Chemistry 102: Acid Equilibrium: Acetylide ion and methanol

Question

part 1). Which structure is the Lewis base?

Choose one:

A.  acetylide

B.  methanol

C.  both acetylide and methanol

D.  neither acetylide not methanol

part 2). Draw the conjugate acid that is formed as a product in this reaction.

Answer 1

The equilibrium reaction between acetylide ion and methanol to form acetylene and methoxide ion can be interpreted as involving both Lewis acid-base and Bronsted-Lowry acid-base chemistry.

In the Lewis sense, acetylide ion is the base (the electron pair donor to the proton of methanol) and when viewed from the Bronsted-Lowry angle it is also a base (a proton acceptor).

Categorized either way the products are acetylene (HC triply bonded to CH) and methoxide ion. Since acetylene is the weaker Bronsted-Lowry acid ( has a larger pKa value than that for methanol), the forward reaction predominates in this equilibrium.