A butyl group is a four-carbon substituent that can be arranged in four different ways, and each form has its own name.
P.S. Check out my 4-hour lesson on nomenclature if you’re not in a rush!
There are four different ways to arrange a four-carbon chain as a substituent, so let’s check them out! I’ve drawn them out on a cyclohexane as the parent molecule. There are more complex ways to name these alkyl groups, but we’ll be focusing on the common IUPAC names here. Remember that we use the suffix -yl here because we're referring to an R-group coming off of a molecule. If these were standalone alkanes, we'd use the suffix -ane (e.g. isobutane instead of isobutyl).
This straight-chain butyl is called butyl, but you might also see it called n-butyl. “n” means straight-chain. It’s got only primary and secondary carbons.
This version, which looks a lot like isopropyl, is called isobutyl. It’s got primary, secondary, and tertiary carbons.
This version, which looks like isopropyl with an extra carbon at the end, is called sec-butyl. It’s got primary, sSec-butylecondary, and tertiary carbons.
This version is called tert-butyl. It’s got a quaternary carbon with three primary carbons coming off of it. Pro-tip: sometimes you'll see this abbreviated as t-bu.
Hopefully this quick guide helps! Check out my videos on common substituents if you need a refresher!