Elimination happens when the nucleophile attacks hydrogen instead of carbon

Elimination happens when the nucleophile attacks hydrogen instead of carbon 

The elimination reaction of f-butyl bromide happens because the nucleophile is basic You will recall from Chapter 12 that there is some correlation between basicity and nucleophilicity strong bases are usually good nucleophiles. But being a good nucleophile doesn t get hydroxide anywhere in the substitution reaction, because it doesn t appear in the first-order rate equation. But being a good base does get it somewhere in the elimination reaction, because hydroxide is involved in the rate-determining step of the elimination, and so it appears in the rate equation. This is the mechanism. 

Now let s look at another sort of elimination. We can approach it again by thinking about an SnI substitution reaction. It is another one you met early in Chapter 17, and it is the reverse of the one at the beginning of this chapter. 

Bromide, the nucleophile, is not involved in the rale-determining step, so we know that the rate of the reaction will be independent of the concentration of Br . But what happens if wc use an acid whose counterion is such a weak nucleophile that it doesn t even attack the carbon of the carhoca-tion Here is an example—f-butanol in sulfuric acid doesn t undergo substitution, but undergoes elimination instead. 

the HSO4 is not involved in the rate-determining step—HSO4 is not at all basic and only behaves as a base (that is, it removes a proton) because it is even more feeble as a nucleophile. The rate equation will not involve the concentration of HSO4, and the rate-determining step is the same as that in the SnI reaction—unimolecular loss of water from the protonated t-BuOH. This elimination mechanism is therefore called El. 

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