Alkenes bond-line structures

The classical Dewar-Chatt-Duncanson model for metal-alkene bonding has been revisited with a combination of X-ray structural data (see Diffraction Methods in Inorganic Chemistry) and DFT calculations (see Molecular Orbital Theory), particularly on complexes of the type (acac)Rh(alkene)2. These indicate the existence of distortions from idealized geometry involving a twist (127), where the axis of the double bond is no longer perpendicular to the molecular plane and a roll (128), where the line... 

FIGURE 14.19 Bond line ring structures with the C s and H s removed, (a) The two cyclic alkanes and (b) the two cyclic alkenes discussed in the text. 

The structure of ethylene and the orbital hybridization model for its double bond were presented m Section 2 20 and are briefly reviewed m Figure 5 1 Ethylene is planar each carbon is sp hybridized and the double bond is considered to have a a component and a TT component The ct component arises from overlap of sp hybrid orbitals along a line connecting the two carbons the tt component via a side by side overlap of two p orbitals Regions of high electron density attributed to the tt electrons appear above and below the plane of the molecule and are clearly evident m the electrostatic potential map Most of the reactions of ethylene and other alkenes involve these electrons... 

Example 4 In Figure 13.30, there is a double bond, represented by the extra vertical line inside the ring structure. You must follow the same rules as for alkenes. That is, the double bond gets priority for the lowest number. This means that one of the carbon atoms, on either end of the double bond, must be carbon number 1. The carbon atom at the other end must be carbon number 2. Next you have to decide in which direction to count so that the branch gets the lowest possible position number. In this compound, the carbon atom on the bottom end of the double bond is carbon number 1. 

This analysis of the simple addition of an electrophilic bromine molecule to a symmetrical alkene or alkyne has highlighted many points. First, there is the induction of a temporary dipole of the soft electrophile by the n electrons of the carbon/carbon double bond. Second, there is the heterolytic fission of the bromine molecule, and the subsequent formation of the cyclic bromonium ion. Third, this cyclic intermediate places certain restrictions on the potential line of attack for the second reagent, and so controls the structural and stereochemical consequences for the product. 

Amphotericin is a fascinating molecule in that one half of the structure is made up of double bonds and is hydrophobic, while the other half contains a series of hydroxyl groups and is hydrophilic. It is a molecule of extremes and as such is ideally suited to act on the cell membrane in the way that it does. Several amphotericin molecules cluster together such that the alkene chains are to the exterior and interact favourably with the hydrophobic centre of the cell membrane. The tunnel resulting from this cluster is lined with the hydroxyl groups and so is hydrophilic, allowing the polar contents of the cell to escape (Fig. 2.6). 

In the transition state leading to an alkene, the C—H and C—Br bonds are partially broken and the double bond is partially formed (partially broken and partially formed bonds are indicated by dashed lines), giving the transition state an alkene-like structure. Because the transition state has an alkene-like structure, any factors that stabilize the alkene will also stabilize the transition state leading to its formation, allowing the alkene to be formed faster. The difference in the rate of formation of the two alkenes is not very great. Consequently, both products are formed, but the more stable of the two alkenes will be the major product of the reaction. 

As discussed in Chapter 5, Section 5.2, the alkyne unit (C-CsC-C) is linear, with all four carbon atoms in a line. The effect on the conformation of a hydrocarbon is to flatten the molecule in the region of the triple bond, to an even greater extent than seen with alkenes. The conformation of 4-hexyne is apparent in 33A, but the flattening effect is quite clear in the space-filling molecular model, 33B. The zigzag structure of the alkyl portion of the molecule... 

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