Which prefix is used to indicate the presence of an ethyl group?

The prefix used to indicate the presence of an ethyl group is "Ethyl-." Ethyl refers specifically to a two-carbon alkyl group, represented chemically as -C2H5. When naming organic compounds, the prefix "Ethyl-" is essential for indicating that there is a two-carbon substituent branching off the main carbon chain. This nomenclature stems from the systematic naming conventions used in organic chemistry, where substituent groups are identified by their carbon count, leading to standardized and unambiguous communication about molecular structure.

For context, "Butyl-" refers to a four-carbon alkyl group (-C4H9), "Propyl-" refers to a three-carbon alkyl group (-C3H7), and "Methyl-" refers to a one-carbon alkyl group (-C1H3). Each of these prefixes corresponds to different carbon chain lengths, making "Ethyl-" the correct choice for a two-carbon substituent.