Double bonds sigma bond

Gassman, P.G. Fentiman Jr., A.F. J. Am. Chem. Soc., 1969, 91, 1545 1970, 92, 2549. For a discussion of the use of the tool of increasing electron demand to probe neighboring-group activity by double bonds, sigma bonds, and aryl rings, see Lambert, J.B. Mark,... 

The anomeric effect in terms of a stabilizing effect can be illustrated by the concept of "double-bond - no-bond resonance" (14, 15) shown by the resonance structures 4 and 2 or by the equivalent modern view (16, 17) that this electronic delocalization is due to the overlap of an electron pair orbital of an oxygen atom with the antibonding orbital of a C —OR sigma bond (12). 

Because single bonds (sigma bonds) are more stable than pi bonds, the most common reactions of double bonds transform the pi bond into a sigma bond. For example, catalytic hydrogenation converts the C = C pi bond and the H—H sigma bond into two C—H sigma bonds (Section 7-7). The reaction is exothermic (AH° = about -80 to -120 kJ/mol or about -20 to -30 kcal/mol), showing that the product is more stable than the reactants. 

Single bonds (sigma bonds) will also be seen in the carbon-oxygen bonds discussed later in the alcohol series, where there is a C-O-H bond. Double bonds (sigma and pi bonds) also occur in aldehydes and ketones where there is a C==0 bond. These homologous series will be looked at in Module 7. 

The hexagonal skeleton is made up of single sigma C-C bonds, at an angle of 120°, and the spare electron from each carbon atom forms a cloud equally distributed between all the carbons. This gives the impression that there are no alternate single and double bonds but bonds of equal length all around. 

In contrast, rotation about the double bond (sigma -I- pi) in 1,2-dichloroethene is restricted, so at room temperature, 1,2-dichloroethene exists in two isomeric forms. 

The carbon—carbon double bond is the distinguishing feature of the butylenes and as such, controls their chemistry. This bond is formed by sp orbitals (a sigma bond and a weaker pi bond). The two carbon atoms plus the four atoms ia the alpha positions therefore He ia a plane. The pi bond which ties over the plane of the atoms acts as a source of electrons ia addition reactions at the double bond. The carbon—carbon bond, acting as a substitute, affects the reactivity of the carbon atoms at the alpha positions through the formation of the aHyUc resonance stmcture. This stmcture can stabilize both positive and... 

You may recall that we discussed the bonding in ethene in Chapter 7. The double bond in ethene and other alkenes consists of a sigma bond and a pi bond. The ethene molecule is planar. There is no rotation about the double bond, since that would require breaking the pi bond. The bond angle in ethene is 120°, corresponding to sp2 hybridization about each carbon atom. The geometries of ethene and the next member of the alkene series, QHg, are shown in Figure 22.6. 

When two p orbitals overlap in a side-by-side configuration, they form a pi bond, shown in Figure 7.7. This bond is named after the Greek letter 7t. The electron clouds in pi bonds overlap less than those in sigma bonds, and they are correspondingly weaker. Pi bonds are often found in molecules with double or triple bonds. One example is ethene, commonly known as ethylene, a simple double-bonded molecule (Figure 7.8). The two vertical p orbitals form a pi bond. The two horizontal orbitals form a sigma bond. 

Carbon atoms (1) and (4) use sp3 hybrid orbitals to form four sigma bonds, three by overlap with the hydrogen Is orbitals and one by overlap with an sp2 orbital from the central carbon (2). The two carbon atoms involved in the double bond undergo sp2 hybridization. They form C-H bonds by overlapping with Is orbitals of the H atoms. The C=C double bond is formed similarly to that described in (a). 

Solution The electron count is 16e (eight from Ru, two from each phosphine, one from each Cl, and two from methylene, assumed to make a Ru=C double bond). Two cu bonds result, leaving four electrons to make the ctruc and 7TruC bonds and two lone pairs. Skeletal sigma-bonding of the parent Lewis-like structure requires three sd2 hybrids (90° angles),... 

All single bonds are sigma (a) in nature. Double bonds contain one sigma bond and one pi (n) bond. Triple bonds contain one sigma bond and two pi (n) bonds. 

In the benzene molecule, there are 9 single bonds (9a) and 3 double bonds (3a and 3tc) for a total of 12 sigma bonds. 

All single bonds between two atoms are sigma (o) bonds. Pi bonds can only be formed after a sigma bond has already been formed. Therefore a double bond contains one o and one n bond, and a triple bond contains one o and two k bonds. 

In ethylene, there are two types of bonds. Sigma (tr) bonds have the overlap of the orbitals on a line between the two atoms involved in the covalent bond. In ethylene, the C-H bonds and one of the C-C bonds are sigma bonds. Pi (ir) bonds have the overlap of orbitals above and below a line through the two nuclei of the atoms involved in the bond. A double bond is always composed of one sigma and one pi bond. A carbon-to-carbon triple bond results from the... 

A double or triple bond is always composed of one sigma bond and the rest pi. 

---