Amine hybridization effects

Nicotine has two nitrogen atoms, one as a cyclic tertiary amine and one in a pyridine ring. The basicities are easily distinguished, in that a pyridine system is much less basic than a simple amine. This is essentially a hybridization effect, the nitrogen lone pair in pyridine being held in an sp orbital. This means the lone pair electrons are held closer to the nitrogen, and are consequently less available for protonation than in an sp -hybridized aliphatic amine. Hence, as mentioned above, pyridine has p Ta approximately 5. It follows that pA"a 8.1 is more appropriate for the pyrrolidine nitrogen. 

Hybridization Effects Our study of terminal alkynes (Section 9-6) showed that electrons are held more tightly by orbitals with more. v character. This principle helps to explain why unsaturated amines tend to be weaker bases than simple aliphatic amines. In pyridine, for example, the nonbonding electrons occupy an sp2 orbital, with greater 5 character and more tightly held electrons than those in the sp3 orbital of an aliphatic amine. Pyridine s nonbonding electrons are less available for bonding to a proton. Pyridine does not lose its aromaticity on protonation, however, and it is a much stronger base than pyrrole. 

Aromatic amines are generally less basic than aliphatic amines.This is true both when the nitrogen atom is part of the aromatic system (as in j pyridine, a hybridization effect), and 1 when the nitrogen atom is bonded to the aromatic ring (as in aniline,... 

Hybridization effects are evident in the acidities of protonated amines also. The pfCa of a protonated nitrile is around -12 (an sp nitrogen), while that of pyridinium is 5.23 (sp nitrogen), and alkyl ammoniums are around 10 to 11 (sp nitrogen). 

The addition product, C QHgNa, called naphthalenesodium or sodium naphthalene complex, may be regarded as a resonance hybrid. The ether is more than just a solvent that promotes the reaction. StabiUty of the complex depends on the presence of the ether, and sodium can be Hberated by evaporating the ether or by dilution using an indifferent solvent, such as ethyl ether. A number of ether-type solvents are effective in complex preparation, such as methyl ethyl ether, ethylene glycol dimethyl ether, dioxane, and THF. Trimethyl amine also promotes complex formation. This reaction proceeds with all alkah metals. Other aromatic compounds, eg, diphenyl, anthracene, and phenanthrene, also form sodium complexes (16,20). 

Resonance effects are also important in aromatic amines. m-Nitroaniline is a weaker base than aniline, a fact that can be accounted for by the —7 effect of the nitro group. But p-nitroaniline is weaker still, though the —I effect should be less because of the greater distance. We can explain this result by taking into account the canonical form A. Because A contributes to the resonance hybrid, " the electron density of the unshared pair is lower in p-nitroaniline than in m-nitroaniline, where a canonical form such as Ais impossible. The basicity is lower in the para compound for two reasons, both... 

The phosphate backbone of DNA molecules often results in undesirable electrostatic interactions with the substrate. Although the electrostatic interactions of DNA can be utilized for physical adsorption of DNA to the surface, this process can also lead to the nonspecific physical adsorption of target DNA on the surface. Rather than sample DNA hybridizing to the probe, it can adsorb to the surface and lead to interferences with the final detection call. Nonspecific adsorption effects have primarily been examined by the microarray community. Blocking strategies have been developed to prevent these nonspecific interactions. Succinic anhydride (SA) and bovine serum albumin (BSA) are two common methods to prevent nonspecific adsorption on amine modified surfaces. Blocking strategies are desired to react with or pas-... 

Takeo Kawabata of the Institute for Chemical Research associated with Kyoto University reports (J. Am. Chem. Soc. 125 13012,2003) that unnatural amino acids can also be used to assemble four-, five-, six-, and seven-membered cyclic amines having quaternary stereogenic centers. Given the conventional wisdom that ester enolates are sp -hybridized, this memory effect is remarkable. 

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