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Carboxylic Acid Derivatives

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Carboxylic Acid Derivatives Block 3 Carboxylic Acids, their Derivatives and Amino Compounds UNIT 10 CARBOXYLIC ACID DERIVATIVES Structure 10.1 Introduction 10.6 Carboxylic Acid Esters Objectives Preparation of Carboxylic Acid Esters 10.2 Structure and Reactivity of Carboxylic Acid Reactions of Carboxylic Acid Derivatives esters 10.3 Physical Properties of Reformatsky Reaction Carboxylic Acid Derivatives 10.7 Amides 10.4 Carboxylic Acid Halides Preparation of Amides Preparation of Carboxylic Acid Reactions of Amides Halides 10.8 Summary Reactions of Carboxylic Acid Halides 10.9 Terminal Questions 10.5 Carboxylic Acid Anhydrides 10.10 Answers Preparation of Carboxylic Acid Anhydrides Reactions of Carboxylic Acid Anhydrides 10.1 INTRODUCTION In the last unit, you have studied about the chemistry of carboxylic acids. There, in Sec. 9.5 under the reactions of carboxylic acids, you came across the formation of derivatives of carboxylic acids. A functional derivative of carboxylic acid is a compound which results on replacement of the hydroxyl group of the carboxylic acid by some other group, L. A characteristic feature of these derivatives is that they regenerate the carboxylic acid on hydrolysis, i.e. 28 Unit 10 Carboxylic Acid Derivatives O O RCL + H2O RCOH + HL Carboxylic acid Water Carboxylic Conjugate derivative acid acid of L Various functional derivatives of carboxylic acids are possible depending upon the nature of L. The functional derivatives which you will study in this unit include carboxylic acid halides also called alkanoyl halides, anhydrides, esters and amides. The general structures of carboxylic acid and these functional derivatives can be represented as follow: O O O O R C OH R C X R C O C R' a carboxylic acid an alkanoyl an anhydride halide O O R C R' R C NH2 an ester an amide O R C You can see that all of these derivatives contain a or alkanoyl group in their structure. While studying this unit, you will realise that there is not only a structural similarity among carboxylic acids and their derivatives but also a close relationship in their chemistry. Expected Learning Outcomes After studying this unit, you should be able to: define carboxylic acid derivatives; give examples of various carboxylic acid derivatives; comment on the acidic and basic behavior of various carboxylic acid derivatives; correlate the reactivities of carboxylic acid derivatives with their structures; outline the synthesis of various carboxylic acid derivatives; explain the reactions of various carboxylic acid derivatives; compare the behavior of various carboxylic acid derivatives, reaction conditions required in various nucleophilic addition-elimination reactions like hydrolysis, formation of amides, etc., and the nature of products obtained; describe the reactions of various carboxylic acid derivatives with organometallic reagents; and explain the reduction reactions undergone by carboxylic acid derivatives. 29 Block 3 Carboxylic Acids, their Derivatives and Amino Compounds 10.2 STRUCTURE AND REACTIVITY OF CARBOXYLIC ACID DERIVATIVES Similar to the structure of carbonyl compounds and carboxylic acids, the derivatives of carboxylic acids have trigonal geometry, i.e., all the bonds to the carbonyl carbon are in the same plane. This is shown in Fig. 10.1. O L C R Fig. 10.1: Planar arrangement of bonds to the carbonyl carbon in carboxylic acid derivatives. An important structural feature of carboxylic acid derivatives is that the atom O R attached to the C group bears an unshared pair of electrons which is capable of interacting with the π electrons of the carbonyl group. This is shown in Fig. 10.2. Individual p orbitals in carboxylic acid Extended orbital system in carboxylic derivatives acid derivatives Fig. 10.2: The extended π electron system in carboxylic acid derivatives. This electron delocalisation can be represented by the following resonance structures. O: : :O: : R C R C + : L L I II The extent of this electron delocalisation depends on the electron donating properties of L. Thus, a less electronegative L will donate the electrons more easily than a more electronegative L. The electron release from L reduces the polarisation of the carbonyl group, thereby, decreasing its electrophilic character. Thus, the greater the electron release from L, the greater is its stabilising effect. Consequently, when L is more electronegative, the extent of resonance decreases and the reactivity increases. Thus, the reactivity of carboxylic acid derivatives towards nucleophilic substitution reactions follows 30 the following order: Unit 10 Carboxylic Acid Derivatives O O O O O A derivative higher in RCCl > RCOCR > RCOR > RCNH this order can be 2 converted to the one alkanoyl halide anhydride ester amide lower but not vice- versa. You will study the nucleophilic substitution reactions of