Sigma bond alkenes

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. 

Substrates for EHC reactions consist of two electron-deficient alkenes tethered to one another. Reduction leads to the formation of an adduct wherein the )S-carbons are joined by a new sigma bond. As illustrated in Table 1, the methodology is exceptionally useful for the construction of three-, five-, and six-membered rings, but not for rings of sizes seven and eight. 

As indicated, these transformations lead to the formation of a new sigma bond between two formally electron-deficient centers [4,22,23], in this instance between the )9-carbon of an electron-deficient alkene and a carbonyl carbon. 

The first examples of what can be categorized as [2 + 2] type cycloaddition product formed by reaction between an alkene and a silicon surface were reported in the late 1980s. Alkenes such as ethylene, as well as the related alkyne molecule acetylene, were reacted with the clean Si(100)-2 x 1 surface in vacuum [196-213]. The adsorption of these unsaturated C2 molecules (ethylene and acetylene) on Si(100)-2 x 1 is also discussed in Chapter 1. The alkenes were found to chemisorb at room temperature, forming stable species that bridge-bonded across the silicon dimers on the surface. The reaction proceeded by formation of two new a bonds between Si and C atoms, hence the bonding was referred to as di-sigma bonding. In addition, it was shown that while the bonds of the alkene and of the Si—Si dimer are... 

A common motif in organometallic chemistry is the agostic interaction, which can act to stabilize low-coordination low-e-count complexes. The requirement is an alkyl group with a / - or a y-C—H bond attached to the metal within reach of (i.e., cis to) an empty coordination site. An attractive interaction occurs with the C—H bond acting as a 2e donor into the low-lying metal valence orbital that occupies that site. In the case of a / -C—H bond, hydride transfer may occur with little activation, resulting in an M—H sigma bond and complex with an alkene as discussed above. 

Born-Oppenheimer approximation, 22, 219 Bound state, 209-210 Bid, with alkenes, 260 Brillouin s theorem, 241 Bromide ion (Br ) effect on Ao, 181 trails effect, 181 as X ligand, 176 Bromine (Br2) sigma bond, 77 Bromochloromethane, 13 ll-Bromo- ii/o-9-chloro-7-... 

An addition reaction is a reaction in which two atoms or ions react with a double bond of an alkene, forming a compound with two new functional groups bonded to the carbons of the original double bond. In these reactions, the existing pi bond is broken and in its place, sigma bonds form to two new atoms. 

Alkenes contain a C=C double bond. The C=C double bond can be described with two different models. According to the most commonly used model, a C=C double bond consists of a <7- and a tr-bond. The bond energy of the a-bond is 83 kcal/mol, about 20 kcal/mol higher than the tr-bond (63 kcal/mol). The higher stability of <7 bonds in comparison to n bonds is due to the difference in the overlap between the atomic orbitals (AOs) that form these bonds. Sigma bonds are produced by the overlap of two spn atomic orbitals (n 1,2,3), which is quite effective because it is frontal. Pi bonds are based on the overlap of 2,pz atomic orbitals, which is not as good because it is lateral or parallel. 

However, the chemical properties of an alkene are dramatically affected by the presence of the double bond. Recall that a carbon-carbon pi bond is considerably weaker than a carbon-carbon or carbon-hydrogen sigma bond. It is possible to selectively cause a reaction to occur at a pi bond under conditions that do not affect the sigma bonds. The pi bond is the weak spot of an alkene, and it is there that most chemical reactions occur. This is why unsaturated fats spoil more readily than saturated fats. Their pi bonds provide a place for reaction with oxygen to occur, which leads to spoilage. 

A cycloaddition reaction most commonly involves two molecules reacting to form two new sigma bonds between the end atoms of their pi systems, resulting in the formation of a ring. The product has two more sigma bonds and two fewer pi bonds than the reactants. The reactions are classified according to the number of pi electrons in each of the reactants. Thus, the reaction of two alkenes to form a cyclobutane derivative is termed a [2 + 2] cycloaddition reaction, and the reaction of a diene with an alkene to form a cyclohexene derivative is termed a [4 + 2] cycloaddition reaction ... 

A wide variety of electrophilic additions involve similar mechanisms. First, a strong electrophile attracts the loosely held electrons from the pi bond of an alkene. The electrophile forms a sigma bond to one of the carbons of the (former) double bond, while the other carbon becomes a carbocation. The carbocation (a strong electrophile) reacts with a nucleophile (often a weak nucleophile) to form another sigma bond. 

The Diels-Alder reaction is called a [4 + 2] cycloaddition because a ring is formed by the interaction of four pi electrons in the diene with two pi electrons of the alkene or alkyne. Since the electron-poor alkene or alkyne is prone to react with a diene, it is called a dienophile ( lover of dienes ). In effect, the Diels-Alder reaction converts two pi bonds into two sigma bonds. We can symbolize the Diels-Alder reaction by using... 

