Elimination by the El Mechanism

It is common practice not to show the weak base that removes H , so that the carbocation seems to fragment spontaneously. 

Common error alert If you do not show the weak base that removes H, be sure that your curved arrow is drawn correctly. The arrow shows how the electrons in the C—H bond move to form a ir bond. It should not show flying ojfinto the wilderness  

The elements of H2O are eliminated. Under acidic conditions, eliminations occur by an El mechanism. First the leaving group is protonated to convert it into a better leaving group, and then it leaves to give a carbocation. Then fragmentation (one of the three typical reactions of carbocations) of a C—H bond occurs to give a diene. 

Unfortunately, this diene is not the desired product. Compare the diene with the desired product. Which cr bonds differ between them One C—H bond has to be made, and another has to be broken. Since the conditions are acidic, first make a new C—H bond by protonating a C=C tt bond. Then fragment the 

Problem 3.6. Draw a reasonable mechanism for the following transformation. 

Elimination can also occur at C(sp3)-X centers under acidic conditions. The mechanism is called El. The leaving group leaves, sometimes after protonation, to give a carbocation. The carbocation then fragments (one of the three reactions 

Elimination of small molecules like water and MeOH is often carried out under acidic conditions. The reaction is often driven to completion by azeotropic distillation with benzene or petroleum ether. 

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Typically elimination by the El mechanism is observed only for tertiary and some secondary alkyl halides and then only when the base is weak or m low con centration Unlike eliminations that follow an E2 pathway and exhibit second order kinetic behavior... 

The reactivity order parallels the ease of carbocation formation Increasing rate of elimination by the El mechanism... 

Direct sulfonation of toluene with concentrated sulfuric acid gives a mixture of ortho and para sulfonic acids from which about 40% of toluene para sulfonic acid can be isolated as the sodium salt. The free acid is important as a convenient solid acid, useful when a strong acid is needed to catalyse a reaction. Being much more easily handled than oily and corrosive sulfuric acid or syrupy phosphoric acid, it is useful for acetal formation (Chapter 14) and eliminations by the El mechanism on alcohols (Chapter 19). It is usually called tosic add, TsOH, or PTSA (para toluene sulfonic acid). 

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