Chemistry · Class 11 · Hydrocarbons · Term 3

Alkenes: Preparation and Reactions

Study the methods for preparing alkenes, such as dehydrohalogenation of alkyl halides, and investigate their characteristic electrophilic addition reactions, including Markovnikov's rule and oxidation reactions.

TL;DR: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

About This Topic

This topic on Alkynes is a fundamental part of the Class 11 Chemistry curriculum, typically covered under the 'Hydrocarbons' unit as per the NCERT framework. It builds directly upon students' prior knowledge of alkanes and alkenes, introducing the third major class of aliphatic hydrocarbons. The central focus is the carbon-carbon triple bond, composed of one strong sigma bond and two weaker pi bonds. This high electron density makes alkynes reactive towards electrophilic addition, but also introduces unique characteristics not seen in alkenes.

A key conceptual leap for students is understanding the acidic nature of terminal alkynes. This is directly linked to the concept of sp hybridisation and the resulting high 's-character' of the C-H bond, a crucial link between structure and property. The topic then delves into the characteristic addition reactions. While some reactions like hydrogenation and halogenation are analogous to those of alkenes, reactions with hydrogen halides (following Markovnikov's rule) and the hydration of alkynes (involving keto-enol tautomerism) introduce more complex mechanisms and regioselectivity. Mastery of these reactions is essential for understanding organic synthesis pathways in subsequent studies.

Key Questions

1. Explain Markovnikov's rule with a suitable example and its mechanism.
2. Compare the products of ozonolysis of propene and but-2-ene.
3. Analyse the reaction of HBr with propene in the presence and absence of peroxide.

Learning Objectives

• Describe the sp hybridisation in alkynes and its effect on their linear geometry and bond properties.
• Explain the acidic character of terminal alkynes by relating it to the stability of the conjugate acetylide anion.
• Predict the products formed from the addition of hydrogen, halogens, hydrogen halides, and water to various alkynes.
• Apply Markovnikov's rule to predict the regiochemistry of addition to unsymmetrical alkynes.
• Illustrate the mechanism of keto-enol tautomerism in the hydration of alkynes to form carbonyl compounds.

Key Vocabulary

Alkyne	An unsaturated hydrocarbon containing at least one carbon-carbon triple bond (C≡C).
Terminal Alkyne	An alkyne where the triple bond is located at the end of the carbon chain, having a hydrogen atom directly attached to a triply bonded carbon.
sp Hybridisation	The mixing of one s and one p orbital to form two identical hybrid orbitals that are oriented linearly at 180 degrees to each other.
Tautomerism	A phenomenon where a single compound exists in two readily interconvertible structures that differ in the position of a proton and a double bond. A common example is keto-enol tautomerism.
Acetylide anion	The conjugate base of a terminal alkyne, formed by the removal of the acidic proton. It is a strong nucleophile.

Watch Out for These Misconceptions

Common Misconception

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

What to Teach Instead

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.

Common Misconception

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

What to Teach Instead

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.

Common Misconception

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

What to Teach Instead

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.

Active Learning Ideas

Collaborative Problem-Solving

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.

25 min·Pairs

Collaborative Problem-Solving

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.

20 min·Small Groups

Collaborative Problem-Solving

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.

15 min·Individual

Real-World Connections

• Oxy-acetylene welding uses the high heat produced from the combustion of ethyne (acetylene) to cut and weld steel.
• Ethyne is a primary starting material in the industrial synthesis of many important organic chemicals, including acetic acid and polymers like PVC.
• Many potent pharmaceuticals and natural products, including some anti-cancer drugs, contain the alkyne functional group in their structure.
• Alkynes are used in 'click chemistry', a set of powerful, specific reactions used in drug development and for attaching fluorescent dyes to biological molecules.
• Historically, carbide lamps used the reaction of calcium carbide with water to produce acetylene gas for lighting in mines.

Assessment Ideas

Exit Ticket

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.

Quick Check

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.

Quick Check

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.

Frequently Asked Questions

Why is mercuric sulphate (HgSO4) needed for the hydration of alkynes?

The triple bond of an alkyne is less reactive to electrophilic addition by water than an alkene's double bond. The Hg2+ ion acts as a potent catalyst by forming a cyclic intermediate that is more easily attacked by the water molecule, thereby speeding up the reaction significantly.

How can we stop the hydrogenation of an alkyne at the alkene stage?

This is achieved by using a 'poisoned' or partially deactivated catalyst. For example, Lindlar's catalyst (palladium on calcium carbonate, deactivated with lead acetate) is used to convert an alkyne to a cis-alkene. Using sodium in liquid ammonia (Birch reduction) produces a trans-alkene.

What is the difference between a terminal and an internal alkyne?

A terminal alkyne has the carbon-carbon triple bond at the end of the carbon chain (C≡C-H). An internal alkyne has the triple bond located somewhere in the middle of the chain, with carbon atoms attached to both sides of it (C-C≡C-C). Only terminal alkynes have an acidic hydrogen.