# How to identify where a bond will break and where a bond will form (acetals)

In the following reaction:

The $\ce{H}$ from $\ce{ROH}$ joins the $\ce{O}$ in the carbonyl.

So the Alcohol breaks at the bond between the $\ce{R'O}$ and the $\ce{H}$.

How do I know this occurs, and that the alcohol doesn't break at the $\ce{R'}$ and the $\ce{OH}$ bond (and then the $\ce{OH}$ joining the $\ce{C}$, and the $\ce{R'}$ joining the carbonyl $\ce{O}$)?

Is it because the electronegativity difference between the $\ce{H}$ and the $\ce{O}$ is greater than the electronegativity difference between the $\ce{R'}$ and the $\ce{O}$?

• I find helpful to look at pKa values. Many of the most common bonds in organic chemistry can be found in a pKa table. Another thing that may help are looking at what constitutes as a good nucleophile, or a good leaving group Sep 10 '17 at 2:14