Ring strain energy

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" . 

Summary Ab initio calculated bond dissociation energies of silicon compounds will be discussed by means of atomic ionization energies and atomic orbital overlap. Ring strain energies of C- as well as Si-rings are estimated by homodesmotic reactions. The hybridization concept is critically examined in the case of silicon compounds. From the most important results a set of basic rules will be presented. 

Ring strain energy can be defined as the energy of the homodesmotic reaction. 

Fig. 3. Calculated ring strain energies per strained angle in silicon and carbon ring compounds using the homodesmotic reaction. The angle a is half of the deviation of the angle between the straight atomic connection lines from the unstrained angle (109.47 deg).

The enhanced propensity of dioxiranes to insert oxygen into unactivated alkane C—H bonds was ascribed initially to the high ring strain energy (SE) of dioxiranes that has sometimes exceeded 30 kcalmol" . Since the SE of these three-membered peroxides has recently been substantially reduced (SE = ca 11-18 kcalmoU ), a different explanation is required . 

Stability. The four-membered ring in the diazabiphenylene series of compounds is strained due to bond angle deformation . The conventional ring strain energy for cyclobutane is 111 kj moP alkyl substituents stabilize the ring by only a few kilojoules per mole. 

The photochemistry of cyclobutanone presents a special case since the Norrish type-I cleavage to give an acylalkyl diradical intermediate releases ring-strain energy. Thus the energy available for subsequent reactions is reduced correspondingly, compared to the energy retained in an acyl radical from an acychc ketone, or less strained cyclic ketones. 

The ring strain energies for 3,3-dimethyloxetane, 3,3-bis(chloromethyl)oxetane and 3-ethyl-3-chloromethyloxetane have been determined from the heats of formation and all have the same value of 93 kJ mol-1, showing a decrease in ring strain energy of 13.3 kJ mol-1... 

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