Structure and Bonding in Alkenes

Ethylene was known to chemists in the eighteenth century and isolated in pure form in 1795. An early name for ethylene was gaz olefiant (French for oil-forming gas ), to describe the fact that an oily liquid product is formed when two gases— ethylene and chlorine—react with each other. 

The term gaz olefiant was the forerunner of the general term olefin, formerly used as the name of the class of compounds we now call alkenes. 

Ethylene occurs naturally in small amounts as a plant hormone. It is formed in a complex series of steps from a compound containing a cyclopropane ring  

Ethylene is the cornerstone of the world s mammoth petrochemical industry and is produced in vast quantities. In a typical 

This dehydrogenation is simultaneously both a source of ethylene and one of the methods by which hydrogen is prepared on an industrial scale. Most of this hydrogen is subsequently used to reduce nitrogen to ammonia for the preparation of fertilizer. 

FIGURE 5.1 (a) The framework of j bonds in ethylene showing bond distances in picometers and bond angles in degrees. All six atoms are coplanar. The carbon-carbon bond is a double bond made up of the T component shown and the IT component illustrated in b. (6) The p orbitals of two sp hybridized carbons overlap to produce a tt bond. An electron pair in the tt bond is shared by the two carbons. 

The double bond in ethylene is stronger than the C—C single bond in ethane, but it is not twice as strong. The C=C bond energy is 605 kJ/mol (144.5 kcal/mol) in ethylene versus 368 kJ/mol (88 kcal/mol) for the C—C bond in ethane. Chemists do not agree on exactly how to apportion the total C=C bond energy between its a and tt components, but all agree that the tt bond is weaker than the a bond. 

There are two different types of carbon-carbon bonds in propene, CH3CH=CH2. The double bond is of the cr + tt type, and the bond to the methyl group is a ct bond formed by sp -sp overlap. 

The simplest arithmetic approach subtracts the C—C cr bond energy of ethane (368 kJ/mol 88 kcal/mol) from the C=C bond energy of ethylene (605 kJ/mol 144.5 kcal/mol). This gives a value of 237 kJ/mol (56.5 kcal/mol) for the TT bond energy. 

PROBLEM 5.3 We can use bond-line formulas to represent alkenes in much the same way that we use them to represent alkanes. Consider the following alkene  

No locants are needed in the absence of substituents it is understood that the double bond connects C-1 and C-2. Substituted cycloalkenes are numbered beginning with the double bond, proceeding through it, and continuing in sequence around the ring. The direction is chosen so as to give the lower of two possible numbers to the substituent. 

Write structural formulas and give the lUPAC names of all the monochloro-substituted derivatives of cyclopentene. 

On the basis of their bond-dissociation enthalpies, the C=C bond in ethylene is stronger than the C—C single bond in ethane, but it is not twice as strong. 

While it is not possible to apportion the C=C bond energy of ethylene between its a and rr components, the data suggest that the tt bond is weaker than the a bond. 

What assumptions would you have to make in order to calculate the Tr-bond strength in ethylene from the bond-dissociation data  

SAMPLE SOLUTION (a) Recall when writing bond-line formulas for hydrocarbons that a carbon occurs at each end and at each bend in a carbon chain. The appropriate number of hydrogens are attached so that each carbon has four bonds. Thus the compound shown is 

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The structures and bonding in substituted alkenes and alkynes can be described through the combined molecular orbital (MO) and VB picture presented in Section 6.5, which uses localized VB bonds to describe the molecular framework and delocalized MOs to describe the tt electrons. Let s apply this method to 2-butene (CH3CHCHCH3) and gain deeper understanding of the isomers discussed earlier. The Lewis diagram for 2-butene is... 

The structures and bonding in substituted alkenes and alkynes can be described by the combined MO-VB picture presented in Section 6.5, in which localized VB bonds describe the molecular framework and delocalized MOs describe the 77 electrons. 

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... 

Other closely related ruthenium-allenylidene were made and evaluated in alkene metathesis [32]. Werner et al. [49] also produced allenylidene complexes of analogous structure to that of the Grubbs catalyst, but containing hemilabile phosphine such as complex X (Scheme 8.9). However, the Ru—O bond may be too stable to initiate the rearrangement into indenylidene, the coordination of alkene and to become a catalyst. 

Cis-trans isomerism (Often called geometric isomerism although this term refers to all stereoisomers) is a form of stereoisomerism and describes the orientation of functional groups at the ends of a bond around which no rotation is possible. Both alkenes and cycloalkanes have restricted rotation around certain bonds. In alkenes, the double bond restricts movement and rotation, as does the looped structure of cycloalkanes. 

The =C-H bonds in alkenes also do not differ by more than 0.004 A in the QC results and by only 0.002 A in the r structure of 1-hutene [34], but the ro structure by Bouchy et al. [35] once again shows results inconsistent with those having =C-H bond lengths varying from 1.081 to 1.106 A. The rather short ro value for =C-H bond length of cis 2-butene (1.0798 A) is also not given in Table 12 and Fig. 12. 

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