Resonance structures carbonyls

Figure 2-51. a) The rotational barrier in amides can only be explained by VB representation using two resonance structures, b) RAMSES accounts for the (albeit partial) conjugation between the carbonyl double bond and the lone pair on the nitrogen atom. 

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... 

In resonance terms electron delocalization map unsaturated carbonyl compounds IS represented by contributions from three principal resonance structures... 

The stabilizing role of other functional groups can also be described in resonance terms. Both electron-attracting groups such as carbonyl and cyano and electron-donating groups such as methoxy and dimethylamino have a stabilizing etfect on a radical intermediate at an adjacent carbon. The resonance structures which depict these interactions indicate delocalization of the unpaired electron onto the adjacent substituents ... 

Protonation of the dienyl carbanion formed by reduction of a linear dienone may occur at a carbanion center either p or S to the carbonyl group as is implied by the resonance structures shown below ... 

A hydrogen attached to the a-carbon atom of a p-keto ester is relatively acidic. Typical P-keto esters have values of about 11. Because the a-carbon atom is flanked by two electron-withdrawing carbonyl groups, a carbanion formed at this site is highly stabilized. The electron delocalization in the anion of a p-keto ester is represented by the resonance structures... 

The reaction starts with the nucleophilic addition of a tertiary amine 4 to the alkene 2 bearing an electron-withdrawing group. The zwitterionic intermediate 5 thus formed, has an activated carbon center a to the carbonyl group, as represented by the resonance structure 5a. The activated a-carbon acts as a nucleophilic center in a reaction with the electrophilic carbonyl carbon of the aldehyde or ketone 1 ... 

The initial step of olefin formation is a nucleophilic addition of the negatively polarized ylide carbon center (see the resonance structure 1 above) to the carbonyl carbon center of an aldehyde or ketone. A betain 8 is thus formed, which can cyclize to give the oxaphosphetane 9 as an intermediate. The latter decomposes to yield a trisubstituted phosphine oxide 4—e.g. triphenylphosphine oxide (with R = Ph) and an alkene 3. The driving force for that reaction is the formation of the strong double bond between phosphorus and oxygen ... 

On reaction with acid, 4-pvrone is protonated on the carbonyl-group oxygen to give a stable cationic product. Using resonance structures and the Hiickel 4n 4- 2 rule, explain why the protonated product is so stable. 

Q Protonation of the carbonyl oxygen atom by an acid catalyst HA yields a cation that can be represented by two resonance structures. 

Q Base removes an acidic hydrogen from the a position of the carbonyl compound, yielding an enolate anion that has two resonance structures. 

The site of reaction on an unsaturated organometallic molecule is not restricted to the most probable position of the metallic atom or cation or to a position corresponding to any one resonance structure of the anion. This has been discussed in a previous section with reference to the special case of reaction with a proton. Although the multiple reactivity is particularly noticeable in the case of derivatives of carbonyl compounds, it is not entirely lacking even in the case of the derivatives of unsaturated hydrocarbons. Triphenylmethyl sodium reacts with triphenylsilyl chloride to give not only the substance related to hexaphenylethane but also a substance related to Chichi-babin s hydrocarbon.401 It will be recalled that both the triphenyl-carbonium ion and triphenylmethyl radical did the same sort of thing. 

The r2 for this line is 0.9995 with a standard deviation of ca 0.9 kJmol 1 (Figure 1). The deviation for R1 = R2 = Ph (129) is some 43 k,I mol 1 below the line. If the exo-methylene/ring bond is quite polar and resonance structures 128 are significant (130 is easily ignorable), then AHf (127, R1, R2) and A//f(0=CR1R2) would be more likely to be linearly related. Another nearly perfect line, that of carbonyls vs fulvenes, can be drawn through the (H,H), (H,Me) and (Me,Me) points (equation 36). 

The RAHB effect may be illustrated by the ubiquitous C=0- -H—N hydrogen bond of protein chemistry. As shown in Section 5.2.2, the simplest non-RAHB prototype for such bonding, the formaldehyde-ammonia complex (5.31c), has only a feeble H-bond (1.41 kcalmol-1). However, when the carbonyl and amine moieties are combined in the resonating amide group of, e.g., formamide, with strong contributions of covalent (I) and ionic (II) resonance structures,... 

We shall see that most of the reactions of simple carbonyl compounds, like formaldehyde, are a consequence of the presence of an electron-deficient carbon atom. This is accounted for in resonance theory by a contribution from the resonance structure with charge separation (see Section 7.1). The second example shows the so-called conjugate acid of acetone, formed to some extent by treating acetone with acid (see Section 7.1). Protonation in this way typically activates acetone towards reaction, and we... 

Figures 10-17 and 10-18 summarize the important reaction features of carbonyl chemistry. Remember that both of these processes are reversible, and note that the carbocation resonance structure is susceptible to nucleophilic attack.

Conjugate addition occurs because there are two sites on the electrophile where a nucleophile can attack. The structure of the resonance hybrid and the two resonance structures contributing to the hybrid cire shown in Figure 11-22. The presence of this resonance is apparent in the infrcired spectrum because the carbonyl stretch shifts to a longer wavenumber. 

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