Amines 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 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,... 

Stork enamine synthesis takes advantage of the fact that an aldehyde or ketone reacts with a secondciry cimine to produce an enamine. Enamines cire resonance stabilized (see Figure 15-25) and have multiple applications. In the first resonance structure, the nitrogen is the nucleophile, while in the second resonance structure, the Ccirbanion is the nucleophile. Some commonly used secondary amines, pyrrolidine, piperidine, and morpholine, are shown in Figure 15-26. 

Basicity of aniline Aniline, like all other amines, is a basic compound ( Tb = 4.2 X 10 °). Anilinium ion has a = 4.63, whereas methylammo-nium ion has a pK = 10.66. Arylamines, e.g. aniline, are less basic than alkylamines, because the nitrogen lone pair electrons are delocalized by interaction with the aromatic ring tt electron system and are less available for bonding to H+. Arylamines are stabilized relative to alkylamines because of the five resonance structures as shown below. Resonance stabilization is lost on protonation, because only two resonance structures are possible for the arylammonium ion. 

Fig. 18.4 Dclocaluiiiion of the hine pair in Irisilyl amine. (si Resonance structures < >) Overlap of /, end pn orhitals.

Ninhydrin (1,2,3-triketohydrindene monohydrate). Widely available chemical, light-sensitive. Ninhydrin reacts with free amines (2 1 molar ratio), giving a purple product (Ruhemann s purple resonance structure). Used for a qualitative test to determine the presence of aliphatic amines on the agarose beads as a 0.2% w/v solution in ethanol (see Note 4). Hazards Harmful if swallowed skin, eye, and respiratory irritant. Toxicity data LD50 78 mg/kg intraperitoneal, mouse. Should be handled in a fume hood with safety glasses and gloves. 

The amide produced in this reaction usually does not undergo further acylation. Amides are stabilized by a resonance structure that involves nitrogen s nonbonding electrons and places a positive charge on nitrogen. As a result, amides are much less basic and less nucleophilic than amines. 

Metal rf-inline complexes with various transition metals [1-10] and lanthanides [11,12] are well known in the literature. Early transition metal if-imine complexes have attracted attention as a-amino carbanion equivalents. Zirconium rf-imine complexes, or zirconaaziridines (the names describe different resonance structures), are readily accessible and have been applied in organic synthesis in view of the umpolung [13] of their carbons whereas imines readily react with nucleophiles, zirconaaziridines undergo the insertion of electrophilic reagents. Accessible compounds include heterocycles and nitrogen-containing products such as allylic amines, diamines, amino alcohols, amino amides, amino am-idines, and amino acid esters. Asymmetric syntheses of allylic amines and a-amino acid esters have even been carried out. The mechanism of such transformations has implications not only for imine complexes, but also for the related aldehyde and ketone complexes [14-16]. The synthesis and properties of zirconaaziridines and their applications toward organic transformations will be discussed in this chapter. 

The zirconaaziridines 2a, 2b, 2d, and 2j show significant elongation of the C-N bond length (Table 1, entry 1) compared to that of a free imine (1.279 A) [46], adopting a bond length much closer to that of a free amine (1.468 A) [46] and confirming that resonance structure B (Fig. 6) is important. The Zr-C bond... 

As the following resonance structures indicate, en amines are electror.-icaLiy simi lar to onoUte ions. Overlap af the nitrogen lone-pair orbital with the double-bond p orbitals leads to an increase in eleetron density en thea carbon atom, making that carbon strongly nucleophihc. An electroeUtic potential map of N, -dimethylaminoethylene shows that electron deonty [Pg.956]

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