Coplanar alignment

The deviation from planarity that is present in a structure such as 1 raises the question of how severely a conjugated system can be distorted from the ideal coplanar alignment of p orbitals and still retain aromaticity. This problem has been analyzed by determining the degree of rehybridization necessary to maximize p orbital overlap in 1. It is found that rehybridization to incorporate fractional amounts of s character can improve orbital alignment substantially. Orbitals with about 6% s character are suggested to be involved... 

In view of the restrictions on the mode of approach of the radical to the double bond, significant strain develops at the transition state, and this requires rotation of the benzylic methylene group out of its preferred coplanar alignment. 

The rather confusing conformational dependences in Fig. 3.52 can be rationalized in a simple way. From the total of 7t-a and a-7t stabilizations at their respective coplanar alignments with the pi system, one can confirm that C—H bonds are better overall hyperconjugating groups than C—I bonds. The most favorable hyperconjugative alignment is therefore to place both C—H bonds maximally out-of-plane (i.e., C—F in-plane) at = 0°, whereas a secondary favorable alignment... 

Figure 4.84 The leading vicinal owh—°wh interaction in equilibrium W2H10 (Fig. 4.83(c)), involving an acutely bent o vh bond with an obtusely oriented vicinal ctwh antibond in approximately coplanar alignment.

The E2 route needs to pass a test and an alignment test. The E2 passes the test, for with the base as acetate, p TabH = 4.8, and the pA aHL of the leaving group is -1.7, the E2 reaction can create a much weaker base. Since the C-N single bond is free to rotate, the N-H and the C-L can assume the coplanar alignment that an E2 requires. Therefore the E2 route is also reasonable. 

The relatively low rate and high activation energy noted for Entry 37 of Table 11.3 also reflects a stereoelectronic effect. The preference for delocalization at the radical center requires coplanarity of the substituents at the radical site, which results in poor alignment. In view of the restrictions on the mode of approach of the radical to the double bond, significant strain develops in the TS and requires rotation of the benzylic methylene group out of its preferred coplanar alignment. ... 

N. Kawatsuki, H. Takatsuka, and T. Yamamoto, Coplanar alignment of mesogenic moiety in photocrosslinked liquid crystalline polymer film containing cinnamoyl groups. Applied Physics Letters 75, 1386 (1999). 

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