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Also, alkyl groups are sterically large, and are most stable when they are far away from each other. In an alkane, the maximum separation is that of the tetrahedral bond angle, 109.5°. In an alkene, the bond angle increases to near 120°. As a result, the separation between alkyl groups is greatest in the most substituted alkene. [7]
Alkenes are relatively stable compounds, but are more reactive than alkanes. Most reactions of alkenes involve additions to this pi bond, forming new single bonds . Alkenes serve as a feedstock for the petrochemical industry because they can participate in a wide variety of reactions, prominently polymerization and alkylation.
The most stable trans-isomers of 10 ring or greater cycloalkenes exhibit 4 irregularities from standard geometric norms. The first irregularity is twisted planes of substituents along the C=C. Using C=C as the stable axis, 2 substituents of 1 carbon can be visualized on the same plane, equally applied to the other carbon.
The least stable alkene (the one with the fewest substituents on the carbons of the double bond), called the Hofmann product, is formed. This tendency, known as the Hofmann alkene synthesis rule , is in contrast to usual elimination reactions, where Zaitsev's rule predicts the formation of the most stable alkene.
The chemical basis for Markovnikov's Rule is the formation of the most stable carbocation during the addition process. Adding the hydrogen ion to one carbon atom in the alkene creates a positive charge on the other carbon, forming a carbocation intermediate.
Cyclohexene is most stable in a half-chair conformation, [11] unlike the preference for a chair form of cyclohexane. One basis for the cyclohexane conformational preference for a chair is that it allows each bond of the ring to adopt a staggered conformation. For cyclohexene, however, the alkene is planar, equivalent to an eclipsed conformation ...
The cis isomer can adopt various conformations, the most stable one being shaped like a ribbon. [1] The most stable conformation of trans-cyclooctene is shaped like the 8-carbon equivalent of the chair conformation of cyclohexane. Longer cycloalkene rings such as the ten-carbon cyclodecene also occur as cis and trans isomers.
Computations indicate that the most stable "crown" conformation has the carbon atoms alternately above and below the plane of the ring. [5] A "half-chair" conformation, with about 6 kcal/mol higher energy, has carbons 2,3,5,6, and 8 on the same side of the plane of carbons 1,4, and 7.