Formulae, Functional Groups and Terminology

Molecular formula. Just the count of each atom. E.g. butane: $\text{C}_{4}\text{H}_{10}$.

Structural formula. Compact form showing connectivity. E.g. butane: $\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{CH}_{3}$ or $\text{CH}_{3}(\text{CH}_{2})_{2}\text{CH}_{3}$.

Displayed formula. Every atom AND every bond drawn out:

    H   H   H   H
    |   |   |   |
H - C - C - C - C - H
    |   |   |   |
    H   H   H   H

When to use which. Cambridge questions specify:

• "Give the molecular formula": just the totals.
• "Give the structural formula": condensed.
• "Draw the displayed formula" or "show all the bonds": every bond.

Worked. Ethanol — give all three forms.

• Molecular: $\text{C}_{2}\text{H}_{6}\text{O}$.
• Structural: $\text{CH}_{3}\text{CH}_{2}\text{OH}$ or $\text{C}_{2}\text{H}_{5}\text{OH}$.
• Displayed: H, H, H bonded to C; that C bonded to another C; second C bonded to H, H, and O–H.

Cambridge tip. Match the form to the question. Drawing a displayed formula when only molecular is asked wastes time; conversely, giving molecular when a structural question is asked loses marks.

Homologous series = a family of compounds where:

• All members have the SAME functional group.
• All members fit a SINGLE general formula.
• Consecutive members differ by a CH₂ unit.
• Properties change GRADUALLY through the series.

Examples.

Series	Functional group	General formula	Example
Alkanes	C-C single bonds only	$\text{C}_{n}\text{H}_{2n+2}$	CH₄, C₂H₆, C₃H₈
Alkenes	C=C double bond	$\text{C}_{n}\text{H}_{2n}$	C₂H₄, C₃H₆, C₄H₈
Alcohols	-OH	$\text{C}_{n}\text{H}_{2n+1}\text{OH}$	CH₃OH, C₂H₅OH, C₃H₇OH
Carboxylic acids	-COOH	$\text{C}_{n}\text{H}_{2n+1}\text{COOH}$	HCOOH, CH₃COOH, C₂H₅COOH

Trends within a homologous series.

• Boiling point INCREASES with chain length (longer chain → stronger intermolecular forces).
• Viscosity INCREASES.
• Volatility DECREASES.
• Reactivity (similar across the series — same functional group).

Worked qualitative. Which has higher b.p., methane or hexane? Hexane — longer chain → stronger intermolecular forces → higher b.p.

Worked. Calculate the formula of decane ($n = 10$ alkane).

• $\text{C}_{n}\text{H}_{2n+2}$: $\text{C}_{10}\text{H}_{22}$.

Structural isomer. Two or more compounds that have the SAME molecular formula but DIFFERENT structural formulae.

Worked. $\text{C}_{4}\text{H}_{10}$ has two isomers:

1. Butane: $\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{CH}_{3}$ — straight chain, 4 carbons in a row.
2. 2-methylpropane (isobutane): $\text{CH}_{3}\text{CH(CH}_{3}\text{)CH}_{3}$ — 3-C main chain with a methyl branch.

Same molecular formula ($\text{C}_{4}\text{H}_{10}$); different structural formulae → different boiling points, slightly different chemical properties.

Worked. $\text{C}_{3}\text{H}_{8}\text{O}$ has three isomers (Extended):

• propan-1-ol: $\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{OH}$.
• propan-2-ol: $\text{CH}_{3}\text{CH(OH)CH}_{3}$.
• methoxyethane: $\text{CH}_{3}\text{O}\text{CH}_{2}\text{CH}_{3}$ (an ether — different functional group).

At Cambridge IGCSE Extended, you're typically asked to:

• Identify whether two formulae represent isomers.
• Draw a small number of structural isomers for a given molecular formula.
• Predict that isomers may have different physical properties.

Cambridge tip. When asked to "draw all isomers", be systematic — start with the longest possible chain, then shorter chains with branches.