Resonance effects, carbonyl compounds

Hydrolysis of an enamine yields a carbonyl compound and a secondary amine. Only a few rate constants are mentioned in the literature. The rate of hydrolysis of l-(jS-styryl)piperidine and l-(l-hexenyl)piperidine have been determined in 95% ethanol at 20°C 13). The values for the first-order rate constants are 4 x 10 sec and approximately 10 sec , respectively. Apart from steric effects the difference in rate may be interpreted in terms of resonance stabilization by the phenyl group on the vinyl amine structure, thus lowering the nucleophilic reactivity of the /3-carbon atom of that enamine. 

The acidity of a-hydrogen atoms of carbonyl compounds is due to the strong electron withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base. 

C=0 Stretching Vibrations (Amide I Band) The C=0 absorption of amides occurs at lower frequencies than normal carbonyl absorption due to the resonance effect (see Section 3.6.10.1). The position of absorption depends on the same environmental factors as the carbonyl absorption of other compounds. 

Substituents with a electron-donating inductive (+1) effect (i.e., alkyl groups) stabilize the C=0 double bond of aldehydes and ketones. They increase the importance of the zwitterionic resonance form by which carbonyl compounds are partly described. The driving force for the formation of addition products from carbonyl compounds therefore decreases in the order H—CH(=Q) > R—CH(=0) > R R2c(=0). 

Alkenyl and aryl substituents stabilize the C=0 double bond of carbonyl compounds even more than alkyl substituents. This is due to their pi electron-donating (+M) effect, which allows one to formulate additional zwitterionic resonance forms for carbonyl compounds of this type. Thus, no hydrates, hemiacetals, oligomers, or polymers can be derived from unsaturated or aromatic aldehydes. 

The equilibrium in these reactions may favor the products or the reactants, depending on the strength of the nucleophile and the structure of the carbonyl compound. Stronger nucleophiles shift the equilibrium toward the products, and very strong nucleophiles give an irreversible reaction, that is, one that proceeds in only one direction, from the reactants to the products. The structure of the aldehyde or ketone exerts its influence through resonance, steric, and inductive effects, as usual. It is easiest to see these effects in examples, so let us proceed to examine some of the nucleophiles that can be used in these addition reactions. 

Protons on the/3 carbon of an a,/3-unsaturated carbonyl compound absorb at very low fields j (about 57) because of the electron- withdrawing resonance effect of the j carbonyl group. 

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. 

Structural studies" show that the cr ring bonds of cyclopropenone are shorter than their cyclopropene counterparts, whilst the carbonyl bond is longer than model compounds. These effects are consistent with extensive n delocalization" and the high dipole moments (4.7-5.1 D), low pK values" and low field ( 9.0 ppm) proton resonances support significant aromatic stabilization. 

The effect is compounded by the geometry of the fused bicyclic ring system. The P-lactam and thiazolidine rings of penicillin do not lie in the same plane (in fact, they lie almost perpendicular to each other), so resonance effects within the cyclic amide are prevented, which leaves the carbonyl carbon atom much more 5+ than expected and hence more liable... 

Analyses of the -decoupled 13C NMR spectrum of the commercial sample of dibutyltin di-isooctylthioglycolate used in this study indicates that its thioglycolate precursor is produced from a mixture of C8 aliphatic alcohols and thioglycolic acid (HSCH2COOH). Consequently, the aliphatic portion of its 13C spectrum (0-60 ppm) is extremely complex. However, only one carbonyl carbon resonance is present (line width at half height = 4.7 Hz), and its chemical shift (173.9 ppm) is separated widely from those of the aliphatic carbons. The carbonyl group in these compounds apparently is not subject to the larger substituent effects well-documented for alkanes (II) and other classes of compounds. Thus, the carbonyl carbon resonance was used to monitor the reaction. 

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. 

Carbanions derived from carbonyl compounds are often referred to as enolates, a name derived from the enol tautomer of carbonyl compounds. The resonance-stabilized enolate anion is the conjugate base of both the keto and enol forms of carbonyl compounds. The anions of nitro compounds are called nitronates and are also resonance stabilized. The stabilization of anions of sulfones is believed to be derived primarily from polar and polarization effects. 

Several factors, then, are important in assessing relative reactivity of carbonyl compounds. Electronegative substituents enhance reactivity by a polar effect, but if they are also tt donors, there is a resonance effect in the opposite direction. Alkyl and aryl substituents decrease reactivity relative to hydrogen by a combination of steric and electronic effects. Protonation or complexation with a Lewis acid at the carbonyl oxygen enhances reactivity. 

The most striking effect is that of the phenyl group in stabilizing the carbonyl compound. This is undoubtedly due to the resonance possible in the carbonyl compound. 

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