Like an alkene, benzene has clouds of pi electrons above and below its sigma bond framework. Although benzene s pi electrons are in a stable aromatic system, they are available to attack a strong electrophile to give a carbocation. This resonance-stabilized carbocation is called a sigma complex because the electrophile is joined to the benzene ring by a new sigma bond. 

The carbonyl carbon atom is sp2 hybridized and bonded to three other atoms through coplanar sigma bonds oriented about 120° apart. The unhybridized p orbital overlaps with a p orbital of oxygen to form a pi bond. The double bond between carbon and oxygen is similar to an alkene C=C double bond, except that the carbonyl double bond is shorter, stronger, and polarized. 

The chemical properties of alkenes are very different from those of alkanes because of the double bond (— C = C —) in the structure. Double bond contains a sigma bond and a pi bond. Since electrons in pi bonds are bonded less strongly than in sigma bonds. This makes alkenes chemically reactive combustion, substitution, oxidation and polymerization reactions are all undergone by alkenes. 

Transfer of a proton to an unsaturated carbon atom occurs more readily because 7r-electron pairs are available to form a new covalent (sigma) bond. A protonated alkene may undergo addition of a nucleophile in the second step, viz. 

Otto Diels and his pupil Kurt Alder received the Nobel Prize in 1950 for their discovery and work on the reaction that bears their names. Its great usefulness lies in its high yield and high stereospecificity. A cycloaddition reaction, it involves the 1,4-addition of a conjugated diene in the s-cis-conformation to an alkene in which two new sigma bonds are formed from two pi bonds. 

In alkanes all the bonds are saturated sigma bonds, whereas in alkenes the carbon to carbon bond is unsaturated. The carbon-to-carbon double bond in an alkene consists of a sigma bond together with a pi bond, as seen in Figure 6.1.6. 

Alkenes are called unsaturated because they contain a C=C double bond and therefore less hydrogen than the corresponding alkanes. They can undergo addition reactions in order to become saturated . The alkanes have no double bonds and all their bonds are sigma bonds between C-C and C-H. They cannot undergo addition reactions because they are saturated already. An even more unsaturated series of hydrocarbons is the alkynes, C H2 -2, which contain a carbon-carbon triple bond, (C=C), made up of a sigma and two pi bonds. The best known member of this series is C2H2, ethyne (also known as acetylene). 

The alkenes are much more reactive than the alkanes by virtue of the double bonds present. The pi bond can react to form two new signui bonds, leaving the original C-C sigma bond intact. These double bonds make the molecules vulnerable to attack by certain reagents, e g. bromine, which can add across the double bond. 

Alkanes are already saturated, ie all their C-H bonds are single sigma bonds, and so they do not have any capacity to take up hydrogen. In alkenes the double bond can be opened up to add on a molecule of hydrogen gas H2, and the corresponding alkane is formed. 

An alkene is an average electron source, and an aromatic compound is usually worse therefore to get electrophilic addition to alkenes and aromatic compounds to occur one needs a good electron sink. Often a loose association of an electrophile with the pi electron cloud (called a pi-complex) occurs before the actual sigma bond formation step. The best electrophiles, carbocations, add easily. For an overview of electrophilic additions to alkenes, see Section 4.4. 

Addition is the characteristic reaction of alkenes and alkynes. Since the carbons of a double or triple bond do not have the maximum number of attached atoms, they can add additional groups or atoms. Double bonds undergo addition once and triple bonds can undergo addition twice. The reactivity of alkenes and alkynes is due to the presence of pi-bonds. Unlike sigma bonds, pi-bonds are directed away from the carbons the electrons are loosely held, very accessible, and quite attractive electron-deficient species (electrophiles) seeking an electron source. 

A qualitative approach will possibly be more fruitful. Fig. 12 illustrates how the dihydrogen addition step (late with respect to heavy atom locations, early with respect to dihydrogen) might appear for the two diastereomeric pathways of the 16-electron route, with CHIRAPHOS as the ligand. In the alternative 14-electron route, dissociation of the alkene is assumed to occur, followed by irreversible H2 addition. The process in then consummated by reformation of the alkene-rhodium bond, or by a sigma bond metathesis which bypasses the dihydride state. 

The alkenes (also called olefins) contain at least one carbon-carbon double bond. Alkenes have the general formula C 2n, where n = 2, 3,... . The simplest alkene is C2H4, ethylene, in which both carbon atoms are ip -hybridized and the double bond is made up of a sigma bond and a pi bond (see Section 10.5). 

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