Conjugation of double bonds

Figure 6.1 The aromatic character of benzene (deriving from the conjugation of double bonds) is an example of an emergent property.

MISCELLANEOUS. Diolefins are superior to the corresponding paraffins (with exceptions). Conjugation of double bonds raises the rating. 

The energetics consequences of the conjugation of double bonds has long been known and will not be reviewed here. What about cyclopropanation of conjugated olefins The... 

Thermodynamic parameters of the ideal-gas hydrogenation reactions for hydrocarbons and fullerene C60 are presented in Table 4.5. The enthalpies of C60 hydrogenation are within (-59 to -86) kJ per mol of H2 (A//"/n). These values are closer to similar values for aromatic hydrocarbons (-68 -f -69 kJmob1) than to those for alkenes and cycloalkenes (-110 5- -119 kJmob1). This fact is probably due to conjugation of double bonds in C60 in spite of non-aromaticity of this compound. The symmetryless hydrogenation entropies ASH /n are close for hydrocarbons and C60 (Table 4.5). 

A number of authors have found that conjugation of double bonds lowers the frequency of N02 bands. Shechter and Shepherd [25], Fieser et al. [32], Eckstein, Kraczkiewicz, T. Urbanski and Wojnowska [33] have all reported that this occurs with nitro-olefins. 

CONJUGATION OF DOUBLE BONDS AND FREE ELECTRON PAIRS... 

A. It has long been known that the conjugation of double bonds, in open chains also, has an important influence on the chemical and physical properties. This is illustrated by various values of the heat of hydrogenation from Table 20. From the... 

Conjugation of double bonds occurs accompanied by isomerization of cis to trans unsamration. 

Intense bands result from tt-tt conjugation of double bonds and carbonyl groups with the aromatic ring. Styrene, for example, has 244 nm... 

It should however be borne in mind that positions 9 and 10 in anthracene are not typically aromatic. They are manifested ]>y a higher reactivity than positions Ur and 0 as established by MO calculation [50]. In addition 9-nitroanthracene shows a non-planar structure with the nitro group out of plane [51] as pointed out by Cerfontain and Telder [48]. This is very similar to the position of the nitro group in o-dinitrobenzene and all derivatives of benzene with two ortho nitro groups. It is well known that the nitro groups in o-dinitrobenzene are not planar and there is no conjugation of double bonds in this compound. Tlie fact is also reflected in ultraviolet-absorption spectrum of o-dinitrobenzene which deviates from those of m- and p-dinitrobenzenes (Vol. I, p. 169, Table 20). 

Nitroalkenes (nitroolefins) are marked by the conjugation of double bonds with those of the nitro group. They show a high intensity absorption band due to JT —transition within the range 220-250 nm. The n — tt band is strongly overlapped there by the intense red shifted absorption band. The... 

The nitro group attached to an aromatic ring takes part, as a rule, in the conjugation of double bonds of the aromatic ring. However, a considerable devi-... 

The difference 0.48 is negative which means an additional stabilization of the butadiene molecule associated with the non-localization of the tt molecular orbitals. This is a consequence of an interaction between the 2pz a.o.s of atoms C(2) and C(3) which prevents the mere existence of two independent tt bonds localized in C(1)C(2) and C(3)C(4). In this and similar situations one refers to conjugation of double bonds, a phenomenon clearly related to the molecular topology, that is to the sequence and geometric arrangement of atoms in the molecule. 

The transformation of an allylic alcohol to an allylic halide (equation 5) or the reverse reaction normally leads to a mixture of isomers of direct and allylic substitution products. Only when tertiary allylic alcohols are employed or a rearrangement leads to extended conjugation of double bonds does a transposition of functionality occur predominantly While a number of procedures have been developed for the conversion of allylic alcohols to halides without rearrangement, there seems to be only one reaction where a cleanly rearranged product is obtainable in a substitution of a primary or secondary allylic (and propargylic) hydroxy group to a chloride, that is by thionyl chloride in ether, probably by a cyclic SNi process. The regioselectivity is worse in other solvents and is lost in the presence of a base. In accordance with the mechanism it is a syn facial substitution, as has been demonstrated in cyclic cases (equa-... 

Conjugation of double bonds is the largest part of what makes aromaticity relevant in organic chemistry, and conjugated double bonds have many other significant impacts on other types of dienes as well. 

Another potential area of application of FTIR spectroscopy is in the determination of the oxidative status or stability of an oil. Autoxidation is a major deteriorative reaction affecting edible fats and oils, and it is of major concern to processors and consumers from the standpoint of oil quality, as the oxidative breakdown products cause marked off flavours in an oil. A wide range of end products are associated with the autoxidative deterioration of fats and oils, the most important being hydroperoxides, alcohols, and aldehydes. Moisture, hydrocarbons, free fatty acids and esters, ketones, lactones, furans, and other minor products may also be produced, with the free fatty acids becoming more important in thermally stressed oils. In addition, there is significant cis to trans isomerisation and conjugation of double bonds in the hydroperoxides formed as an oil oxidises. 

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