Bonds conjugated system of double

Carotenoids are natural pigments characterized by a tail to tail linkage between two C20 units and an extended conjugated system of double bonds They are the most widely dis tributed of the substances that give color to our world and occur m flowers fruits plants insects and animals It has been estimated that biosynthesis from acetate produces approximately a hundred million tons of carotenoids per year The most familiar carotenoids are lycopene and (3 carotene pigments found m numerous plants and easily isolable from npe tomatoes and carrots respectively... 

Annulene (Section 11.20) Monocyclic hydrocarbon characterized by a completely conjugated system of double bonds. Annulenes may or may not be aromatic. 

In an electrocyclic ring-closure reaction a new o-bond is formed between the end atoms of a conjugated system of double bonds. Consider the cyclisation of buta-1,3-diene ... 

Heterocyclic carboxylic esters (708) are as reactive as formate and acetate esters. Extended to 1-substituted biguanides, the reaction is promoted by metal alkoxides. Since the heterocyclic ring must be five-or six-membered and contains a conjugated system of double bonds, the reaction resembles formally that involving a,p-unsaturated esters (see below). Several successful examples are on record (599, 606). 

Unsaturated Molecules with Conjugated System of Double Bond... 

A surprising prediction is that it is the cis configuration (rather than the trans) about the triple bond (and the two adjacent single bonds) that is the stable one in conjugated systems of double bonds and a... 

The ring system of vitamin B12, like that of porphyrins (Fig. 16-5), is made up of four pyrrole rings whose biosynthetic relationship to the corresponding rings in porphyrins is obvious from the structures. In addition, a number of "extra" methyl groups are present. A less extensive conjugated system of double bonds is present in the corrin ring of vitamin B12 than in porphyrins, and as a result, many chiral centers are found around the periphery... 

Why is the excited state of a conjugated system of double bonds stabilized more, relative to the ground state, than for a nonconjugated system Resonance theory provides an explanation (see Section 6-5). Of the several conventional valence-bond structures that can be written for 1,3-butadiene, four of which are shown here, 2a-2d, only structure 2a has a low enough energy to be dominant for the ground state of 1,3-butadiene ... 

Although all molecules are capable of absorption, only a few of them are capable of fluorescence. The phenomenon is generally observed in those organic molecules which have a rigid framework and not many loosely coupled substituents through which vibrionic energy can flow out. Fluorescence occurs mostly in molecules with conjugated system of double bonds such as C = C, N = O, - N = N, - C = O, - C = S. These parts of the molecules are called fluorophores. Some substituents enhance... 

The product is yellow because of the extended conjugated system of double bonds. 

The isomerization proceeds via an intermediate compound that is symmetrical at the C-8 position and that exhibits a continuous conjugated system of double bonds stretching from the enol double bond up to the indole system. Thus, isomerization of dihydrolysergic acids, in... 

These do not, so far, constitute industrially important monomers. Nevertheless, they do have some technical importance which is documented by the number of published studies of their polymerization. Acetylenes yield chains with a conjugated system of double bonds with semiconducting properties. It is probably just this possibility of conjugation in the generated chain that prevents formation of three-dimensional structures. Acetylene as such is a potentially tetrafunctional monomer. 

The molecule of this substance contains thirteen double bonds. It is seen that eleven of these double bonds are related to one another in a special way—they alternate regularly with single bonds. A regular alteration of double bonds and single bonds in a hydrocarbon chain is called a conjugated system of double bonds. The existence of this... 

The refraction of molecules may be calculated by the addition of the appropriate atomic refractions. It is necessary to add to the sum so obtained additional quantities corresponding to the multiplicity of the bonds involved. For a large number of molecules, the calculated and measured values of the molecular refraction agree, within the limits of experimental error. In molecules containing a conjugated system of double bonds, however, deviations from the additivity rule occur, the experimental value always being greater than the calculated. The difference, may... 

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