Double bonds, in alkenes

TT bond (Section 2 20) In alkenes a bond formed by overlap of p orbitals in a side by side manner A tt bond is weaker than a u bond The carbon-carbon double bond in alkenes con sists of two sp hybridized carbons joined by a a bond and a TT bond... 

In principle, cri-2-butene and fran5-2-butene may be interconveited by rotation about the C-2=C-3 double bond. However, unlike rotation about the C-2—C-3 single bond in butane, which is quite fast, interconversion of the stereoisomeric 2-butenes does not occur under normal circumstances. It is sometimes said that rotation about a car bon-carbon double bond is restricted, but this is an understatement. Conventional laboratory sources of heat do not provide enough energy for rotation about the double bond in alkenes. As shown in Figure 5.2, rotation about a double bond requues the p orbitals of C-2 and C-3 to be twisted from their stable parallel alignment—in effect, the tt component of the double bond must be broken at the transition state. 

These pentahydrides have attracted attention as catalysts for hydrogenation of the double bond in alkenes. IrH5(PPr3)2 catalyses vinylic H-D exchange between terminal alkenes and benzene, the isomerization of a,f3-ynones, isomerization of unsaturated alcohols and dehydrogenation of molecules such as secondary alcohols [176],... 

The carhon-carbon double bond in alkenes is more reactive than carbon-carbon single bonds and gives alkenes their characteristic properties. As we saw in Section 3.4, a double bond consists of a a-bond and a 7r-bond. Each carbon atom in a double bond is sp2 hybridized and uses the three hybrid orbitals to form three cr-bonds. The unhvbridized p-orbitals on each carbon atom overlap each other and form a Tr-bond. As we saw in Section 3.7, the carbon-carbon 7r-bond is relatively weak because the overlap responsible for the formation of the 7r-bond is less extensive than that responsible for the formation of the a-bond and the enhanced electron density does not lie directly between the two nuclei. A consequence of this weakness is the reaction most characteristic of alkenes, the replacement of the 77-bond by two new a-bonds, which is discussed in Section 18.6. 

The double bonds in alkenes can be generated by elimination reactions. 

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. 

The location of the position of double bonds in alkenes or similar compounds is a difficult process when only very small amounts of sample are available [712,713]. Hass spectrometry is often unsuited for this purpose unless the position of the double bond is fixed by derivatization. Oxidation of the double bond to either an ozonide or cis-diol, or formation of a methoxy or epoxide derivative, can be carried out on micrograms to nanograms of sample [713-716]. Single peaks can be trapped in a cooled section of a capillary tube and derivatized within the trap for reinjection. Ozonolysis is simple to carry out and occurs sufficiently rapidly that reaction temperatures of -70 C are common [436,705,707,713-717]. Several micro-ozonolysis. apparatuses are commercially available or can be readily assembled in the laboratory using standard equipment and a Tesla coil (vacuum tester) to generate the ozone. Reaction yields of ozonolysis products are typically 70 to 95t, although structures such as... 

Levsen, K. Weber, R. Borchers, F. Heimbach, H. Beckey, H.D. Determination of Double Bonds in Alkenes by Field Ionization Mass Spectrometry. Anal. Chem. 1978,50, 1655-1658. 

Isolated double bonds in alkenes and cycloalkenes are best reduced by catalytic hydrogenation. Addition of hydrogen is very easy and takes place at room temperature and atmospheric pressure over almost any noble metal catalyst, over Raney nickel, and over nickel catalysts prepared in special ways such as P-1 nickel [13] or complex catalysts, referred to as Nic [49]. 

In addition to electron-transfer reactions such as (39)—(42), N03 can also react in solution with a variety of organics. For example, addition to the double bond in alkenes is quite fast, with rate constants of the order of 109 L mol-1 s-1 at room temperature the reactions... 

Even stronger polarizations of double bonds in alkenes are induced by electron withdrawing substituents, as present in enol ethers, enones, and enamines (Sections 4.6.2, 4.7, and 4.9.2). Deshielding of C-7 in norbornadiene (75.5 ppm, Table 4.12) is understood as arising from interaction of antibonding n orbitals at the olefinic carbon atoms with o orbitals of the bridgehead bonds [214, 216]. Spiroconjugation in spiro[4.4]nonatetraene is interpreted similarly [242]. 

The overall ozonization reaction sequence provides an excellent means for locating the positions of double bonds in alkenes. The potentialities of the method may be illustrated by the difference in reaction products from the 1-and 2-butenes ... 

To describe carbon-carbon double bonds in alkenes, we use the pattern provided by ethene, CH2=CH2. We know from experimental data that all six atoms in ethene lie in the same plane, with H—C—H and C—C—H bond angles of 120°. This angle suggests a trigonal planar electron arrangement and sp2 hybridization for each C atom (46). 

