Bond-angle change

The C-C-R bond angle changes significantly in the 0° conformation of vinyl alcohol and in vinyl amine. In all of these vinyl systems, the substituent shortens the C-H bond (on the carbon to which the substituent is attached), most dramatically in the case of the substituents NHj and OH. 

Fig. 21 Bond-angle changes at the migration origin in four oxime structures [105] (see text) as a function of the lengthening of the N-OX bond. Reprinted with permission from Edwards et al. (1987).

No single bond distance change or bond angle change provides an adequate description. Some combination of motions is required, the exact nature of which is not at all apparent. 

On the whole, compared with Pm compounds, quaternization of phosphorus produces a large 31P downfield chemical shift which parallels the bond angle changes, including those due to the steric effects coming from phosphorus substituents (shielding y-effect) (cf. Table 2). 

What bond angle would you expect for M — O—R in an alkoxide complex How might this bond angle change as it donation from the p orbitals of oxygen increases0... 

Formaldehyde is an extremely reactive aldehyde as it has no substituents to hinder attack—it is so reactive that it is rather prone to polymerization (Chapter 52). And it is quite happy to move from sp2 to sp3 hybridization because there is very little increased steric hindrance between the two hydrogen atoms as the bond angle changes from 120° to 109° (p. 139). This is why our aqueous solution of formaldehyde contains essentially no CH2O—it is completely hydrated. A mechanism for the hydration reaction is shown below. Notice how a proton has to be transferred from one oxygen atom to the other, mediated by water molecules. 

Cyclopropanones—threc-membered ring ketones—are also hydrated to a significant extent, but for a different reason. You saw earlier how acyclic ketones suffer increased steric hindrance when the bond angle changes from 120° to 109° on moving from sp2 to sp3 hybridization. Cyclopropan ones... 

Lcl there be an energy change Cq(S I/cIY for every bond-length change 6d (3 bonds per atom pair, some dd s arc different) and for vcry bond-angle change Sij/ (six angles... 

Cyclobutane derivatives are usually, but not always, nonplanar. Cy-clobutane itself exists as two butterflylike conformers that are easily interconvertible by inversion. Cyclopentane is a cyclic five-membered ring structure that can exist as a set of half-chair (twist-boat) forms (C2) and a set of envelope (C5) conformations. These are low-energy conformations and are readily interconverted by twists about bonds without any bond angle changes, only changes in torsion angles. These interconversions are called pseudorotations. 

---