{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

Chapter 11 - Chapter 11 CARBOXYLIC ACIDS AND THEIR...

Info iconThis preview shows pages 1–9. Sign up to view the full content.

View Full Document Right Arrow Icon
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Background image of page 2
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Background image of page 4
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Background image of page 6
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Background image of page 8
Background image of page 9
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Chapter 11 CARBOXYLIC § ACIDS AND THEIR DERIVATIVES IN THIS CHAPTER: I/ Nomenclature t/ Carboxyiic Acid Derivatives I/ Preparation of Carboxy/ic Acids V Acidity of Carboxylic Acids t/ Reactions of Carboxylic Acids I/ Polyfunctionai Carboxylic Acids I/ Reactions of Acid Derivatives t/ Solved Problems Nomenclature Carboxylic acids (RCOQH or ArCOZH) have the structure shown below. Some have names derived from acetic acid; e.g., (CH3)3CCOZH and CGHSCHQCOEH, are Lrimethylacetic acid and phenylacetic acid, respectively. Occasionally they are named as carboxylic acids, e.g., the compound here is cyclohexanecarboxylic acid. 91 92 ORGANIC CHEMISTRY For IUPAC names, replace the —e of the c0r~ responding alkane with -oic acid: thus, CH3CH2C02H is propanoic acid. The carbons are 002H numbered: the carbon of COQH is numbered 1. Carboxylic Acid Derivatives The common types of acid derivatives are given in the table. with con- ventions of nomenclature that involve changes of the name of the cor— responding carboxylic acid. Preparation of Carboxylic Acids Oxidation of 1° Alcohols, Aldehydes, and Arenes. Carboxylic acids can be prepared by oxidizing primary alcohols or aldehydes with CrO3 in H2804 (Jones oxidation). Aromatic carboxylic acids can be made by side chain oxidation of substituted benzenes using KMnO4. Oxidative Cleavage of Alkenes and Alkynes. Carboxylic acids can be prepared by oxidative cleavage of alkenes or alkynes using KMnO4 in acid. CH3\C/CH3 CH3\ /CH3 KMnO4, 14* C 0 ll —> H | C c 0 CH3/ \H CH3/ \OH Grignard Reagent and C02. Addition of a Grignard reagent to CO: followed by acid workup leads to carboxylic acids. RMgBr + 002 —> RCOEH Hydrolysis of Acid Derivatives and Nitriles. Hydrolysis of carboxylic CHAPTER 11: Carboxylic Acids and Their Derivatives 93 5Y0uNeed to Know / ' Acetyl Ciilqlrid‘“! " ,‘(Although nitril _ , 7 , are grouped, ith acid derivative. because, they are readily hydrolyzed to RCO H.) 94 ORGANIC CHEMISTRY acid derivatives (acid chlorides, esters, amides, anhydrides. and nitriles) using either acidic or basic water produces a carboxylic acid. Acidity of Carboxylic Acids The H of COzH is acidic because RCO; is delocalized over both oxy— gens and is more stable and a weaker base than RO‘. whose charge is localized on only one oxygen. 9 ® RC02H + HgO——> RC02 + H30 RCOQH forms carboxylate salts with bases; when R is a long alkyl chain, these salts are called soaps. e G) RCOgH + KOH—> R002K + H20 The influence of substituents on acidity is best understood in terms of the conjugate base, RCOZ‘, and can be summarized as follows. Electron- withdrawing groups stabilize the carboxylate anion, strengthening the acid. Electron-donating groups destabilize RC0; and weaken the acid. Like all halogens, Cl is electronegative, electron-withdrawing, and acid-strengthening. Since F is more electronegative than Cl. it is a bet- ter Withdrawing group and a better acid strengthener. Inductive effects diminish as the number of CS between C] and the O’s increases. ClCH2C02H is a stronger acid than ClCHzCI-IECOZH, since the chlorine is closer to the negative charge on the anion that it sta— bilizes. Two Cl’s are more electron—withdrawing than one Cl. so C12CHCOZH is a stronger acid than ClCHgCOgH. Reactions of Carboxylic Acids Acid Chloride Formation. Carboxylic acids give acid chlorides when they are treated with thionyl chloride. RCOQH + socu ———-> RCOCI + HCI + $02 CHAPTER 11: Carboxylic Acids and Their Derivatives 95 Reaction with SOCl2 is particularly useful because the two gaseous products SO2 and HCI are readily separated from RCOCl. Ester Formation. Carboxylic acids react with alcohols to give esters. An acid catalyst is required. H4- RCO2H + R'OH —> HCOZR' In this reaction, the oxygen of the C=O is protonated, which increases the electrophilicity of the carbonyl carbon and renders it more easily attacked in the slow step by the weakly nucleophilic R‘OH. The tetra- hedral intermediate undergoes a sequence of fast deprotonations and protonations. the end result being the loss of H+ and H20 and the for- mation of the ester. (9 H :oz/NHG') : 5 TH Cl —> ICE ——> FI—C—OH R/ \OH Ft/J\OH @é -- HO\H' R'QH /" B: B: » H\@/H [email protected]‘\. R\<~9(“/H Cl) .