Bond angle strain

The intermediate configuration b would involve complete resonance between I and II. Since the resonance energy is assumed to be very small, and this configuration corresponds to bond-angle strain for both I and II, the configuration would be less stable than a or c. 

C-C(=0)-C Bond Angle Strain C-C(02)-C Bond Angle Strain ... 

The ion 96 carries a positive charge which can be distributed over both rings formation of 96 involves compensation of bond angle strain, leading to facilitation of n-n repulsion strain between the neutral benzene nuclei in the starting compound. The twisting of the system in 96 corresponds to the somewhat twisted crystal structure of the [2.2]-paracyclophane molecule. 

Is there any ground-state destabilization of the substrate by bond-angle strain or binding in a high-energy conformation ... 

In cyclic peptides the cis- and frans-peptide bonds occur, with the cis form preferred in small cycles, and the trans form preferred in large systems (75AX(B)2035). The peptide bond is expected (68MI6) to deviate slightly from planarity in order to alleviate short-range interactions and bond-angle strain. 

Studies of hydrogen bonding and iodine complexation on alkyl-substituted oxetanes have shown that 2-alkyl and 2,2-dialkyl substitution enhances the electron donor ability, while 3,3-dialkyl substitution substantially diminishes it. This has been attributed to a decrease in the bond angle strain at the oxygen atom, due to a Thorpe-Ingold effect at C-3, since the effect increases with the size of the alkyl substituent at that position (71BSF4576). 

While the isomerization did not take place above 120°C, the usual boiling range of azoalkanes, it could be carried out at reduced pressure. It appears critical that a minimum amount of heat be applied to the system to cause refluxing by judicious use of low pressures and careful insulation of the apparatus. The method fails in an attempt to isomerize cyclooctanone methylhydrazone. The bond angle strain is said to effect the ease of rearrangement of the hydrazone... 

It is probably significant that for these structures only the four stable. v,bit-ale ol the silicon valence shell m jd to be used. The bond-angle strain is considerably less than for pure xp orbital, since d character ",a,n he introduced with iiTie promotion energy Some contribution may be made ai30 by the six sfcrucsun s of type C ... 

The fully saturated rings share with cyclohexane the property of being able to adopt one or more conformations which are virtually free of torsion or bond angle strain. 

We expect that the total strain in cycloalkanes of the type (CH2) should decrease rapidly in the order = 2>ra = 3> = 4. However, the data of Table 12-3 show that the order actually is 3 = 4 > 2. This difference in order often is disguised by dividing the heats of combustion by the numbers of CH2 groups and showing that the heats of combustion per CH2 are at least in the order expected from bond-angle strain. This stratagem does not really solve the problem. 

There is another conformation of cyclohexane that you might have made that looks like this. This conformation is know as the boat conformation. In this conformation there are still four carbon atoms in one plane, but the other two are both above this plane. Now all the carbon atoms are not the same—the four in the plane are different from the ones above. However, this is not a stable conformation of cyclohexane, even though there is no bond angle strain (all the angles are 109.5°). In order to understand why not, we must go back a few steps and answer our other question why is cyclopentane strained even though a planar conformation has virtually no angle strain ... 

In fact, in any planar conformation all the C-H bonds will be eclipsed with their neighbours. In cyclobutane, the ring distorts from a planar conformation in order to reduce the eclipsing interactions, even though this reduces the bond angles further and so increases the bond angle strain. 

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