Carboxylic Acids and Esters

Transcription

1 24 Carboxylic Acids and Esters The sour tang in fruit juice comes from carboxylic acids. Introduction to General, Organic, and Biochemistry, 10e John Wiley & Sons, Inc Morris Hein, Scott Pattison, and Susan Arena

2 Course 24.1 Carboxylic Acids 24.2 Nomenclature and Sources of Aliphatic Carboxylic Acids 24.3 Physical Properties of Carboxylic Acids 24.4 Classification of Carboxylic Acids 24.5 Preparation of Carboxylic Acids 24.6 Chemical Properties of Carboxylic Acids 25.7 Nomenclature of Esters 2

3 Course 24.8 Occurrence and Physical Properties of Esters 24.9 Polyesters: Condensation Polymers Chemical Properties of Esters Glycerol Esters Soaps and Synthetic Detergents Esters and Anhydrides of Phosphoric Acid 24 Summary 3

4 Carboxylic Acids Carboxylic acids are organic compounds that contain the carboxyl functional group as shown here. These are three different ways to represent the carboxyl group. Carboxylic acids can be aliphatic or aromatic... 4

5 Carboxylic Acids This is the general formula for an aliphatic carboxylic acid. acetic acid ( an aliphatic carboxylic acid) This is the general formula for an aromatic carboxylic acid. benzoic acid (an aromatic carboxylic acid) 5

6 Nomenclature and Sources of Aliphatic Carboxylic Acids IUPAC Rules for Naming Carboxylic Acids 1. Name the longest continuous carbon chain of carbon atoms containing the COOH group. 2. The COOH carbon is the #1 carbon atom because the carboxyl group is always at the beginning of the carbon chain. 6

7 Nomenclature and Sources of Aliphatic Carboxylic Acids 3. Drop the e from the name of the parent alkane and add the suffix oic acid as shown below. 7

8 Nomenclature and Sources of Aliphatic Carboxylic Acids Carboxylic acids can be named using the IUPAC system or given common names. Sometimes Greek letters are used to in common names to locate groups attached to the molecules. 8

9 Nomenclature and Sources of Aliphatic Carboxylic Acids Table 24.1 on the next slide is a list of the common and IUPAC names for many other carboxylic acids... 9

10 10

11 Your Turn! Write formulas for 3-chloro-4-methylhexanoic acid and α-aminobutyric acid. 11

12 Your Turn! Write formulas for 3-chloro-4-methylhexanoic acid and α-aminobutyric acid. CH 3 O O CH 3 CH 2 CHCHCHC OH CH 3 CH 2 CHC OH Cl 3-chloro-4-methylhexanoic acid NH 2 α-aminobutyric acid 12

13 Physical Properties of Carboxylic Acids The solubility of carboxylic acids is determined by which of the two groups (the nonpolar R group or the polar COOH group) of the molecule is the most dominant in the chemical structure. Carboxylic acids with: 1-4 carbon atoms are water soluble 5-7 carbon atoms are slightly water soluble 8 and more carbon atoms are water insoluble

14 Physical Properties of Carboxylic Acids The relative high boiling points of carboxylic acids are due to intermolecular attractions resulting from hydrogen bonding. 14

15 Physical Properties of Carboxylic Acids Acid-Base Properties of Carboxylic Acids Carboxylic acids and phenols like mineral acids ionize in water to produce hydronium ions and anions. 15

16 Physical Properties of Carboxylic Acids Carboxylic acids are generally weak acids; that is, they are only slightly ionized in water. Phenols are, in general, even weaker acids than carboxylic acids. 16

17 Physical Properties of Carboxylic Acids Carboxylic Acids in Biochemistry Carboxylic acids are released in the blood by the liver in an excessive amount during ketoacidosis which is a condition associated with uncontrolled diabetes. Normal amounts of carboxylic acid can be controlled by blood buffers and by respiration rate and kidney function. 17

18 Physical Properties of Carboxylic Acids L-Dopa (levodopa) is a carboxylic acid that is a derivative of dopamine and is used in the treatment of Parkinson s disease. 18

