Chemistry · Class 11

Active learning ideas

Alkenes: Preparation and Reactions

Let's explore alkynes, the hydrocarbons featuring a reactive carbon-carbon triple bond. This topic uncovers their unique linear geometry and the surprising acidity of terminal alkynes.

• CBSE Learning Outcomes
• NCERT Class 11 Chemistry: Unit 13 - Hydrocarbons

• 15–25 min
• Pairs → Whole Class
• 3 activities

Activity 01

• Collaborative Problem-Solving
• 25 min · Pairs

Hybridisation Model Building

Using molecular model kits, students build models of ethane, ethene, and ethyne. They compare the bond angles, linearity, and rotation around the carbon-carbon bonds to physically grasp the structural differences arising from sp3, sp2, and sp hybridisation.

Explain Markovnikov's rule with a suitable example and its mechanism.

Facilitation Tip

Ask students to explain why the ethyne model is linear while the ethane model is tetrahedral.

What to look for

Give students an exit slip with the structure of pent-1-yne. Ask them to predict the major product upon reaction with a) H2/Lindlar's catalyst and b) H2O, H2SO4, HgSO4.

• Apply
• Analyze
• Evaluate
• Create
• Relationship Skills
• Decision-Making
• Self-Management

Activity 02

• Collaborative Problem-Solving
• 20 min · Small Groups

Predict the Product Challenge

The teacher writes various alkynes (e.g., propyne, but-2-yne) and reagents (HBr, Br2, H2O/H2SO4,HgSO4) on the board. In small groups, students race to draw the correct major products, justifying their answers with Markovnikov's rule or tautomerism.

Compare the products of ozonolysis of propene and but-2-ene.

Facilitation Tip

Include both symmetrical and unsymmetrical alkynes to check for a thorough understanding of regioselectivity.

What to look for

Include a question in the unit test that requires students to devise a two-step synthesis, such as converting 1,2-dibromopropane into propanone, which involves alkyne formation and hydration.

• Apply
• Analyze
• Evaluate
• Create
• Relationship Skills
• Decision-Making
• Self-Management

Activity 03

• Collaborative Problem-Solving
• 15 min · Individual

Acidity Ranking Ladder

Students are given cards with structures of an alkane, alkene, terminal alkyne, and water. They must arrange them in order of increasing acidity and write a one-line justification for their order, focusing on the stability of the conjugate base.

Analyse the reaction of HBr with propene in the presence and absence of peroxide.

Facilitation Tip

Prompt students to connect the percentage s-character of the carbon atom to its electronegativity.

What to look for

Provide a worksheet with various alkyne reactions. Students solve it and then check their answers against a provided key to identify which reaction types they need to revise.

• Apply
• Analyze
• Evaluate
• Create
• Relationship Skills
• Decision-Making
• Self-Management

A few notes on teaching this unit

Start by visually comparing models of alkanes, alkenes, and alkynes to solidify the concept of hybridisation and geometry. Use guided practice to walk through the mechanism of an addition reaction, like HBr to propyne, step-by-step. When teaching hydration, explicitly draw the unstable enol intermediate before showing its rearrangement to the stable ketone, as this is a critical and often confusing step.

By the end of this topic, your students will be able to explain why some alkynes act as weak acids and confidently predict the outcomes of their key addition reactions.

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Watch Out for These Misconceptions

• Adding water to an alkyne produces a stable alcohol, just like with alkenes.

  The hydration of an alkyne first forms an unstable intermediate called an enol (a compound with a hydroxyl group on a double-bonded carbon). This enol immediately rearranges into a more stable carbonyl compound (a ketone or aldehyde) through a process called keto-enol tautomerism.

• All hydrogen atoms attached to carbons in a hydrocarbon are non-acidic.

  The hydrogen atom attached to a triply bonded carbon in a terminal alkyne is weakly acidic. This is because the sp-hybridised carbon is highly electronegative (50% s-character) and can stabilise the negative charge of the resulting acetylide anion.

• Adding one mole of HBr to propyne will add the Br to the first carbon.

  According to Markovnikov's rule, the negative part of the reagent (Br-) adds to the carbon atom of the multiple bond that has fewer hydrogen atoms. In propyne, the middle carbon has zero hydrogens, so the Br attaches there to form 2-bromopropene.

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Methods used in this brief

• Collaborative Problem-Solving