Alkenes structural isomers

The first three members of the olefin series are ethylene, propylene, and butylene (or butene). Structural isomers exist when n > 4, as a consequence of the positioning of the double bond in normal alkenes as a result of branching in branched alkenes. In addition, geometric isomers may be possible owing to restricted rotation of atoms about the C=C bond. For instance, C H (butene) has four possible isomers instead of the expected three ... 

As we saw earlier, there are three structural isomers of the alkene C4H8. You may be surprised to learn that there are actually/owr different alkenes with this molecular formula. The extra compound arises because of a phenomenon called geometric isomerism. There are two different geometric isomers of the structure shown on the left, on page 597, under (1). 

Geometric, or cis-trans, isomerism is common among alkenes. It occurs when both of the double-bonded carbon atoms are joined to two different atoms or groups. The other two structural isomers of C4H8 shown under (1) on page 597 do not show cis-trans isomerism. In both cases the carbon atom at the left is joined to two identical hydrogen atoms. 

Synthesis of Four-membered Cyclic Nitronates Conjugated nitro alkenes (42) (Scheme 3.47) can be considered as structural isomers of... 

Both the 1 1 and the 1 2 bonded alkenes should show structural isomers, with, for the ethylene adduct, nonequivalence of the two protons bonded to the carbon. For substituted olefins, it is often possible to detect the presence of both isomeric forms, especially with the osmium derivatives. However, all these molecules are fluxional and the flux-... 

Triple bonds in side chains of aromatics can be reduced to double bonds or completely saturated. The outcome of such reductions depends on the structure of the acetylene and on the method of reduction. If the triple bond is not conjugated with the benzene ring it can be handled in the same way as in aliphatic acetylenes. In addition, electrochemical reduction in a solution of lithium chloride in methylamine has been used for partial reduction to alkenes trans isomers, where applicable) in 40-51% yields (with 2,5-dihydroaromatic alkenes as by-products) [379]. Aromatic acetylenes with triple bonds conjugated with benzene rings can be hydrogenated over Raney nickel to cis olefins [356], or to alkyl aromatics over rhenium sulfide catalyst [54]. Electroreduction in methylamine containing lithium chloride gives 80% yields of alkyl aromatics [379]. 

Alkynes, unlike alkenes, are not hydrated readily in aqueous acid unless a mercuric salt is present as a catalyst. Also, the products that are isolated are either aldehydes or ketones instead of alcohols. Even though the addition of one molecule of water to ethyne probably gives ethenol (vinyl alcohol) initially, this compound is unstable relative to its structural isomer (ethanal) and rapidly rearranges ... 

Alkadienes also have structural isomers as well as ordinary alkenes (Table 4). 

Under proper conditions, isobutylene is converted by sulfuric or phosphoric acid into a mixture of two alkenes of molecular formula CgHig. Hydrogenation of either of these alkenes produces the same alkane, 2,2,4-trimethylpentane (Sec. 3.30). The two alkenes are isomers, then, and differ only in position of the double bond. Problem Could they, instead, be cis-trans isomers ) When studied by the methods discussed at the end of this chapter (Sec. 6.29), these two alkenes are found to have the structures shown ... 

The carbenoid reaction between a-diazo ketones and simple alkenes or styrenes leads to acylcyclopropanes. (For the enantioselective cyclopropanation of styrene with 2-diazo-5,5-dimethylcyclohexane-l,3-dione, see Section 1.2.1.2.4.2.6.3.2.). With ketene acetals, 2,3-dihyd-rofurans are obtained. In contrast, l-acyl-2-oxycyclopropanes or 2-oxy-2,3-dihydrofurans can be formed in reactions with enol ethers and enol acetates the result depends strongly on the substitution pattern of both reaction partners.Whereas simple diazo ketones usually lead to cyclopropanes (Table 15), 3-diazo-2-oxopropanoates and 2-diazo-l,3-dicarbonyl compounds, such as 2-diazoacetoacetates, 3-diazopentane-2,4-dione, and 2-diazo-5,5-dimethylcy-clohexane-1,3-dione, yield 2,3-dihydrofurans and occasionally acyclic structural isomers thereof when reacted with these electron-rich oxy-substituted alkenes. 

It is apparent that release of strain and steric hindrance to attack of the proton are negligible for these isomers. Similar results have been obtained in the protonation of 1-methylcyclopentene (274) and 1-methylcyclobutene (275), where the small rate difference leads to the conclusion that little change in alkene structure at the transition state has occurred (237). 

Alkenes and cycloalkanes are structural isomers of each other. Give an example of each using C4EI8. Another common feature of alkenes and cycloalkanes is... 

Problem 12.10. Are cis and trans isomers possible for all alkene structures ... 

Ans. No. Cis and trans isomers are possible for an alkene structure only when each carbon of the double bond is bonded to two different substituents. Cis and trans isomers are not possible when either doubly bonded carbon is bonded to two of the same substituents. Alkenes of the following types cannot exist as cis and trans isomers... 

Alkenes are unsaturated hydrocarbons that contain at least one C=C bond. The simplest alkene is CH2=CH2, called ethene (lUPAC) or ethylene, which plays important roles as a plant hormone in seed germination and fruit ripening. The next member of the series is CH3 — CH = CH2, called propene or propylene. Alkenes with four or more carbon atoms have several isomers. For example, the alkene C4H8 has the four structural isomers shown in A FIGURE 24.7. Notice both their structures and their names. 

The number of structural isomers increases rapidly as the number of carbons increases because, besides variations in chain length and branching, variations occur in the position of the donble bond. lUPAC nomenclature is extremely useful in differentiating among these many alkene compounds. 

Explain the difference between geometric and structural isomers of alkenes. 

In the alkene series, the possibility of locating the double bond between two different carbon atoms creates additional structural isomers. Ethene and propene have only one possible location for the double bond. However, the next alkene in the series, butene, has two possible locations for the double bond. 

Since 1971 Maxwell s group at Bristol used these methods to separate and identify the stereochemistry of isoprenoidic acyclic alkanes . Whereas most acyclic isoprenoids in nature appear functionalized (alcohols, acids, alkenes, etc.), they are found in the geoenvironment as fossil alkanes. Hence it is important not only to analyse the structural isomers, but also to determine the stereochemistry. 

A Figure 24.7 The alkene C4H8 has four structural Isomers. 

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