19 Classification of Carboxylic Acids There are five major types of carboxylic acids in addition to the saturated monocarboxylic acids like acetic acid (CH 3 COOH ). These acids are summarized and discussed in this section... 19

20 Classification of Carboxylic Acids These major types are: Unsaturated carboxylic acids Aromatic carboxylic acids Dicarboxylic acids Hydroxy acids Amino acids 20

21 Classification of Carboxylic Acids An unsaturated carboxylic acid contains one or more C=C bonds. The C=C bond affects the physical and chemical properties of the acid. For example stearic acid, CH 3 (CH 2 ) 16 COOH, and oleic acid, CH 3 (CH 2 ) 7 CH=CH(CH 2 ) 7 COOH, both contain 18 carbon atoms. Stearic acid has no C=C bonds and melts at 70 C where as oleic acid, CH 3 (CH 2 ) 7 CH=CH(CH 2 ) 7 COOH, has one C=C bond and melts at 16 C. 21

22 Classification of Carboxylic Acids Aromatic acids have a COOH group bonded to a carbon in the aromatic ring. Benzoic acid and the three isomers of toluic acid are examples. 22

23 Classification of Carboxylic Acids Dicarboxylic acids have two carboxyl ( COOH) groups. Examples of some dicarboxylic acids are shown below. 23

24 Classification of Carboxylic Acids Dicarboxylic acids are subject to decarboxylation (loss of CO 2 ) to form a monocarboxylic acid. They will also undergo internal dehydration to form anhydrides when heated. 24

25 Classification of Carboxylic Acids Citric acid is a hydroxytricarboxylic acid. It is 5-8% of lemon juice and widely distributed in plant and animal tissues 25

26 Classification of Carboxylic Acids Hydroxy acids have hydroxyl ( OH) and carboxyl ( COOH) functional groups. Two important α-hydroxy acids are lactic acid and salicylic acid. 26

27 Classification of Carboxylic Acids Lactic acid is found in sour milk as well as in muscle tissue after strenuous exercise. Salicyclic acid and its derivatives are found in analgesics like aspirin. 27

28 Classification of Carboxylic Acids Malic acid and tartaric acid are two other common α- hydroxy acids found in apples and grapes respectively. 28

29 Classification of Carboxylic Acids Amino acids have a carboxyl group ( COOH) and an amino group ( NH 2 ). The COOH group acts as an acid while the NH 2 acts as a base. Most amino acids in nature have the NH 2 in the α-position as shown here. 29

30 Classification of Carboxylic Acids Amino acid units are the building blocks of proteins. Approximately 20 amino acids are biologically significant. In amino acids the R group does not necessarily represent an alkyl group. 30

31 Your Turn! Write the structural formulas of the following. 1) A dicarboxylic acid with three carbon atoms. 2) An α-amino acid with three carbon atoms. 31

32 Your Turn! Write the structural formulas of the following. 1) A dicarboxylic acid with three carbon atoms. O O HO C CH 2 C OH 2) An α-amino acid with three carbon atoms. O CH 3 CHC OH NH 2 32

33 Preparation of Carboxylic Acids Carboxylic acids can be prepared by various methods. Three methods we will discuss are the: Oxidation of primary alcohols or aldehydes Oxidation of alkyl groups attached to aromatic rings Hydrolysis of nitriles 33

34 Preparation of Carboxylic Acids Primary alcohols are oxidized to carboxylic acids. (general reaction) 34

35 Preparation of Carboxylic Acids Primary alcohols like benzyl alcohol can also be oxidized to a carboxylic acid. 35

36 Preparation of Carboxylic Acids Aldehydes also oxidize to carboxylic acids ( general reaction) 36

37 Preparation of Carboxylic Acids Alkyl benzenes are oxidized in basic solution to the carboxylate salt and then protonated with acid to form benzoic acid. 37

38 Preparation of Carboxylic Acids Nitriles can be hydrolyzed to form carboxylic acids. Nitriles can be prepared by reacting an alkyl halide and a cyanide salt. 38

39 Preparation of Carboxylic Acids Nitriles can also be prepared by reacting an aldehyde or a ketone with HCN. The specific types of molecules formed in these reactions are cyanohydrins. 39

