Homologous Series — Organic Chemistry Mnemonic
Target: mnemonic for homologous series organic chemistry
Why is this hard to memorize?
A homologous series is a family of organic compounds with the same general formula, where each member differs from the next by –CH₂–. The three main series students must know are: Alkanes (CₙH₂ₙ₊₂, single bonds only), Alkenes (CₙH₂ₙ, one C=C double bond), and Alkynes (CₙH₂ₙ₋₂, one C≡C triple bond). Understanding these formulas is fundamental — Class 10 boards, NEET, and JEE all test your ability to write formulas, predict molecular masses, and identify which series a compound belongs to based on its molecular formula.
Classic mnemonics you should know
"Alkane: 2n+2, Alkene: 2n, Alkyne: 2n-2 — "Plus 2, Zero, Minus 2""
For n carbons: Alkane has 2n+2 hydrogens (saturated, all single bonds). Alkene has 2n hydrogens (one double bond removes 2 H). Alkyne has 2n−2 hydrogens (one triple bond removes 4 H). Each degree of unsaturation removes 2 H.
"-ane = single(ANd), -ene = double(ENough bonds), -yne = triple(whYNot three)"
Alkane(-ane) = all single bonds. Alkene(-ene) = one double bond. Alkyne(-yne) = one triple bond. The suffixes are also used in IUPAC naming: ethane, ethene, ethyne.
"Each CH₂ added = +14 g/mol, +1 C, +2 H — "14 grams per step""
Moving from one member to the next in any homologous series adds CH₂ (mass 14). Methane(16) → Ethane(30) → Propane(44): each jump is exactly 14. This pattern helps calculate molecular mass instantly.
The complete list
- Alkane (CₙH₂ₙ₊₂) — single bonds
- Alkene (CₙH₂ₙ) — one double bond
- Alkyne (CₙH₂ₙ₋₂) — one triple bond
- Alcohol (CₙH₂ₙ₊₁OH)
- Carboxylic acid (CₙH₂ₙ₊₁COOH)
- Each member differs by CH₂ (14 g/mol)
Frequently asked questions
What is a homologous series?
A family of organic compounds with the same general formula, same functional group, and similar chemical properties. Each consecutive member differs by one –CH₂– unit (14 g/mol). Examples: methane-ethane-propane (alkane series), ethene-propene-butene (alkene series).
How do I find which homologous series a compound belongs to?
Count the C and H atoms and check against formulas: If H = 2C+2 → alkane. If H = 2C → alkene (or cycloalkane). If H = 2C−2 → alkyne. If it has oxygen, check for –OH (alcohol: CₙH₂ₙ₊₂O) or –COOH (acid: CₙH₂ₙO₂).
Why do members of a homologous series have similar chemical properties?
Same functional group → same reactive site → same type of reactions. The only difference between members is chain length (more CH₂ units), which mainly affects physical properties (boiling point, solubility) but not the type of chemical reactions. Ethane and propane both undergo combustion and substitution the same way.
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