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Steric hindrance is often exploited to control selectivity, such as slowing unwanted side-reactions. Steric hindrance between adjacent groups can also affect torsional bond angles. Steric hindrance is responsible for the observed shape of rotaxanes and the low rates of racemization of 2,2'-disubstituted biphenyl and binaphthyl derivatives.
Van der Waals strain, or steric strain, occurs when atoms are forced to get closer than their Van der Waals radii allow. [ 4 ] : 5 Specifically, Van der Waals strain is considered a form of strain where the interacting atoms are at least four bonds away from each other. [ 5 ]
cyclobutane (26.3 kcal/mol), C 4 H 8 — if cyclobutane were completely square planar, its bond angles would be 90° whereas tetrahedral 109.5° bond angles are expected. [6] However, the actual C-C-C bond angle is 88° because it has a slightly folded form to relieve some torsional strain at the expense of slightly more angle strain. [6]
The chair conformation minimizes both angle strain and torsional strain by having all carbon-carbon bonds at 110.9° and all hydrogens staggered from one another. [2] The conformational changes that occur in a cyclohexane ring flip take place over several stages. Structure D (10.8 kcal/mol) is the highest energy transition state of the process.
For the 'small rings' (3- and 4- membered), the slow rates is a consequence of angle strain experienced at the transition state. Although three-membered rings are more strained, formation of aziridine is faster than formation of azetidine due to the proximity of the leaving group and nucleophile in the former, which increases the probability that they would meet in a reactive conformation.
This serves to weaken C-H and C-X bond, both of which are broken in an E 2 reaction. It also sets up the molecule to more easily move its σ C-H electrons into a π C-C orbital (Figure 10). Figure 8: In an E 2 mechanism, the breaking C–H bond and the leaving group are often anti-periplanar.
Also of importance is the cone angle, which in diphosphines is defined as the average of the cone angle for the two substituents attached to the phosphorus atoms, the bisector of the P–M–P angle, and the angle between each M–P bond. [7] Larger cone angles usually result in faster dissociation of phosphine ligands because of steric crowding.
Atropisomers of 6,6'-dinitro-2,2'-diphenic acid were first experimentally described case, by Christie and Kenner (1922). Atropisomers are stereoisomers arising because of hindered rotation about a single bond, where energy differences due to steric strain or other contributors create a barrier to rotation that is high enough to allow for isolation of individual rotamers.