40 Preparation of Carboxylic Acids Once formed nitriles can be hydrolyzed to form carboxylic acids. 40

41 Your Turn! What primary alcohol can be oxidized to form pentanoic acid? 41

42 Your Turn! What primary alcohol can be oxidized to form pentanoic acid? 1-Pentanol is a primary alcohol that oxidizes to pentanoic acid. O CH 3 CH 2 CH 2 CH 2 CH 2 OH [O] CH 3 CH 2 CH 2 CH 2 C OH 1-pentanol pentanoic acid 42

43 Your Turn! 1-Chlorobutane reacts with KCN. The product of this reaction is hydrolyzed to form a carboxylic acid. Write the equations for these reactions. 43

44 Your Turn! 1-Chlorobutane reacts with KCN. The product of this reaction is hydrolyzed to form a carboxylic acid. Write the equations for these reactions. CH 3 CH 2 CH 2 CH 2 Cl + KCN CH 3 CH 2 CH 2 CH 2 CN + KCl 1-chlorobutane a nitrile O CH 3 CH 2 CH 2 CH 2 CN + 2 H 2 O H + CH 3 CH 2 CH 2 CH 2 C OH + NH 4 + a nitrile pentanoic acid Notice that the resulting carboxylic acid has one more carbon atom than the starting alkyl halide. 44

45 Chemical Properties of Carboxylic Acids Carboxylic acids undergo two broad classes of reactions which are acid-base reactions and substitution reactions. 1. Acid-base reactions occur because the OH on the COOH group can act as a proton donor 2. The C=O on the COOH group serves as a site for substitution reactions. 45

46 Chemical Properties of Carboxylic Acids Acid-Base Properties Low molar-mass carboxylic acids release H + ions in solution and as a result have the following properties unique to acids in general. a. Sour taste b. Blue litmus paper changes to red in an acidic solution c. Forms solutions with a ph < 7 d. Reacts with bases in neutralization reactions 46

47 Chemical Properties of Carboxylic Acids The characteristics listed on the previous slide are not true of higher molar-mass carboxylic acids because the dominant nonpolar R group makes these higher molarmass carboxylic acids insoluble in water... 47

48 Chemical Properties of Carboxylic Acids However the higher molar-mass carboxylic acids do react with strong bases like NaOH. The sodium and potassium salts of the carboxylic acids are soluble in water. 48

49 Chemical Properties of Carboxylic Acids There are four major types of substitution reactions of carboxylic acids. 49

50 Chemical Properties of Carboxylic Acids Acid Chloride Formation Acid chlorides are prepared by reacting thionyl chloride (SOCl 2 ) with a carboxylic acid. 50

51 Chemical Properties of Carboxylic Acids Acid chlorides are very reactive and will hydrolyze back to the carboxylic acid if exposed to moisture. 51

52 Chemical Properties of Carboxylic Acids Acid chlorides can be used to prepare esters like methyl acetate or amides like acetamide. 52

53 Chemical Properties of Carboxylic Acids Acid Anhydride Formation Acid anhydrides can be prepared by reacting two molecules of a carboxylic acid and eliminating a molecule of water. 53

54 Chemical Properties of Carboxylic Acids Acid anhydrides can also prepared by reacting an acid chloride with a carboxylate anion. 54

55 Chemical Properties of Carboxylic Acids Ester Formation Esters can be prepared by a reacting a carboxylic acid and an alcohol or a phenol in the presence of a strong acid catalyst. 55

56 Chemical Properties of Carboxylic Acids The specific reaction of formic acid and ethanol is shown here. 56

57 Nomenclature of Esters Naming Esters 1. Name the alkoxy alkyl group. This is the alcohol portion of the molecule. 2. Name the carboxylic acid portion of the molecule. Change the -ic in the name of the acid to ate. 57

58 Nomenclature of Esters 3. The alcohol portion is named first followed by the name of the acid portion. The IUPAC name for this ester is methyl ethanoate. The common name is methyl acetate. 58

59 Nomenclature of Esters The ester formed by the reaction of propanoic acid and methanol is methyl propanoate. The common name is methyl propionate. Table 24.3 on the next slide lists the IUPAC and common names of other esters... 59

