SolutionsOther Electrophilic Additions
1. The electrophile and the nucleophile
In the electrophilic addition of HBr to propene, which is the electrophile, which the nucleophile and which the slow step?
The proton of HBr is the electrophile that attacks the π cloud of the double bond, which acts as the nucleophile. The slow step is this protonation, which gives the carbocation; attack of bromide on the carbocation is fast.
2. Hydration of isobutene
Write the mechanism of the acid-catalysed hydration of isobutene and give the product.
1) Slow protonation of the terminal CH2, giving the tert-butyl carbocation. 2) Fast attack of water on the carbocation. 3) Deprotonation of the oxonium ion, which regenerates the acid. The product is tert-butanol, (CH3)3COH.
The acid is a catalyst: it is consumed in the first step and regenerated in the last.
3. Why are they exothermic?
Why are almost all additions to a double bond exothermic?
Because a π bond (about 63 kcal/mol) and the X–Y bond of the reagent are broken, and two σ bonds, C–X and C–Y, which together are stronger, are formed. The sum of the energies of the bonds formed exceeds that of the bonds broken.