Reacciones de alquenos

4

Reactions of Alkenes

W

e have seen that an alkene such as 2-butene undergoes an electrophilic addition reaction with HBr (Section 3.6). The first step of the reaction is a relatively slow addition of the electrophilic proton to the nucleophilic alkene to form a carbocation intermediate. In the second step, the positively charged carbocation intermediate (an electrophile) reacts rapidlywith the negatively charged bromide ion (a nucleophile).
C C + H Br
slow

A cyclic bromonium ion

C
+

C H

+

Br

−

fast

C

C

Br H

a carbocation intermediate

In this chapter, we will look at a wide variety of alkene reactions. You will see that some of the reactions form carbocation intermediates like the one formed when HBr reacts with an alkene, some form otherkinds of intermediates, and some don’t form an intermediate at all. At first, the reactions covered in this chapter might appear to be quite different, but you will see that they all occur by similar mechanisms. So as you study each reaction, notice the feature that all alkene reactions have in common: The relatively loosely held p electrons of the carbon–carbon double bond are attracted to anelectrophile. Thus, each reaction starts with the addition of an electrophile to one of the sp 2 carbons of the alkene and concludes with the addition of a nucleophile to the other sp 2 carbon. The end result is that the p bond breaks and the sp 2 carbons form new s bonds with the electrophile and the nucleophile.
C C + Y+ + Z− C C

Y Z
the double bond is composed of a σ bond and a π bondelectrophile the π bond has broken and new σ bonds have formed nucleophile

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CHAPTER 4

Reactions of Alkenes

This reactivity makes alkenes an important class of organic compounds because they can be used to synthesize a wide variety of other compounds. For example, alkyl halides, alcohols, ethers, and alkanes all can be synthesized from alkenes by electrophilic addition reactions.The particular product obtained depends only on the electrophile and the nucleophile used in the addition reaction.

4.1

Addition of Hydrogen Halides

If the electrophilic reagent that adds to an alkene is a hydrogen halide (HF, HCl, HBr, or HI), the product of the reaction will be an alkyl halide:
CH2 CH2 + HCl CH3CH2Cl
ethyl chloride

ethene

H3C
Synthetic Tutorial: Addition of HBrto an alkene.

CH3 C C CH3 + HBr

CH3 CH3 CH3CH CCH3 Br
2-bromo-2,3-dimethylbutane

H3C

2,3-dimethyl-2-butene

+
cyclohexene

HI I
iodocyclohexane

Because the alkenes in the preceding reactions have the same substituents on both of the sp 2 carbons, it is easy to determine the product of the reaction: The electrophile (H +) adds to one of the sp 2 carbons, and the nucleophile (X-) adds to the other sp 2 carbon. It doesn’t make any difference which sp 2 carbon the electrophile attaches to, because the same product will be obtained in either case. But what happens if the alkene does not have the same substituents on both of the sp 2 carbons? Which sp 2 carbon gets the hydrogen? For example, does the addition of HCl to 2-methylpropene produce tert-butyl chloride or isobutylchloride?
CH3 CH3C CH2 + HCl CH3 CH3CCH3 Cl
2-methylpropene tert-butyl chloride isobutyl chloride

CH3 or CH3CHCH2Cl

To answer this question, we need to look at the mechanism of the reaction. Recall that the first step of the reaction—the addition of H + to an sp 2 carbon to form either the tert-butyl cation or the isobutyl cation—is the rate-determining step (Section 3.7). If there is anydifference in the rate of formation of these two carbocations, the one that is formed faster will be the preferred product of the first step. Moreover, because carbocation formation is rate determining, the particular carbocation that is formed in the first step determines the final product of the reaction. That is, if the tert-butyl cation is formed, it will react rapidly with Cl- to form...