NDA Chemistry · Carbon and Its Compounds

Hydrocarbons and Organic Classification

Hydrocarbons are compounds of only carbon and hydrogen, grouped into homologous series (alkanes, alkenes, alkynes) each with its own general formula and trends.

All Carbon and Its Compounds notes NDA Chemistry strategy

Why this matters

Small but reliable — the bank asks which homologous series a formula belongs to, orders boiling points by chain length, and tests the historic organic-vs-inorganic boundary that Wöhler's urea synthesis erased. The general formulas are the one piece of pure recall here.

Concept 1 of 2

Homologous series and general formulas

Intuition

A homologous series is a family of hydrocarbons that share a general formula and differ from the next member by one CH₂ unit. Knowing the general formula lets you classify any hydrocarbon on sight: count the carbons, check the hydrogens.

Definition

The three open-chain series and their general formulas (n = number of carbon atoms):

Alkanes (single bonds, saturated): CₙH₂ₙ₊₂ — e.g. CH₄, C₂H₆.
Alkenes (one C=C double bond): CₙH₂ₙ — e.g. C₂H₄, C₄H₈.
Alkynes (one C≡C triple bond): CₙH₂ₙ₋₂ — e.g. C₂H₂, C₃H₄.

Boiling point rises with chain length — bigger molecules have stronger forces between them.

General formulas of hydrocarbon series

\text{Alkane: } \text{C}_n\text{H}_{2n+2} \qquad \text{Alkene: } \text{C}_n\text{H}_{2n} \qquad \text{Alkyne: } \text{C}_n\text{H}_{2n-2}

Worked example

To which homologous series does C₃H₄ belong?

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Carbon and Its CompoundsMODERATE

\text{C}_4\text{H}_8

belongs to the homologous series of

[Q110 · Sep · 2022]

CₙH₂ₙ is shared by alkenes and cycloalkanes

An open-chain CₙH₂ₙ is an alkene, but a ring (cycloalkane) has the same formula. For the bank's classification question on an open-chain formula like C₄H₈, the intended answer is alkene.

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Concept 2 of 2

Organic vs inorganic — and Wöhler's synthesis

Intuition

'Organic' once meant 'made by living things' and was thought impossible to make in a lab — until Wöhler made urea from an inorganic salt in 1828, breaking the vital-force theory. Today organic = carbon compounds (with a few inorganic exceptions like carbonates, CO and CO₂). The bank tests the odd-one-out: which listed compound is NOT organic.

Definition

The historic boundary and the bank's examples:

Marsh gas = methane (CH₄) — organic.
Urea — organic; first synthesised by Wöhler (1828) from ammonium cyanate, disproving the vital-force theory.
Cane sugar (sucrose) — organic (a carbohydrate).
Ammonium cyanate (NH₄OCN) — an inorganic ionic salt (Wöhler's starting material).
Carbon forms the largest number of compounds of any element, because of catenation and tetra-valency.

Compound	Organic or inorganic	Note
Marsh gas (methane, CH₄)	Organic	Simplest alkane
Urea	Organic	First lab-synthesised organic compound (Wöhler, 1828) Wöhler made urea FROM ammonium cyanate — the product is organic, the starting salt is inorganic.
Cane sugar (sucrose)	Organic	A carbohydrate
Ammonium cyanate (NH₄OCN)	Inorganic	An ionic salt — Wöhler's precursor

Practice this concept4 quick reps

From the bank · past-year question

Example 2Carbon and Its CompoundsHARD

Which one of the following is NOT an example of organic compound?

[Q88 · Sep · 2023]

The starting salt is inorganic, the product is organic

Ammonium cyanate (NH₄OCN) is the inorganic answer to 'which is NOT organic', even though Wöhler turned it into urea (organic). Don't let the urea connection fool you into calling the salt organic.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Homologous series and general formulas
General formulas of hydrocarbon series
$\text{Alkane: } \text{C}_n\text{H}_{2n+2} \qquad \text{Alkene: } \text{C}_n\text{H}_{2n} \qquad \text{Alkyne: } \text{C}_n\text{H}_{2n-2}$

Reference tables (1)

Organic vs inorganic — and Wöhler's synthesis4 rows

Compound	Organic or inorganic	Note
Marsh gas (methane, CH₄)	Organic	Simplest alkane
Urea	Organic	First lab-synthesised organic compound (Wöhler, 1828) Wöhler made urea FROM ammonium cyanate — the product is organic, the starting salt is inorganic.
Cane sugar (sucrose)	Organic	A carbohydrate
Ammonium cyanate (NH₄OCN)	Inorganic	An ionic salt — Wöhler's precursor

Watch out for (2)

CₙH₂ₙ is shared by alkenes and cycloalkanes
→ Homologous series and general formulas
The starting salt is inorganic, the product is organic
→ Organic vs inorganic — and Wöhler's synthesis

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