(IDH 0—0 ‘— R—C—OH <— R—C—OH OR. | | OR' OR“ \ R C: 96 ORGANIC CHEMISTRY Reductioiiio Carboxylic ACIdS ' A6ids are beSt reduce ,_ by LiAIH4. NaBH4i is n enough reducing 45599.” , carboxylic acids ’ Polyfunctional Carboxylic Acids Dicarboxylic Acids. [HOZC(CHQ)n C02H] The chemistry of dicar— boxylic acids depends on the value of n. For n = l, decarboxylations can occur upon heating the diacid. When n = 2 or 3, the diacid forms cyclic anhydrides when heated. Longer-chain 0t, (o—dicarboxylic acids usually undergo intermolecular dehydration on heating to form long-chain polymeric anhydrides. Hydroxyacids: Lactones. Reactions of hydroxycarboxylic acids, HO(CHZ)HC02H, also depend on value of n. In acid solutions. y—hydroxy— carboxylic acid (11 : 3) and 5-hydroxycarboxylic acid (H = 4) form cyclic esters (lactones) with five—membered and six—membercd rings, respectively. lntramolecular nucleophilic displacements. such as those in lactone formation, have faster reaction rates than intermolecular 8N2 reactions because the latter require two species to collide. Reactions of Acid Derivatives The more reactive derivatives are readily converted to the less reactive ones. Because acetic anhydride reacts less Violently, it is often used instead of the more reactive acetyl chlou'de to make derivatives of acetic acid. Since other anhydrides are not readily available, the acid chlorides are used to make acid derivatives. The reactivity order is acid chlorides > anhydridcs > ester > amide. CHAPTER 11: Carboxylic Acids and Their Derivatives 97 0 e0 0 (ll —. Ll —. t R/ \CI R/|t_c;| Ft/ \Nuc J Nuc N 6) UC N ucleophilic substitution of acyl compounds takes place readily if the incoming group (Nu:' or Nuz) is a stronger base than the leaving group (G:—) or if the final product is a resonance—stabilized RCOI‘. The reactions of acid derivatives generally involve nucleophilic attack at the carbonyl carbon. Nucleophilic substitutions of RCOG, such as RCOCl. occur in two steps. The first step (addition) resembles nucleophilic addition to ketones and aldehydes and the second step (elimination) is loss of G. in this case. Cl—. Reactions of this type can be carried out either in acid or in base. For example. in hydrolysis. the protonation of carbonyl 0 makes C more electrophilic and hence more reactive toward weakly nucleophilic H20. Strongly basic ~OH readily attacks the carbonyl C. Unlike acid hydrolysis. this reaction is irreversible, because ~OH removes H“ from RCOql-I to form resonance-stabilized RCOZ‘. Acid Chlorides. Acid chlorides are used in Friedel-Craft acylations of benzene rings with AlCl3 catalyst, discussed in Chapter 7. They are also readily converted to other acid derivatives by reaction with the appro— priate nucleophile. Acid Anhydrides. Heating dicarboxylic acids, H03(CH2)“COQH (n = 2 or 3), forms cyclic anhydrides by intramolecular dehydration. Intermolecular dehydration of carboxylic acids is used to prepare acetic anhydride, but other anhydrides are not readily formed using this method. Although they are less reactive than acid chlorides, anhydrides resemble acid halides in their reactions. Acid anhydrides can also be used to acylate aromatic rings in electrophilic substitutions. Esters. Esters react more slowly than acid chlorides or anhydrides. They can be used to prepare amides by reaction with an amine. 98 ORGANIC CHEMISTRY Reduction of esters with LiAlH4 gives alcohols, as discussed in Chapter 9. Fats and Oils. Fats and oils are mixtures of esters of glycerol. HOCHQCHOHCHZOH. with acyl groups from carboxylic acids, usual- ly with long carbon Chains. These triacylglycerols, also called triglyc- erides, are types of lipids because they are naturally occurring and sol— uble only in nonpolar solvents. The acyl groups may be identical. or they may be different. Amides. Unsubstituted amides may be prepared by careful partial hydrolysis of nitriles. Amides are slowly hydrolyzed under either acidic or basic conditions. 0 O H+ I H+ I R—CEN ———-> C —> C H 0 H20 2 R/ \NHQ R/ \OH Imides. The hydrogen on N of the imides is acidic because the negative charge on N of the conjugate base is delocalized to each 0 of the two C=O groups, thereby stabilizing the anion. CHAPTER 11: Carboxylic Acids and Their Derivatives 99 0 (30 C// C/ Base \N—H \\N / / C C \ \\ O 0 You Need to Know ° Preparation of carboxylic acids ' Theiresonance stabilization of the carboxylate anion - Formation and reactions of acid derivatives Solved Problems Problem 11.1 Describe the electronic effect of CSH5 on acidity, if the acid strengths of CfiHSCOzH and H COEH are 6.4 x 10’5 and 1.7 x 10“, respectively. The weaker acidity of CfiH5 COQH shows that the electron releasing resonance effect of CfiH5 outweighs its electron attracting inductive effect. Problem 11.2 Give the products from reaction of benzamide. PhCONHE. with (a) LiAiH4. followed by H30+ (b) hot aqueous NaOH (:1) PhCHzNHZ (b) PhCOzNa + NH3 ...
View Full Document

{[ snackBarMessage ]}