Alkenes chemical reactivity

The chemical properties of alkenes are very different from those of alkanes because of the double bond (— C = C —) in the structure. Double bond contains a sigma bond and a pi bond. Since electrons in pi bonds are bonded less strongly than in sigma bonds. This makes alkenes chemically reactive combustion, substitution, oxidation and polymerization reactions are all undergone by alkenes. 

Isopentenyl pyrophosphate and dimethylallyl pyrophosphate are structurally sim liar—both contain a double bond and a pyrophosphate ester unit—but the chemical reactivity expressed by each is different The principal site of reaction m dimethylallyl pyrophosphate is the carbon that bears the pyrophosphate group Pyrophosphate is a reasonably good leaving group m nucleophilic substitution reactions especially when as in dimethylallyl pyrophosphate it is located at an allylic carbon Isopentenyl pyrophosphate on the other hand does not have its leaving group attached to an allylic carbon and is far less reactive than dimethylallyl pyrophosphate toward nucleophilic reagents The principal site of reaction m isopentenyl pyrophosphate is the carbon-carbon double bond which like the double bonds of simple alkenes is reactive toward electrophiles... 

The chemical reactivity of these two substituted ethylenes is in agreement with the ideas encompassed by both the MO and resonance descriptions. Enamines, as amino-substituted alkenes are called, are vety reactive toward electrophilic species, and it is the p carbon that is the site of attack. For example, enamines are protonated on the carbon. Acrolein is an electrophilic alkene, as predicted, and the nucleophile attacks the P carbon. 

Cyclopentane has the low chemical reactivity which is typical of saturated hydrocarbons, while 2-pentene is much more reactive. Similarly, ring structures containing double bonds, called cyclo-alkenes, can be shown to be isomeric with alkynes. 

As observed with alkenes, bromine addition to sterically hindered dienes shows a peculiar behavior. Highly substituted dienes, existing predominantly in non-planar conformations, often present a chemical reactivity distinctly different from that of planar... 

This chapter begins, thus, with a short introduction to the chemical reactivity of epoxides. We continue with a description of the epoxides hydrolases and their biochemistry, and devote most of its length to a systematic discussion of the substrates hydrated by these enzymes. Arene oxides and diol epoxides will be presented first, followed by a large variety of alkene and cy-cloalkene oxides. 

As was the case for alkenes, alkyl substituents lower the enthalpy of the unsaturated molecule. Hence, ketones, with two R s, have lower enthalpies than aldehydes, with one R. The electron-releasing R s diminish the electrophilicity of the carbonyl C, lessening the chemical reactivity of ketones. Furthermore, the R s, especially large bulky ones, make approach of reactants to the C more difficult. 

Overall, fullerenes and especially Ceo show a chemical reactivity very similar to that of bulky electron-deficient alkenes. They readily react with many electron-rich metal centers to form stable or a complexes. With either bulky or less electron-rich centers, they show a reduced reactivity and form much less stable complexes. 

Alkene complexes can be prepared with platinum in a divalent or a zerovalent oxidation state. The electron density at the platinum center exerts significant changes in bonding between the alkene and platinum. These effects exhibit themselves in both structural features and chemical reactivities. 

The 1,2 bonds in both napthalene and anthracene are in fact shorter than the other ring bonds, whereas the 9,10 bond in phenanthrene closely resembles an alkene double bond in both its length and chemical reactivity. 

It is hardly surprising that different chemical reactivity might be expected from the exciplex and the radical ion pair formed by complete electron exchange. Lewis observation (50) that in the excited state interaction of trans-stilbene with either electron-rich or electron-poor alkenes cycloaddition is more efficient from the relatively less polar exciplex than from radical ion pairs is typical for many such cycloadditions. 

Benzene is a flat, symmetrical molecule that is often represented as a six-membered ring with three double bonds. The problem with this representation, however, is that it gives the wrong impression about benzene s chemical reactivity. Because benzene appears to have three double bonds, we might expect it to react with H2,Br2, and FFO to give the same kinds of addition products that alkenes do. In fact, though, benzene and other aromatic compounds are much less reactive than alkenes and don t normally undergo addition reactions. 

The double bond makes alkenes more reactive than alkanes in chemical reactions. For example, hydrogen adds across the double bond of ethene, under suitable conditions, forming ethane (Figure 14.12). 

Discuss the chemical reactivity of alkenes. Why are they chemically reactive or unreactive ... 

To compare the chemical reactivity of an alkane, an alkene, and an aromatic compound. 

Since alkylidene- and alkylidynephosphanes show an alkene- or alkyne-like chemical reactivity, they are often called phosphaalkenes and phosphaalkynes. 

The two most fundamental properties affecting the chemical reactivity of cyclopropane and its derivatives are the subject of this chapter. The only previous review treating both topics, acidity and basicity of cyclopropanes, appears to be that of Charton in an earlier volume of this series. The Chemistry of Alkenes, Volume 2. In effect, the present review may be considered a detailed critical update of developments in these two related areas. Literature coverage is not intended to be complete and apologies are extended to those authors whose work of topical importance may not have been included. While there is some variability between sections, the literature surveyed is inclusive to June, 1985. 

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