Strain energies oxiranes

It appears that oxiranes known to give predominantly a-deprotonation in basic media (cyclopentene, cyclooctene and exo-norbomene oxide) are also the more strained (Table 1 entries 3, 6, 7). On the other hand, oxiranes that give mainly -deprotonation (butene, cyclohexene oxide) have lower strain energies and higher a-anion stabilities (Table 1 ... 

Entry Oxirane Strain energies a-Anion energies... 

Three membered rings are all strained and the parent systems may be endothermic (AH°f. cyclopropane 35 oxirane -78 aziridine 92 thiirane 52 kJ/mole). Some molecules with small distorted rings (of high strain energy) are explosively unstable. Individual entries are ... 

Epoxy adhesives get their name from the portion of the adhesive containing 1,2-epoxy, epoxide, or oxirane ring. This three-member ring consists of two carbon atoms joined to an oxygen atom. The highly strained geometry of this moiety with a strain energy of 114 kJ/mole accounts for its reactivity with many nucleophilic or electrophilic compounds. Typical... 

From the known heat of formation, the bond separation energy of oxirane can be obtained. This is — 14.0 kcal/mol after zero-point correction and is partly a measure of the ring strain. By this criterion, there is a steady reduction in strain energy from cyclopropane to aziridine to oxirane. This trend is reproduced by the theory at the 6-31G level (Table 5). However, even with 7-functions included, the theoretical value for oxirane is still about 5 kcal/mol too negative. 

TABLE 3. RING-STRAIN ENERGY OF OXIRANE AND ITS ANALOGUES... 

At low pressures, the isomerization of oxiranes will cause initiation, as release of the ring strain energy combined with other rearrangements produces chemically activated intermediates which decompose into radicals unless the intermediate is stablized [90]. 

For oxiranes, isomerization occurs exothermically due mainly to the strain energy in the ring (ca 120kJmol ). It is probable that oxirane isomerization proceeds via chemically activated CH3CHO. 

The epoxide (oxirane) functional group has special features that allow it to play very important roles in synthesis and as an electrophilic species in biologically reactive intermediates. The epoxide group has an estimated strain energy of 27kcalmol-1,19 and its reactions with both acidic and nucleophilic reagents result in epoxide ring... 

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