Back double-bonding

Chapter 4 examines the group frequencies of triple bonds and cumulated double bonds. The alkyne hydrocarbons (acetylenes, —C=C—) are treated in I and the important nitrile group, —C=N, in detail in II. In the case of the alkynes the amount and type of substitution can often be identified from vibrational data (I, D, E). Systems containing cumulative double bonds (so-called back-to-back double bonds, >C=C=C<) such as the allenes in the hydrocarbon series are considered in III. A number of them contain hetero-atoms as for example, the ketenes (>C=C=0, III, B). Section F presents a problem that allows the reader to determine which of two possible structures is the correct one based on infrared and Raman data. 

Analysis We must go back to the corresponding hicarbonyl compound, writing CHO or C02Et for CH2OH. Then we see that the 1,5-dicarbonyl relationship is no use as there isn t room for a double bond (e.g. 3-4) in the precursor we would have to write. We have to use the 1,4 relationships ... 

Strategy a can lead back to a Dicls-Alder reaction if we put in another double bond - a trivial step since we are going to reduce out all double bonds at the end ... 

The iodine number of fats and oils provides a quantitative measurement of the degree of unsaturation. A solution containing a 100% excess of IGl is added to the sample, reacting across the double-bonded sites of unsaturation. The excess IGl is converted to I2 by adding KI. The resulting I2 is reacted with a known excess of Na2S203. To complete the analysis the excess 8203 is back titrated with coulometrically generated I2. 

Enolizalion of conjugated or /3,y-unsatiirated enones and dienones in O-deiiterated solvents facilitates the introduction of deuterium labels into positions as far as three and five carbon atoms away from a given ketone function. Exchange of the activated hydrogens in androst-4-en-3-one (12) provides a good illustration of the potential of this method. Saturation of the double bond (section V) in the deuterated enone (13) followed by back exchange of the a-deuteriums (section II-B) proves to be an excellent method for the preparation of 6,6-d2-5a-androstan-3-one (15). ... 

Cyclohexatriene to benzene displays a sequence of structures from 1,3,5-cyclohexatriene (withCC single and double bonds initially set to 1.5 and 1.3 A, respectively) to benzene (witb all CC bonds set to 1.4 A) and back to cyclohexatriene. Plot energy (vertical axis) vs. CC bond length (horizontal axis). How many energy minima are there Do the minima look more like 1,3,5-cyclohexatriene or benzene What is the correct interpretation of the resonance picture ... 

Shortly after the tetravalent nature of carbon was proposed, extensions to the Kekule-Couper theory were made w7hen the possibility of multiple bonding between atoms was suggested. Emil Erlenmeyer proposed a carbon-carbon triple bond for acetylene, and Alexander Crum Brown proposed a carbon-carbon double bond for ethylene. In 1865, Kekule provided another major advance when he suggested that carbon chains can double back on themselves to form rings of atoms. 

Stearic acid is a saturated fatty acid. This means it has only single bonds between its carbon atoms. This means it can coil up and form into random shapes. Double bonds between carbon atoms restrict the bending of the molecule at the point of the bond, like a hinge that lets a door swing back and forth but not up and down. Triple bonds are even more restrictive, locking the joint in place three-dimensionally, like the legs of a tripod. 

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... 

With the Robinson annelation in mind, we can get back ic a cyclohexadienone (2) by a Wittig disconnection. One of the double bonds could be put in by quinone oxidation leaving cyclohexenonc (3), which we have already made by a Rotjinson annelation and by another route (p 380). 

So let s go back to our example above with the addition of Br and Br across a double bond. This reaction is an anti addition, so we get only the set of enantiomers that has the two Br groups on opposite sides of the ring ... 

The concept of electrons not belonging to any particular atom in a molecule brings us back to resonance structures. The electrons in a metal are also delocalized. An electron in a bar of sodium is not associated with any particular atom, just as the electrons in the double bonds of benzene are not associated with any particular atom. 

Compounds with cis double bonds in the side chain were in general found to be more potent and efficacious than their triple-bond congeners, both in in vivo and in in vitro functional assays [98, 106, 107]. QSAR models have been generated for the compounds with unsaturated [108] and l, l -dimethyl [96] side chains to determine more precisely the pharmacophoric requirements of the receptor. It is postulated that for optimum potency, the side chain must be of a suitable length and flexibility to have the ability to loop back so that its terminus is in proximity to the phenolic ring. The widely used, potency enhancing 1 - and 2 -methyl substituents would be expected to increase the tendency of the side chain to adopt a looped back, rather than an extended conformation. 

In the case of r)2-coordination of the exocyclic C=C bond, it becomes substantially elongated compared with the double bond of free alkenes, as a result of back donation from the metal to the 7t orbitals of the double bond. For instance, in complex 17b the coordinated bond length is 1.437 A (see Fig. 3.2).18 This is also reflected in the loss of planarity around the quaternary exocyclic carbon, the methylenic carbon being bent out of the ring plane by 10.78°.18 Similar structural features were also observed with other P2Pd conjugated olefin complexes.39... 

The classic problem of the silicon-silicon double bond dates back at least to the early part of this century, when F. S. Kipping and his students attempted unsuccessfully to synthesize disilenes. Evidence for the probable transient existence of disilenes began to appear in the 1970s, but it was the isolation of the stable disilene 1 in 19811 that opened up modem disilene chemistry. The early history of this discovery has been recounted in a review2 several other reviews covering Si=Si double bonds have been published.2 5... 

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