In: Chemistry
Consider the hydroboration of 1-pentene. Predict the major outcome of this reaction through a transition state analysis and construct a reaction coordinate diagram to justify your answer. Note: the oxidation step after hydroboration does not have any impact on regioselectivity.
The hydroboration of 1-pentene will give 1-pentanol as the major product and 2-pentanol as the minor product. The detailed mechanism is given in the picture below.
The T.S. (I) below is less sterically crowded then T.S. (II) as shown below. Thus the T.S. (I) is more favoured and more stable. Addition of BH3 occurs by Markovnikov's rule. Whereas, T.S. (II) is formed by an anti-Markovnikov's rule of addition of BH3 to the alkene. Hence it is less stable and least favoured T.S..
The reaction coordinate diagram also shows higher energy of activation for T.S. (II) than T.S. (I). Therefore, 1-pentanol will be the preferred product of this reaction. Below are shown, the hydroboration reaction, the two T.S.'s formation and reaction coordinate diagram.