19.5
Struttura delle ammine
Nelle ammine l'atomo di azoto ibridato possiede una coppia solitaria di elettroni ed è legato a tre sostituenti con un angolo di legame di circa 108°,…
Le ammine sono costituite da un atomo di azoto ibridato sp3 che mostra una geometria molecolare piramidale trigonale con azoto all'apice e i tre gruppi legati che formano la base piramidale.
Se gli elettroni di non legame sono considerati come il quarto gruppo, la geometria dell'ammina di azoto è approssimativamente tetraedrica, con angoli di legame di 108° e una lunghezza di legame C-N di 147 pm.
Le ammine che sopportano tre diversi sostituenti sono chirali con l'azoto come centro stereogenico. Esistono come enantiomeri e agli elettroni non leganti viene assegnata la priorità più bassa durante la denominazione.
I due enantiomeri possono interconvertirsi rapidamente attraverso l'inversione piramidale.
Durante l'inversione, l'enantiomero passa attraverso uno stato di transizione planare ibridato sp2, seguito da una riibridazione per dare una configurazione tetraedrica invertita.
A causa della bassa barriera energetica, questa interconversione può avvenire a temperatura ambiente e i due enantiomeri sono difficili da risolvere, risultando in una miscela racemica.
Al contrario, gli ioni ammonio quaternario chirale, senza coppie solitarie per l'inversione, sono facilmente risolvibili.
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Q1: Why does nitrogen in amines have a trigonal pyramidal geometry?
Nitrogen in amines is sp3 hybridized with three bonded groups and one lone pair of electrons. The lone pair occupies the apex of the pyramid while the three substituents form the triangular base. This geometry results in C–N–H bond angles of approximately 108°, which is less than the standard tetrahedral angle of 109.5° due to lone pair repulsion.
Q2: How does the C–N bond length in amines compare to other functional groups?
The C–N bond length in amines is 147 pm, which is longer than the C–O bond in alcohols (143 pm) but shorter than the C–C bond in alkanes (154 pm). This intermediate bond length reflects nitrogen's electronegativity and hybridization state relative to oxygen and carbon atoms in other organic compounds.
Q3: What makes amines with three different substituents chiral?
Amines bearing three different substituents are chiral because the nitrogen atom becomes a stereogenic center. The lone pair is assigned the lowest priority in stereochemical nomenclature, and the three bonded groups create two nonsuperposable mirror images called enantiomers that differ in their three-dimensional spatial arrangement.
Q4: What is pyramidal inversion and why does it prevent amine enantiomer resolution?
Pyramidal inversion is the rapid interconversion between amine enantiomers through an sp2-hybridized planar transition state, followed by rehybridization to an inverted tetrahedral configuration. With an energy barrier of only 25 kJ/mol, this process occurs readily at room temperature, causing the two enantiomers to rapidly interconvert into a racemic mixture that cannot be easily resolved.
Q5: Why can quaternary ammonium salts be easily resolved into enantiomers?
Quaternary ammonium salts have four different substituents and no lone pair electrons. Without a lone pair, they cannot undergo pyramidal inversion, so the stereochemical configuration remains fixed. This stability allows quaternary ammonium salts to be easily separated and resolved into distinct enantiomers, unlike simple chiral amines.
Q6: How does the lone pair affect bond angles in amine structures?
The lone pair on nitrogen exerts greater repulsive force than bonded electron pairs, compressing the C–N–C bond angles to approximately 108°. However, C–N–H bond angles are slightly larger at 112° because hydrogen is smaller and exerts less steric hindrance, allowing the lone pair to push the C–N bonds closer together.
Q7: Which amines can be separated into enantiomers despite having a lone pair?
Amines with chiral carbons or those unable to attain an sp2-hybridized transition state during pyramidal inversion can be separated into enantiomers. Additionally, preparation of 1° amines through reductive amination of aldehydes and ketones can yield chiral products that may be resolved if structural constraints prevent rapid inversion.