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Chapter 20: Carboxylic Acids and Nitriles the Importance of Carboxylic Acids

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Chapter 20: Carboxylic Acids and Nitriles the Importance of Carboxylic Acids CHEM 213 Exam 4 Part 1 Professor Kelly Boebinger Chapter 20: Carboxylic Acids and Nitriles The Importance of Carboxylic Acids (RCO2H) Carboxylic acids and their derivatives are carbonyl compounds in which the acyl group is bonded to electronegative atom such as oxygen, halogen, nitrogen, or sulfur. In contrast to aldehydes and ketones, the acyl group bonded to the substituent that can act as a leaving group in substitution reactions. Starting materials for acyl derivatives (esters, amides, and acid chlorides) Abundant in nature from oxidation of aldehydes and alcohols in metabolism 3 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 4 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 5 Problems: Draw the structures for the following A. 2,3-dimethylhexanoic acid B. 2-cyclohexenecarboxylic acid C. butanedioic acid D. 2-aminopropanoic acid (alanine) (amino = NH2) E. 2-hydroxypropanoic acid (lactic acid) A. B. C. E. D. 6 Give the IUPAC name A. CH3CH=CHCH2CH2COOH B. (CH3)2CHCH2COOH C. CH3CH(Br)CH2CH2COOH A. B. C. 7 20.2 Structure and Physical Properties of Carboxylic Acids Carboxyl carbon sp2 hybridized: planar, 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 8 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 9 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) Weaker acids than mineral acids but stronger than alcohols. Weak acids slightly dissociate. 10 20.4 Substituent Effects on Acidity Electronegative substituents promote formation of the carboxylate ion 11 Substituent Effects Carboxylic acids differ in acid strength. Electron-withdrawing groups stabilize carboxylate anions and increase acidity. An electron-withdrawing group attached to the α-carbon of a carboxylic acid inductively withdraws electron density, thereby delocalizing the negative charge, thus stabilizes the carboxylate anion thus increasing acidity. An electron-donating group destabilizes the carboxylate anion and decreases acidity 12 Examples of Inductive Effects on Acidity Fluoroacetic, chloroacetic, bromoacetic, and iodoacetic acids are stronger acids than acetic acid since more of the acid is in the dissociated form. 13 20.5 Substituent Effects in Substituted Benzoic Acids .Groups that are deactivating in electrophilic aromatic substitution reactions increase the acidity of substituted benzoic acids. .The acidity of benzoic acids can be used to predict electrophilic reactivity, since measuring acidity is easier. 14 20.6 Preparation of Carboxylic Acids Oxidation of a substituted alkylbenzene with KMnO4 or Na2Cr2O7 gives a substituted benzoic acid (see Section 16.10) 1° and 2° alkyl groups can be oxidized, but NOT 3o 15 From Alkenes Oxidative cleavage of an alkene with KMnO4 gives a carboxylic acid if the alkene has at least one vinylic hydrogen (see Section 7.8) O O O O KMnO4 H3C CH CH C OH H3C C OH HO C C OH + + H3O 16 From Alcohols & Aldehydes Oxidation of a primary alcohol or an aldehyde H CrO O R C OH 3 R C OH + H H3O O O Ag2O R C H R C OH NH4OH 17 Hydrolysis of Nitriles (RCN) 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 18 Carboxylation of Grignard Reagents Grignard reagents react with dry CO2 to yield a metal carboxylate Limited to alkyl halides that can form Grignard reagents (see 17.6) O ether 1. CO2, ether o R-Br + Mg R-Mg-Br R C OH + 2. H3O 19 20.7 Reactions of Carboxylic Acids: A Preview Carboxylic acids transfer a proton to a base to give anions, which are good nucleophiles in SN2 reactions Like ketones, carboxylic acids undergo addition of nucleophiles to the carbonyl group In addition, carboxylic acids undergo other reactions characteristic of neither alcohols nor ketones 20 20.7 Reactions of Carboxylic Acids: A Preview 21 20.8 Reduction of Carboxylic Acids Reduced by to yield primary alcohols Carboxylic acids can be reduced to primary alcohols with either LiAIH4 or BH3 (but not by NaBH4). LiAlH4 is difficult and often requires heating in tetrahydrofuran solvent to go to completion H O 1. LiAlH4, THF, ∆ R C OH R C OH + 2. H3O H BH3 is a more selective reagent, since the reaction occurs rapidly at room temperature. H O 1. BH3, THF R C OH R C OH + 2. H3O 22 H Nitriles, RCN (Covered in chapter 21.8 McMurry 5th ed.) Closely related to carboxylic acids named by adding -nitrile as a suffix to the alkane name, with the nitrile carbon as C1 Complex nitriles are named as derivatives of carboxylic acids. Replace -ic acid or -oic acid ending with -onitrile 23 20.9 