Alkenes double bond position

The trends in alkene isomer stability can be easily seen in plots of the enthalpies of formation versus the number of carbons, nc (equation 4). For example, in Figure 3 which includes the gaseous 1-, cis- and trans-2-, and the cis- and trans-2-n-alkenes, the relatively more negative enthalpies of formation for the 2- and 3 -n-alkenes show them to be more stable than their 1-n-alkene double bond positional isomers. (Only the C4-C8 1-n-alkene members are shown for clarity.) Another well-known relationship is depicted as well acyclic trans hydrocarbons are more stable than their cis isomers. 

The reaction is illustrated by the intramolecular cycloaddition of the nitrilimine (374) with the alkenic double bond separated from the dipole by three methylene units. The nitrilimine (374) was generated photochemically from the corresponding tetrazole (373) and the pyrrolidino[l,2-6]pyrazoline (375) was obtained in high yield 82JOC4256). Applications of a variety of these reactions will be found in Chapter 4.36. Other aspects of intramolecular 1,3-dipolar cycloadditions leading to complex, fused systems, especially when the 1,3-dipole and the dipolarophile are substituted into a benzene ring in the ortho positions, have been described (76AG(E)123). 

The benzylic position of an alkylbcnzene can be brominated by reaction with jV-bromosuccinimide, and the entire side chain can be degraded to a carboxyl group by oxidation with aqueous KMnCfy Although aromatic rings are less reactive than isolated alkene double bonds, they can be reduced to cyclohexanes by hydrogenation over a platinum or rhodium catalyst. In addition, aryl alkyl ketones are reduced to alkylbenzenes by hydrogenation over a platinum catalyst. 

Initial bond formation occurs between the ketene carbonyl and the more nucleophilic end of the alkene double bond. This is related to the charge separation in the TS and results in the second bond being formed between the terminal ketene carbon and the carbon that is best able to support positive character.174... 

Cyclic olefins and diolefins form much more aerosol than 1-alkenes that have the same number of carbon atoms (for example, cyclohexene 1-hexene, and 1,7-octadiene 1-octene). The same effect of chain length and double-bond position is observed for diolefins (1,7-octadiene > 1,6-heptadiene > 1,5-hexadiene, and 1,7-octadiene 2,6-octadiene). Heavier unsaturated cyclic compounds, such as indene and terpenes, form even more aerosol. 

A cyclic alkene is named by a prefix cyclo- to the name of the acyclic alkene. Double bonded carbons are considered to occupy positions 1 and 2. 

In addition to the alkene isomers that exist because of double-bond position, alkene isomers can also exist because of double-bond geometry. For instance, there are two geometrical, or cis-trans isomers, of 2-butene, which differ in their geometry about the double bond. The cis isomer has its two -CH3 groups on the same side of the double bond, and the trans isomer has its two -CH3 groups on opposite sides. Like other kinds of isomers we ve discussed, the individual cis and trans isomers of an alkene are different substances with different physical properties and different (although often similar) chemical behavior. cfs-2-Butene boils at 4°C, for example, but trans-2-butene boils at 0.9°C. 

Coulombic forces will determine the regioselectivity of the ortho addition [189], In the charge-transfer complexes of monosubstituted benzenes with alkenes, the charge (positive or negative) on the arene is largely located at the carbon atoms ipso and (to a lesser extent) para to the substituent. The carbon atoms of the alkene double bond will preferentially be located in the neighborhood of either the ipso carbon or (to a lesser extent) the para carbon atom of the monosubstituted benzene. This would explain the 1,2 and 3,4 selectivity in the ortho photocycloaddition. 

The second, even more convenient, method to determine double-bond positions in unsaturated hydrocarbons is the use of dimethyldisulfide derivatives (Francis and Veland, 1981 Carlson et al, 1989 Howard, 1993). In this approach, alkenes are dissolved in a carbon disulfide and iodine solution and kept overnight. The reaction yields a derivative with methyl sulfide substituents on each of the carbons that comprised the double bond. If the double bonds are separated by four or more methylene groups, the reaction proceeds... 

The double bond positional isomerization of alkenes is catalyzed by many transition metals and often occurs as an undesired side reaction. Equations 4 show a simplified scheme of the alkyl-hydride isomerization mechanism in which all the catalytic intermediates have the metal atom in the same oxidation state. 

Another class of mthenium aUcene complexes contains those derived from the hexaaqua ion [Ru(H20)6] +. The thermodynamically stable complex [(cod)Ru(H20)4] + (74) forms directly from [Ru(H20)6] + and cod in alcohol at ambient temperature (equation 14). In (74), the redox potential of Ru has shifted more positive for the oxidation to Ru and more negative for the reduction to Ru or Ru°, so as to impose a high stability towards disproportionation see Disproportionation) (in contrast to the readily disproportionating aqua ion [Ru(H20)6] +). The X-ray crystal structure see X-ray Crystallography) of the Tosylate (Ots) salt disclosed quite different R-OH2 distances of 2.095(2) and 2.156(2) A for water gauche or trans to the alkene double bond, showing the structural trans effect see Trans Effect) of the latter on a a- (and tv-) donor ligand trans... 

The electron-rich alkene double bond repels the electrons in the bromine molecule to create a partial positive charge on the bromine atom near the double bond. An intermediate bromonium ion is formed, which reacts to give the /ram-dibromide derived from anti- addition (i.e. the two Br groups add to the alkene from the opposite sides). 

Ethenolysis can also be applied to difficult analytical problems it provides a simple and accurate method for locating double bonds and determining isomer purity of internal (poly)olefins with or without functional groups, via analysis of the terminal alkenes produced. Direct chromatographic separation of positional isomers is often impossible, while other methods for determining the double-bond position, such as ozonization followed by subsequent reduction, are far more complicated. 

Although typical radicals are neutral, they are electron deficient in the sense that they are one electron short of having a full octet. Reaction between the it electrons of an alkene double bond can take place, completing the octet around the radical atom. Unlike additions of positively charged electrophiles, however, the result of this addition is not a carbocation but another radical. Like the processes we studied in Chapter 3. these reactions follow radical chain mechanisms. 

Hydroboration occurs by a concerted process and takes place through a four-membered cyclic transition state, formed by addition of a polarized B—H bond (boron is the more positive) to the alkene double bond (5.2). This is supported by the fact that the reaction is stereospecific, with syn addition of the boron and hydrogen atoms. The reaction can also be stereoselective, with hydroboration taking place preferentially on the less hindered side of the double bond. Stereospecific addition of borane to a 1-alkylcycloalkene such as 1-methylcyclohexene, gives, after oxidation of the organoborane product (see Scheme 5.21), almost exclusively the trans alcohol product (5.3). 

From the third member (butene ) of the alkene family onward, there is a need to specify the location of the double bond to recognize the correct structure denoted by the name. When naming alkenes, the numbering should begin from the carbon chain end which gives the double bond position the lowest number. 

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