Electrophilic addition to unsymmetrical alkenes is regioselective

Here are two more syntheses of alkyl bromides, but this time we need td ask our question about which end of the alkene is attacked, because the alkenes are unsymmetrical (they have different substituents at each end). First, the results. 

In each case, the bromine atom ends up on the more substituted carbon, and the mechanism explains why. There are the two possible outcomes for protonation of styrene by HBr, but you should immediately be able to spot which is preferred, even if you don t know the outcome of the reaction. Protonation at one end gives a stabilized, benzylic cation, while protonation at the other would give a highly unstable primary cation, and therefore does not take place. The benzylic cation gives the benzylic alkyl bromide. 

You get the same result with isobutene the more stable, tertiary cation leads to the product the alternative primary cation is not formed. 

There is a traditional mnemonic called Markovnikov s rule for electrophilic additions of H-X to alkenes, which can be stated as The hydrogen ends up attached to the carbon of the double bond that had more hydrogens to start with/ We don t suggest you learn this rule, though you may hear it referred to. As with ail rules it is much 

The protonation of alkenes to give carbocations is quite general. The carbocations may trap a nucleophile, as you have just seen, or they may simply lose a proton to give back an alkene. This is just the same as saying the protonation is reversible, but it needn t be the same proton that is lost. A more stable alkene may be formed by losing a different proton, which means that acid can catalyse the isomerization of alkenes—both between Zand-E geometrical isomers and between regioisomers, isomerization of an alkene in acid 

Protonation at the other end would give a highly unstable primary cation, and therefore does not take place. 

Interactive mechanisms for electrophilic additions to unsymmetrical alkenes 

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See Table 6.6. Hydrogen halide addition to alkenes proceeds by electrophilic attack of the reagent on the tt electrons of the double bond. Carbocations are intermediates. Addition to unsymmetrical alkenes is regioselective. 

Rates of radical additions to alkenes are controlled mainly by the enthalpy of the reaction, which is the origin of regioselectivity in additions to unsymmetrical systems, with polar effects superimposed when there is a favorable match between the electrophilic or nucleophilic character of the radical and that of the radico-phile. For example, in the addition of an alkyl radical to methyl acrylate (2), the nucleophilic alkyl radical interacts favorably with the resonance structure 3. Polar effects are apparent in the representative rate constants shown in Figure 4.14 for additions of carbon radicals to terminal alkenes. Addition of the electron-deficient or electrophilic rert-butoxycarbonylmethyl radical to the electron-deficient molecule methyl acrylate is 10 times as fast as addition of... 

When the same substituents are at each end of the double or triple bond, it is called symmetrical. Unsymmetrical means different substituents are at each end of the double or triple bond. Electrophilic addition of unsymmetrical reagents to unsymmetrical double or triple bonds follows Markovnikov s rule. According to Markovnikov s rule, addition of unsymmetrical reagents, e.g. HX, H2O or ROH, to an unsymmetrical alkene proceeds in a way that the hydrogen atom adds to the carbon that already has the most hydrogen atoms. The reaction is not stereoselective since it proceeds via a planar carbocation intermediate. However, when reaction proceeds via a cyclic carbocation intermediate, it produces regiospecific and stereospecific product (see below). A regioselective reaction is a reaction that can potentially yield two or more constitutional isomers, but actually produces only one isomer. A reaction in which one stereoisomer is formed predominantly is called a stereoselective reaction. 

Electrophilic addition to terminal alkynes (unsymmetrical) is regioselective and follows Markovnikov s rule. Hydrogen halides can be added to alkynes just like alkenes, to form first the vinyl halide, and then the geminal alkyl dihalide. The addition of HX to an alkyne can be stopped after the first... 

Preparatively, it is important that mineral acids, carboxylic acids, and terf-carbenium ions can be added to alkenes via carbenium ion intermediates. Because of their relatively low stability, primary carbenium ions form more slowly in the course of such reactions than the more stable secondary carbenium ions, and these form more slowly than the even more stable tertiary carbenium ions (Hammond postulate ). Therefore, mineral and carboxylic acids add to unsymmetrical alkenes regioselectively to give Markovnikov products (see Section 3.3.3 for an explanation of this term). In addition, these electrophiles add most rapidly to those alkenes from which tertiary carbenium ion intermediates can be derived. 

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