Chemistry · Class 11 · Hydrocarbons · Term 3

Alkenes: Structure and Isomerism

Understand the structure of the carbon-carbon double bond in alkenes, consisting of a sigma and a pi bond, and learn about geometrical (cis-trans) isomerism that arises due to restricted rotation.

TL;DR:Let's investigate the unique nature of the carbon-carbon double bond. We will discover how its structure locks atoms in place, leading to a fascinating type of isomerism with real-world consequences.

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

About This Topic

This topic, Alkenes: Structure and Isomerism, is a cornerstone of organic chemistry in the Class 11 curriculum, as outlined in the NCERT syllabus under Unit 13: Hydrocarbons. It builds directly upon the foundational concepts of chemical bonding and hybridization. The primary focus is to move students from the simple, freely rotating sigma bonds of alkanes to the more rigid and complex structure of the carbon-carbon double bond. Understanding the formation of a double bond, comprising one strong sigma (σ) bond from the head-on overlap of sp² hybrid orbitals and one weaker pi (π) bond from the sideways overlap of unhybridized p-orbitals, is crucial. This unique structure is the key to explaining the planarity of the ethene molecule and, most importantly, the restricted rotation around the C=C axis.

The concept of restricted rotation directly leads to a new form of stereoisomerism known as geometrical or cis-trans isomerism. This is often a student's first encounter with isomerism where connectivity is the same, but the spatial arrangement differs. It is essential to contextualise this by establishing the two necessary conditions for an alkene to exhibit this phenomenon: the presence of a C=C double bond and that each carbon atom of the double bond must be attached to two different groups. Using but-2-ene as a primary example helps illustrate how these different spatial arrangements result in two distinct compounds, cis-but-2-ene and trans-but-2-ene, which have different physical properties like boiling points, melting points, and dipole moments. This connection between molecular structure and macroscopic properties is a vital theme in chemistry.

Key Questions

1. Explain why rotation around a carbon-carbon double bond is restricted.
2. Compare the physical properties of cis-but-2-ene and trans-but-2-ene.
3. Identify which of the given alkenes will exhibit geometrical isomerism and draw their structures.

Learning Objectives

• Describe the formation of sigma and pi bonds in ethene using the concept of sp² hybridization.
• Explain why the presence of a pi bond restricts rotation around the carbon-carbon double bond.
• Identify the structural conditions necessary for an alkene to exhibit geometrical isomerism.
• Draw and name the cis and trans isomers for given alkenes like but-2-ene and pent-2-ene.
• Compare the relative stabilities and physical properties, like boiling point and dipole moment, of cis and trans isomers.

Key Vocabulary

Alkene	An unsaturated hydrocarbon containing at least one carbon-carbon double bond (C=C).
Pi (π) bond	A covalent bond formed by the sideways overlap of unhybridized p-orbitals, with electron density concentrated above and below the plane of the bonded atoms.
Geometrical Isomerism	A type of stereoisomerism in which molecules differ in the spatial arrangement of groups around a double bond or a ring structure.
Stereoisomers	Isomers that have the same molecular formula and connectivity of atoms but differ in the three-dimensional orientation of their atoms in space.
Restricted Rotation	The prevention of free rotation around a bond, typically a double or triple bond, due to the presence of a pi bond.

Watch Out for These Misconceptions

Common Misconception

Any alkene with a double bond will show cis-trans isomerism.

What to Teach Instead

Geometrical isomerism only occurs if each carbon atom of the double bond is attached to two different groups. For example, propene (CH₂=CH-CH₃) does not show this isomerism because one of the double-bonded carbons is attached to two identical hydrogen atoms.

Common Misconception

Cis and trans isomers are just different ways to draw the same molecule.

What to Teach Instead

They are distinct molecules called stereoisomers with the same formula and connectivity but different spatial arrangements. This difference leads to unique physical properties like different boiling points, making them separate, isolable compounds.

Common Misconception

Rotation around a double bond is completely impossible.

What to Teach Instead

Rotation is not impossible, but it is highly restricted because it requires enough energy (about 250 kJ/mol) to break the pi bond. Under normal laboratory conditions, this energy is not available, so the isomers do not interconvert.

Active Learning Ideas

Simulation Game

Build-an-Alkene: Cis vs. Trans

Using molecular model kits, students construct models of ethene and but-2-ene. They physically attempt to rotate the double bond to feel the restriction and then build both the cis and trans isomers to compare their shapes.

25 min·Pairs

Simulation Game

Geometrical Isomer Hunt

Provide a worksheet with structures of various alkenes (e.g., propene, pent-2-ene, 2-methylbut-2-ene). Students must identify which ones can exhibit geometrical isomerism and draw the cis and trans forms for those that can.

20 min·Individual

Simulation Game

Property Comparison Chart

In small groups, students research and create a T-chart comparing the physical properties (boiling point, melting point, dipole moment) of cis-but-2-ene and trans-but-2-ene. They then write a short explanation for the observed differences.

30 min·Small Groups

Real-World Connections

• The molecule responsible for vision, retinal, changes from a cis isomer to a trans isomer when light hits the eye, triggering a nerve signal to the brain.
• Unsaturated fats in our diet can be 'cis fats' (found naturally in vegetable oils) or 'trans fats' (often formed during industrial processing). Trans fats are known to be harmful to cardiovascular health.
• Many insects communicate using pheromones, which are often alkenes. The biological response is highly specific to one geometrical isomer, for example, the sex pheromone of the silkworm moth.
• Natural rubber is a polymer of the cis-isomer of isoprene, which gives it its characteristic elasticity. The trans-isomer, known as gutta-percha, is a hard and non-elastic substance.

Assessment Ideas

Exit Ticket

Exit Ticket: Give students the structures of three alkenes (e.g., hex-1-ene, hex-2-ene, 2,3-dimethylbut-2-ene). Ask them to identify which will show geometrical isomerism and to justify their answer.

Quick Check

A short quiz with questions that require students to draw and name cis/trans isomers, explain the difference in boiling points between an isomer pair, and describe the orbital overlap in a C=C double bond.

Quick Check

Provide a worksheet where students must identify if a given alkene is cis, trans, or cannot exhibit this isomerism. They can check their answers against a provided key to gauge their own understanding.

Frequently Asked Questions

Why does the trans isomer usually have a higher melting point than the cis isomer?

The trans isomer is generally more symmetrical. This allows its molecules to pack more efficiently and tightly in a crystal lattice, leading to stronger intermolecular forces which require more energy to break, resulting in a higher melting point.

Why does the cis isomer usually have a higher boiling point?

In the cis isomer, similar groups are on the same side, often creating a net molecular dipole moment, making the molecule polar. The trans isomer is often non-polar due to its symmetry where individual bond dipoles cancel out. The polar cis isomers experience stronger dipole-dipole interactions, requiring more energy to separate, hence a higher boiling point.

Can alkynes show geometrical isomerism?

No, alkynes cannot show geometrical isomerism. The carbon-carbon triple bond and the atoms attached to it are arranged in a straight line (180° bond angle), so there is no possibility for different spatial arrangements of groups.