Methoxy group

An oxygen atom separated by an sp³ carbon from a centre of high electron density shows its electronegative properties. This can be seen by comparing the acidity of acetic acid (pK_a = 4.74) with that of methoxyacetic acid (pK_a = 3.54), more than one order of magnitude higher.

That is, methoxyacetic acid is more than ten times stronger than acetic acid, because the oxygen of the methoxy group, separated by an sp³ carbon from the carboxylate, stabilizes it through its electronegativity.