Bonding in alkenes

The next group of hydrocarbons that we will study is the alkenes (old name olefins), compounds containing one or more carbon-carbon double bonds. The first member of the class is ethene (3.1) in which there is a double bond between the two carbon atoms. The old name for this compound was ethylene, and this is still widely used in the polymer industry. There are two carbon-carbon bonds along the same direction—so clearly, we cannot use sp hybrid orbitals, which we used to make alkanes, for this molecule. 

In order to describe a carbon-carbon double bond, we need to return to the electronic configuration of carbon  

FIGURE 3.2 Sigma bond framework of ethene, showing orbitals. 

TABLE 3.1 Bond Strengths and Lengths in Hydrocarbons Bond Bond Strength (kJ mohM C-C Distance (A) Bonds 

How is this concept useful Let us say that we have isolated a hydrocarbon of molecular formula QHg. If this had been an acyclic alkane, we would have expected 2 + 2 hydrogens, in this case 14. So we are six hydrogens short. So we must have three rings or double bonds, for example, 3.3, 3.4, or 3.5. We can generalize this to say that if we have carbon atoms and m hydrogen atoms, then (DBE) = 2n + 2) - m /2. Halogens count as if they were hydrogens. 

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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. 

Section 5.2 Bonding in alkenes is described according to an sp orbital hybridization model. The double bond unites two 5/r -hybridized cabon atoms and is made of a a component and a tt component. The a bond arises by overlap of an sp hybrid orbital on each cabon. The tt bond is weaker than the a bond and results from a side-by-side overlap of p orbitals. 

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],... 

Features (2) and (3) are explicable in terms of the Dewar-Chatt-Duncanson model for bonding in alkene complexes (Figure 3.63), which involves... 

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... 

The carbene route to bridgehead olefins is a well established reaction and has been developed to a major method for the generation of bridgehead alkenes. The field has been recently reviewed by one of the main contributors to this area.1 The reversed reaction, the formation of carbenes from distorted olefins, has also been known for a long time. Distortion of the tt bond in alkenes is easily affected by photoexitation. In connection with those reactions, carbene chemistry has been observed and the field has also been reviewed some years ago.2... 

The separate resonance structures for the aromatic hydrocarbons show C=C bonds. These may appear the same as similar bonds in alkenes, but resonance negates any similarity in behavior. 

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. 

All alkenes contain a carbon-to-carbon double bond. We can use ethene as a typical example to explain the bonding in alkenes. On each carbon atom of the double bond, the 2s orbital mixes with two of the 2p orbitals to form three degenerate sp hybrid orbitals. The remaining 2p orbital is left unhybridised. 

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]. 

Fig. 8.1. Representation of n bonding in alkene-transition-metal complexes.

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... 

It is possible to add reagants across the Si—Si double bond in some ways analogous to the C=C bond in alkenes ... 

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). 

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