1. A carbon's class = how many other carbons are bonded to it.
Count only the neighboring carbons: one gives primary (1°), two secondary (2°), three tertiary (3°), four quaternary (4°).
2. An alkyl halide is named for the carbon bearing the halogen.
The bromine-bearing carbon in each structure above sets the class, so these are a 1°, 2° and 3° bromide respectively.
3. A quaternary carbon has four carbons and no H — it can't bear a leaving group.
Neopentane's central carbon is bonded to four CH₃ groups, so it is quaternary and unreactive in substitution.
4. Amines are classed by the carbons on nitrogen, not on carbon.
Count the carbon groups attached to N: one is a 1° amine, two a 2° amine, three a 3° amine.
5. Class predicts reactivity, so it's worth spotting fast.
SN2 favors 1°/methyl; SN1 and E1 favor 3° via stable carbocations; 3° alcohols resist oxidation.
6. Summary
Count carbons on the carbon · 1 = 1° · 2 = 2° · 3 = 3° · 4 = 4° (quaternary) · amines count carbons on N · SN2 likes 1° · SN1/E1 like 3° · 3° alcohols don't oxidize.
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Secondary (2°) — that carbon is bonded to two other carbons.
It is bonded to four carbons (four CH₃ groups) and no hydrogen.
The 1° (primary) halide; less steric bulk lets the nucleophile attack the back side.
Tertiary (3°) — three carbon groups are attached to nitrogen.
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.