffix -carboxylic acid The CO2H carbon is not itself numbered in this system Use common names for formic acid (HCOOH) and acetic acid (CH3COOH) – see Table 20.1 Based on McMurry, Organic Chemistry, Chapter 20, 6th edition, (c) 2003 Dr.Mohanad Al-Hachamii 4 5 • Greek letters are used to designate the location of substituents in common names. • The carbon adjacent to the COOH is called the carbon, followed by the carbon, followed by the carbon, the carbon and so forth down the chain. • The last carbon in the chain is sometimes called the carbon. • The carbon in the common system is numbered C2 in the IUPAC system. 6 • Compounds containing two carboxy groups are called diacids. Diacids are named using the suffix –dioic acid. • Metal salts of carboxylate anions are formed from carboxylic acids in many reactions. To name the metal salt of a carboxylate anion, put three parts together: 7 Figure 19.2 Naming the metal salts of carboxylate anions 8 Nitriles, RCN Closely related to carboxylic acids named by adding -nitrile as a suffix to the alkane name, with the nitrile carbon numbered C1 Complex nitriles are named as derivatives of carboxylic acids. Replace -ic acid or -oic acid ending with -onitrile Based on McMurry, Organic Chemistry, Chapter 20, 6th edition, (c) 2003 Dr.Mohanad Al-Hachamii 9 20.2 Structure and Physical Properties of Carboxylic Acids Carboxyl carbon sp2 hybridized: carboxylic acid groups are planar with C–C=O and O=C–O bond angles of approximately 120° Carboxylic acids form hydrogen bonds, existing as cyclic dimers held together by two hydrogen bonds Strong hydrogen bonding causes much higher boiling points than the corresponding alcohols Based on McMurry, Organic Chemistry, Chapter 20, 6th edition, (c) 2003 Dr.Mohanad Al-Hachamii 10 Carboxylic Acids and the Acidity of the O—H Bond Structure and Bonding: • Carboxylic acids are compounds containing a carboxy group (COOH). • The structure of carboxylic acids is often abbreviated as RCOOH or RCO2H, but keep in mind that the central carbon atom of the functional group is doubly bonded to one oxygen atom and singly bonded to another. 11 Structure and Bonding: • The C—O single bond of a carboxylic acid is shorter than the C—O bond of an alcohol. • This can be explained by looking at the hybridization of the respective carbon atoms. • Because oxygen is more electronegative than either carbon or hydrogen, the C—O and O—H bonds are polar. 12 Physical Properties: • Carboxylic acids exhibit dipole-dipole interactions because they have polar C—O and O—H bonds. • They also exhibit intermolecular hydrogen bonding. • Carboxylic acids often exist as dimers held together by two intermolecular hydrogen bonds. Figure 19.3 Two molecules of acetic acid (CH3COOH) held together by two hydrogen bonds 13 Spectroscopic Properties: • Carboxylic acids have very characteristic IR and NMR absorptions. • In the IR: -The C=O group absorbs at ~ 1710 cm-1. -The O—H absorption occurs from 2500-3500 cm-1. • In the 1H NMR: -The O—H proton absorbs between 10-12 ppm. -The protons absorb between 2-2.5 ppm. • In the 13C NMR, the C=O appears at 170-210 ppm. 14 Figure 19.4 The IR spectrum of butanoic acid, CH3CH2CH2COOH 15 20.3 Dissociation of Carboxylic Acids •Carboxylic acids are proton donors toward weak and + strong bases, producing metal carboxylate salts, RCO2 M •Carboxylic acids with more than six carbons are only slightly soluble in water, but their conjugate base salts are water-soluble Based on McMurry, Organic Chemistry, Chapter 20, 6th 16 edition, (c) 2003 Dr.Mohanad Al-Hachamii Acidity Constant and pKa + •Carboxylic acids transfer a proton to water to give H3O + and carboxylate anions, RCO2 , but H3O is a much stronger acid -5 •The acidity constant, Ka,, is about 10 for a typical carboxylic acid (pKa ~ 5) Based on McMurry, Organic Chemistry, Chapter 20, 6th Dr.Mohanad Al-Hachamii edition, (c) 2003 17 Acidity Compared to Alcohols •Carboxylic acids are better proton donors than are alcohols (The pKa of ethanol is ~16, compared to ~5 for acetic acid) •In an alkoxide ion, the negative charge is localized on oxygen while in a carboxylate ion the negative charge is delocalized over two equivalent oxygen atoms, giving resonance stabilization 18 Based on McMurry, Organic Chemistry, Chapter 20, 6th edition, (c) 2003Dr.Mohanad Al-Hachamii 20.4 Substituent