Formulas Gerais Dos Hidrocarbonetos

Hydrocarbon General Formulas — The Actual Things You Need to Know

The general formulas for hydrocarbons come in a small set that repeats across almost every introductory course. Knowing them helps, but the formulas alone will not protect you from a trick question on an exam. Here is what actually works in practice. The core set looks like this: Alcanos (parafinas): C_nH_2n+2

Alquenos (olefinas): C_nH_2n Alquinos: C_nH_2n-2 Cicloalcanos: C_nH_2n

Arenos monossúbmicos (série homóloga do benzeno): C_nH_2n-6 That last one trips people up constantly. It is not a mistake — it is the rule for a monocyclic aromatic ring with one benzene unit and no additional unsaturation outside the ring. Once you add more rings, the formula shifts again. I remember grading a lab report where someone wrote C₁₀H₂₀ for decalin and flagged it wrong. The compound was bicyclic, so the expected formula under the standard alkane derivation is C₁₀H₁₈. The student had applied the alkene formula blindly. That is the kind of error that shows up when you memorize instead of deriving.

How to derive them without memorizing every variant

Start from the saturated acyclic alkane: C_nH_2n+2. Every degree of unsaturation removes two hydrogens. A double bond removes two. A ring removes two. A triple bond removes four. That is the entire system. Apply it step by step: One double bond subtract 2H C_nH_2n (alkene). One ring subtract 2H C_nH_2n (cycloalkane). One triple bond subtract 4H C_nH_2n-2 (alkyne). Benzene ring three double bonds plus one ring inside a six-carbon loop subtract 6H from C₆H₁₄ C₆H₆. Extend that pattern by adding CH₂ groups for each homolog C_nH_2n-6.

This derivation takes about thirty seconds and eliminates the need to memorize the arene formula separately. I used to make students do this on the first day of organic chemistry because the percentage of errors on multiple-choice questions dropped noticeably after that exercise.

The part most textbooks skip

The general formulas assume ideal, neutral, fully saturated or partially unsaturated hydrocarbons only. They break down the moment you introduce heteroatoms, which is almost immediately in any real problem set. Here is the practical workaround: convert heteroatoms to their hydrogen equivalents before applying the degree-of-unsaturation formula. Halogens count as one hydrogen. Oxygen and sulfur are ignored. Nitrogen adds one to the carbon count and one to the hydrogen count in the adjustment. So C₄H₉Cl behaves like C₄H₁₀, which is an alkane. C₄H₈O could be an alkene or a cyclic ether — the formula alone will not tell you, and that is a feature, not a bug.

I once spent twenty minutes trying to reconcile a mass spectrum because I treated an oxygenated fragment as if the hydrocarbon formula should fit cleanly. It did not. The compound was an unsaturated ketone, and the C:H ratio was misleading until I accounted for the oxygen properly.

Common pitfalls

Using C_nH_2n for everything with one degree of unsaturation without checking whether the structure is cyclic or contains a double bond. The formula gives you the count, not the connectivity. Two isomers can share the same molecular formula and have completely different chemical behavior. Applying C_nH_2n-6 to polycyclic aromatics. Naphthalene is C₁₀H₈, not C₁₀H₁₄. The monosubstituted benzene formula only works when there is exactly one aromatic ring and no additional rings fused to it. Any more rings and you need to recalculate from first principles. Confusing empirical and molecular formulas. C₂H₄ and C₃H₆ both reduce to CH₂, but they are different compounds. The general formulas give molecular relationships, not empirical shortcuts.

Quick reference table

Class	General Formula	Degree of Unsaturation
Alkane	CnH2n+2	0
Alkene / Cycloalkane	CnH2n	1
Alkyne / Dienes / Cyclic alkene	CnH2n-2	2
Arene (monocyclic)	CnH2n-6	4

Notice that degree of unsaturation is the common thread. It is more useful than the formula itself because it transfers directly to spectroscopy problems, where you will be asked to determine structure from IR and NMR data rather than just writing a molecular formula. If you need a downloadable summary sheet, search for "fórmulas gerais dos hidrocarbonetos pdf" and pick one from a university chemistry department page. Avoid the generic homework sites — their tables often miss the polycyclic exceptions and list incorrect arene formulas for fused ring systems.