Organic Compounds and representation

Organic chemistry is the chemistry of carbon compounds. Carbon is special because each carbon atom can join to up to four other atoms — and it can join to other carbon atoms to build long chains, branches and rings. That is why there are millions of carbon compounds but only a handful of compounds for most other elements.

Hydrocarbon = a compound that contains HYDROGEN and CARBON ONLY — no other elements at all.

• ✅ IS a hydrocarbon: methane (CH₄), ethane (C₂H₆), ethene (C₂H₄), propane (C₃H₈), hexane (C₆H₁₄). Just C and H.
• ❌ NOT a hydrocarbon: methanol (CH₃OH — contains O), chloromethane (CH₃Cl — contains Cl), ethanoic acid (CH₃COOH — contains O). One extra element and it is no longer a hydrocarbon.

Most hydrocarbons come from crude oil (a mixture of mainly hydrocarbons) and natural gas (mostly methane). They are the basis of our fuels — petrol, diesel, natural gas — and the raw material for plastics.

The golden rule — carbon forms FOUR bonds.

Every carbon atom in a stable organic molecule has exactly four bonds coming off it (no more, no fewer). This single rule lets you build a structure and check it: count the lines on each carbon — there must be four. Hydrogen, in contrast, forms only one bond.

Hydrocarbons are sorted into two groups by the kind of carbon–carbon bonds they contain.

Saturated. A compound that contains ONLY single C–C bonds. Every carbon is bonded to the maximum number of atoms it can hold — the molecule is 'full up'.

• Examples: methane (CH₄), ethane (C₂H₆), propane (C₃H₈). These are alkanes.

Unsaturated. A compound that contains AT LEAST ONE C=C double bond.

• Example: ethene (C₂H₄). These are alkenes.
• The word 'unsaturated' means the molecule is not full — it could add more atoms across the double bond.

How to tell them apart from a structure. Look at the bonds between carbon atoms:

• All single lines between carbons → saturated.
• Spot a double line (=) between two carbons → unsaturated.

The C=C double bond is shown as two parallel lines in a displayed formula. Spotting it is the whole skill — and it is also the key to the bromine water test (covered in the alkenes subtopic): an alkene decolourises orange-brown bromine water; an alkane does not.

There are three formulae you must be able to write, read and convert between. They describe the same molecule at different levels of detail.

(1) Molecular formula. Just the count of each atom — how many carbons and how many hydrogens. It tells you nothing about how the atoms are joined.

• Ethane → C₂H₆. Ethene → C₂H₄. Propane → C₃H₈.

(2) Displayed (structural) formula. Shows EVERY atom and EVERY bond drawn out in full, with each bond as a line. A single bond = one line; a double bond = two parallel lines. This is where you apply the rule each carbon has four bonds.

(3) Condensed structural formula. A shorthand that groups each carbon with its hydrogens, written in a line. It shows how atoms are connected without drawing every bond.

• Ethane → CH₃CH₃. Ethene → CH₂=CH₂ (the = keeps the double bond visible). Propane → CH₃CH₂CH₃.

Worked comparison — ethane, three ways.

Representation	Ethane	What it shows
Molecular formula	C₂H₆	how many of each atom
Condensed structural	CH₃CH₃	which atoms join which
Displayed formula	(every C–H and C–C drawn as lines)	every atom AND every bond

Converting between them.

• Molecular → displayed: lay out the carbon 'skeleton', join the carbons, then add H atoms until every carbon has four bonds. For C₂H₆: draw C–C (that uses one bond on each carbon), then add 3 H to each carbon → 6 H in total → C₂H₆. ✓
• Displayed → molecular: just count the atoms in your drawing. Count carbons, count hydrogens, write Cₓ Hᵧ.

Common displayed/condensed formulae to know:

Compound	Molecular	Condensed	Bonds between carbons
Methane	CH₄	CH₄	(only one carbon)
Ethane	C₂H₆	CH₃CH₃	single C–C
Propane	C₃H₈	CH₃CH₂CH₃	single C–C
Ethene	C₂H₄	CH₂=CH₂	double C=C

Drawing a correct displayed formula is a high-frequency exam skill. Follow a fixed routine and you will not lose marks.

The routine.

1. Draw the carbon chain — write the carbons and join them with single lines (or a double line where there is a C=C).
2. Give each carbon four bonds — add hydrogen atoms (each on a single line) until every carbon has exactly four lines coming off it.
3. Check every hydrogen has one bond and every carbon has four.

Worked check — ethene (C₂H₄). Draw C=C between the two carbons. That double bond uses two of the four bonds on each carbon, leaving two bonds free on each. Add 2 H to each carbon → 4 H total → C₂H₄. ✓ Every carbon now has four bonds (two in the C=C, two to H).

The most common mistakes the routine catches:

• A carbon with only three lines (a missing bond) — add a hydrogen.
• A carbon with five lines (one too many) — usually a forgotten double bond drawn as well as too many H.
• Drawing the C=C as a single line — that turns an unsaturated molecule into a saturated one (wrong formula AND wrong classification).

Reading a structure you are given. To interpret a displayed formula:

• Classify it: scan the carbon–carbon bonds — any double line means unsaturated, otherwise saturated.
• Write its molecular formula: count all the C atoms and all the H atoms.

This 'draw, then check the four bonds' habit is the single most reliable way to score full marks on structure questions.