1. Four groups share the formula C₄H₉, so the prefix does the work.
"Butyl" alone is ambiguous — n-, iso-, sec-, and tert- tell you how the four carbons are arranged and where the group attaches.
2. n-Butyl is a straight chain attached at its end carbon, which is 1°.
The attachment carbon (–CH₂–) is bonded to just one other carbon, so n-butyl is a primary group.
3. Isobutyl branches at the next carbon but still attaches through a 1° CH₂.
The group attaches at a CH₂ sitting next to a CH(CH₃)₂ branch point, so the attachment carbon is still primary.
4. sec-Butyl attaches at a carbon bearing one methyl and one ethyl, making it 2°.
That attachment carbon is bonded to two other carbons, so sec-butyl is a secondary group.
5. tert-Butyl attaches at the central carbon of (CH₃)₃C, which is 3°.
The attachment carbon holds three methyl groups and no hydrogen, so tert-butyl is tertiary and highly branched.
6. Summary
Same formula C₄H₉ · n-butyl = straight chain, 1° · isobutyl = branched, 1° · sec-butyl = 2° attachment · tert-butyl = 3° attachment · the prefix encodes branching + degree.
Quiz yourself
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n-butyl and isobutyl — both attach at a CH₂.
Tertiary (3°) — it is the central carbon of (CH₃)₃C, bonded to three carbons and no H.
n-butyl is a straight chain; isobutyl has a methyl branch (a CH(CH₃)₂ unit) one carbon in.
sec-butyl — its attachment carbon carries one methyl and one ethyl, so it is 2°.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.