Double bond, rigidity

All pIC50 s and the pEC50 were colinear (r = 0.905-0.993), despite two orders of magnitude in the concentration response. Correlations are weak (r = 0.571-0.696) for this set (n = 9), but the major negative factor is the double-bond rigidity of the crotonates and musci-mols (coefficients = -0.77 to -1.46). 

Structure aroma Since only little is known about the molecular structure of the appropriate receptors, empirically determined relationships between odor and molecular structure still provide the basis for the specific development of aroma chemicals The most important factors are molecular shape and size (vapor pressure) and the nature and position of polar, so-called osmophoric groups such as carbonyl, ester, and hydroxy groups or double bonds. Rigid molecules such as, e. g., camphor, exhibit only one specific odor type while flexible molecules, especially those with differing molecular parts, have complex odors since they can apparently influence several different types of receptors. 

In the previous chapter, we concentrated on conformational changes enabled by rotation around a single bond and presented just a few selected examples of reactivity. For substituents at a double bond, the conformational changes are less important (with the exception of imines and similar compounds). Because the conformational equilibrium is usually frozen by the double bond rigidity, the stereoelectronic effects are more often observed as effects on reactivity and relative stability. 

However, the descriptors cannot be considered independently as there is no free rotation around the double bond, In order to take account of this rigidity, the descriptors of the two units have to be multiplied to fix a descriptor of the complete stereoisomer. 

To conclude this computer project, we shall lirst search the potential surface for rotation of u-butane about its 23 C C bond, for which we think we know the answei, then seai ch the potential sutface foi I-butene, foi which we do not. In I -butene, the double bond establishes a rigid plane but the methyl group can take up several d i ffe ren t positions re I at i ve to i t by rotation ab ou t th e 2 - 3 s i n g I e bo n d,... 

LB Films of Polymerizable Amphiphiles. Stxidies of LB films of polymerizable amphiphiles include simple olefinic amphiphiles, conjugated double bonds, dienes, and diacetylenes (4). In general, a monomeric ampbipbile can be spread and polymerization can be induced either at tbe air—water interface or after transfer to a soHd substrate. Tbe former polymerization results in a rigid layer tbat is difficult to transfer. 

Polyester Resins. Reinforced polyester resins are thermosets based on unsaturated polyesters from glycols and dibasic acids, either or both of which contain reactive double bonds. The ratio of saturated to unsaturated components controls the degree of cross-linking and thus the rigidity of the product (see Polyesters, unsaturated). Typically, the glycols and acids are esterified until a viscous Hquid results, to which an inhibitor is added to prevent premature gelation. Addition of the monomer, usually styrene, reduces the viscosity to an easily workable level. 

The prime function of the saturated acid is to space out the double bonds and thus reduce the density of cross-linking. Phthalic anhydride is most commonly used for this purpose because it provides an inflexible link and maintains the rigidity in the cured resin. It has been used in increasing proportions during the past decade since its low price enables cheaper resins to be made. The most detrimental effect of this is to reduce the heat resistance of the laminates but this is frequently unimportant. It is usually produced by catalytic oxidation of o-xylene but sometimes naphthalene and is a crystalline solid melting at 131°C. 

The double bonds found in fatty acids are nearly always in the cis configuration. As shown in Figure 8.1, this causes a bend or kink in the fatty acid chain. This bend has very important consequences for the structure of biological membranes. Saturated fatty acid chains can pack closely together to form ordered, rigid arrays under certain conditions, but unsaturated fatty acids prevent such close packing and produce flexible, fluid aggregates. 

The rearrangement proceeds from the Si-state of the 1,4-diene 1. The Ti-state would allow for different reactions like double bond isomerization. Rigid systems like cyclic dienes, where EfZ -isomerization of a double bond is hindered for steric reasons, can react through the Ti-state. When the rearrangement proceeds from the Si-state, it proves to be stereospecific at C-1 and C-5 no -isomerization is observed. Z-l,l-Diphenyl-3,3-dimethyl-l,4-hexadiene 5 rearranges to the Z-configured vinylcyclopropane 6. In this case the reaction also is regiospecific. Only the vinylcyclopropane 6 is formed, but not the alternative product 7. ... 

The shape of the ethylene molecule has been learned by a variety of types of experiments. Ethylene is a planar molecule—the four hydrogen and the two carbon atoms all lie in one plane. The implication of this experimental fact is that there is a rigidity of the double bond which prevents a twisting movement of one of the CHj groups relative to the other. Rotation of one CHt group relative to the other—with the C—C bond as an axis—must be energetically restricted or the molecule would not retain this flat form. 

The diastereoselectivity of the reaction may be rationalized by assuming a chelation model, which has been developed in the addition of Grignard reagents to enantiomerically pure a-keto acetals7,8. Cerium metal is fixed by chelation between the N-atom, the methoxy O-atom and one of the acetal O-atoms leading to a rigid structure in the transition state of the reaction (see below). Hence, nucleophilic attack from the Si-face of the C-N double bond is favored4. 

Single bonds are flexible, and can bend and rotate easily. Double bonds are more rigid. A rigid bond does not allow the molecules to fit together easily, so they can t pack tightly into a solid. 

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... 

The double bond in alkenes makes them more rigid than alkanes. Some of the atoms of alkene molecules are locked into a planar arrangement by the TT-bond hence, they cannot roll up into a ball as compactly as alkanes can. Because they do not pack together as compactly as alkanes do, they have lower boiling and melting points. 

Intramolecular cycloadditions of furans are a useful method for creating an oxygenated cyclohexane ring in rigid cycloadducts. Thus, a MeAICI2-catalyzed intramolecular reaction [40] of compounds 34 leads stereoselectively to tricyclic cycloadducts (Equation 3.8). The reaction yield is strongly dependent on the quantity of the catalyst and the type of substituent at the olefmic double bond. Cycloadduct 35 (R = R2 = Me, Ri = R3 = R4 = H) was then converted [40b] into 1,4-epoxycadinane (36). 

The largest single use of maleic anhydride is in the preparation of unsaturated polyester resins. It is first esterihed with a polyalcohol (two or more hydroxyls) and then the double bond is copolymerized (crosslinked) with a vinyl monomer such as styrene to form a rigid structure. Such resins are usually reinforced with hberglass (FRP). Maleic anhydride is also used to make oil additives and agricultural chemicals. 

The formation of ROO and RO radicals, and M" =0 species is expected when phthalocyanines and porphyrins are used as catalysts (21, 22). The formation of the epoxide, Fignre 49.1, may be associated with the attack of metal oxo species (Fe = O) to the double bond (23). For a-pinene this attack is possibly favored by its rigid structure that causes an orbital overlapping, making the allylic hydrogen abstraction difficult (24). 

Because of the necessity of spatially appropriate overlap of the 2p. .-orbital on boron with the adjacent jp3-hybridized carbon orbital (12) or with the tt-MO of the double bond (13), any such ir-bonding should be most sensitive to molecular conformation. Hence, the occurrence of such 7r-bonding might be more evident in certain highly rigid cyclic structures, such as 14 and 15. 

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