carboxylic acid derivatives in detail in the later sections of this unit. The degree of resonance stabilisation is also reflected in the structural parameters and spectral characteristics of carboxylic acid derivatives about which you will study in the next section. But before that let us understand the reactivity of carboxylic acid derivatives as acids and bases. Basicity and Acidity of Carboxylic Acid Derivatives Carboxylic acid derivatives are weakly basic at the carbonyl oxygen which can be protonated using strong acids. This property is particularly useful in some of the acid-catalysed reactions of esters and amides. .. .. .. H H H .. .. .. + O .. O O O H+ C C + C .. .. C .. R L R L R L R L+ pKa values of the The pKa values for the conjugate acids of carboxylic acid derivatives show that conjugate acids of alkanoyl halides are the weakest bases as their conjugate acids have the carboxylic acid derivatives. lowest pKa and are, therefore, strongest acids. Esters are about as basic as : H : H + + + : H carboxylic acids whereas amides are the most basic. O O O : : : RCCl : < RCOR ' < RCNH The acidity of the -hydrogens next to the carbonyl group shows the following : : 2 pKa ~ 9 ~ 10 ~ 0 order amongst carboxylic acid derivatives. O O O CH CCl < CH COCH < 3 3 3 CH3CN(CH3)2 pK ~ 16 pK ~ 25 pK ~ 30 a a a Primary and secondary amides are deprotonated at nitrogen to give an amidate ion which is resonance stabilised in the same way as the carboxylate ion. .. .. .. .. .. .. O .. O O C H+ + C C .. .. .. R R NH2 NH R NH pK ~ 15 an amidate ion a 31 Block 3 Carboxylic Acids, their Derivatives and Amino Compounds 10.3 PHYSICAL PROPERTIES OF CARBOXYLIC ACID DERIVATIVES It was pointed out in the earlier section that the extent of resonance is reflected in the structural parameters. This can be understood when we compare the C-L bond lengths in various carboxylic acid derivatives with the C-L bond lengths in the compounds of the type R-L. These bond lengths are listed in Table 10.1. Table 10.1: C-L Bond lengths of some carboxylic acid derivatives and compounds of R-L type O L R C L (pm) R L (pm) Cl 179 178 OCH3 136 143 NH2 136 147 The bond lengths shown in Table 10.1 indicate that as we go from the most reactive alkanoyl halides to the much less reactive esters and amides, the C-L bond becomes shorter as compared to the normal C-L single bond. Thus, in amides, the contribution of the dipolar structure II as discussed earlier, is strong enough to impart some double bond character to the carbon-nitrogen bond. The double bond character is also indicated by a barrier of 75 to 84 kJ mol1 to the rotation of the carbon-nitrogen bond. O R' R R' C N C N R R" O R" The other physical properties for various carboxylic acid derivatives are briefly stated below. Alkanoyl Halides and Anhydrides The lower members of these derivatives are dense, water-insoluble liquids with piercing odours. Their boiling points are not very different from those of other polar molecules of similar weight and shape. Some examples are given below: O O O CH3 O C C C C CH CH CH C Cl O CH CH3 3 3 CH3 3 b.p. 413 K b.p. 403 K b.p. 324 K O O O CH CH C Cl C OCH CH3 C O 2 3 3 b.p. 330 K b.p. 470 K b.p. 486 K 32 Unit 10 Carboxylic Acid Derivatives Esters: The lower members of this class are volatile, fragrant liquids having lower density than water. Most esters are not soluble in water. Amides: The lower members are water-soluble, polar in nature and have high boiling points. Primary and secondary amides associate to form hydrogen bonded dimers or higher aggregates in solid and liquid state. O C CH3 NH2 ethanamide m.p. 355 K O b.p. 494 K CH3 HCN CH3 A number of amides have high dielectric constants. N, N-Dimethylmethanamide (commonly known as N, N-dimethylfomamide, N,N-Dimethylmethanamide abbreviated as DMF) is widely used as a polar aprotic solvent. SAQ 1 Which carboxylic acid derivative is least a) reactive towards nucleophilic substitution reactions? b) basic? 10.4 CARBOXYLIC ACID HALIDES Carboxylic acid halides are important class of compounds belonging to carboxylic acid derivatives. In this section, we will be dealing with the preparation and the reactions of this class of carboxylic acid derivatives. 10.4.1 Preparation of Carboxylic Acid Halides Carboxylic acid halides can be prepared from carboxylic acids using the acid chlorides of inorganic acids such as PCl5 (acid chloride of phosphoric acid), PCl3 (acid chloride of phosphorous acid) and SOCl2 (acid chloride of sulphurous acid). The general reactions can be represented as shown below: O O RCOH + PCl5 RCCl + POCl3 + HCl phosophrous pentachloride O O 3 RCOH + PCl3 3 RCCl + H3PO3 phosophrous trichloride O O RCOH + SOCl2 RCCl + SO2 + HCl thionyl chloride 33 Block 3 Carboxylic Acids, their Derivatives and Amino Compounds Carboxylic acid fluorides, bromides and iodides are prepared from carboxylic acid chlorides by reaction with HF, HBr or HI, respectively.
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