Alkenes characteristic chemical properties

Since alkenes evidently contain less than the maximum quantity of hydrogen, they arc referred to as unsaturated hydrocarbons. This unsaturation can be satisiied by reagents other than hydrogen and gives rise to the characteristic chemical properties of alkenes. 

The characteristic chemical property of a C=C structural unit is susceptibility to attack by electrophiles. Electrons flow from the tt component of the double bond toward the electrophile and ultimately become a shared-electron pair in a covalent bond. WeTl see numerous other examples of electrophilic addition to alkenes in this chapter. First, however, we need to extend our discussion of hydrogen halide addition to alkenes of various types. 

Addition to the double bond, of which catal5dic hydrogenation is but one example, is the most characteristic chemical property of alkenes. In many of these reactions the attacking reagent is a polar molecule such as a hydrogen halide. Addition occurs rapidly in a variety of solvents, including pentane, benzene, chchloromethane, chloroform, and acetic acid. 

A functional gronp is a reactive group of bonded atoms that appears in all the members of a chemical family. Each functional group reacts in a characteristic way. Thus, functional groups help to determine the physical and chemical properties of compounds. For example, the reactive double bond is the functional group for an alkene. In this course, you will encounter many different functional groups. 

Hydrocarbons — Organic chemical compounds composed only of the elements carbon and hydrogen. Hydrocarbons are the principal constituents of crude oils, natural gas, and refined petroleum products and include four major classes of compounds (alkanes, alkenes, naphthenes, and aromatics) each with characteristic structural arrangements of hydrogen and carbon atoms, as well as different physical and chemical properties. (See also Alkanes, Alkenes, Aromatics, Naphthenes, Olefins, Paraffin, Saturate group.)... 

Let us imagine ourselves in the mid-nineteenth century and examine the evidence on which chemists attempted to build a model for the structure of benzene. First, because the molecular formula of benzene is CeHe, it seemed clear that the molecule must be highly unsaturated. Yet benzene does not show the chemical properties of alkenes, the only unsaturated hydrocarbons known at that time. Benzene does undergo chemical reactions, but its characteristic reaction is substitution rather than addition. When benzene is treated with bromine in the presence of ferric chloride as a catalyst, for example, only one compound with the molecular formula CeHsBr forms ... 

All alkenes have very similar chemical and physical properties, primarily determined by the carbon-carbon double bond. When a small section of an organic molecule is largely responsible for the molecule s chemical and physical characteristics, that section is called a functional group. 

---