functional derivatives of carboxylic acid

Reaction of an excess of these reagents with acyl chlorides, anhydrides and esters leads to alcohol products, in the same fashion as the hydride reductions. Two additional functional group derivatives will be considered in this chapter, viz., amides and acid chlorides. Since acyl chlorides and anhydrides are expensive and time consuming to prepare, acids and esters are the most commonly used reactants for this transformation. Step 6 is thus an essential part of the mechanism. This is futher demonstrated by the last reaction, in which a nitrile is preferentially reduced in the presence of a carbonyl group and two benzene rings. Even if one were to use a large exces of the alkoxide anion, it would be unable to add to this functionality at an appreciable rate. (CH3(CH2)2CO)2O is butanoic anhydride   &   CH3COOCOCH2CH3 is ethanoic propanoic anhydride (or acetic propionic anhydride). 1. q      NOTE: To this point, the conversion is from an amide to a carboxylic acid. This is the neutralization of the carboxylic acid by the hydroxide anion (just as in the case of base-promoted ester hydrolysis). Specifically, these are the carbonyl carbon, its attached oxygen, the atom (R) attached to the carbonyl carbon, the nitrogen and the two atoms (R’,R’’) attached to nitrogen. q      In contrast, the nitrogen-protonated conjugate acid has no resonance stabilization at all (there is only one valid resonance structure). In this two-stage mechanism bond formation occurs before bond cleavage, and the carbonyl carbon atom undergoes a hybridization change from sp2 to sp3 and back again. In the illustration on the right, R and Z represent the remainder of a benzene ring. The strongest resonance effect occurs in amides, which exhibit substantial carbon-nitrogen double bond character and are the least reactive of the derivatives. In the following examples the IUPAC names are color coded, and common names are given in parentheses. Nomenclature Three examples of acyl groups having specific names were noted earlier. Carboxylic acids have a hydroxyl group bonded to an acyl group, and their functional derivatives are prepared by replacement of the hydroxyl group with substituents, such as halo, alkoxyl, amino and acyloxy. and drives the equilibrium to completion. When these substituents are attached to an sp2 carbon that is part of a π-electron system, a similar inductive effect occurs, but p-π conjugation moves electron density in the opposite direction. q      It should be noted that the hydroxide ion is consumed in the final step of the reaction, so that the reaction is not base-catalyzed, but rather requires a stoichiometric amount of the base. q      The overall reaction, starting from the amide, goes to completion because the carboxylate anion has greater resonance stabilization than the amide. The only other reduction of a carboxylic acid derivative that is widely used is that of nitriles to 1º-amines. The exceptional reactivity of acyl halides, on the other hand, facilitates their reduction under mild conditions, by using a poisoned palladium catalyst similar to that used for the partial reduction of alkynes to alkenes. The first three examples concern reactions of acyl chlorides, the most reactive acylating reagents discussed here. • Amides: The name of the related acid is used first and the oic acid or ic acid suffix is replaced by amide (only for 1º-amides). A temperature of -78 ºC is easily maintained by using dry-ice as a coolant. This analysis also predicts the influence these substituent groups have on the reactivity of carboxylic acid derivatives toward nucleophiles (Z = O in the illustration). . Also, a specific example of acyl chloride formation from the reaction of a carboxylic acid with thionyl chloride will be shown. These functional groups are listed in Table 15.1 “Organic Acids, Bases, and Acid Derivatives” , along with an example (identified by common and International Union of Pure and Applied Chemistry [IUPAC] names) for each type of compound. Addition of hydride produces a tetrahedral intermediate, shown in brackets, which has a polar oxygen-aluminum bond. For a summary overview of the reducing reagents discussed above. q      In the case of acid chlorides, the third resonance structure is the least favorable of all of the acyl compounds, placing a positive charge on the highly electronegative chlorine atom. We can also consider the conjugate base of a carboxylic acid, a. , as a functional derivative of the carboxylic acid. q      Condensation of a Carboxylic Acid with An Amine via Carbodiimide Catalysis. q      Although the unavoidable consumption of the base requires that we use a mole for mole amount of sodium or potassium hydroxide, there is also a favorable aspect of this final, neturalization, step. As in the reductions of aldehydes and ketones, the first step in each case is believed to be the irreversible addition of hydride to the electrophilic carbonyl carbon atom. Lithium tri-tert-butoxyaluminohydride   (LtBAH), LiAl[OC(CH3)3]3H :   Soluble in THF, diglyme & ether. • Acid Halides: The acyl group is named first, followed by the halogen name as a separate word. q      The mechanism of the base-promoted ester hydrolysis is illustrated below. The most important such reaction is hydrolysis, and this normally requires heat and strong acid or base catalysts. e.g. The amide group has a carboxyl group joined to an amino group. The acyl compounds as a group all contain a carbon atom (the acyl carbon) in the +3 oxidation state, so that their interconversions, which are the special emphasis of this chapter, are not redox reactions. As illustrated by the following equations (shaded box), this occurs by sequential addition-elimination-addition reactions, and finishes with hydrolysis of the resulting alkoxide salt. These are often used in common names of compounds. o    Since acid chlorides are much less thermodynamically stable than carboxylic acids, or any of the other acyl derivatives, their reactions with nucleophiles can proceed rapidly, at much lower temperatures (room temperature) and without the need for catalysis. Examples of these reductions are provided in the following diagram. The third reviews three common reactions, applied to eight carbonyl compounds, including aldehydes and ketones. Even if one were to use a large exces of the alkoxide anion, it would be unable to add to this functionality at an appreciable rate. As with aldehydes and ketones, carboxylic acid formulas can be written to show the carbon-to-oxygen double bond explicitly, or the carboxyl group can be written in condensed form on one line. It must be used in an equimolar amount. Because of the very strong pi bonding between the nitrogen atom and the carbonyl carbon in amides, the trigonal plane of nitrogen atom is required to be, with the trigonal plane of the carbonyl carbon atom (also, of course, sp. Such a method is the room temperature condensation between carboxylic acids and amines catalyzed by dicyclohexylcarbodimide (R’’’ = cyclohexyl). The overall reaction, starting from the amide, goes to completion because. Furthermore, such substitution reactions of alcohols and ethers are rare, except in the presence of strong mineral acids. These are designated by "N-alkyl" term(s) at the beginning of the name. The positive charge is also localized on an atom (N) which is directly attached to a highly electron deficient carbonyl carbon (electrostatic repulsion). Acyl Group Substitution This is probably the single most important reaction of carboxylic acid derivatives. Overlap of this type of orbital with the 2p AO on the carbonyl carbon would be much less efficient (recall that pi bonding is most efficient between p type orbitals). The aldehyde or ketone product of this elimination then adds a second equivalent of the reagent. In contrast, the nitrogen-protonated conjugate acid has no resonance stabilization at all (there is only one valid resonance structure). From the previous discussions you should be able to predict the favored product from each of the following reactions. Also, the 3p orbital of chlorine is too large to overlap very efficiently with the much smaller 2p AO on the carbonyl carbon. In practice, both reagents are used in equimolar amounts, and usually at temperatures well below 0 ºC. CHAPTER 18: FUNCTIONAL DERIVATIVES OF CARBOXYLIC ACIDS . Esters are less reactive acylating reagents than anhydrides, and the ester exchange reaction (#6) requires a strong acid or base catalyst. In contrast to the usefulness of lithium aluminum hydride in reducing various carboxylic acid derivatives, sodium borohydride is seldom chosen for this purpose. Note in particular that the step in which the carbonyl pi bond is broken is the “slow” step.

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