60 60

61 Your Turn! Give IUPAC and common names for the following esters. O O CH 3 CH 2 CH 2 O CCH 2 CH 2 CH 3 O CCH 3 61

62 Your Turn! Give IUPAC and common names for the following esters. O O CH 3 CH 2 CH 2 O CCH 2 CH 2 CH 3 O CCH 3 IUPAC: propyl butanoate Common: propyl butyrate IUPAC: phenyl ethanoate Common: phenyl acetate 62

63 Occurrence and Physical Properties of Esters Low molar mass esters are volatile nonpolar liquids at room temperature. For example esters such as ethyl acetate, butyl acetate, and isoamyl acetate are used extensively in paints, varnishes, and lacquers. 63

64 Occurrence and Physical Properties of Esters Low- and intermediate-molar-mass esters have characteristic fragrant or fruity odors. The difference in properties between an acid and its esters is remarkable. O O CH 3 CH 2 CH 2 C OH The carboxylic acid butyric acid has the unpleasant odor of rancid butter. CH 3 CH 2 CH 2 C OCH 2 CH 3 The ester ethyl butyrate has the pleasant odor of pineapple. 64

65 Occurrence and Physical Properties of Esters High molar mass esters are nonpolar solids at room temperature. Many of these esters are waxes. Carnauba wax is an ester that can have a 28-carbon fatty acid chain and a 34-carbon alcohol chain. High molar mass esters are used in furniture and automobile wax preparations. Very high molar mass esters include polyesters such as Dacron. 65

66 Polyesters: Condensation Polymers A polyester is a polymer formed between an alcohol monomer and a carboxylic acid monomer. Polyesters are classified as condensation polymers. An example of a general polymerization reaction to produce a polyester is shown here. 66

67 Polyesters: Condensation Polymers This is a polymerization reaction that produces the polyethylene terephthalate (PETE) polyester. 67

68 Polyesters: Condensation Polymers Polyethylene terephthalate is a common polyester found in Mylar films and soft drink bottles. This is the plastic recycle code for polethylene terephthalate. 68

69 Polyesters: Condensation Polymers When trifunctional acids or alcohols are used as monomers, cross-linked thermosetting polyesters are obtained. One common example is the reaction of glycerol and o- phthalic acid. The polymer formed is one of a group of polymers known as alkyd resins. 69

70 Polyesters: Condensation Polymers Glycerol has three functional groups, and phthalic acid has two functional COOH groups making the formation of the cross-linked polymer possible. Glycerol o-phthalic acid 70

71 Chemical Properties of Esters The principal reaction of esters is hydrolysis. Ester hydrolysis is either acid-catalyzed (acid hydrolysis) or base-promoted (alkaline hydrolysis/saponification). 71

72 Chemical Properties of Esters An ester hydrolyzed in the presence of an acid catalyst forms an alcohol and a carboxylic acid. The acid is a catalyst so it isn t consumed during the reaction. 72

73 Chemical Properties of Esters Specific examples of acid-catalyzed ester hydrolysis are shown here. 73

74 Chemical Properties of Esters The ester hydrolysis in the presence of a strong base to form an alcohol and a salt is called saponification. 74

75 Chemical Properties of Esters The carboxylic acid may be obtained by reacting the salt obtained from the hydrolysis with a strong acid. Notice that in saponification, the base is a reactant, not a catalyst. 75

76 Your Turn! Write equations for the acid hydrolysis and the base hydrolysis of ethyl butyrate. 76

77 Your Turn! Write equations for the acid hydrolysis and the base hydrolysis of ethyl butyrate. O O CH 3 CH 2 CH 2 C OCH 2 CH 3 H + + H 2 O CH 3 CH 2 CH 2 C OH + CH 3 CH 2 OH Acid hydrolysis O O CH 3 CH 2 CH 2 C OCH 2 CH 3 H 2 O + NaOH CH 3 CH 2 CH 2 C heat O - Na + + CH 3 CH 2 OH Base hydrolysis 77

