Carboxylic acid derivatives resonance structures

Because carboxylic acid derivatives (RCOZ) all contain an atom Z with a nonbonded electron pair, three resonance structures can be drawn for RCOZ, compared to just two for aldehydes and ketones (Section 20.1). These three resonance structures stabilize RCOZ by delocalizing electron density. In fact, the more resonance structures 2 and 3 contribute to the resonance hybrid, the more stable RCOZ is. 

This resonance interaction affects the geometry that is adopted by some of the derivatives of the carboxylic acids. Draw the structure of a general carboxylic acid ester. 

At this point we want to consider the relative reactivity of carboxylic acid derivatives and other carbonyl compounds in general terms. We return to the subject in more detail in Chapter 7. Let us first examine some of the salient structural features of the carbonyl compounds. The strong polarity of the C=0 bond is the origin of its reactivity toward nucleophiles. The bond dipole of the C-X bond would be expected increase carbonyl reactivity as the group X becomes more electronegative. There is another powerful effect exerted by the group X, which is resonance electron donation. 

The second effect derives from the relative resonance stabilization of the carboxylic acid derivatives. As follows, each derivative can be written with contributing structures that will be stabilizing to some extent. The second contributing structure that we show for each carboxylic acid derivative has a positive charge on the carbonyl carbon. This structure reflects the electrophilicity of these carbons. However, for ea derivative, it is the other contributing structures that reflect the relative resonance stabilization of the derivatives. 

The extent of resonance can be observed directly in the structures of carboxylic acid derivatives. In the progression from acyl halides to esters and amides, the C-L bond becomes progressively shorter, owing to increased double-bond character (Table 20-1). The NMR spectra of amides reveal that rotation about this bond has become restricted. For example, W,N-dimethylformamide at room temperature exhibits two singlets for the two methyl groups, because rotation about the C-N bond is very slow on the NMR time scale. The evidence points to considerable tt overlap between the lone pair on nitrogen and the carbonyl carbon, as a result of the increased importance of the dipolar resonance form in amides. The measured barrier to this rotation is about 21 kcal moF (88 kJ moF ). 

Infrared spectroscopy can also be used to probe resonance in carboxylic acid derivatives. The dipolar resonance structure weakens the C=0 bond and causes a corresponding decrease in the carbonyl stretching frequency (Table 20-2). The IR data for carboxylic acids reported in Section 19-3 refer to the common dimeric form, in which hydrogen bonding reduces the stretching frequencies of both the 0-H and C=0 bonds to about 3000 and 1700 cm respectively. A special technique—vapor deposition at very low temperature—allows the IR spectra of carboxylic acid monomers to be measured, for direct comparison with the spectra of carboxylic acid derivatives. Monomeric acetic acid displays vc=o at 1780 cm similar to the value for carboxylic anhydrides, higher than that for esters, and lower than that of halides, consistent with the degree of resonance delocalization in carboxylic acids. 

In Summary The relative electronegativity and the size of L in RCL controls the extent of resonance of the lone electron pair(s) and the relative reactivity of a carboxylic acid derivative in nucleophilic addition-elimination reactions. This effect manifests itself structurally and spectroscopically, as well as in the relative acidity and basicity of the a-hydrogen and the carbonyl oxygen, respectively. 

The reaction is less selective than the related benzoylation reaction (/pMe = 30.2, cf. 626), thereby indicating a greater charge on the electrophile this is in complete agreement with the greater ease of nuclophilic substitution of sulphonic acids and derivatives compared to carboxylic acids and derivatives and may be rationalized from a consideration of resonance structures. The effect of substituents on the reactivity of the sulphonyl chloride follows from the effect of stabilizing the aryl-sulphonium ion formed in the ionisation step (81) or from the effect on the preequilibrium step (79). 

Structures III and IV assist ionisation of the C-X bond, whereas structure II facilitates nucleophilic addition and consequently a bimolecular displacement of X. The various derivatives of carboxylic acids form a series with varying degrees of resonance stabilisation decreasing in the following order ... 

Carbocation rearrangements, 94 Carbocation reactions, 45, 114 Carbonic acid derivatives, 358 Carbonyl group, reduction of, 261 resonance structures, 333 Carboxylic acids, summary of chemistry, 356 Catechol, 432 Center of symmetry, 69 Chair and boat forms, 168 Chemical shift, 2ilff Chiral center, 71... 

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