tructure* is the key to everything in chemistry. The properties of a substance depend on the atoms it contains and the way the atoms are connected. What is less 0 obvious, but very powerful, is the idea that someone who is trained in chemistry can look at a structural formula of a substance and tell you a lot about its properties

Ikenes are hydrocarbons that contain a carbon-carbon double bond. A car- bon-carbon double bond is both an important structural unit and an important functional group in organic chemistry. The shape of an organic molecule is influ- enced by the presence of this bond, and the double bond is the site of most of the chem- ical reactions that alkenes undergo. Some representative alkenes include isobutylene(an industrial chemical), a-pinene (a fragrant liquid obtained from pine trees), and farnesene (a naturally occurring alkene with three double bonds

N ow that we're familiar with the structure and preparation of alkenes, let's look at their chemical reactions. The characteristic reaction of alkenes is addition to the double bond according to the general equation:

he Greek word stereos means\solid, \and stereochemistry refers to chemistry in three dimensions. The foundations of organic stereochemistry were laid by Jacobus van't Hoff* and Joseph Achille Le Bel in 1874. Independently of each other, van't Hoff and Le Bel proposed that the four bonds to carbon were directed toward the cor- ners of a tetrahedron. One consequence of a tetrahedral arrangement of bonds to carbon

W hen we discussed elimination reactions in Chapter 5, we learned that Lewis base can react with an alkyl halide to form an alkene. In the present chapter, you will find that the same kinds of reactants can also undergo a different reaction, one in which the Lewis base acts as a nucleophile to substitute for the halide substituent on carbon

H ydrocarbons that contain carbon-carbon triple bond are called alkynes. Non- cyclic alkynes have the molecular formula C, H2n-2. Acetylene (HCCH) is th simplest alkyne. We call compounds that have their triple bond at the end of a carbon chain(RC=CH) monosubstituted, or terminal, alkynes. Disubstituted alkynes (RCCR') are said to have internal triple bonds. You will see in this chapter that a car- bon-carbon triple bond is a functional group, reacting with many of the same reagents

ot all the properties of alkenes are revealed by focusing exclusively on the func- tional group behavior of the double bond. A double bond can affect the proper- ties of a second functional unit to which it is directly attached. It can be a sub- stituent, for example, on a positively charged carbon in an allylic carbocation, or on a carbon that bears an unpaired electron in an allylic free radical, or it can

this chapter and the next we extend our coverage of conjugated systems to include arenes. Arenes are hydrocarbons based on the benzene ring as a structural unit. Ben- are zene, toluene, and naphthalene, for example, are arenes

the preceding chapter the special stability of benzene was described, along with reac- tions in which an aromatic ring was present as a substituent. In the present chapter we move from considering the aromatic ring as a substituent to studying it as a functional

ntil the second half of the twentieth century, the structure of a substance-a newly discovered natural product, for example-was determined using information obtained from chemical reactions. This information included the identification of functional groups by chemical tests, along with the results of experiments in which the substance was broken down into smaller, more readily identifiable fragments. Typical of this approach is the demonstration of the presence of a double bond in an alkene by cat- nd subsequent