IMAT Chemistry · Organic Chemistry

Hydrocarbons: Families, Benzene, Names and Unsaturation

Hydrocarbons contain only carbon and hydrogen; their family, their name and their number of rings and multiple bonds can all be read from the formula.

All Organic Chemistry notes

Why this matters

The 2026 paper asked which family ethene belongs to and the 2024 paper asked which of five named compounds has the most hydrogen atoms. Older papers asked for formulas of aromatic compounds and for the IUPAC name of a drawn molecule.

Concept 1 of 4: Hydrocarbon families and their general formulas

Start from an alkane, where every carbon holds as many hydrogens as it can. Each double bond or ring costs two hydrogens, and a triple bond costs four. So the general formula of a family tells you how many multiple bonds and rings it has.

Definition

A hydrocarbon contains only C and H.

Saturated: only C-C single bonds (alkanes, cycloalkanes). Unsaturated: at least one C=C or C≡C (alkenes, alkynes, arenes).
A compound with two C=C bonds is a diene (general formula $\mathrm{C_nH_{2n-2}}$ , like an alkyne).
Alkanes are fairly unreactive: they burn, and react with halogens in UV light. Alkenes and alkynes are much more reactive because of the π bond.
A homologous series is a family whose members differ by $\mathrm{CH_2}$ ; they share a general formula and similar chemistry.

Family	General formula	Key feature	Example	Name ending
Alkane	$\mathrm{C_nH_{2n+2}}$	Only single bonds, open chain	Propane $\mathrm{C_3H_8}$	-ane
Cycloalkane	$\mathrm{C_nH_{2n}}$	Single bonds in one ring	Cyclohexane $\mathrm{C_6H_{12}}$	cyclo-, then -ane
Alkene	$\mathrm{C_nH_{2n}}$	One C=C double bond	Propene $\mathrm{C_3H_6}$	-ene
Alkyne	$\mathrm{C_nH_{2n-2}}$	One C≡C triple bond	Propyne $\mathrm{C_3H_4}$	-yne
Arene (aromatic)	$\mathrm{C_nH_{2n-6}}$ for one benzene ring	A benzene ring	Methylbenzene $\mathrm{C_7H_8}$	-benzene

Alkenes and cycloalkanes share

\mathrm{C_nH_{2n}}

; a formula alone cannot tell them apart.

Practice this conceptself-check · 4 quick reps

Ethene is an alkene, not an alkyne

The letters before the ending count the carbons (eth = 2) and the ending gives the bond: -ane single, -ene double, -yne triple. Ethene

\mathrm{CH_2{=}CH_2}

has one double bond, so it is an alkene. A diene has two C=C bonds and needs at least three carbons.

Concept 2 of 4: Benzene and aromatic compounds

Benzene looks as if it should have three double bonds in a ring, but it does not behave that way. Its six π electrons spread evenly around the whole ring. This makes the ring flat, very stable and unwilling to give up its π electrons in addition reactions.

Definition

Benzene is $\mathrm{C_6H_6}$ : a ring of six carbons, each carrying one H.

Every carbon is sp² and joined to three atoms, so the molecule is planar with 120° angles.
The π electrons are delocalised over the ring. All six C-C bonds are the same length, between a single and a double bond. The ring is often drawn as a hexagon with a circle inside.
Aromatic compounds contain a benzene ring. Benzene reacts mainly by substitution (an H is replaced), because addition would destroy the stable ring.
Common aromatics: methylbenzene (toluene) $\mathrm{C_6H_5CH_3}$ , phenol $\mathrm{C_6H_5OH}$ , benzoic acid $\mathrm{C_6H_5COOH}$ . The group $\mathrm{C_6H_5}$ is called phenyl.
Rings that share an edge are fused. Each extra fused ring adds 4 C and 2 H.

Property	Benzene	Cyclohexane, for contrast
Formula	$\mathrm{C_6H_6}$	$\mathrm{C_6H_{12}}$
Hybridisation of every carbon	$\mathrm{sp^2}$	$\mathrm{sp^3}$
Shape	Flat hexagon, 120° angles	Puckered ring (chair or boat shape)
C-C bonds	Six equal bonds, delocalised π electrons	Six single bonds
Typical reaction	Substitution (keeps the ring)	Burns; substitution with halogens in UV light

Practice this conceptself-check · 4 quick reps

Benzene is flat; the chair and boat shapes belong to cyclohexane

Benzene's carbons are all sp² and the ring is planar. Cyclohexane's carbons are sp³, so its ring folds into chair or boat shapes. Options that mix the two (sp² but chair-shaped, or some sp³ carbons in benzene) are wrong.

Concept 3 of 4: IUPAC names for alkanes and alkenes

An IUPAC name is a set of instructions for drawing the molecule. The stem gives the longest carbon chain, the ending gives the bond type, and numbered prefixes say which branches hang off which carbon. Number the chain so that the numbers come out as small as possible.

