Pyrrolidine, strain energy

The strain energies of these five-membered heterocycles are relatively small with values of 23.5, 24.8 and S.SkJmoF estimated for tetrahydrofuran, pyrrolidine and tetrahy-drothiophene respectively (74PMH(6)199). The closeness of the values for the two former compounds reflects the almost identical covalent radii of oxygen (0.66 A) and nitrogen (0.70 A) atoms. The sulfur atom with a much larger covalent radius of 1.04 A causes a... 

Strain energies of 23.5, 24.8 and 8.3 kJ mol 1 were estimated for tetrahydrofuran, pyrrolidine and tetrahydrothiophene, respectively (74Pmh(6)199). The larger sulfur covalent radius of 1.04 A lowers angular strain. 

The pyrrolidine molecule is practically without strain, nonplanar and conformationally mobile. As in the case with tetrahydrofuran (see p 67), the twist and envelope conformations are preferred. The activation energy for pseudorotation is 1.3 kJ mol". The chemical shifts in the NMR spectra lie in the region characteristic for cycloalkanes and dialkylamines. 

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