Lesson 2: IUPAC Naming & Isomers | CHE 2.5

Lesson at a Glance | He Tirohanga Whakamua

Do NowWhy 2-methylpropane and butane share the formula

C_{4} H_{10}

10 min

IUPAC Naming RulesLongest chain, locant numbers & alphabetization15 min

Structural IsomerismChain isomers vs Positional isomers15 min

Portfolio EntryWrite Section 2: IUPAC Naming & Isomer Identification10 min

Exit DrillName 3 complex branched structures5 min

Ngā Whāinga Ako | Learning Intentions

Students will know

The systematic 4-step IUPAC naming system (Prefix root: meth- to oct-; suffix for functional group; locant numbers for branches & functional group position).
The definition of Structural Isomers: molecules with the same molecular formula but different structural arrangements of atoms.
The distinction between Chain Isomers (variations in carbon backbone branching) and Positional Isomers (variations in locant position of the functional group).

Students will demonstrate

By drawing and naming all structural isomers of $C_{4} H_{9} Cl$ and $C_{5} H_{12}$ .
By completing Section 2 of their Level 2 Organic Chemistry Mastery Portfolio.

Curriculum alignment

NZC (2007) · Science · Level 7 · Material World: “Relate properties of matter to structure and bonding.”

Do Now | Tīmatanga Whakaaro (10 min)

Isomerism Prompt:

"If two bottles of gas in a chemistry lab both read $C_{4} H_{10}$ on the label, but one boils at -0.5°C and the other boils at -11.7°C, how is that chemically possible?"

Unpack: They are structural isomers! Straight-chain butane ( ${CH}_{3} {CH}_{2} {CH}_{2} {CH}_{3}$ ) has greater surface area contact between molecules, leading to stronger intermolecular dispersion forces and a higher boiling point than branched 2-methylpropane ( ${CH}_{3} CH ({CH}_{3}) {CH}_{3}$ ).

4-Step IUPAC Naming Algorithm (15 min)

Step 1: Longest Carbon Chain

Find the longest continuous carbon chain containing the main functional group. (1=meth, 2=eth, 3=prop, 4=but, 5=pent, 6=hex, 7=hept, 8=oct).

Step 2: Numbering the Chain

Number the carbons from the end that gives the primary functional group (e.g. $- O H, - C O O H, C = C$ ) the lowest possible locant number.

Step 3: Identify Branch & Halo Substituents

Identify methyl ( $- C H_{3}$ ), ethyl ( $- C H_{2} C H_{3}$ ), chloro ( $- C l$ ), bromo ( $- B r$ ), iodo ( $- I$ ), and amino ( $- N H_{2}$ ) groups with their locant numbers.

Step 4: Alphabetize & Assemble

List substituents alphabetically (e.g. 2-bromo-3-methylpentane). Use di-, tri- prefixes if multiple identical groups exist.

Structural Isomerism (15 min)

Where this usually goes wrong: students count any redrawing as a new isomer. Rotating a bond on paper does not make a different compound — isomers must have the same molecular formula and a genuinely different connectivity.

Chain isomers: the skeleton changes

Same formula, different carbon backbone. $C_{4} H_{10}$ gives butane (a straight chain) and methylpropane (a branched one). Branching lowers the boiling point, because a more compact molecule has less surface contact with its neighbours.

Positional isomers: the group moves

Same skeleton, same functional group, different attachment point. $C_{3} H_{7} O H$ gives propan-1-ol and propan-2-ol. The IUPAC locant number exists precisely to tell these apart, which is why naming and isomerism are one topic and not two.

Do this now: draw every chain isomer of $C_{5} H_{12}$ . There are three. If you find a fourth, check whether you have merely rotated one you already have.

📁 Organic Chemistry Portfolio — Section 2: IUPAC Naming & Structural Isomers

Students open their Level 2 Chemistry Portfolio and complete Section 2:

Section 2 Requirements:

1. IUPAC Naming Practice: Draw structural formulas and provide systematic IUPAC names for 6 challenging molecules (e.g. 2,3-dichlorobutane, 3-methylbutan-2-ol, 2-methylpropanoic acid).

2. Structural Isomer Grid ( $C_{4} H_{9} Cl$ ): Draw and name all 4 haloalkane isomers: 1-chlorobutane, 2-chlorobutane (positional), 1-chloro-2-methylpropane, and 2-chloro-2-methylpropane (chain).

3. Excellence Isomer Distinction: 1-paragraph explanation of why 2-methylpropan-2-ol is a tertiary alcohol while its positional isomer butan-1-ol is a primary alcohol.

Exit Verification | Ka Mutu Hoki (5 min)

Exit Check:

"My Section 2 correctly identifies all 4 isomers of C₄H₉Cl and uses correct IUPAC hyphen/comma punctuation."

Teacher Planning & NCEA Alignment

NCEA Level 2 Chemistry Alignment (4 Credits External):

IUPAC Nomenclature: Name and draw structural formulas for organic molecules up to 8 carbons in the main chain.
Structural Isomerism: Draw and identify chain isomers and positional isomers.

Vocabulary: IUPAC nomenclature, prefix root, suffix, locant number, structural isomerism, chain isomer, positional isomer, branch chain.

Other teaching approach: Guided Viewing & Practical Chemistry →

Lesson 2: IUPAC Nomenclature Rules & Structural Isomerism