Double atoms 421 bond

For compounds containing only double-bonded atoms, the reference plane contains the double-bonded atoms and is perpendicular to the plane containing these atoms and those directly attached to them. It is customary to draw the formulas so that the reference plane is perpendicular to that of... 

This type of isomerism is also possible with other double bonds, such as C=N, N=N, or even C=S, though in these cases only two or three groups are connected to the double-bond atoms. In the case of imines, oximes, and other C=N compounds, if W = Y 60 may be called syn and 61 anti, though ( ) and (Z) are often used here too. In azo compounds there is no ambiguity. Compound 62 is always syn or (Z) regardless of the nature of W and Y, and 63 is E. 

We now turn to molecules containing double-bonded atoms, such as those with the substructure -C(=Y)-X-. Carboxylic acid derivatives with X = OR, NR2, or Hal and Y = O, S, or NR belong to this category. The sp2 carbon atoms in such compounds are significantly shielded (ca. 20-30 ppm) compared with those in corresponding ketone or aldehyde analogs. This has been ascribed... 

In addition to these general requirements, the distance between the y-hydrogen and the double-bonded atom must be less than 1.8 x 10" m [87,88] and the Cy-H bond must be in plane with the acceptor group. [89]... 

Many molecules that have several double bonds are much less reactive than might be expected. The reason for this is that the double bonds in these structures cannot be localized unequivocally. Their n orbitals are not confined to the space between the double-bonded atoms, but form a shared, extended Tu-molecular orbital. Structures with this property are referred to as resonance hybrids, because it is impossible to describe their actual bonding structure using standard formulas. One can either use what are known as resonance structures—i. e., idealized configurations in which n electrons are assigned to specific atoms (cf pp. 32 and 66, for example)—or one can use dashed lines as in Fig. B to suggest the extent of the delocalized orbitals. (Details are discussed in chemistry textbooks.)... 

Double bonds are not freely rotatable (see p.4). If double-bonded atoms have different substituents, there are two possible orientations for these groups. In fumaric acid, an intermediate of the tricarboxylic acid cycle (see p. 136), the carboxy groups lie on different sides of the double bond (trans or E position). In its isomer maleic acid, which is not produced in metabolic processes, the carboxy groups lie on the same side of the bond (cis or Z position). Cis-trans isomers (geometric isomers) have different chemical and physical properties—e.g., their melting points (Fp.) and pl

Electrons on an aromatic ring, double bonded atoms, and triple bonded atoms deshield attached hydrogens. 

Isomerism (geometric) Configuration around double bonds is specified by the characters / and indicating relative directionality between the connected (by double bond) atoms—for example,... 

Scheiner, S., and Wang, L., Effect of bond multiplicity upon hydrogen bonding and proton transfers. Double bonded atoms, J. Am. Chem. Soc. 114, 3650-3655 (1992). 

Carbon atoms can bond to each other not only by single covalent bonds but also by double and triple covalent bonds. In a double bond, atoms share two pairs of electrons in a triple bond, they share three pairs of electrons. The following diagram shows Lewis structures and structural formulas for single, double, and triple covalent bonds. 

Predict the arrangement of the atoms about the sulfur atom in P4SPO, assuming that double-bonded atoms require more space than single-bonded atoms. 

Z-E isomerism about a double bond is a more general designation than cis-trans and is determined by examining the bond s substituents and sequencing them in the maimer prescribed by lUPAC. The special relationship between the higher priority substituent on each end of the double bond is examined relative to a reference plane including the two double bonded atoms and drawn perpendicular to the plane of the four substituents. If the two higher priority substituents he on the same side of this reference plane, the isomerism is denoted as Z if they he on opposite sides, it is E. 

Figure 3. Stereoscopic view of the molecular packing of 2 (P2,/a) on (001) the molecules are tilted by 43" (in the almost identical next layer the angle is —39 ) the double-bond atoms are shown with grids and shadings.

Two pairs of double-bonded atoms joined by a single bond are called a conjugate system of double bonds. The type formula is... 

Atoms attached by double or triple bonds are assigned single bond equivalences. Every double-bonded atom is duplicated, and every triple-bonded atom is triplicated. 

When an alkene and an alkyne are in the same molecule, the priority is not necessarily straightforward. In general, numbers as low as possible are given to carbon atoms bearing double and triple bonds. If there is a further choice, the double-bond atoms are assigned the lowest nrunbers. En-yne 18 can be named either 3-propylhept-l-en-5-yne or 5-propylhept-6-en-2-yne. The former is the lower number for both groups, so 18 is named 3-propylhept-l-en-5-yne. For 19, however, the possible names are 3-propylhex-l-en-5-yne or 5-propylhex-5-en-l-yne. There is no clear choice and, in such a case, the C=C unit receives the lower number thus, 19 is named 3-propylhex-l-en-5-yne. 

From the Lewis formula, determine the hybrid orbitals on the double-bonded atoms. Do this by counting the number of electron groups about each atom, and from this determine the kind of hybrid orbitals used. Form a cr bond between the double-bonded atoms by overlapping a hybrid orbital of one atom with a hybrid orbital of the other atom. Also, form cr bonds from these atoms to other atoms. Use any unhybridized p orbitals on the central atoms to form tt bonds between them. 

Also double-bonded atoms, such as an oxygen, count twice, thus... 

From the point of view of color the second case is of greatest importance, because the conjugation of jc-bonds can connect several chromophoric centers and create a sufficient condition for absorption in the visible region. In addition to groups with double bonds, atoms and groups with free-electron pairs also play a very important role in the molecules of colored substances, because they can take part in mesomeric shifts in the molecule and thus have a substantial influence on the color (auxochromes). However, groups can also exist which produce an opposite effect (so-called anti-auxochromes). 

If no oxygen is present, the free radical then reacts with the carbon of the double bonded atom of the... 

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