Amine hybrid orbitals

An orbital hybridization description of bonding m methylamme is shown m Figure 22 2 Nitrogen and carbon are both sp hybridized and are joined by a ct bond The unshared electron pair on nitrogen occupies an sp hybridized orbital This lone parr IS involved m reactions m which amines act as bases or nucleophiles The graphic that opened this chapter is an electrostatic potential map that clearly shows the concentration of electron density at nitrogen m methylamme... 

For the sp3 hybridization encountered in the amines, the orbital expressions are somewhat more complicated but for symmetrical compounds the coupling can be still expressed in a simple form... 

As in pyrrole, the lone pair on the indole N is part of the aromatic sextet. One N in histidine is like that in pyridine, with the lone pair in an sp2 hybrid orbital. 24.9. Reductive amination of (a) CH3COCOOH ... 

The amide (a) is the least basic (lone pair conjugated with the carbonyl group). The pyridine (c) nitrogen, with the lone pair in an sp- hybridized orbital, is less basic than the piperidine (d). The most basic is the quinuclidine (b), which is a tertiary amine in which the ring system ties the alkyl groups back, exposing the lone pair. 

The hybridization of the orbital that contains an amine s lone pair also affects its basicity. This is illustrated by comparing the basicity of piperidine and pyridine, two nitrogen heterocycles. The lone pair in piperidine resides in an sp hybrid orbital that has 25% s-character. The lone pair in pyridine resides in an sp hybrid orbital that has 33% s-character. 

Pyridine is a weaker base than piperidine because its nonbonded pair of electrons resides in an sp hybrid orbital. Although pyridine is an aromatic amine, its lone pair is not part of the delocalized 7C system, so its basicity is determined by the hybridization of its N atom. As a result, the pXa value for the conjugate acid of pyridine is much lower than that for the conjugate acid of piperidine, making pyridine the weaker base. 

An important outgrowth of this electronic configuration of bismuth is that trivalent bismuthines exhibit little basicity [94JCS(P1)3479]. The lack of basicity is due to the high s-character of the lone pair electron. All three valence electrons and two lone pair electrons occupy the sp hybrid orbital in amines, whereas three valence electrons lie in almost pure p orbitals and lone pair electrons occupy the remaining s orbital in bismuthines. 

In common with other first-row elements, nitrogen has only four orbitals available for bond formation, and a maximum of four 2c-2e bonds may be formed. However, since formation of three electron-pair bonds completes the octet, N( R)3, and the nitrogen atom then possesses a lone pair of electrons, four 2c-2e bonds can only be formed either (a) by coordination, as in donor-acceptor complexes, e.g., F3B—N(CH3)3, or in amine oxides, e.g. (CH3)3N—6, or (b) by loss of an electron, as in ammonium ions NH, NR 4. This loss of an electron gives a valence-state configuration for nitrogen (as N+) with four unpaired electrons in sp3 hybrid orbitals analogous to that of neutral carbon, while, as noted above, gain of an electron (as in... 

Proton transfer from water or other acid to pyridine does not involve the electrons of the aromatic sextet. Why, then, is pyridine a considerably weaker base than aliphatic amines The answer is that the unshared pair of electrons on the pyridine nitrogen lies in a relatively electronegative sp hybrid orbital, whereas in aliphatic amines, the unshared pair lies in an sp hybrid orbital. This effect decreases markedly the basicity of the electron pair on an spi hybridized nitrogen compared with that on an sp hybridized nitrogen. 

The nitrogen atom of an amine is typically sp hybridized, with the lone pair occupying an yj -hybridized orbital. Consider trimethylamine as an example ... 

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