As shown by the two simple compounds in Figure 1.11, the two carbon atoms connected by a double bond in alkenes cannot rotate relative to each other. For this reason, another kind of isomerism, called cis-trans, is possible for alkenes. Cis-trans isomers have different parts of the molecule oriented differently in space, although these parts occur in the same order. Both alkenes illustrated in Figure 1.11 have a molecular formula of C4H8. In the case of m-2-butcnc, the two CH3 (methyl) groups attached to the C=C carbon atoms are on the same side of the molecule, whereas in trans-2-butene they are on opposite sides. 

Alkenes can be dihydroxylated cis-selectively by reaction with a stoichiometric amount of yV-mcthylmorpholinc-yV-oxidc (NMO for a preparation, see Figure 14.30), a catalytic amount of a suitable Os(VIII) reagent, and in the presence of water. This reaction, the cis-vic dihydroxylation, was not discussed in the section on cis-selective additions to C=C double bonds in alkenes (Section 3.3), and it also was not discussed in the context of cycloadditions (Chapter 12). From a preparative point of view, this reaction is closely related to oxidative cleavages, and for this reason it is introduced now (Figure 14.16). 

One of the most common uses of ozonolysis has been for determining the positions of double bonds in alkenes. For example, if we were uncertain of the position of the methyl group in a methylcyclopentene, the products of ozonolysis-reduction would confirm the structure of the original alkene. 

The double bond in alkenes has important consequences for their chemical properties. Alkenes are much more reactive than alkanes. For example, alkenes react with halogens in the absence of light, but alkanes do not. 

Mk. You have seen that isomers result from rearranging carbon atoms and double bonds in alkenes. Another type of isomer results from the presence of a double bond. It is called a cis-trans isomer (or geometric isomer). Cis-trans isomers occur when different groups of atoms are arranged around the double bond. Unlike the single carbon-carbon bond, which can rotate, the double carbon-carbon bond remains fixed. 

An important reaction of HSiCl3 and HGeCl3 is their addition across double bonds in alkenes. This reaction is known as the Speier reaction, and it leads to alkyl trichloro derivatives ... 

Unsymmetrical diorganyltellurium dihalides are formed upon condensation of aryltellurium trichlorides with activated aromatic compounds and with ketones. The addition of the trichloride across carbon-carbon double bonds in alkenes, as well as the reaction with aryl(trimethyl)silane, hexaphenyldilead, and aryhnercury chlorides leads to the transfer of the aryl group to the tellurium atom. 

The most important functional groups that participate in chain-growth polymerizations are the carbon-carbon double bond in alkenes and the carbon-oxygen double bond in aldehydes and ketones. In such polymerizations the active species A adds to one atom of the double bond and produces a new active species on the other atom ... 

The areas in which work on alkyne photochemistry has been most prolific are those involving cycloaddition to carbon-carbon double bonds in alkenes, aromatics and related compounds. The simplest type of reaction involves formation of a cyclobutene from an alkene and an alkyne (equation 41) . The cyclobutene product may itself be photolabile, and if radiation is used which is absorbed more strongly by the cyclobutene, the product isolated may be the 1,3-diene derived from it by electrocyclic ring opening (equation 42) . 

The formal double bond in carbene complexes may be compared with the double bond in alkenes. In the case of a carbene complex, the metal must use a d orbital (rather than a p orbital) to form the tt bond with carbon, as illustrated in Figure 13-40. 

Metabolic epoxidation of carbon-carbon double bonds in alkenes and arenes is a fundamentally important biotransformation of foreign compounds. The primary epoxide (oxirane) metabolites formed generally undergo further biotransformation to more polar and readily excreted metabolites via conjugation with glutathione or epoxide-hydrolase-mediated hydrolysis to diols. Thus, epoxidation can be considered the first step in a metabolic detoxification scheme. On the other hand, epoxidation is also a... 

The carbon-carbon double bond in alkenes is shorter than the carbon-carbon single bond in alkanes because four electrons bind more tightly than two. But in... 

Isomerization. With Pd catalysts, and, to a lesser extent, with Pt catalysts, a mixture of isomeric products may be obtained due to positional isomerization of double bonds during hydrogenation. As illustrated below, 5yn-addition of H2 to either face of the double bond in alkene A furnishes cw-decalin C. However, 5yn-addition of Hj to the isornerized alkene B can produce the cw-decalin C and/or trans-decalin D, depending on which face of the double bond undergoes addition by H2. In fact, hydrogenation of A in the presence of Pt furnishes 80% of the thermodynamically more stable trans-decalin and only 20% of cw-decalin. 

Jennings [59a] has shown that other mixtures, such as Nj (or CH ) +N2O to form 0 , may be used. These ions abstract H or according to the position of the double bond in alkene compounds. With this reagent it is possible to demonstrate if the a carbon(s) of a nitrile or ketone function is (are) secondary (or not). In this case, an intense [M-Hj]" ion appears. 

Addition of dialkyl aluminum hydrides (dialkyl alanes) to C=C double bonds in alkenes (hydroalumination) leads to trialkyl alanes (291, 298). For the preparation of higher trialkyl alanes it is proposed to add diethyl-alane to higher 1-alkenes and from the resulting ethyl-alkyl alane mixtures (analogous to the organoboron compounds) (142) to remove the ethyl groups as triethylalane by distillation (241) ... 

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