Carbonyl groups effects

Oxidized Starches. Alkaline hypochlorite treatment introduces carboxyl and carbonyl groups, effects some depolymerization, and produces whiter (bleached) products that produce softer, clearer gels. Ammonium persulfate is used in some paper mills with continuous thermal cookers to prepare in situ bigb solids, low viscosity dispersions. Most of the hypochlorite-oxidized starch and all the ammonium persulfate-oxidized starch is used in the paper industry. The low solution viscosity and good binding and adhesive properties of these products make them especially effective in high solids, pigmented... 

Tris(pentafluorophenyl)borane (35) is a powerful Lewis acid with a hard boron center. It interacts strongly with hard Lewis basic centers and abstracts hard anion such as methyl anion and hydride to initiate selective reactions that are otherwise difficult to achieve. The B(CgF5)3-promoted early transition-metal-catalyzed olefin polymerizations are typical reactions [16]. The B(C6F5)3-Ph3SiH system is useful for hydro-silylation of the carbonyl group, in which the boron of 35 activates the Si—H bond rather than the carbonyl group (see 40 in Scheme 3.39) and reduces even the ester carbonyl group effectively [17]. A number... 

Another way to stabilize an eclipsed or gauche conformation is to coordinate heteroatom substituents with a metal ion via chelation. Oishi and co-workers reduction of 153 with zinc borohydride proceeds via a chelated species, 154. 2 Chelation of zinc to the hydroxyl and carbonyl groups effectively locks the conformation into that shown in the transition state required for reaction. The methyl and hydrogen are held in place, and the hydride is delivered from the less hindered face (over the hydrogen in 154) to complete the reaction (see secs. 4.4.B and 4.7.B). Since transition metal salts usually behave as Lewis acids, the presence of a heteroatom with... 

Equilibrium constants with BF3 adducts have been determined by NMR at room temperature (Table 1), and the corresponding AG values can be derived. The diethyl ether present in the commercial Et20 BF3 complex competes with the carbonyl group effectively. At room temperature in CDCI3, BF3 rapidly interchanges its position on ether and benzaldehyde, and only 31% of benzaldehyde is found in the complexed form 18 (entry 1). Most aromatic and aliphatic aldehydes are predominantly in the uncomplexed form at room temperature (8 8% of complex), which translates into a positive AG. Therefore, counterintuitively, cooling the mixture only shifts the equilibrium towards the left, and as a result there is less complex present at low temperatures. For benzaldehyde (AG = 0.93 kcal/mol) the proportion of complex 18 is calculated to decrease from 31% at room temperature to 23% at -78 °C. /7-Anisaldehyde (entry 2) is the only exception among the examples reported in Table 1, where 73% of the p-anisaldehyde is actually coordinated to BF3. 

Reagent A is particularly useful for the treatment of the lower aliphatic aldehydes and ketones which are soluble in water cf. acetaldehyde, p. 342 acetone, p. 346). The Recent is a very dilute solution of the dinitrophenylhydrazine, and therefore is used more to detect the presence of a carbonyl group in a compound than to isolate sufficient of the hydrazone for effective recrystallisation and melting-point determination. 

These substances, as well as the parent compound, are p-keto esters and undergo hydrol3rtio cleavage in two directions. One type of cleavage, ketonlc hydrolysis, is effected by the action of dilute caustic alkali in the cold, followed by acidification and boiling the free acetoacetic acid produced has a carboxyl and carbonyl group on the same carbon atom and therefore readily undergoes decarboxylation to yield a ketone, for example ... 

The rate of the Lewis-acid catalysed Diels-Alder reaction in water has been compared to that in other solvents. The results demonstrate that the expected beneficial effect of water on the Lewis-acid catalysed reaction is indeed present. However, the water-induced acceleration of the Lewis-add catalysed reaction is not as pronounced as the corresponding effect on the uncatalysed reaction. The two effects that underlie the beneficial influence of water on the uncatalysed Diels-Alder reaction, enforced hydrophobic interactions and enhanced hydrogen bonding of water to the carbonyl moiety of 1 in the activated complex, are likely to be diminished in the Lewis-acid catalysed process. Upon coordination of the Lewis-acid catalyst to the carbonyl group of the dienophile, the catalyst takes over from the hydrogen bonds an important part of the activating influence. Also the influence of enforced hydrophobic interactions is expected to be significantly reduced in the Lewis-acid catalysed Diels-Alder reaction. Obviously, the presence of the hydrophilic Lewis-acid diminished the nonpolar character of 1 in the initial state. 

