Benzaldehyde, resonance structures

Because the carbon atom attached to the ring is positively polarized a carbonyl group behaves m much the same way as a trifluoromethyl group and destabilizes all the cyclo hexadienyl cation intermediates m electrophilic aromatic substitution reactions Attack at any nng position m benzaldehyde is slower than attack m benzene The intermediates for ortho and para substitution are particularly unstable because each has a resonance structure m which there is a positive charge on the carbon that bears the electron withdrawing substituent The intermediate for meta substitution avoids this unfavorable juxtaposition of positive charges is not as unstable and gives rise to most of the product... 

For example, seven resonance structures can be drawn for benzaldehyde (C HsCHO). Because three of them place a positive charge on a carbon atom of the benzene ring, a CHO group withdraws electron density from a benzene ring by a resonance effect. 

A -Alkylated thiazolium and benzothiazolium salts also experience base-promoted deprotonation at the 2-position to form ylides. Such compounds, often referred to as TV-heterocyclic carbene (NHC), are nucleophilic catalysts in benzoin condensation. In 1943, Ugai and co-workers reported that thiazolium salts catalyze self-condensation of benzaldehyde to generate benzoin via an umpoulong process. Breslow at Columbia University in 1958 proposed thiazolium ylide as the actual catalyst for this transformation. In this mechanism, the catalytically active species was represented as a thiazolium zwitterion, the resonance structure of an NHC, and the reaction was postulated to ensue via the enaminol or the Breslow intermediate. ... 

Write contributing resonance structures and the resonance hybrid for the arenium ion formed when benzaldehyde undergoes nitration at the meta position. 

Such a compound is found to be stable in water and the hydrogen acidic enough to be exchanged with H20. The acidity is ascribed to the carbene resonance structure which helps stabilize the carbanion intermediate. However, the compound has no catalytic activity toward benzaldehyde. Instead, an addition product is formed which can be explained by the following mechanism (301) ... 

Draw second-order resonance structures of the aromatic starting material (benzaldehyde) that can be used to justify why the EAS reaction in the box yields mostly meta-bromobenzaldehyde. 

The idea that azolium salts could serve as nucleophilic catalysts dates back to the pioneering work of Ugai, who in 1943 demonstrated that thiamine (vitamin Bl) isolated from natural sources catalyzed the benzoin condensation of benzaldehyde/ The elegant mechanistic work of Breslow established the currently understood mechanism for thiamine s catalytic reaction, and provided the first suggestion of the resonance structure represented by the N-heterocyclic carbene (NHC) (Scheme 14.1). ... 

Chromium (and tungsten) imino carbene complexes 113 are found to be synthetic equivalents for nitrile ylides on the basis of the resonance structure (Scheme 5.23) [34]. The reaction of the imino carbene complex 113 with alkyne 114 affords pyrrole 115. Imidazoles 116 and 120, pyrrolidines 118, and oxazoline 119 are also obtained by the [3+2] cycloaddition of the imino carbene complexes 113 with benzonitrile, alkenes 117, and benzaldehyde, respectively. Although the regiochemical outcome of these reactions is unpredictable, the regioselectivity can be correlated with the resonance structure in the case of the reaction with 3-hexyn-2-one. 

Until recently it was thought that mono-substituted hydrazines invariably reacted with 5-methylisothiouronium salts to give compounds of type (XVII) [129, 192, 193] however, examples have now come to light in which the alternative isomer (XVI) has been isolated from this reaction [21, 131, 194, 195). This formulation is based on (a) the failure of the products to react with benzaldehyde, [21, 129, 131, 194], (b) the unambiguous synthesis of the two possible alternative structures [21, 129, 194], and (c) an examination of the nuclear magnetic resonance spectra of the mono- and di-protonated forms of some... 

In benzaldehyde and many other similar molecules, on the other hand, the resonance effect directs toward the meta positions, this resulting whenever the substituted group R contains an electronegative atom and a double or triple bond conjugated with the benzene ring (R = COOH, CHO, NO, COCHa, SOaH, CN, etc.). The structures leading to this effect, F, G, and H, are of the types... 

We said earlier that we can never prove a mechanism—only disprove it. Unfortunately, just as the correct mechanism seems to be found, there are some observations that make us doubt this mechanism. In Chapter 39 you saw how a technique called electron spin resonance (ESR) detects radicals and gives some information about their structure. When the Cannizzaro reaction was carried out with benzaldehyde and a number of substituted benzaldehydes in an ESR spectrometer, a radical was detected. For each aldehyde used, the ESR spectrum proved to be identical to that formed when the aldehyde was reduced using sodi-... 

Finally, the inductive and resonance effects in compounds having the general structure C6Hs-Y=Z (with Z more electronegative than Y) are both electron withdrawing in other words, the two effects reinforce each other. This is true for benzaldehyde (C HsCHO) and all other compounds that contain a carbonyl group directly bonded to the benzene ring. 

Consequently if form XXXV is to contribute to the structure of benzaldehyde, all the atoms of the resonating system must be in the same plane. 

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