78 Glycerol Esters Esters of glycerol (glycerol esters) are known as triacylglycerols and triglycerides. 78

79 Glycerol Esters Fats and oils are triacylglycerols. Fats originating from animal sources are solids at room temperature because they have a higher percentage of saturated fatty acid derivatives in their triacylglycerol structure. Oils originating from plant sources are liquids at room temperature because they have a higher percentage of unsaturated fatty acid derivatives in their triacylglycerol structure. 79

80 Glycerol Esters Some common unsaturated fatty acid derivatives found in oils include the corresponding parent fatty acids oleic acid, linoleic acid, and linolenic acid. Table 24.4 is a list of the types of fatty acids in various fats and oils by percent composition... 80

81 81

82 Glycerol Esters Triacylglycerols undergo two basic types of reactions. Reduction which includes hydrogenation and hydrogenolysis. Acid and basic hydrolysis. 82

83 Glycerol Esters Hydrogenation of Glycerides Oils can be partially hydrogenated using a metal catalyst and hydrogen gas to obtain a solid like the shortening found in Crisco. 83

84 Glycerol Esters During partial hydrogenation some of the cis bonds change to trans bonds which is the basis for the term trans fat. Research has shown these unnatural fats to be unhealthy and to increase the risk of heart disease. 84

85 Glycerol Esters Hydrogenolysis Long-chain alcohols are prepared by reducing triacylglycerols with hydrogen gas and a copper chromite catalyst. 85

86 Glycerol Esters Acid Hydrolysis Fatty acids can be prepared by hydrolyzing triacylglycerols with either enzymes or mineral acid catalysts. 86

87 Glycerol Esters Alkaline Hydrolysis (Saponification) Soap (i.e. carboxylate salts) is prepared by the alkaline hydrolysis of triacylglycerols. 87

88 Your Turn! Write the structure of a triacylglycerol that has one unit each of stearic, palmitic and lauric acids. How many isomers of this molecule exists? 88

89 Your Turn! Write the structure of a triacylglycerol that has one unit each of stearic, palmitic and lauric acids. How many isomers of this molecule exists? O CH 2 O C C 17 H 35 Stearic There are three isomers of this molecule. CH O O C C 15 H 31 Palmitic O CH 2 O C One isomer C 11 H 23 Lauric 89

90 Your Turn! Write the structure of a triacylglycerol that has one unit each of stearic, palmitic and lauric acids. How many isomers of this molecule exists? O O CH 2 O C C 17 H 35 Stearic CH 2 O C C 11 H 23 Lauric O O CH O C C 11 H 23 Lauric CH O C C 17 H 35 Stearic O O CH 2 O C Second isomer C 15 H 31 Palmitic CH 2 O C Third isomer C 15 H 31 Palmitic 90

91 Soaps and Synthetic Detergents Soaps are salts of long-chain fatty acids that are prepared by reacting fat or oil with sodium hydroxide. Sodium palmitate is a soap that is the sodium salt of palmitic acid. 91

92 Soaps and Synthetic Detergents The hydrocarbon tail of the carboxylate ion is nonpolar and dissolves the nonpolar grease. The hydrocarbon tail is hydrophobic (i.e. water-fearing). The polar head of the carboxylate ion allows the ion to remain soluble in water which is also polar. The polar head is hydrophilic (i.e. water-loving). Figure 24.1 shows the cleansing action of the hydrocarbon tails and the polar heads of the carboxylate ions... 92

93 Soaps and Synthetic Detergents 93

94 Soaps and Synthetic Detergents The cleansing action of soaps in hard water is limited because hard water contains ions that react with carboxylate ions to form insoluble salts (soap scum) as shown in the reaction below. These ions include Ca 2+, Fe 3+, and Mg

95 Soaps and Synthetic Detergents Soaps are also ineffective in acidic solutions because insoluble fatty acids are formed. 95

96 Soaps and Synthetic Detergents Synthetic detergents (syndets) are synthetic organic products that act as cleansing agents. These synthetic detergents are effective in soft water and hard water. Two examples are shown below. 96

97 Soaps and Synthetic Detergents Synthetic detergents can be ionic (either anionic or cationic) or nonionic. Sodium lauryl sulfate is an anionic detergent because it contains long-chain negatively-charged ions. 97

