Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Unlike ethers, esters have a carbonyl group. Unlike carboxylic acids, esters have no acidic hydrogen atom; they have a hydrocarbon group in its place. Answer: hydrocarbon group
Carboxylic acids have an acidic hydrogen atom, but esters do not. what do esters have in place of an acidic hydrogen atom?