Alkene, electrophilic addition

Limonene, a fragrant hydrocarbon found in lemons and oranges, is bio-synthesized from geranyl diphosphate by the following pathway. Add curvec arrows to show the mechanism of each step. Which step involves an alkene electrophilic addition (The ion 0P2064- is the diphosphate ion, and "Base is an unspecified base in the enzyme that catalyzes the reaction.)... 

Practically everything we ve said in previous chapters has been stated without any proof. We said in Section 6.8, for instance, that Markovnikov s rule is followed in alkene electrophilic addition reactions and that treatment of 1-butene with HC1 yields 2-chJorobutane rather than 1-chlorobutane. Similarly, we said in Section 11.7 that Zaitsev s rule is followed in elimination reactions and that treatment of 2-chlorobutane with NaOH yields 2-butene rather than 1-butene. But how do we know that these statements are correct The answer to these and many thousands of similar questions is that the structures of the reaction products have been determined experimentally. 

Fluoromethanol (4) and symmetrical bis(fluoromethyl) ethers (5) are obtained from paraformaldehyde in hydrogen fluoride.205,209 Dissociation of fluoromethanol creates the electrophilic species [HOCH2 + ]F, which then attacks the 7r-bond of an alkene (electrophilic addition).209... 

Step 2, in which the double bond electrons add to the carbocation, is an alkene electrophilic addition. 

Steps 1 and 2 are alkene electrophilic additions, and steps 3 and 4 involve carbocation rearrangements. 

Well nee more details about alkene electrophilic addition reactions ahortly, but fia the present wc can imugine the readioa as takii place by Che pathway shown in Figure 5.4, Hie reaction begins when the alkene donates a pair of electrons from its OC bond to HBr to form a new C-H bond... 

Most reactions of carbonyl groups occur by one of four general mechanisms nucleophilic addition, nucleophilic acyl substitution, alpha substitution, am carbonyl condensation. These mechanisms have many variations, just a alkene electrophilic addition reactions and 8 2 reactions do, but the varia tions are much easier to learn when the fundamental features of the mechanisms are understood. Let s see what the four mechanisms are and what kinds of chemistry carbonyl groups undergo. 

Now that we understand the mechanisms of alkenes reactions, we can devise a rule that applies to all alkene electrophilic addition reactions The electrophile adds to the sp carbon that is bonded to the greater number of hydrogens. This is the rule you should remember because all electrophilic addition reactions follow this rule, so it will keep you from having to memorize which reactions follow Markovnikov s rule and which ones don t. 

Thus alkynes, like alkenes, undergo electrophilic addition reactions. We will see that the same electrophilic reagents that add to alkenes also add to alkynes and that— again like alkenes—electrophilic addition to a terminal alkyne is regioselective When an electrophile adds to a terminal alkyne, it adds to the sp carbon that is bonded to the hydrogen. The addition reactions of alkynes, however, have a feature that alkenes do not have Because the product of the addition of an electrophilic reagent to an alkyne is an alkene, a second electrophilic addition reaction can occur. 

The reactions of a conjugate diene reflect the fact that a conjugated diene should be viewed as a functional group in its own right, rather than as two separate alkenes. Electrophilic addition to a conjugated diene results in a mixture of two possible products arising from 1,2-addition and 1,4 addition ... 

The following reaction takes place in high yield. Use your general knowledge of alkene electrophilic additions to propose a mechanism, even though you ve never seen the exact reaction before. 

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