we introduced the enolates of aldehydes and ketones (review) and looked at their reactions as C nucleophiles (review).  These enolates were formed by treating the aldehyde or ketone with a suitable base :

Now we will investigate another group of carbonyl containing compounds, the esters, which behave in a very similar fashion…..

When preparing ester enolates, the base is normally chosen to match the alcohol portion of the ester…
So if you have:
a methyl ester, RCOOCH₃ you use methoxide, ^–OCH₃, or,
an ethyl ester, RCOOCH₂CH₃ you use ethoxide, ^–OCH₂CH₃

This avoids problems caused by transesterification (conversion of one ester into another)……

or hydrolysis……

Note that in both cases the problem arises because the base reacts as a “nucleophile“