Question 26.SIE.1: In an example of an electrophilic addition reaction 1-butene...
In an example of an electrophilic addition reaction 1-butene reacts with hydrogen bromide to give 2-bromobutane. (a) Draw out the mechanism of this reaction including the carbocation intermediate. Show all the atoms attached to the double bond and use curved arrows to indicate electron-pair movements. (b) What are the hybridization and molecular geometry of the two atoms C1 and C2 in the starting material, intermediate and product? (c) Indicate the stereogenic carbon center in the product with *. (d) Explain why the product is formed as a racemic mixture.
Analyse In this problem we examine the mechanism of an electrophilic addition reaction and how the shape of the intermediate relates to the chirality of the product.
Plan First identify the structure of the starting material and product from the names given. Then draw the mechanism of the reaction and identify how the molecular geometry of C1 and C2 changes as the reaction progresses. Finally identify the features of the geometry change that result in a racemic mixture being produced.
Solve
(a)
(b)
table 1
(c)
(d) The carbocation is sp² hybridized and trigonal planar in shape. The positive charge may be thought of as an empty p orbital pointing above and below the plane containing the carbocation. The final stage of this reaction involves attack by the bromide ion on the carbocation and there is equal probability of this occurring to the top or the bottom face of the intermediate.
table 1
\begin{array}{lclcl}\hline & \text{Hybridization-C1}& \text{Geometry-C1}& \text{Hybridization-C2}& \text{Geometry-C2}\\\hline \text{Starting material}& s p^2 & \text{Trigonal planar}& s p^2 & \text{Trigonal planar}\\\text{Intermediate}& s p^3 & \text{Tetrahedral}& s p^2 & \text{Trigonal planar}\\\text{Product}& s p^3 & \text{Tetrahedral}& s p^3 & \text{Tetrahedral}\\\hline\end{array}