Polarization, of carbonyl group

Thus, the solvent effect on five-membered ring formation could be reasonably understood because in polar media the polarization of carbonyl groups would be higher and consequently, the electrostatic repulsion between cyclic anhydride units would be enhanced. 

After metal ions interact with specific peptide side chain functional groups, they act as a Lewis acid to enhance the reactivity of proximal peptide bonds through coordination and polarization of carbonyl groups [49]. Meanwhile, metal ions can also increase the concentration of nucleophilic hydroxide ions by forming metal hydroxides (M-OH). In this way, peptide bond cleavage can be facilitated at specific residues bearing high affinity with metal ions. 

Whereas you can construct a 1,3-difunctional relationship in any number of ways, it will always be possible to construct this relationship using the normal polarity of carbonyl groups (no A-functions needed). This is a valuable consideration, BUT NOT A RULE. Sometimes the use of an A-function (in a complex setting) will have advantages. 

Let s consider possible syntheses of compound 16 from monofunctional compounds. Polar disconnection of bond a leads to an acyl cation and an enolate . Real world versions of these polar species would be benzoate 17 and ester 18 (which would be deprotonated to provide the enolate). Thus, a logical way to contruct 16 might use a crossed Claisen condensation of 17 and 18. Disconnection of bond b leads to a crossed condensation between acetophenone (19) and diethyl carbonate (20). Both of these approaches to construction of the 1,3-difunctional relationship in 16 rely on the normal polarity of carbonyl groups. No A-function is needed. 

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... 

Polarity Parameter. Despite their appareat simplicity, these parameters, ( ), show a good correlatioa with plasticizer activity for nonpolymeric plasticizers (10). The parameter is defiaed 2ls (j) = [M A j Po)]/1000 where M = molar mass of plasticizer, = number of carboa atoms ia the plasticizer excluding aromatic and carboxyHc acid carbon atoms, and Pg — number of polar (eg, carbonyl) groups present. The 1000 factor is used to produce values of convenient magnitude. Polarity parameters provide useful predictions of the activity of monomeric plasticizers, but are not able to compare activity of plasticizers from different families. 

O Protonation of the carbonyl oxygen strongly polarizes the carbonyl group and. . . ... 

There is little doubt that the zinc ion acts as an electrophilic catalyst to polarize the carbonyl group and stabilize the negative charge that develops on the oxygen (Chapter 2, section B7).151 The ionized carboxylate of Glu-270 is impli-cated in catalysis from the pH-rate profile.152... 

Another factor that influences the reactivity of two polar reactants, acylperoxyl radical with aldehyde, is the polar interaction of carbonyl group with reaction center in the transition state. Aldehydes are polar compounds, their dipole moments are higher than 2.5 Debye (see Section 8.1.1). The dipole moment of the acylperoxyl radical is about 4 Debye (/jl = 3.87 Debye for PhC(0)00 according to the quantum-chemical calculation [54]). Due to this, one can expect a strong polar effect in the reaction of peroxyl radicals with aldehydes. The IPM helps to evaluate the increment Ain the activation energy Ee of the chosen reaction using experimental data [1], The results of Acalculation are presented in Table 8.10. 

The experimental evidence shows that the carbon of terminal carbonyl groups is positively polarized (or polarizable) and, contrary to the behaviour of free carbon monoxide, is easily attacked by strong nucleophiles (OH-, OR-) a behaviour which is general in the chemistry of metal carbonyls. Moreover, the negative polarisation (or polarizability) of oxygen atoms of carbonyl groups, particularly bridging carbonyls, is illustrated by the facile formation of adducts with Lewis acids as shown inEq. (16) 7. ... 

Fig. 3.2. Common catalytic groups of hydrolases involved in ester and amide bond hydrolysis (Z+ = electrophilic component polarizing the carbonyl group Y = nucleophilic group attacking the carbonyl C-atom H-B = proton donor transforming the -OR or -NR R" moiety into...

When considering the reaction of carbonyl groups, remember the polarity of the carbon-oxygen bond, the hybridization of the carbon atom (sp ), and the bond angles of the planar group. Figure 10-16 summarizes these features. 

The conformation of the [18]crown-6 ring is similar to that observed in other complexes and side chains are in axial conformations on both sides of the macrocyclic ring. The polar amide carbonyl groups are oriented toward the internal space while the apolar groups composed of methyl and methylenes lie on the outside. 

There are two classes of aldolases. Class I aldolases, found in animals and plants, use the mechanism shown in Figure 14-5. Class II enzymes, in fungi and bacteria, do not form the Schiff base intermediate. Instead, a zinc ion at the active site is coordinated with the carbonyl oxygen at C-2 the Zn2+ polarizes the carbonyl group... 

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