Carbonyl group in carboxylic acids

The most significant change in these reactions is the formation of the carbon-nncleophile bond so, in both types of mechanism, the reaction is termed a nucleophilic addition. It should be noted that the polarization in the carbonyl group leads to nucleophilic addition, whereas the lack of polarization in the C=C donble bond of an alkene leads to electrophilic addition reactions (see Chapter 8). Carbonyl groups in carboxylic acid derivatives undergo a similar type of reactivity to nucleophiles, but the... 

N -(C=O)- in-carboxyiic-acid, ketone, aldehyde is the total number of carbonyl groups in carboxylic acid, ketone, and aldehyde. 

The carboxyl function does absorb ultraviolet radiation, but the wavelengths at which this occurs are appreciably shorter than for carbonyl compounds such as aldehydes and ketones, and, in fact, are out of the range of most commercial ultraviolet spectrometers. Some idea of how the hydroxyl substituent modifies the absorption properties of the carbonyl group in carboxylic acids can be seen from Table 18-2, in which are listed the wavelengths of maximum light absorption (Amax) and the extinction coefficients at maximum absorption (emax) of several carboxylic acids, aldehydes, and ketones. 

Carboxylic acids contain a carbonyl group, but it does not undergo the type of addition reactions that occur with the aldehydes and ketones. The carbonyl group in carboxylic acids, esters, amides or acyl chlorides has the electronegative atoms O, N or Cl next to the C=0, and these stop it from acting as a proper C=0 group should (Figure 7.2.15). 

N[-(C=0)- in carboxylic acid, ketone or aldehyde] denotes the total number of carbonyl groups in carboxylic acid (-COOH), ketone [R-(C=0)-R with R H and R H] and aldehyde [R-(C=0)-H with R H] environments. For example, poly(oxy-l,4-phenylene-oxy-l,4-phenylene-carbonyl-1,4-phenylene) (Figure 2.4) and poly(acrylic acid) (Figure 4.2) each have one such carbonyl group, in a ketone and a carboxylic acid environment, respectively. 

C. The carbonyl group in carboxylic acids are more electrophilic than that of ketones. 

FIGURE 3.10 Maps of electrostatic potential at approximately the bond density surface for acetic acid and ethanol. The positive charge at the carbonyl carbon of acetic acid is evident in the blue color of the electrostatic potential map at that position, as compared to the hydroxyl carbon of ethanol. The inductive electron-withdrawing effect of the carbonyl group in carboxylic acids contributes to the acidity of this functional group. 

You learned in Chapter 17 that nucleophilic addition to aldehydes and ketones is one of the fundamental reaction types of organic chemistry, then in Chapter 19 you saw that addition to carbonyl groups in carboxylic acid derivatives can lead to nucleophilic acyl substitution. In this chapter, you ll encounter a third pattern of carbonyl reactivity—one that involves the enol tautomers of aldehydes, ketones, and esters and the conjugate bases of enols known as enolates. 

The carbonyl group in carboxylic acids undergoes nucleophilic displacement via the addition-elimination pathway. Addition of a nucleophile gives an unstable tetrahedral intermediate that decomposes by elimination of the hydroxy group to give a carboxylic acid derivative. 

With pyridinium dichromate3V, the oxidation reaction of PCTFE-1-propanol produced PCTFE-propanoic acid as illustrated in Equation 9. Its IR spectrum showed the presence of the carboxylic acid moiety at 3439 (broad, O-H stretching) and 1656 (broad, perhaps asymmetric stretching of two carbonyl groups of carboxylic acid salt) cm-1. Furthermore, the disappearance of C-0 and =C-H bands was clearly observed in the IR spectrum. 

Lithium aluminum hydride usually reduces carbonyl groups without affecting carbon-carbon double bonds. It is, in addition, a good reducing agent for carbonyl groups of carboxylic acids, esters, and other acid derivatives, as will be described in Chapter 18. 

Below are shown a few examples of the types of complex structures that can be assembled by intramolecular free-radical cyclization. Note the presence of a great many polar functional groups present in the cyclization substrates which are compatible with the process. While the examples shown do not need protecting groups, a great number of other free-radical cyclizations are known which have unprotected alcohols, carbonyl groups, and carboxylic acids in the cyclization precursor. 

FTIR and NMR Analysis of Oxidation Products. As expected the FTIR spectra of the soluble oxidation products are dominated by the strong absorptions due to the hydroxyl and carbonyl groups of carboxylic acids. There are absorptions in the spectra which occur in the regions expected for sulfones (1350-1310 and 1160-1120 cm-1) and sulfonic acids (1420-1330 and 1200-1145 cm-1), but these are probably due to non-sulfur containing functional groups, especially carbon-oxygen bonds which are presumably present in much higher concentrations. 

The most obvious feature in the infrared spectrum of a carboxylic acid is the intense carbonyl stretching absorption. In a saturated acid, this vibration occurs around 1710 cm-1, often broadened by hydrogen bonding involving the carbonyl group. In conjugated acids, the carbonyl stretching frequency is lowered to about 1690 cm-1. 

The addition reactions of Grignard and organolithium reagents with carbonyl groups and carboxylic acid derivatives are summarized in Table 3.2. 

Assigning the peaks to individual carbons in a C SSNMR experiment is not always trivial, as peaks can vary by more than 10 ppm from their solution values. In a broad sense, peaks from 160 to 180 ppm are due to carbonyl groups of carboxylic acid derivatives, 200-220 ppm are due to ketone carbonyls, 100-160 ppm are from aromatic and olefinic carbons, 50-100 ppm are from sp -hydridized carbons attached to heteroatoms, and 10-40 ppm are typically aliphatic carbons attached to other carbons and/or hydrogens. These are purely estimates of the basic functionalities found in most organic molecules, and exceptions to these ranges are not uncommon. Many crystalline systems also possess more than one crystallographically inequivalent molecule per unit cell, which is also easily detectable in SSNMR experiments and can make interpretation of spectra more complicated. For instance, if two peaks exist in the C SSNMR spectrum for each carbon, there are two molecules in the unit cell, although not every peak in each inequivalent molecule is always resolved. 

The carbonyl (—C— ) group in carboxylic acids leads to keto acids. 

The carbonyl groups of carboxylic acids, esters, and amides are less reactive, so they are harder to reduce than the carbonyl groups of aldehydes and ketones (Section 18.5). They cannot be reduced by catalytic hydrogenation (except under extreme conditions). They can, however, be reduced by a method we will discuss later in this section. 

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