How to draw structural formulae of organic compounds

Drawing a structural formula is where organic marks are won or lost, because a single missing bond or an over-bonded carbon turns a correct answer into a wrong one. In carbon compounds, you are regularly asked to draw the displayed formula of a named compound, or to draw its isomers. This guide gives you one rule and a repeatable method so your structures are always valid.

The formulae you need to know apart

SPM uses several kinds of formula, and questions specify which one they want:

• Molecular formula, how many of each atom, e.g. C₂H₆ for ethane.
• Structural (displayed) formula, every atom and every bond drawn out in full, so each bond is a line.
• Condensed structural formula, the bonds implied, written on one line, e.g. CH₃CH₃ for ethane or CH₃CH₂OH for ethanol.

When a question says “draw the structural formula”, it usually wants the displayed version with all bonds shown. Read the command word carefully.

The one rule: valency

Every structure you draw must obey the valencies, the fixed number of bonds each atom forms:

• Carbon forms 4 bonds.
• Hydrogen forms 1 bond.
• Oxygen forms 2 bonds.

That is the whole secret. If any carbon in your drawing has more or fewer than four lines coming off it, the structure is wrong. Checking every carbon for exactly four bonds is the single most valuable habit you can build, and it catches almost every error before the examiner does.

A step-by-step method

1. Draw the carbon skeleton first, just the carbon atoms in a chain (or with a branch), joined by single bonds, ignoring hydrogen for now.
2. Insert the functional group if there is one, a C=C double bond for an alkene, an –OH for an alcohol, a –COOH for a carboxylic acid.
3. Add hydrogens to fill every carbon up to four bonds. Count the bonds already on each carbon and add H atoms until it reaches four.
4. Check every atom’s valency, each C has 4 bonds, each O has 2, each H has 1. If it all balances, the structure is valid.

Worked drawings

Ethane, C₂H₆. Skeleton: C–C. No functional group. Each carbon already has one bond (to the other carbon), so each needs three more hydrogens: CH₃–CH₃. Displayed, that is two carbons each bonded to three H and to each other, every carbon has four bonds. Correct.

Propene, C₃H₆. This is an alkene, so it must contain a C=C. Skeleton with the double bond: C=C–C. The two double-bonded carbons: the first C=C carbon has the double bond (counts as 2) plus needs 2 H; the middle carbon has the double bond (2) plus a single bond to the third carbon (1) plus 1 H; the end carbon has one bond to the middle carbon and needs 3 H. Result: CH₂=CH–CH₃. Check each carbon has four bonds, it does.

Ethanol, C₂H₅OH. Skeleton C–C, then attach –OH to one carbon: C–C–OH. Fill hydrogens: the CH₃ end carbon needs 3 H; the middle carbon is bonded to the CH₃, to the O, and needs 2 H. The oxygen has one bond to carbon and one to hydrogen (2 bonds, correct). Result: CH₃–CH₂–OH.

Ethanoic acid, CH₃COOH. The –COOH group is one carbon double-bonded to one oxygen and single-bonded to an –OH. Draw CH₃– joined to that carboxyl carbon: CH₃–COOH, where the acid carbon shows C=O and C–O–H. Check: the carboxyl carbon has a single bond to CH₃, a double bond to O, and a single bond to the O of OH, four bonds. Correct.

Drawing isomers

Isomers are compounds with the same molecular formula but different structural formulae. A classic task is to draw the isomers of a formula. Take C₄H₁₀: you can draw a straight chain, CH₃–CH₂–CH₂–CH₃ (butane), or a branched chain, CH₃–CH(CH₃)–CH₃ (2-methylpropane). Both are C₄H₁₀; both obey the valency rule; they differ only in how the carbons are arranged. Alkenes give positional isomers too, but-1-ene has the double bond at the end, CH₂=CH–CH₂–CH₃, while but-2-ene has it in the middle, CH₃–CH=CH–CH₃. When you draw isomers, change the carbon arrangement or the position of the functional group, never the number of atoms. The isomerism glossary entry has the precise definition to quote.

Common mistakes to avoid

The most frequent error is a carbon with five bonds or with only three, always recount. The second is forgetting that an alkene must show the double bond as two lines; writing it as a single bond turns it into an alkane. The third is drawing –COOH wrongly, missing either the C=O or the O–H. Because the double bond you draw changes how a molecule reacts, an alkene’s C=C is exactly what lets it undergo the addition reactions of alkenes, an accurate structure is not just neat, it is chemically meaningful.

How to get fluent

Structural formulae reward doing, not reading, so practise drawing every compound in the syllabus from its name until the four-bond check is automatic. If your structures keep coming back marked wrong for a valency slip you cannot spot yourself, a teacher looking over your drawing catches the pattern immediately. Our online one-to-one lessons run in English from RM50 an hour with a paid one-hour trial. Once drawing valid structures is second nature, the rest of organic chemistry, naming, isomers, reactions, has something solid to stand on.