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Ch_322b_20.02 - Chiral tetravalent ammonium...

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Physical Properties and Structure of Amines Amines are moderately polar compounds because of the greater electronegativity of nitrogen relative to carbon and hydrogen. Their boiling points are higher than alkanes of similar size but lower than comparable alcohols. CH 3 NH 2 OH methylcyclohexane cyclohexylamine cyclohexanol MW 98 99 100 BP 100 o C 134 o C 161.5 o C insoluble in water slightly soluble in water soluble in water (3.6g/100 mL)
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Structure The nitrogen atom in most amines is similar to that in ammonia and can be considered to be sp 3 hybridized. The nonbonding electron pair is in an sp 3 orbital. N R' R'' R''' : sp 3 hybridized N The geometry is described as trigonal pyramidal . Bond angles are close to 109.5 o .
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Stereochemical Features Chiral Tetravalent Nitrogen Compounds
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Unformatted text preview: Chiral tetravalent ammonium salts (quaternary ammonium salts) display stereochemical features similar to chiral tetravalent carbon. Interconversion of enantiomers requires bond breaking. N : R' R'' R''' fast N : R' R'' R''' interconversion of amine enantiomers by fast inversion at the nitrogen center When there are three different groups attached to the nitrogen, the structure is chiral . But because of very fast inversion at the stereocenter (the inversion rate in NH 3 is close to 10 11 s-1 ), it is not possible to resolve a chiral amine . N R' R'' R''' N R' R'' R''' X R R + + chiral quaternary ammonium salts can be resolved C R' R'' R''' C R' R'' R''' X R R chiral carbon compounds can be resolved...
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