E2 and El mechanisms

Now that you have seen a few examples of elimination reactions, it is time to return to our discussion of the two mechanisms for elimination. To summarize what we have said so far  

With less hindered alkyl halides hydroxide would not be a good choice as a base for an elimination because it is rather small and still very good at S]sf2 substitutions (and even with tertiary alkyl halides, substitution outpaces elimination at low concentrations of hydroxide). So what are good alternatives  

We have already mentioned the bulky f-butoxide—ideal for promoting E2 as it s both bulky and a strong base (pJCgH - 18). Here it is at work converting a dibromide to a diene with two successive E2 eliminations. Since dibromides can be made from alkenes (you will see how in the next chapter), this is a useful two-step conversion of an alkene to a diene, synthesis of a diene by a double E2 elimination 

The product of the next reaction is a ketene acetal —you met ketene, CH2=C=0, in Chapter 15. Unlike most acetals, this one can t be formed directly from ketene (ketene is too unstable), so 

The loss of the leaving group and removal of the proton are concerted. 

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These reactions are often promoted by a strong base, which assists the departure of the proton. X is the leaving group. Both El and E2 mechanisms are known, as is a variant designated Elcb, for unimolecular elimination from the conjugate base of the substrate. ... 

The El reactions can involve ion pairs, just as is true for S l reactions (p. 398), This effect is naturally greatest for nondissociating solvents it is least in water, greater in ethanol, and greater still in acetic acid. It has been proposed that the ion-pair mechanism (p. 400) extends to elimination reactions too, and that the S l, Sn2, El, and E2 mechanisms possess in common an ion-pair intermediate, at least occasionally. ... 

Dehydrohalogenation is the elimination of a hydrogen and a halogen from an alkyl halide to form an alkene. In Sections 6-17 through 6-21 we saw how dehydrohalogenation can take place by the El and E2 mechanisms. The second-order elimination (E2) is usually better for synthetic purposes because the El has more competing reactions. 

The El and E2 mechanisms both involve the same number of bonds broken and formed. The only diflierence is the timing. 

El and E2 mechanisms, 130 Electron-dot structures, 2 Electronegativity, 21 Electrophiles, 35, 44, 47 Electrophilic substitution of arenes, 205 in napthalene, 213 Elimination, 34 1,2-Elimination, 91 Enantiomers, 69 conditions for, 80 conformational, 80 Enamines, 315, 378 Enolsilanes, 396 Enthalpy, 36 diagrams, 40, 49 Entropy, 36... 

Phosphates are also active dehydration catalysts [6,8,9]. Although a mixed (El and E2) mechanism has been reported [24], most of these materials and particularly BPO4 have typical El behavior [25-27]. The involvement of the carbocatio-nic intermediate results in a complex mixture of isomeric alkenes [27,28] and intramolecular and intermolecular dehydration (alkene and ether formation, respectively) are often parallel processes. For example, aluminum phosphates with P/Al < 1 yield a mixture of alkenes and ethers whereas those with P/Al > 1 give alkenes selectively [6,29]. Dehydration activity usually correlated with surface acidity [29-31]. The strongly acidic sites of AIPO4 were found to promote alkene formation [26]. 

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