Definition

Steps:

Find the longest carbon chain. It may turn through a branch as written. Stems: meth 1, eth 2, prop 3, but 4, pent 5, hex 6, hept 7, oct 8, non 9, dec 10.
For an alkene, the chain must contain the C=C, and the C=C gets the lowest possible number: but-1-ene, but-2-ene. Two double bonds: buta-1,3-diene.
Number from the end that gives the branches (after the C=C) the lowest locants.
Name the branches: methyl $\mathrm{CH_3}$ , ethyl $\mathrm{C_2H_5}$ , chloro, bromo. Use di-, tri- for repeats, and list branches in alphabetical order (ignore di- and tri- when sorting).
Commas between numbers, hyphens between numbers and letters: 2,3-dimethylpentane.
A ring takes cyclo-: methylcyclohexane. Older names put the number first (2-butene for but-2-ene); IMAT uses both.

Order of an IUPAC name

\text{locants-branches} + \text{stem} + \text{ending}

branchesalphabetical, with their carbon numbers
stemthe number of carbons in the longest chain
ending-ane, -ene (with its number), -yne

Worked example

Name the alkane

\mathrm{CH_3CH(CH_3)CH(C_2H_5)CH_2CH_3}

Practice this conceptself-check · 4 quick reps

The longest chain is not always the one written in a straight line

Condensed and skeletal formulas often bend the main chain through a branch. Check every path from one end carbon to another; a molecule written as a butane with an ethyl branch may really be a pentane or hexane chain.

Concept 4 of 4: Degrees of unsaturation: rings and multiple bonds from a formula

An open-chain alkane holds the most hydrogens a carbon skeleton can carry. Every ring or π bond removes two of them. So by counting how many hydrogens are missing compared with the alkane, you know how many rings plus π bonds the molecule has, without seeing its structure.

Definition

The degree of unsaturation (DoU) is the number of rings plus π bonds.

A C=C or C=O counts 1, a C≡C or C≡N counts 2, a ring counts 1, a benzene ring counts 4 (one ring, three π).
Halogens count like H. Nitrogen adds one place for H. Oxygen and sulfur do not change the count.
To compare hydrogen counts of named compounds: each ring or double bond removes 2 H from the alkane value $2n + 2$ .

Degree of unsaturation

\text{DoU} = \frac{2C + 2 + N - H - X}{2}

$C, H, N$ numbers of carbon, hydrogen and nitrogen atoms
$X$ number of halogen atoms (F, Cl, Br, I)

Worked example

Find the degree of unsaturation of

\mathrm{C_5H_8}

and of

\mathrm{C_4H_9Br}

, and say what each could contain.

Practice this conceptself-check · 4 quick reps

A ring removes two hydrogens just like a double bond

Cyclohexane

\mathrm{C_6H_{12}}

has two fewer H than hexane

\mathrm{C_6H_{14}}

even though it has no double bond. When ranking named compounds by hydrogen count, a 'cyclo' or a 'methylcyclo' costs 2 H, exactly like an '-ene'.

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 (2)

IUPAC names for alkanes and alkenes
Order of an IUPAC name
$\text{locants-branches} + \text{stem} + \text{ending}$
Degrees of unsaturation: rings and multiple bonds from a formula
Degree of unsaturation
$\text{DoU} = \frac{2C + 2 + N - H - X}{2}$

Reference tables (2)

Hydrocarbon families and their general formulas5 rows

Family	General formula	Key feature	Example	Name ending
Alkane	$\mathrm{C_nH_{2n+2}}$	Only single bonds, open chain	Propane $\mathrm{C_3H_8}$	-ane
Cycloalkane	$\mathrm{C_nH_{2n}}$	Single bonds in one ring	Cyclohexane $\mathrm{C_6H_{12}}$	cyclo-, then -ane
Alkene	$\mathrm{C_nH_{2n}}$	One C=C double bond	Propene $\mathrm{C_3H_6}$	-ene
Alkyne	$\mathrm{C_nH_{2n-2}}$	One C≡C triple bond	Propyne $\mathrm{C_3H_4}$	-yne
Arene (aromatic)	$\mathrm{C_nH_{2n-6}}$ for one benzene ring	A benzene ring	Methylbenzene $\mathrm{C_7H_8}$	-benzene

Alkenes and cycloalkanes share

\mathrm{C_nH_{2n}}

; a formula alone cannot tell them apart.

Benzene and aromatic compounds5 rows

Property	Benzene	Cyclohexane, for contrast
Formula	$\mathrm{C_6H_6}$	$\mathrm{C_6H_{12}}$
Hybridisation of every carbon	$\mathrm{sp^2}$	$\mathrm{sp^3}$
Shape	Flat hexagon, 120° angles	Puckered ring (chair or boat shape)
C-C bonds	Six equal bonds, delocalised π electrons	Six single bonds
Typical reaction	Substitution (keeps the ring)	Burns; substitution with halogens in UV light

Watch out for (4)

Ethene is an alkene, not an alkyne
→ Hydrocarbon families and their general formulas
Benzene is flat; the chair and boat shapes belong to cyclohexane
→ Benzene and aromatic compounds
The longest chain is not always the one written in a straight line
→ IUPAC names for alkanes and alkenes
A ring removes two hydrogens just like a double bond
→ Degrees of unsaturation: rings and multiple bonds from a formula

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