OH in Carboxylic Acids

Very important compounds are the carboxylic acids and their derivatives, which can be formally obtained by exchanging the OH group for another group. In fact, derivatives of this type are formed by nucleophilic substitutions of activated intermediate compounds and the release of water (see p. 14). Carboxylic acid esters (R-O-CO-R ) arise from carboxylic acids and alcohols. This group includes the fats, for example (see p.48). Similarly, a carboxylic acid and a thiol yield a thioester (R-S-CO-R ). Thioesters play an extremely important role in carboxylic acid metabolism. The best-known compound of this type is acetyl-coenzyme A (see p. 12). 

In carboxylic acids, the functional group consists only of the —OH and =0 groups. 

Carboxylic acids are generally nonreactive in their anionic form. The reactivity of the acid form is strongly enhanced by electron withdrawing groups (Peter and Neta, 1972 Micic and Markovic, 1972). It was concluded, therefore, that the electron adds predominantly to a positive centre on the OH oxygen and not to the carbonyl double bond. The reactivity of nitriles toward eaq is affected by substituents similarly to the effect in carboxylic acids (Draganic et al., 1973). 

Acid derivatives differ in the nature of the nucleophile bonded to the acyl carbon —OH in the acid, —Cl in the acid chloride, —OR in the ester, and —NH2 (or an amine) in the amide. Nucleophilic acyl substitution is the most common method for interconverting these derivatives. We will see many examples of nucleophilic acyl substitution in this chapter and in Chapter 21 ( Carboxylic Acid Derivatives ). The specific mechanisms depend on the reagents and conditions, but we can group them generally according to whether they take place under acidic or basic conditions. 

In carboxylic acids, A is hydrogen or an alkyl group, and B must be an OH group. 

Direct ester fomnation from alcohols (R OH) and carboxylic acids (R C02H) works in acid solution v - but does not work at all in basic solution. Why not By contrast, ester formation from alcohols (R OH) and-carboxylic acid anhydrides, (R C0)20, or acid chlorides, RCOCI, is commonly earned-out in the presence of amines such as pyridine or EfsN. Why does this work ... 

Carboxylic acids react with phenyliminovinylidenetriphenylphosphorane (equation 106) via the intermediates (1) with formation of the alkylidenephosphoranes (2), which, on heating, rearrange in an intramolecular acyl migration to ylides (3). By heating (2) in the presence of an alcohol the acyl ylides (4) and N-phenylurethanes are formed. - The reaction sequence allows the replacement of the OH group in carboxylic acids by the ylide function. 

The carbon atom bearing the — OH group in carboxylic acids is not saturated. Carboxylic acids do not show properties of alcohols. 

Nhydroxyl denotes the total number of -OH moieties in alcohol or phenol environments. Unlike the term proportional to Non used in Section 3.B, the -OH groups in carboxylic acid (-COOH) or sulfonic acid (-SO3H) environments are not counted in Nhydroxyi- This dependence on the environment of the -OH group causes to be a group correction term, while Nqh... 

The -OH groups in carboxylic acid (-COOH) and sulfonic acid (-SO3H) moieties do not contribute to Nbbar. 

Zn(SH)(Tp Bu>Me)] does not react with esters, phosphates, or CO2, e.g., for thiolytic cleavage reactions, whereas acidic organic X-OH compounds (carboxylic acids, trinitrophenol, hexafluoroacetylace-tone) replace the SH groups with the formation of [Zn(OX)-(Tp Bu Me)].103 The reaction of [Zn(OH)(Tp,Bu Me)] with alcohols has been followed by combined experimental and computational investigations in order to determine the factors influencing the formation of tetrahedral alkoxide complexes. The [Zn(OR)(Tp Bu,Me)l derivatives have been found unstable toward hydrolysis.104... 

Formation of H2 from OH systems 1, 1.2.3.2 In carboxylic acid synthesis from olefins 16, 14.6.1.4... 

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