Strain in cyclobutane

The double bond in cyclobutene has about 17 kJ/mol of extra ring strain (in addition to the ring strain in cyclobutane) by virtue of the small ring. The 90° bond angles in cyclobutene compress the angles of the sp2 hybrid carbons (normally 120°) more than they compress the sp3 hybrid angles (normally 109.5°) in cyclobutane. The extra ring strain in cyclobutene makes its double bond more reactive than a typical double bond. 

The C(F)2(CF2)2 group increment allows evaluation of the strain energy in octa-fluorocyclobutane. There are three published experimental studies providing AHf (g) data for this compound The first value (taken from Pedley and Rylance s thermochemical archives ) is — 368.7 kcal mol" and is based upon reaction with sodium while the O Neal and Benson value ( — 367.8 kcal mol" if one employs the archival value for C2F4) is based upon the experimental equilibrium with tetrafluoroethylene. The third value, based upon combustion measurements, is - 365.2 kcal mol" if one employs archival values for the products. The strain energy is thus between 14.5 and 18 kcal mol" some 8.5-12 kcal mol" lower than in the parent hydrocarbon. Why is this value low One explanation may follow recent work by Wiberg . He concludes that a large part of the strain in cyclobutane is due to repulsion between non-bonded carbons and... 

Tables 3-1 through 3-3 contain data on which empirical calculations of chemical shifts can be made. The tables represent a fraction of the data available on the fundamental alkane, alkene, and aromatic structures. Moreover, corrections must be applied in order to avoid nonadditivity caused primarily by steric effects. Thus, three groups on a single carbon atom, two large groups cis to each other on a double bond, or any two ortho groups can cause deviations from the parameters listed in the tables. If sufficient model compounds are available, the corrections shown can be applied. Further empirical calculations are possible for any structural entity, so that the eclipsing strain in cyclobutanes, the variety of steric interactions in cyclopentanones, or the variations in angle strain in norbornanes may be taken into account.

Azetidine itself has been studied by electron diffraction, which reveals a non-planar structure (Figure 1) (73CC772). The enhanced length of the bonds reflects the strain in the ring and the angle between the CCC and CNC planes of 37° is similar to that found for cyclobutane (35°), but quite different from that for oxetane (4°). 

C-C bond Fluonnation increases the bond strengths in cycloalkanes, including cyclobutanes [75, 94], but by contrast, it decreases C-C bond strengths and increases nng strain in cyclopropanes and other three-membered nng compounds [75 94. 9S]... 

The Origin of Ring Strain in Cyclopropane and Cyclobutane Angle Strain and torsional Strain... 

We know that all cycloalkanes do not have the same relative stability. Cyclohexane is most stable while cyclopropane and cyclobutane are much less stable, because they have a ring strain in their molecules. 

As an illustration, the ring strain in cyclopropane, cyclobutane and cyclohexane and cyclodecane has been calculated ... 

Archibald and co-workers have explored the synthesis of polynitrocyclobutanes and then-derivatives. The synthesis of these compounds via the nucleophilic substitution of cyclobutyl halides with nitrite anion was ruled out at an early stage because displacement in this system is too slow for practical use. This is a consequence of the molecular strain in the cyclobutane ring, which causes carbon atoms to deviate from sp hybridization towards sp character. 

Polynitro derivatives of pentacyclo[5.4.0.0 .0 °.0 ]undecane have attracted interest as potential high-energy explosives. Molecular strain in this caged system could arise from both the constrained norbomyl moiety and the cyclobutane ring. Additional strain would be expected from nonbonding interactions if the S-endo and 1 l-endo positions were substituted with gm-dinitro groups. 

If the C s of the cyclobutane ring were coplanar, they would form a rigid square with internal bond angles of 90°. The deviation from 109.5° would not be as great as that for cyclopropane, and there would be less angle strain in cyclopropane. However, this is somewhat offset by the fact that the eclipsing strain involves four pairs of H s, one pair more than in cyclopropane. 

Due to the ring strain in the cyclobutane ring, /(-carbonyl cyclobutanones made by oxidation of methylenecyclobutane in methanol will readily undergo retro-Claisen-type reactions. For ozonolysis, this can be avoided by changing the solvent from methanol to dichloromethane treatment of 6-methyl-8-methylene-m-3-oxabicyclo[4.2.0]octan-2-one with ozone in methanol gave methyl 2-(4-methyl-2-oxo-4-tetrahydropyranyl)acetate (1), while the same treatment in dichloromethane gave 6-methyl-m-3-oxabicyclo[4.2.0]octane-2,8-dione (2).16... 

Because cyclopentane and cyclobutane (Sections 12-3E and 12-3F) also have nonplanar carbon rings, it is clear that the Baeyer postulate of planar rings is not correct. Nonetheless, the idea of angle strain in small rings is important. There is much evidence to show that such strain produces thermodynamic instability and usually, but not always, enhanced chemical reactivity. 

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