To understand the effect of a carbonyl group attached directly to the ring consider Its polarization The electrons m the carbon-oxygen double bond are drawn toward oxy gen and away from carbon leaving the carbon attached to the nng with a partial posi tive charge Using benzaldehyde as an example... 

Electronegativity and hybridization effects combine to make the carbon of a carbonyl group especially deshielded Normally the carbon of C=0 is the least shielded one m... 

The carbonyl carbon of a ketone bears two electron releasing alkyl groups an aldehyde carbonyl group has only one Just as a disubstituted double bond m an alkene is more stable than a monosubstituted double bond a ketone carbonyl is more stable than an aldehyde carbonyl We 11 see later m this chapter that structural effects on the relative stability of carbonyl groups m aldehydes and ketones are an important factor m then rel ative reactivity... 

A striking example of an electronic effect on carbonyl group stability and its rela tion to the equilibrium constant for hydration is seen m the case of hexafluoroacetone In contrast to the almost negligible hydration of acetone hexafluoroacetone is completely hydrated... 

Steric and electronic effects influence the rate of nucleophilic addition to a proton ated carbonyl group m much the same way as they do for the case of a neutral one and protonated aldehydes react faster than protonated ketones... 

Conjugation of the newly formed double bond with the carbonyl group stabilizes the a p unsaturated aldehyde provides the driving force for the dehydration and controls Its regioselectivity Dehydration can be effected by heating the aldol with acid or base Normally if the a p unsaturated aldehyde is the desired product all that is done is to carry out the base catalyzed aldol addition reaction at elevated temperature Under these conditions once the aldol addition product is formed it rapidly loses water to form the a p unsaturated aldehyde... 

Aldol condensation offers an effective route to a p unsaturated aldehydes and ketones These compounds have some interesting properties that result from conjugation of the carbon-carbon double bond with the carbonyl group As shown m Figure 18 6 the rr systems of the carbon-carbon and carbon-oxygen double bonds overlap to form an extended rr system that permits increased electron delocalization... 

The resonance effect of the carbonyl group Electron delocalization expressed by resonance between the following Lewis structures causes the negative charge in acetate to be shared equally by both oxygens Electron delocalization of this type IS not available to ethoxide ion... 

For many years resonance m carboxylate 10ns was emphasized when explaining the acidity of carboxylic acids Recently however it has been suggested that the indue tive effect of the carbonyl group may be more important It seems clear that even though their relative contributions may be a matter of debate both play major roles... 

Carboxylic acids are weak acids and m the absence of electron attracting substituents have s of approximately 5 Carboxylic acids are much stronger acids than alcohols because of the electron withdrawing power of the carbonyl group (inductive effect) and its ability to delocalize negative charge m the carboxylate anion (resonance effect)... 

Thioesters Like chlorine sulfur is a third row element with limited ability to donate a pair of 3p electrons into the carbonyl tt system With an electronegativ ity that IS much less than Cl or O however its destabilizing effect on the carbonyl group IS slight and thioesters he m the middle of the group of carboxylic acid derivatives m respect to reactivity... 

The negatively charged oxygen substituent is a powerful electron donor to the carbonyl group Resonance m carboxylate anions is more effective than resonance m carboxylic acids acyl chlorides anhydrides thioesters esters and amides... 

Conversions of acid anhydrides to other carboxylic acid derivatives are illustrated m Table 20 2 Because a more highly stabilized carbonyl group must result m order for nucleophilic acyl substitution to be effective acid anhydrides are readily converted to carboxylic acids esters and amides but not to acyl chlorides... 

Section 20 13 Thioesters undergo reactions analogous to those of esters but at faster rates A sulfur atom stabilizes a carbonyl group less effectively than an oxygen... 

Conjugation with olefinic or acetylenic groups lowers the frequency and raises the intensity. Conjugation with carbonyl groups usually has little effect on the position of absorption. 

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