Alkanes, alkenes and alkynes

The three hydrocarbon families side by side

A general formula lets you write any member of a family once you know how many carbons it has. Count the carbons, substitute for n, and you have the molecular formula.

Family	C–C bond	General formula	Saturated?
Alkane	single	$C_nH_{2n+2}$	saturated
Alkene	one double	$C_nH_{2n}$	unsaturated
Alkyne	one triple	$C_nH_{2n-2}$	unsaturated

• n is the number of carbon atoms in the molecule.
• Each step from alkane to alkene to alkyne removes two hydrogen atoms, because forming an extra bond between the carbons uses up a bond each carbon was using for hydrogen.

Using a general formula

• Find the number of carbons.
• Substitute that number for n.
• Work out the hydrogen count.

Worked through for a five-carbon molecule:

• Pentane (alkane): $n = 5$, so $H = 2(5) + 2 = 12$ → $C_5H_{12}$.
• Pentene (alkene): $n = 5$, so $H = 2(5) = 10$ → $C_5H_{10}$.
• Pentyne (alkyne): $n = 5$, so $H = 2(5) - 2 = 8$ → $C_5H_8$.

The first eight members

The stem of the name comes from the carbon count, and you need all eight:

Carbons	Stem	Alkane	Alkene	Alkyne
1	meth-	methane	—	—
2	eth-	ethane	ethene	ethyne
3	prop-	propane	propene	propyne
4	but-	butane	butene	butyne
5	pent-	pentane	pentene	pentyne
6	hex-	hexane	hexene	hexyne
7	hept-	heptane	heptene	heptyne
8	oct-	octane	octene	octyne

• There is no methene or methyne — a C=C or C≡C needs two carbons.

Why the difference matters

• Alkanes are unreactive. They are saturated: there is no double bond to open, so the only reactions they undergo are combustion and substitution with halogens under UV light.
• Alkenes and alkynes are reactive. The multiple bond can open, so they undergo addition reactions readily.
• This single difference is the basis of the bromine water test that tells them apart, and the reason alkenes are the starting point for making alcohols, haloalkanes and plastics.