Chemistry of Nitriles RC≡N Nitriles and carboxylic acids both have a carbon atom with three σ bonds to an electronegative atom, and both contain a bond Both both are electrophiles Preparation of Nitriles by Dehydration Reaction of primary amides RCONH2 with SOCl2 or POCl3 (or other dehydrating agents) Not limited by steric hindrance or side reactions (as is the reaction of alkyl halides with NaCN) O SOCl2, benzene R C N + SO2 + 2 HCl R C NH2 80 °C 24 Reactions of Nitriles 25 Reactions of Nitriles RC≡N Hydrolysis: Conversion of Nitriles into Carboxylic Acids Hydrolyzed in with acid or base catalysis to a carboxylic acid and ammonia or an amine Reaction of Nitriles with Organometallic Reagents Grignard reagents add to give an intermediate imine anion that is hydrolyzed by addition of water to yield a ketone 26 Reactions of Nitriles RC≡N Reduction: of a nitrile with LiAlH4 gives a primary amine Mechanism Nucleophilic addition of hydride ion to the polar CN bond, yields an imine anion The C=N bond undergoes a second nucleophilic addition of hydride to give a dianion, which is protonated by water 27 20.10 Spectroscopy of Carboxylic Acids and Nitriles. Infrared Spectroscopy O–H bond of the carboxyl group gives a very broad absorption 2500 to 3300 cm1 C=O bond absorbs sharply between 1710 and 1760 cm1 Free carboxyl groups absorb at 1760 cm1 Commonly encountered dimeric carboxyl groups absorb in a broad band centered around 1710 cm1 Nitriles show an intense CN bond absorption near 2250 cm1 for saturated compounds and 2230 cm1 for aromatic and conjugated molecules 28 13CNMR Carboxyl 13COOH signals are at 165 to 185 Aromatic and ,b-unsaturated acids are near 165 and saturated aliphatic acids are near 185 13C N signal 115 to 130 29 1HNMR The acidic CO2H proton is a singlet near 12 When D2O is added to the sample the CO2H proton is replaced by D causing the absorption to disappear from the NMR spectrum Note that the carboxyl proton absorption occurs at 12.0 30 CHEM 213 Exam 4 Part 2 Professor Kelly Boebinger Chapter 21. Carboxylic Acid Derivatives and Nucleophilic Acyl Substitution Reactions Carboxylic Compounds General reaction pattern: Nucleophilic acyl substitution 33 21.1 Naming Carboxylic Acid Derivatives Acid Halides, RCOX Derived from the carboxylic acid name by replacing the -ic acid ending with -yl or the - carboxylic acid ending with –carbonyl and specifying the halide 34 Naming Acid Anhydrides, RCO2COR' If symnmetrical replace “acid” with “anhydride” based on the related carboxylic acid (for symmetrical anhydrides) Unsymmetrical anhydrides— cite the two acids alphabetically From substituted monocarboxylic acids: use bis- ahead of the acid name 35 Naming Amides, RCONH2 With unsubstituted NH2 group. replace -oic acid or -ic acid with -amide, or by replacing the -carboxylic acid ending with –carboxamide If the N is further substituted, identify the substituent groups (preceded by “N”) and then the parent amide 36 Naming Esters, RCO2R Name R’ and then, after a space, the carboxylic acid (RCOOH), with the “-ic acid” ending replaced by “- ate” 37 Name the following B. C. A. O O CH3 CH3 O C H3C C Br CH CH CH C O CH Cl 3 2 2 3 D. O O A. acetyl bromide B. benzoyl chloride CH3 CH2 CH2 C O C CH3 C. methyl 2,3-dimethylbutanoate D. butanoic ethanoic anhydride E. E. 2-butenenitrile CH3 CH CH C N F. butanamide O F. CH CH CH C NH2 3 2 2 38 Give the structure of the following A. A. methyl ethanoate B. (methyl acetate) B. propanoic anhydride C. benzamide C. (benzenecarboxamide) D. D. N,N-dimethylformamide E. 1-methylcyclobutanecarboxamide E. F. ethyl benzoate G. 2-chlorobutanoyl chloride F. G. 39 Reactions of Carboxylic Acids 40 21.2 Nucleophilic Acyl Substitution The substitution of a nucleophile to a polar C=O bond is a key step in 3 of the major 4 reactions of carbonyl groups. Carboxylic acid derivatives have an acyl carbon bonded to a group Y that can leave A tetrahedral intermediate is formed and the leaving group is expelled to generate a new carbonyl compound, leading to substitution O O O + :Nu- (or :Nu-H) + :Y- C C C R Nu R1 Y 1 R1 Nu Y Y is a leaving group = -OR, -NR2, -Cl 41 Relative Reactivity of Carboxylic Acid Derivatives Nucleophiles react more readily with unhindered carbonyl groups More electrophilic carbonyl groups are more reactive to addition (acyl halides are most reactive, amides are least) The intermediate with the best leaving group decomposes fastest 42 Substitution in Synthesis It is possible to convert a more reactive acid derivative into a less reactive one 43 General Reactions of Carboxylic Acid Derivatives Hydrolysis: reaction with water to yield a carboxylic acid.
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