Thermochemistry and conformation of saturated heterocycles

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 effect on strain energy of introducing unsaturation into these rings has been evaluated in the cases of 2,3- and 2,5-dihydrothiophene, where the additional values are 18 and 15.8 kJ mol 1. 

All of the parent heterocycles possess some degree of aromaticity, as based upon chemical behavior such as their proclivity to undergo substitution reactions with electrophilic reagents. Quantification of the relative aromaticities of these heterocycles is less easily achieved. The wide range of potential criteria available for this purpose has been surveyed in Section 2.2.4.2. 

Structure of Five-membered Rings with One Heteroatom 

According to structural indices AN and I, the aromaticity of non-condensed heterocycles varies in the sequence thiophene pyrrole selenophene tellurophene furan. 

Calculation of group increments for oxygen, sulfur and nitrogen compounds has allowed the estimation of conventional ring-strain energies (CRSE) for saturated heterocycles from enthalpies of formation. For 1,3-dioxolane, CRSE is about 20 kJ mol . In 2,4-dialkyl-l,3-dioxolanes the cis form is always thermodynamically the more stable by approximately 1 kJ mol . 

3-dithiolanes the ring is flexible and only small energy differences are observed between the diastereoisomeric 2,4-dialkyl derivatives. The 1,3-oxathiolane ring is less mobile and pseudoaxial 2- or 5-alkyl groups possess conformational energy differences (cf. 113 114) see also the discussion of conformational behavior in Section 4.01.4.3. 

This subject has been dealt with in (74AHC( 17)255), which should be consulted for further details and references to the original literature. 

Using the bond energy terms reported by Cottrell. 

Results obtained using Pauling s bond energy terms. 

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The next main section deals with thermodynamic aspects. It starts by consideration of the intramolecular forces between heterocyclic molecules which influence melting and boiling points, solubility and chromatographic characteristics. This is followed by a section on stability and stabilization, including thermochemistry and conformation of the saturated ring systems, and then a discussion of aromaticity. 

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