Question

The sec-butyl cation can react as both a Brønsted-Lowry acid (a proton donor) and a Lewis acid (an electron pair acceptor) in the presence of a water- sulfuric acid mixture. In each case, however, the product is different. The two reactions are as follows: (1) $\mathrm{CH}_{3}-\stackrel{+}{\mathrm{CH}}-\mathrm{CH}_{2}-\mathrm{CH}_{3}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{CH}_{3}-\stackrel{\mathrm{CH}}{+}-\mathrm{CH}_{2}-\mathrm{CH}_{3}$ sec-Butyl cation (2) $\mathrm{CH}_{3}-\mathrm{CH}_{-}^{+}-\mathrm{CH}_{2}-\mathrm{CH}_{3}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{CH}_{3}-\mathrm{CH}^{2} \mathrm{CH}-\mathrm{CH}_{3}+\mathrm{H}_{3} \mathrm{O}^{+}$ sec-Butyl cation (a) In which reaction(s) does this cation react as a Lewis acid? In which reaction(s) does it react as a Bronsted-Lowry acid? (b) Write Lewis structures for reactants and products and show by the use of curved arrows how each reaction occurs.

Short answer

Draw the Lewis structures for the reactants and products and show the reaction mechanism using curved arrows. Answer: In Reaction (1), the sec-butyl cation acts as a Lewis acid, whereas in Reaction (2), it acts as a Brønsted-Lowry acid. In Reaction (1), the Lewis structure shows the lone pair of electrons on the oxygen atom of water forming a bond with the carbon atom in the sec-butyl cation, and the reaction mechanism involves the curved arrow starting from the lone pair on the oxygen and going towards the positive carbon atom. In Reaction (2), the Lewis structure shows the formation of a bond between the hydrogen atom of the sec-butyl cation and the oxygen atom of water, and the reaction mechanism involves the curved arrow starting from the lone pair on the oxygen atom of water and going towards the hydrogen atom of the sec-butyl cation.

Step-by-step solution

Determine the Acid Type in each Reaction

In Reaction (1): The sec-butyl cation reacts with water (H2O). As a Brønsted-Lowry acid, the cation would donate a proton to water. Since it does not have any proton to donate, the sec-butyl cation is not acting as a Brønsted-Lowry acid here. As a Lewis acid, the cation would accept an electron pair from water to form a bond. In this reaction, the positive charge on the sec-butyl cation reacts with the lone pair on the oxygen atom of water. This reaction shows the sec-butyl cation accepting an electron pair and behaving as a Lewis acid. In Reaction (2): The sec-butyl cation reacts with water (H2O). As a Brønsted-Lowry acid, the cation would donate a proton to water. In this reaction, the proton (H+) from the sec-butyl cation is transferred to water forming H3O+. The sec-butyl cation is acting as a Brønsted-Lowry acid here. As a Lewis acid, the cation would accept an electron pair from water to form a bond. Since there is no bond formation between the sec-butyl cation and water in this reaction, it is not behaving as a Lewis acid here.

Draw the Lewis Structures and Reaction Mechanisms

For Reaction (1): Reactants: Sec-Butyl cation: CH3-CH+(CH2-CH3) Water: O with lone pairs and bonded to two hydrogens Products: CH3-CH2-CH2-CH3 (with a positive charge on the second carbon) Reaction Mechanism: The lone pair of electrons on the oxygen atom of water forms a bond with the carbon atom that has a positive charge in the sec-butyl cation. The curved arrow starts from the lone pair on the oxygen and goes towards the positive carbon atom. For Reaction (2): Reactants: Sec-Butyl cation: CH3-CH+(CH2-CH3) Water: O with lone pairs and bonded to two hydrogens Products: CH3CH2CHCH3 Hydronium ion: H3O+ Reaction Mechanism: The proton from the sec-butyl cation transfers to water, forming a bond between the hydrogen atom and the oxygen atom with a lone pair. The curved arrow starts from the lone pair on the oxygen atom of water and goes towards the hydrogen atom of the sec-butyl cation.