Effects on Acidity •Electronegative substituents promote formation of the carboxylate ion Based on McMurry, Organic Chemistry, Chapter 20, 6th edition, (c) 2003 Dr.Mohanad Al-Hachamii 19 Substituent Effects •An electronegative group will drive the ionization equilibrium toward dissociation, increasing acidity •An electron-donating group destabilizes the carboxylate anion and decreases acidity Based on McMurry, Organic Chemistry, Chapter 20, 6th edition, (c) 2003 Dr.Mohanad Al-Hachamii 20 Examples of Inductive Effects on Acidity •Fluoroacetic, chloroacetic, bromoacetic, and iodoacetic acids are stronger acids than acetic acid •Multiple electronegative substituents have synergistic effects on acidity Based on McMurry, Organic Chemistry, Chapter 20, 6th Dr.Mohanad Al-Hachamii edition, (c) 2003 21 20.5 Substituent Effects in Substituted Benzoic Acids Based on McMurry, Organic Chemistry, Chapter 20, 6th Dr.Mohanad Al-Hachamii 22 edition, (c) 2003 Aromatic Substituent Effects •An electron-withdrawing group (-NO2) increases acidity by stabilizing the carboxylate anion, and an electron-donating (activating) group (OCH3) decreases acidity by destabilizing the carboxylate anion •We can use relative pKa’s as a calibration for effects on relative free energies of reactions with the same substituents Based on McMurry, Organic Chemistry, Chapter 20, 6th Dr.Mohanad Al-Hachamii edition, (c) 2003 23 20.6 Preparation of Carboxylic Acids 1- Oxidation of arenes Oxidation of a substituted alkylbenzene with KMnO4 or Na2Cr2O7 or H2Cr2O7 gives a substituted benzoic acid (see Section 16.10) 1° and 2° alkyl groups can be oxidized, but tertiary groups are not ( the alkylbenzene must contain at least one benzylic hydrogen atom. Therefore, tert-butylbenzene is not oxidized). Based on McMurry, Organic Chemistry, Chapter 20, 6th Dr.Mohanad Al-Hachamii 24 edition, (c) 2003 Based on McMurry, Organic Chemistry, Chapter 20, 6th Dr.Mohanad Al-Hachamii 25 edition, (c) 2003 KMnO4, heat CH3 COOH toluene benzoic acid COOH CH3 KMnO4, heat HOOC H3C p-xylene terephthalic acid KMnO4, heat COOH + CO CH2CH3 2 ethylbenzene benzoic acid Based on McMurry, Organic Chemistry, Chapter 20, 6th Dr.Mohanad Al-Hachamii 26 edition, (c) 2003 • Benzene ring affected by donating group such as OH and NH2… making it easy to oxidizing • Converting OH or NH2 into weak donating group such as acetamido to protecting the benzene ring Based on McMurry, Organic Chemistry, Chapter 20, 6th Dr.Mohanad Al-Hachamii 27 edition, (c) 2003 2- Oxidation of alkenes and alkynes Oxidative cleavage of an alkene with KMnO4 gives a carboxylic acid if the alkene has at least one vinylic hydrogen (see Section 7.8) Based on McMurry, Organic Chemistry, Chapter 20, 6th 28 edition, (c) 2003 Dr.Mohanad Al-Hachamii Also, can be Oxidation of alkene by Ozone Based on McMurry, Organic Chemistry, Chapter 20, 6th Dr.Mohanad Al-Hachamii 29 edition, (c) 2003 •Oxdation of Alkynes •Strong oxidizing reagents (O3 or KMnO4) cleave internal alkynes, producing two carboxylic acids •Terminal alkynes are oxidized to a carboxylic acid and carbon dioxide Based on McMurry, Organic Chemistry, Chapter 20, 6th Dr.Mohanad Al-Hachamii 30 edition, (c) 2003 O3; then Zn, H2O KMnO4 C C CH3 COOH + HOOCCH3 KMnO4, heat Based on McMurry, Organic Chemistry, Chapter 20, 6th edition, (c) 2003 Dr.Mohanad Al-Hachamii 3- Oxidation of primary alcohols or aldehyde •Oxidation of a primary alcohol or an aldehyde with CrO3 in aqueous acid Based on McMurry, Organic Chemistry, Chapter 20, 6th Dr.Mohanad Al-Hachamii 32 edition, (c) 2003 Based on McMurry, Organic Chemistry, Chapter 20, 6th Dr.Mohanad Al-Hachamii 33 edition, (c) 2003 4- Hydrolysis of nitriles:Conversion of Nitriles into Carboxylic Acids •Nitriles and carboxylic acids both have a carbon atom with three bonds to an electronegative atom, and both contain a bond •Both both are electrophiles Based on McMurry, Organic Chemistry, Chapter 20, 6th Dr.Mohanad Al-Hachamii 34 edition, (c) 2003 Hot acid or base yields carboxylic acids Conversion of an alkyl halide to a nitrile (with cyanide ion) followed by hydrolysis produces a carboxylic acid with one more carbon (RBr RCN RCO2H) Best with primary halides because elimination reactions occur with secondary or tertiary alkyl halides Based on McMurry, Organic Chemistry, Chapter 20, 6th Dr.Mohanad Al-Hachamii 35