Coplanar

An atom lhai hassp h yhridi/alion lends lo be coplanar wilh its attached aloms. This effect isaccoiinled for by improper torsions in Olher force fields and by oiil-of-plane-bending inlcraclions in... 

Various other ways to incorporate the out-of-plane bending contribution are possible. For e3plane bend involves a cakulation of the angle between a bond from the central atom and the plane defined by I he central atom and the other two atoms (Figure 4.10). A value of 0° corresponds to all four atoms being coplanar. A third approach is to calculate the height of the central atom above a plane defined by the other three atoms (Figure 4.10). With these two definitions the deviation of the out-of-plane coordinate (be it an angle or a distance) can be modelled Lt ing a harmonic potential of the form... 

Boron trifluoride is a trigonal planar molecule There are six electrons two for each B—F bond associated with the valence shell of boron These three bonded pairs are farthest apart when they are coplanar with F—B—F bond angles of 120°... 

Section 2 20 Carbon is sp hybridized in ethylene and the double bond has a ct com ponent and a rr component The sp hybridization state is derived by mix mg the 2s and two of the three 2p orbitals Three equivalent sp orbitals result and their axes are coplanar Overlap of an sp orbital of one car bon with an sp orbital of another produces a ct bond between them Each carbon still has one unhybridized p orbital available for bonding and side by side overlap of the p orbitals of adjacent carbons gives a rr bond between them... 

Conformational analysis is far simpler m cyclopropane than m any other cycloalkane Cyclopropane s three carbon atoms are of geometric necessity coplanar and rotation about Its carbon-carbon bonds is impossible You saw m Section 3 4 how angle strain m cyclopropane leads to an abnormally large heat of combustion Let s now look at cyclopropane m more detail to see how our orbital hybridization bonding model may be adapted to molecules of unusual geometry... 

In fhe envelope confermafion four of fhe carbon afoms are coplanar The fiflh car bon IS ouf of fhe plane of fhe ofher four There are fhree coplanar carbons m fhe half chair confermafion wifh one carbon atom displaced above fhaf plane and anofher below if In bofh fhe envelope and fhe half chair confermafions m plane and ouf of plane carbons exchange posifions rapidly Equihbrafion befween confermafions of cyclopenfane is very fasf and occurs af rafes similar to fhaf of rofafion abouf fhe carbon-carbon bond of efhane... 

The properties of tert butyl cation can be understood by focusing on its structure which IS shown m Figure 4 9 With only six valence electrons which are distributed among three coplanar ct bonds the positively charged carbon is sp hybridized The unhybridized 2p orbital that remains on the positively charged carbon contains no elec Irons Its axis is perpendicular to the plane of the bonds connecting that carbon to the three methyl groups... 

Although both stereoisomers yield 4 tert butylcyclohexene as the only alkene they do so at quite different rates The cis isomer reacts over 500 times faster than the trans The difference in reaction rate results from different degrees of rr bond develop ment in the E2 transition state Since rr overlap of p orbitals requires their axes to be parallel rr bond formation is best achieved when the four atoms of the H—C—C—X unit he in the same plane at the transition state The two conformations that permit this are termed syn coplanar and anti coplanar... 

Because adjacent bonds are eclipsed when the H—C—C—X unit is syn coplanar a transition state with this geometry is less stable than one that has an anti coplanar rela tionship between the proton and the leaving group... 

Effects that arise because one spatial arrangement of electrons (or orbitals or bonds) IS more stable than another are called stereoelectronic effects There is a stereoelec tromc preference for the anti coplanar arrangement of proton and leaving group in E2 reactions Although coplanarity of the p orbitals is the best geometry for the E2 process modest deviations from this ideal can be tolerated In such cases the terms used are syn periplanar and anti periplanar... 

The per/ n periplanar means almost or nearly The coplanar/periplanar dis tinction IS discussed in the October 2000 issue of the Journal of Chemical Educa tion p 1366... 

Dehydrohalogenation of alkyl halides (Sections 5 14-5 16) Strong bases cause a proton and a halide to be lost from adjacent carbons of an alkyl halide to yield an alkene Regioselectivity is in accord with the Zaitsev rule The order of halide reactivity is I > Br > Cl > F A concerted E2 reaction pathway is followed carbocations are not involved and rearrangements do not occur An anti coplanar arrangement of the proton being removed and the halide being lost characterizes the transition state... 

Section 5 16 The preceding equation shows the proton H and the halogen X m the anti coplanar relationship that is required for elimination by the E2 mechanism... 

Additional evidence for electron delocalization m 1 3 butadiene can be obtained by considering its conformations Overlap of the two rr electron systems is optimal when the four carbon atoms are coplanar Two conformations allow this coplananty they are called the s cis and s trans conformations... 

Section 17 2 The carbonyl carbon is sp hybridized and it and the atoms attached to It are coplanar Aldehydes and ketones are polar molecules Nucleophiles attack C=0 at carbon (positively polarized) and electrophiles especially protons attack oxygen (negatively polarized)... 

FIGURE 27 20 Heme shown as (a) a structural drawing and as (b) a space filling model The space filling model shows the coplanar arrangement of the groups surrounding iron... 

Envelope (Section 3 6) One of the two most stable conforma tions of cyclopentane Four of the carbons in the envelope conformation are coplanar the fifth carbon lies above or be low this plane... 

An atom that has sp hybridization tends to be coplanar with its attached atoms. This effect is accounted for by improper torsions in other force fields and by out-of-plane-bending interactions in... 

Value may be decreased markedly by steric hindrance to coplanarity. 

The ortho hydrogen atoms surrounding the central carbon atom show considerable steric overlap. Therefore, it can be assumed that the three aryl groups in the dye are not coplanar, but are twisted in such a fashion that the shape of the dye resembles that of a three-bladed propeller (9). Substitution in the para position of the three aryl groups determines the hue of the dye. When only one amino group is present, as in fuchsoriimine hydrochloride [84215-84-9] = 440 nm (2), the shade is a weak orange-yeUow. 

The triaLkoxy(aryloxy)boranes are typically monomeric, soluble in most organic solvents, and dissolve in water with hydrolysis to form boric acid and the corresponding alcohol and phenol. Although the rate of hydrolysis is usually very fast, it is dependent on the bulk of the alkyl or aryl substituent groups bonded to the boron atom. Secondary and tertiary alkyl esters are generally more stable than the primary alkyl esters. The boron atom in these compounds is in a trigonal coplanar state with bond hybridization. A vacantp orbital exists along the threefold axis perpendicular to the BO plane. 

Figure 2 illustrates the two primary classes of PCBs that exhibit this type of activity, namely the coplanar PCBs and their monoortho coplanar analogues. 

Fig. 2. Toxic PCB congeners coplanar (a) and monoortho coplanar compounds (b). See Table 2.

The structure of lumazine has been studied more precisely by X-ray analysis (72AX(B)659). The crystal structure is built up of almost coplanar, hydrogen-bonded dimers of lumazine with the oxygens of the pyrimidine moiety in the keto form and the observed bond distances indicating the pyrazine ring electrons to be delocalized. 

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