Final answer:
Propanone, also known as acetone or dimethyl ketone, is the open chain ketone with a methyl substituent. According to IUPAC naming, ketones are named by replacing the -e in the alkane name with -one, making propanone the correct answer.
Step-by-step explanation:
The student has asked about the open chain ketone with a methyl substituent. By definition, ketones are organic compounds that have a carbonyl group (C=O) bonded to two alkyl or aryl groups. According to IUPAC nomenclature, ketones are named by dropping the -e from the alkane name and adding the ending -one. Therefore, the IUPAC name for acetone is propanone, which is a ketone with three carbon atoms in the longest continuous chain and two methyl groups on either side of the carbonyl group.
Another common name for propanone is dimethyl ketone, reflecting the two methyl groups attached to the carbonyl. Meanwhile, butanone has four carbon atoms and its structure is that of ethyl methyl ketone. For the purpose of this question, which asks specifically for a ketone with a methyl substituent, the answer is acetone, also known as propanone or dimethyl ketone.