Hydrocarbons and Alcohols
High-Yield Summary
- Alkanes: CₙH₂ₙ₊₂, saturated (all single bonds), suffix -ane.
- Haloalkanes: halogens named as prefixes fluoro-/chloro-/bromo-/iodo-, alphabetized with other substituents.
- Alkenes (-ene) and alkynes (-yne): number the chain so the double/triple bond gets the lowest locant.
- Alcohols (-ol, replacing the parent's -e ending) outrank alkenes, alkynes, and alkanes for numbering priority because the hydroxyl carbon has a higher oxidation state. When an even higher-priority group is present, the hydroxyl becomes a hydroxy- prefix instead.
- Diols: geminal = both -OH on the same carbon (pentane-3,3-diol); vicinal = -OH on adjacent carbons (ethane-1,2-diol); name by adding -diol to the full parent hydrocarbon name.
Key Terms
- Saturated
- Contains only single bonds between carbons — alkanes are the saturated hydrocarbon class.
- Geminal diol
- Two hydroxyl groups on the same carbon, e.g. pentane-3,3-diol.
- Vicinal diol
- Two hydroxyl groups on adjacent carbons, e.g. ethane-1,2-diol.
- Hydroxy- prefix
- Used to name an -OH group as a substituent instead of the -ol suffix, when a higher-priority functional group (e.g. a ketone) is present.
Suffix by Hydrocarbon/Alcohol Class
| Class | Suffix / Naming Rule |
|---|---|
| Alkane | -ane (all single bonds, CₙH₂ₙ₊₂) |
| Haloalkane | fluoro-/chloro-/bromo-/iodo- prefix, alphabetized |
| Alkene | -ene, lowest locant to the double bond |
| Alkyne | -yne, lowest locant to the triple bond |
| Alcohol | -ol (replaces -e), outranks ene/yne/ane for numbering |
| Diol | -diol added to full parent name, both -OH locants numbered |
Common MCAT Trap
- The -OH group wins numbering priority over a double/triple bond even if that pushes the double/triple bond's locant higher — e.g. pent-4-en-2-ol, not pent-1-en-4-ol.
- If a higher-priority group than alcohol is present (like a ketone), -OH loses the -ol suffix and becomes the hydroxy- prefix instead.
- Don't confuse geminal (same carbon) with vicinal (adjacent carbons) diols — the prefix depends on relative position, not just presence of two -OH groups.
Quick Recall
What is the general formula for alkanes, and what does it tell you about bonding?
Why does the hydroxyl group get numbering priority over alkenes/alkynes/alkanes?
Name a molecule with an -OH on carbon 2 and a double bond between carbons 4 and 5 of a 5-carbon chain.
What's the difference between a geminal and a vicinal diol?