Ch 1 · Formulae, functional groups and the naming of organic compounds · AS & A Level Chemistry 9701

Topic 1 of 4 in An introduction to AS Level organic chemistry — AS & A Level Chemistry 9701 — 58 lessons.

Introduction

Organic chemistry is built on a small set of drawing and naming conventions that let any compound, however large, be represented unambiguously. Before any mechanism or reaction can be studied, you must be able to move between the different types of formula and apply IUPAC nomenclature accurately, since examiners penalise imprecise or ambiguous names and formulae throughout the AS and A Level papers.

Key ideas — Hydrocarbons and functional groups

A hydrocarbon is a compound made up of carbon and hydrogen only, for example C_3H_8 (propane) or C_2H_4 (ethene). Most organic molecules also contain a functional group: a specific atom or group of atoms within a molecule that is responsible for its characteristic physical and chemical reactions. The functional group dictates how a molecule behaves regardless of the length of its hydrocarbon chain; for example, every alcohol containing an -OH group undergoes broadly similar reactions, such as oxidation and esterification, whatever its chain length.

🧑‍🏫 Teacher's explanation

Think of an organic molecule like a car. The 'hydrocarbon' part is the chassis and the body—it provides the basic shape and size. However, the 'functional group' is like the engine. No matter how big or small the car is, if it has a specific type of engine, it will behave in a specific way (like how fast it goes or how it consumes fuel). In chemistry, the hydrocarbon chain determines the physical properties like boiling point, but the functional group determines the chemical 'personality'.

📖 Definitions

🔍 Worked example — Easy

Draw the displayed formula for butane and determine its molecular formula.

🔍 Worked example — Standard

Provide the IUPAC name for the following molecule: CH_3CH(OH)CH_2CH_3

🔍 Worked example — Exam-style

A compound has the skeletal formula representing a 3-carbon chain with a double bond between C1 and C2 and a chlorine atom on C3. Write its molecular formula and IUPAC name.

⚠️ Common errors

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🎯 Exam question

(a) Draw the displayed formula for this compound. [2] (b) Give the IUPAC name for this compound. [1] [3 marks]

Key ideas — Types of formula

Several kinds of formula represent the same molecule at different levels of detail:

🧑‍🏫 Teacher's explanation

Think of organic chemistry like a map. A molecular formula is like saying 'this city is 50km from here'—it tells you the distance but nothing about the route. A structural formula is like a simplified road map showing main highways. A displayed formula is like a high-resolution satellite image showing every single house and driveway. Finally, a skeletal formula is like a subway map; it only shows the essential connections (the lines) and ignores the small details (the houses) to make the system easy to read.

📖 Definitions

🔍 Worked example — Easy

A compound has the molecular formula C_3H_8. Write its structural formula.

🔍 Worked example — Standard

A molecule has a molecular formula of C_4H_10. Determine its empirical formula.

🔍 Worked example — Exam-style

A skeletal formula shows a zig-zag line with 5 vertices (including ends). How many hydrogen atoms are attached to the carbon at the second vertex from the left?

⚠️ Common errors

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🎯 Exam question

Key ideas — Homologous series and naming stems

A homologous series is a family of compounds with the same functional group and general formula, in which successive members differ by CH_2 and show a gradual trend in physical properties, such as boiling point. Systematic (IUPAC) names are built from a stem showing the number of carbon atoms in the longest chain (meth-, eth-, prop-, but-, pent-, hex-) and a suffix or prefix showing the functional group, for example -ane (alkane), -ene (alkene), -ol (alcohol), -al (aldehyde), -one (ketone), -oic acid (carboxylic acid) and chloro-/bromo-/iodo- (halogenoalkane).

🧑‍🏫 Teacher's explanation

Think of a homologous series like a family of cars. Every car in the 'SUV family' has the same basic structure (four wheels, a steering wheel, an engine), but as you go from a small SUV to a massive one, the size increases predictably. In organic chemistry, the 'family' is the homologous series. Every member has the same functional group—the part of the molecule that actually 'does' the chemistry—but they get slightly larger as you add CH_2 units.

📖 Definitions

🔍 Worked example — Easy

Identify the homologous series and the general formula for the following series of molecules: CH_4, C_2H_6, C_3H_8.

🔍 Worked example — Standard

Write the IUPAC name for a straight-chain molecule containing 5 carbon atoms, with a double bond between the second and third carbons.

🔍 Worked example — Exam-style

A compound has the molecular formula C_3H_6O. It contains a carbonyl group (C=O) at the end of the carbon chain. Draw the structural formula and name it.

⚠️ Common errors

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🎯 Exam question

Key ideas — Rules for systematic nomenclature

To name an aliphatic compound:

🧑‍🏫 Teacher's explanation

Think of naming organic molecules like giving someone directions to a specific house in a long street. If you just say 'the blue house', it's not helpful. You need to know which street (the longest carbon chain), which house number (the locant), and what special features it has, like a red door (substituents).

📖 Definitions

🔍 Worked example — Easy

Name the following compound: CH_3CH_2CH_2Cl

🔍 Worked example — Standard

Name the following compound: CH_3C(CH_3)_2CH_2OH

🔍 Worked example — Exam-style

Provide the IUPAC name for a molecule with a 5-carbon chain, an - extOH group on carbon 2, and a methyl group on carbon 4.

⚠️ Common errors

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🎯 Exam question

(a) Draw the displayed formula for this compound. (b) Provide the systematic IUPAC name for this compound. [3 marks]

Key ideas — Deducing formulae from a diagram

Given a structural, displayed or skeletal formula, the molecular formula is found by counting every atom of each element present. The empirical formula is then obtained by dividing each of these numbers by their highest common factor. For example, hex-1-ene, mathrmC_6H_12, has an empirical formula of CH_2, since dividing both subscripts by 6 gives the simplest whole-number ratio.

🧑‍🏫 Teacher's explanation

Think of an organic molecule like a Lego model. The 'displayed formula' is like the instruction manual showing every single brick and how they are clicked together. The 'structural formula' is like a simplified blueprint showing how the pieces are connected, and the 'skeletal formula' is like a quick sketch where we don't bother drawing every single brick, just the main frame.

Ch 1 · Formulae, functional groups and the naming of organic compounds · AS & A Level Chemistry 9701

All chapters in An introduction to AS Level organic chemistry