98 Soaps and Synthetic Detergents A cationic detergent has a long hydrocarbon chain and a positive charge. 98

99 Soaps and Synthetic Detergents Nonionic detergents are molecular substances. The molecule of a nonionic detergent contains a greasesoluble component and a water-soluble component. 99

100 Soaps and Synthetic Detergents Biodegradable organic substances are those that can be readily decomposed by microorganisms in the environment into simple inorganic ions and molecules. Detergents with straight-chain alkyl benzenes are biodegradable. 100

101 Soaps and Synthetic Detergents Detergents with branched-chain alkyl benzenes are not biodegradable. 101

102 Esters and Anhydrides of Phosphoric Acid Phosphoric acid and a carboxylic acid have similar structures. Because of this structural similarity phosphoric acid can form esters and anhydrides just like a carboxylic acid. 102

103 Esters and Anhydrides of Phosphoric Acid Here a phosphate ester is formed by the reaction of phosphoric acid and ethanol in the presence of a strong acid catalyst. 103

104 Esters and Anhydrides of Phosphoric Acid Because phosphoric acid has more than one OH, it can form more than one ester. 104

105 Esters and Anhydrides of Phosphoric Acid In living cells, the phosphoric acid often connects different biochemicals by two ester bonds (phosphodiesters). Our cells genetic material (DNA and RNA) is linked by phosphodiesters. 105

106 Esters and Anhydrides of Phosphoric Acid Pyrophosphoric acid is a phosphoric acid anhydride formed by the reaction of two molecules of phosphoric acid and the elimination of a water molecule. Phosphoric acid anhydrides are biologically important because they temporarily store metabolic energy in the form of molecules like ATP. 106

107 Esters and Anhydrides of Phosphoric Acid The energy currency of the cell, adenosine triphosphate (ATP), carries its energy in phosphoric acid anhydride bonds. 107

108 24 Summary Carboxylic acids contain the carboxyl group. The IUPAC naming of carboxylic acids follows a similar process to that used for other organic molecules. Many carboxylic acids are known by common names. Each common name generally refers to a natural source for the carboxylic acid. The polar carboxyl group causes the small carboxylic acids to be water-soluble. 108

109 24 Summary Carboxyl groups can hydrogen-bond to each other at two places. This strong noncovalent bonding causes high boiling points for carboxylic acids. Carboxylic acids are weak acids. Strong oxidizing agents will convert aldehydes and primary alcohols to carboxylic acids. Alkyl chains on benzene rings can be oxidized by strong oxidizing agents to form benzoic acid. 109

110 24 Summary Nitriles, RCN, will react with water plus hydrogen ion to yield carboxylic acids. Carboxylic acids have a sour taste, affect ph indicators, form water solutions with a ph of less than 7, and undergo neutralization reactions. Carboxylic acids react with bases to form salts. Many common carboxylic acid reactions involve substitution of the hydroxyl group for another atom or group of atoms. 110

111 24 Summary An ester is formed when an alcohol or phenol substitutes for the carboxylic acid hydroxyl group. Esters are named as alcohol derivatives of carboxylic acids. Condensation polymers are produced when monomers combine with the loss of a small molecule. Polyesters are formed when alcohol monomers react with carboxylic acid monomers with the loss of water. 111

112 24 Summary The most important reaction of esters is hydrolysis. Fats and oils are esters of glycerol and long-chain carboxylic acids (fatty acids). A triacylglycerol or triglyceride is a glycerol that has been esterified with three fatty acids molecules. 112

113 24 Summary Fats are solids at room temperature, while oils are liquids. Triacylglycerols are the principal energy-storage molecules in the human body. Hydrogenation of unsaturated fatty acids results in the addition of hydrogen to carbon carbon double bonds. Hydrogenolysis breaks a triacylglycerol into glycerol and three long-chain primary alcohols. 113

114 24 Summary Triacylglycerols can be hydrolyzed in acid to form fatty acids and glycerol or in base to form soaps and glycerol. A synthetic detergent is an organic molecule that has been synthesized in a chemical factory and acts as a cleansing agent. Soaps are salts of long-chain fatty acids. 114