Carbonyl group, addition reactions reactivity

The reaction of NO3 with unsaturated ethers proceeds by addition to the >C=C< double bond. Both steric and inductive effects are known to influence the reactivity of NO3 with unsaturated alcohols, e.g. 2-methyl-3-butene-2-ol is slightly less reactive than 1-butene. Unsaturated ethers are more reactive than unsaturated alcohols towards NO3. Comparison of the measured rate constants shows that, similar to the reaction with O3, the presence of the carbonyl group decreases the reactivity of NO3 toward the >C=C< double bond. The reaction of NO3 with unsaturated oxygenates proceeds by addition to either one of the carbon atoms of >C=C< double bond, preferentially to the most substituted radical. In the presence of NOx, the reaction may ultimately lead to organic nitrates among the first generation products. 

The double bonds in bismalemide are highly electron-deficient due to two flanking carbonyl groups, and are reactive towards a bimolecular addition reaction. Hence the maleimide groups of a bismaleimide monomer or chain-extended prepolymer can undergo homopolymerisation to produce 3D network structures. To manipulate the structure and properties, different types of maleimide monomer or prepolymer can be used. The reactivity of such resins depends on their chemical structure and UPE. 

A second common reaction theme of a carbonyl group is reaction with a proton or another Lewis acid to form a resonance-stabilized cation. Protonation increases the electron deficiency of the carbonyl carbon and makes it more reactive toward nucleophiles. The reaction is followed by removal of a proton to give a tetrahedral carbonyl addition compound. 

The mechanism of this reaction is only slightly more complicated than the usual reaction of nucleophiles with acid chlorides (p. 854). Diazo compounds are nucleophiles at carbon and add to the carbonyl group of the reactive acid chloride. Chloride ion is expelled in the elimination phase of this normal addition-elimination process. In the only new step of the reaction, chloride removes the newly acidic hydrogen to give the diazo ketone (Fig. 18.64).This hydrogen is acidic because the conjugate base is stabilized by resonance. 

The reaction that prevails when the reaction is under kinetic control is the one that is faster. Therefore, the product depends on the reactivity of the carbonyl group. Compounds with reactive carbonyl groups form primarily direct addition products because for those compounds, direct addition is faster. Compounds with less reactive carbonyl groups form conjugate addition products because for those compounds, conjugate addition is faster. 

In the preceding chapter you learned that nucleophilic addition to the carbonyl group IS one of the fundamental reaction types of organic chemistry In addition to its own reactivity a carbonyl group can affect the chemical properties of aldehydes and ketones m other ways Aldehydes and ketones having at least one hydrogen on a carbon next to the carbonyl are m equilibrium with their enol isomers... 

Methacryhc acid and its ester derivatives are Ctfjy -unsaturated carbonyl compounds and exhibit the reactivity typical of this class of compounds, ie, Michael and Michael-type conjugate addition reactions and a variety of cycloaddition and related reactions. Although less reactive than the corresponding acrylates as the result of the electron-donating effect and the steric hindrance of the a-methyl group, methacrylates readily undergo a wide variety of reactions and are valuable intermediates in many synthetic procedures. 

A very important relationship between stereochemistry and reactivity arises in the case of reaction at an 5 carbon adjacent to a chiral center. Using nucleophilic addition to the carbonyl group as an example, it can be seen that two diastereomeric products are possible. The stereoselectivity and predictability of such reactions are important in controlling stereochemistry in synthesis. 

Because the pK s of the aldehyde and water are similar, the solution contains significant quantities of both the aldehyde and its enolate. Moreover, their reactivities are complementary. The aldehyde is capable of undergoing nucleophilic addition to its carbonyl group, and the enolate is a nucleophile capable of adding to a carbonyl group. And as shown in Figure 18.4, this is exactly what happens. The product of this step is an alkoxide, which abstracts a proton from the solvent (usually water or ethanol) to yield a (3-hydroxy aldehyde. A compound of this type is known as an aldol because it contains both an aldehyde function and a hydroxyl group (aid + ol = aldol). The reaction is called aldol addition. 

The organozinc compound 2 is less reactive than an organomagnesium compound the addition to an ester carbonyl group is much slower than the addition to an aldehyde or ketone. Nevertheless the addition of 2 to the carbonyl group of unreacted a-halo ester 1 is the most frequently observed side-reaction ... 

One further comparison aromatic aldehydes, such as benzaldehyde, are less reactive in nucleophilic addition reactions than aliphatic aldehydes because the electron-donating resonance effect of the aromatic ring makes the carbonyl group less electrophilic. Comparing electrostatic potential maps of formaldehyde and benzaldehyde, for example, shows that the carbonyl carbon atom is less positive (less blue) in the aromatic aldehyde. 

Acetal formation is similar to the hydration reaction discussed in Section 19.5. Like water, alcohols are weak nucleophiles that add to aldehydes and ketones only slowly under neutral conditions. Under acidic conditions, however, the reactivity of the carbonyl group is increased by protonation, so addition of an alcohol occurs rapidly. 

Both the initial addition step and the subsequent elimination step can affect the overall rate of a nucleophilic acyl substitution reaction, but the addition step is generally the rate-limiting one. Thus, any factor that makes the carbonyl group more reactive toward nucleophiles favors the substitution process. 

Alkyl substituents accelerate electrophilic addition reactions of alkenes and retard nucleophilic additions to carbonyl compounds. The bonding orbital of the alkyl groups interacts with the n bonding orbital, i.e., the HOMO of alkenes and raises the energy (Scheme 22). The reactivity increases toward electron acceptors. The orbital interacts with jt (LUMO) of carbonyl compounds and raises the energy (Scheme 23). The reactivity decreases toward electron donors. 

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