Chapter 7, Problem 7.1P

Interpretation Introduction

Interpretation:

The electron-poor H atom in methanol is to be identified, and the mechanism by which methanol acts as an acid in a proton transfer reaction with ${\text{H}}_{\text{2}}{\text{N}}^{-}$ is to be drawn.

Concept introduction:

An electron poor atom or a molecule is an atom or a site in a molecule that has fewer electrons than the number required for stability. In a proton transfer reaction, a proton is transferred from a Bronsted-Lowry acid (proton donor) to a Bronsted–Lowry base (proton acceptor). In an elementary step, electrons tend to flow from an electron-rich site to an electron-poor site. The curved arrow notation shows the movement of valence electrons.

Answer to Problem 7.1P

The electron-poor H atom in methanol is:

The mechanism by which methanol acts as an acid in a proton transfer reaction with ${\text{H}}_{\text{2}}{\text{N}}^{-}$ is:

Explanation of Solution

Atoms in ${\text{CH}}_{\text{3}}\text{OH}$ do not bear full charge. However, the high electronegativity of oxygen leads to a partial positive charge on the H to which it is bonded. Therefore, H is electron poor. ${\text{H}}_{\text{2}}{\text{N}}^{-}$ bears full negative charge, and therefore, it is electron rich. The curved arrow from a lone pair on N to the H atom on O represents the flow of electrons from the electron-rich site ${\text{(H}}_{\text{2}}{\text{N}}^{-})$ to the electron-poor site $\left({\text{CH}}_{\text{3}}\text{OH}\right)$. A second curved arrow is needed to make sure H has only one bond, not two.

Conclusion

The curved arrows are drawn from the electron-rich site ${\text{(H}}_{\text{2}}{\text{N}}^{-})$ to the electron-poor site $\left({\text{CH}}_{\text{3}}\text{OH}\right)$.