Lesson 5: Hydrocarbon Reactions | CHE 2.5

Lesson at a Glance | He Tirohanga Whakamua

Do NowBromine water test: Why does an alkene decolourise it instantly?10 min

Substitution vs AdditionAlkane +

B r_{2} / U V

vs Alkene +

B r_{2}

15 min

Markovnikov's Rule"The rich get richer" Major vs Minor products15 min

Portfolio EntryWrite Section 5: Hydrocarbon Reactions & Markovnikov Map10 min

Exit DrillPredict major product for propene +

H B r

5 min

Ngā Whāinga Ako | Learning Intentions

Students will know

The difference between Substitution (swapping one atom/group for another; e.g. alkane + $B r_{2} \overset{U V l i g h t}{\to}$ haloalkane + $H B r$ ) and Addition (breaking $C = C$ double bond to add two atoms across the carbons).
The 4 key alkene addition reactions: Hydrogenation ( $H_{2} / N i$ ), Halogenation ( $B r_{2} (a q)$ ), Hydration ( $H_{2} O / H^{+}$ ), and Hydrohalogenation ( $H X$ ).
Markovnikov's Rule: When adding an unsymmetrical reagent ( $H X$ or $H_{2} O$ ) to an unsymmetrical alkene, Hydrogen adds to the double-bonded carbon with MORE hydrogens attached, forming the Major Product.

Students will demonstrate

By predicting structural formulas for major and minor products of propene + $H B r$ .
By completing Section 5 of their Level 2 Organic Chemistry Mastery Portfolio.

Curriculum alignment

NZC (2007) · Science · Level 7 · Material World: “Investigate and measure the chemical and physical properties of a range of groups of substances, for example, acids and bases, oxidants and reductants, and selected organic and inorganic compounds.”

Do Now | Tīmatanga Whakaaro (10 min)

Bromine Water Test Prompt:

"When orange bromine water ( $B r_{2} (a q)$ ) is shaken with hexane in the dark, nothing happens. When shaken with hex-1-ene in the dark, the orange colour disappears instantly. Why?"

Unpack: Hexane is an alkane; it requires UV light to undergo slow substitution. Hex-1-ene has a reactive $C = C$ double bond that undergoes rapid addition, breaking the double bond to absorb $B r_{2}$ and form colorless 1,2-dibromohexane instantly!

Markovnikov's Rule: Major & Minor Products (15 min)

Reaction: Propene ( ${CH}_{3} CH = {CH}_{2}$ ) + Hydrogen Bromide ( $HBr$ )

🌟 Major Product (~90%): 2-bromopropane

${CH}_{3} CH (Br) {CH}_{3}$
Hydrogen adds to C1 (which has 2 H's); Bromine adds to C2 (which has 1 H). "The rich get richer!"

⚠️ Minor Product (~10%): 1-bromopropane

${CH}_{3} {CH}_{2} {CH}_{2} Br$
Hydrogen adds to C2; Bromine adds to C1. Occurs less frequently due to lower carbocation stability.

Substitution versus Addition (15 min)

Where this usually goes wrong: students reach for the wrong reaction type because they did not check saturation first. Decide whether the molecule is saturated before choosing anything else.

Alkanes substitute, and need UV

Saturated, so there is nothing to add across. A halogen swaps for a hydrogen: $C H_{4} + B r_{2} \to C H_{3} B r + H B r$ , and it needs ultraviolet light to start. Without UV, nothing happens — which is itself a diagnostic observation.

Alkenes add, and need no UV

The $C = C$ opens and both halogen atoms attach: $C_{2} H_{4} + B r_{2} \to C_{2} H_{4} B r_{2}$ . This is why bromine water is decolourised instantly by an alkene in the dark. That single observation separates the two families in one test.

Do this now: predict the product when bromine water is added to hexane and to hexene, in the dark. State what a student would SEE in each tube, not just the equation.

📁 Organic Chemistry Portfolio — Section 5: Hydrocarbon Reactions

Students open their Level 2 Chemistry Portfolio and complete Section 5:

Section 5 Requirements:

1. Hydrocarbon Reaction Map: Equations and conditions for Alkane $+ B r_{2} / U V$ , Alkene $+ H_{2} / N i$ , Alkene $+ B r_{2}$ , Alkene $+ H_{2} O / H^{+}$ , and Alkene $+ H C l$ .

2. Markovnikov Application: Draw and name Major and Minor products for 2 unsymmetrical reactions: 1) But-1-ene $+ H B r$ , and 2) 2-methylbut-2-ene $+ H_{2} O / H^{+}$ .

3. Excellence Diagnostic Comparison: 1-paragraph explanation of how orange $B r_{2} (a q)$ water distinguishes alkanes from alkenes in light vs dark conditions.

Exit Verification | Ka Mutu Hoki (5 min)

Exit Check:

"My Section 5 correctly identifies 2-propanol as the major product of propene hydration and explains Markovnikov's Rule."

Teacher Planning & NCEA Alignment

NCEA Level 2 Chemistry Alignment (4 Credits External):

Hydrocarbon Reactions: Predict reagents, conditions, and structural formulas for substitution and addition reactions.
Markovnikov's Rule: Justify major and minor products for addition of unsymmetrical reagents to unsymmetrical alkenes.

Vocabulary: Substitution reaction, addition reaction, hydrogenation, halogenation, hydration, hydrohalogenation, Markovnikov's Rule, major product, minor product, bromine water decolourisation.

Other teaching approach: Guided Viewing & Practical Chemistry →

Lesson 5: Alkanes, Alkenes & Alkynes: Addition & Substitution Reactions