Double bonds molecular shape

Some of the more remarkable examples of this form of topologically controlled radical polymerization were reported by Percec et cii.231 234 Dendron maeromonomers were observed to self-assemble at a concentration above 0.20 mol/L in benzene to form spherical micellar aggregates where the polymerizable double bonds are concentrated inside. The polymerization of the aggregates initiated by AIBN showed some living characteristics. Diversities were narrow and molecular weights were dictated by the size of the aggregate. The shape of the resultant macroniolecules, as observed by atomic force microscopy (ATM), was found to depend on Xn. With A, <20, the polymer remained spherical. On the other hand, with X>20, the polymer became cylindrical.231,232... 

Double bonds and their influence on molecular shape are vitally important for living organisms. For instance, they enable you to read these words. Vision depends on the shape of the molecule retinal in the retina of the eye. cis-Retinal is held rigid by its double bonds (41). When light enters the eye, it excites an electron out of the iT-bond marked by the arrow. The double bond is now weaker, and the molecule is free to rotate about the remaining o-bond. When the excited electron falls back, the molecule has rotated about the double... 

Robert Curl, Richard Smalley, and Harold Kroto were awarded the Nobel prize in chemistry in 1996 for the discovery of the soccer-ball-shaped molecule C60. This fundamental molecule was the first of a new series of molecular allotropes of carbon. The enthalpy of combustion of C60 is —25 937 kj-mol, and its enthalpy of sublimation is +233 kj-mol There are 90 bonds in C60, of which 60 are single bonds and 30 are double bonds. Like benzene, C60 has... 

U shape. This has profound significance on molecular packing in membranes and on the positions occupied by fatty acids in more complex molecules such as phospholipids. Trans double bonds alter these spatial relationships. Trans fatty acids are present in certain foods, arising as a by-product of the samration of fatty acids during hydrogenation, or hardening, of natural oils in the manufacture of margarine. An additional small... 

The carbon atom in CO2 has two groups of electrons. Recall from our definition of a group that a double bond counts as one group of four electrons. Although each double bond includes four electrons, all four are concentrated between the nuclei. Remember also that the VSEPR model applies to electron groups, not specifically to electron pairs (despite the name of the model). It is the number of ligands and lone pairs, not the number of shared eiectrons, that determines the steric number and hence the molecular shape of an inner atom. 

In this chapter, we develop a model of bonding that can be applied to molecules as simple as H2 or as complex as chlorophyll. We begin with a description of bonding based on the idea of overlapping atomic orbitals. We then extend the model to include the molecular shapes described in Chapter 9. Next we apply the model to molecules with double and triple bonds. Then we present variations on the orbital overlap model that encompass electrons distributed across three, four, or more atoms, including the extended systems of molecules such as chlorophyll. Finally, we show how to generalize the model to describe the electronic structures of metals and semiconductors. 

A more traveled route to the absolute configuration represented by cyclohexa-1,4-diene 8 involves Birch reduction-alkylation of benzoxazepinone 9.2.5 heterocycle is best prepared by the base-induced cyclization of the amide obtained from 2-fiuorobenzoyl chloride and (5)-pyrrolidine-2-metha-nol. o The molecular shape of enolate 10 is such that the hydrogen at the stereogenic center provides some shielding of the a-face of the enolate double bond. Thus, alkylation occurs primarily at the 3-face of 10 to give 11 as the major diastereomer. The diastereoselectivity for alkylation with methyl iodide is only 85 15, but with more sterically demanding alkyl halides such as ethyl iodide, allyl bromide, 4-bromobut-1-ene etc., diastereoselectivities are greater than 98 2. 

Number of Bonded Pairs on the Central Atom (Double and Triple Bonds Count Only as 1 Pair) Number of Lone Electron Pairs on the Central Atom Molecular Shape (Molecular Geometry)... 

As with alkanes, the boiling points and melting points of alkenes decrease with increasing molecular weight, but show some variations that depend on the shape of the molecule. Alkenes with the same molecular formula are isomers of one another if the position and the stereochemistry of the double bond differ. For example, there are four different acyclic structures that can be drawn for butene (C4H8). They have different b.p. and m.p. as follows. 

When there is a total of four electrons, Hund s rule predicts that two will be in the lowest orbital but the other two will be unpaired, so that the system will exist as a diradical rather than as two pairs. The degeneracy can be removed if the molecule is distorted from maximum molecular symmetry to a structure of lesser symmetry. For example, if 44 assumes a rectangular rather than a square shape, one of the previously degenerate orbitals has a lower energy than the other and will be occupied by two electrons. In this case, of course, the double bonds are essentially separate and the molecule is still not aromatic. Distortions of symmetry can also occur when one or more carbons are replaced by hetero atoms or in other ways.124... 

So far we have looked at molecular orbitals in a simplified way as electron pairs that try to seek locations of minimum potential energy. Now that double bonds are under consideration, it will pay us to examine in greater detail the characteristics of both atomic and molecular orbitals. You recall that an atomic orbital is a volume element oriented with respect to the nucleus of the atom where there is a high probability of finding (at the most) two electrons that are identical in quantum numbers except for direction of spin. The orbital of each type of electron (5, p, [Pg.134]

Likewise for the N()2 ion, the two 0 atoms must be treated the same way as participating in double-bond formation. All three atoms must use sp- orbitals, with end-overlap between each 0 and N to form cr bonds. And of course the three p orbitals that rise perpendicularly from their common plane will side-overlap and merge to give a delocalized split-banana-shaped n molecular orbital, with one half-banana above the plane of the molecule and the other half below (Figure 9-28). Four of the 18 valence electrons are held in the two cr bonds, 10 are held as five lone pairs in the sp- orbitals (two on each O, and one on the N), and the remaining four are delocalized over the length of the molecule in the n bond (two above the plane and two below). 

If the bond formed between two adjacent atoms is symmetric along the axis between the two nuclei, we refer to the bond as a o-bond. In contrast, the bond that is formed between two electrons in the p-orbitals is called a ji-bond. These molecular orbitals have a very different shape. The single bond between two carbon atoms (-C-C-) is formed between electrons in the sp3-orbitals (o-bond) of two adjacent carbon atoms (Figure la). A double bond between two C-atoms (-C=C-) consists of a o-bond formed between electrons in the sp2 orbitals of two adjacent C-atoms, and a 7i-bond formed between two 2p-orbitals (Figure 2-1). 

---