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Alkenes and the C=c Bond — Sigma and Pi
Sigma: overlap along the line. Pi: above and below. No free rotation about C=C (cis-trans). Hybridisation sp2 named (trigonal, 120 school). Ethene C2H4 simplest; test: decolourises bromine water. This page separates the idea from the wider topic “ALKENES AND THE C=C BOND” so it can be revised and assessed on its own.
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Alkenes and the C=c Bond — Sigma and Pi
What is it?
Sigma: overlap along the line. Pi: above and below. No free rotation about C=C (cis-trans). Hybridisation sp2 named (trigonal, 120 school). Ethene C2H4 simplest; test: decolourises bromine water. This page separates the idea from the wider topic “ALKENES AND THE C=C BOND” so it can be revised and assessed on its own.
In simple words
- Sigma: overlap along the line.
- Ethene planar
- Exam link: pi is reactive.
Important terms
SIGMA AND PI: Sigma: overlap along the line.
ALKENES AND THE C=C BOND: Alkenes: C=C, unsaturated, CnH2n (one double bond, open).
What you should learn
- Define sigma and pi using accurate chemistry terms.
- Explain why sigma and pi matters within alkenes and the c=c bond.
- Apply the learning unit to an FBISE-style example, comparison, diagram, or calculation.
Examples
- Ethene planar
- Cannot spin the double bond
- sp2 named
KEY IDEA
Sigma: overlap along the line. Pi: above and below. No free rotation about C=C (cis-trans). Hybridisation sp2 named (trigonal, 120 school). Ethene C2H4 simplest; test: decolourises bromine water.
EXAM APPLICATION
Exam: why no rotation about C=C? The pi bond would have to break. That costs energy at room T. So cis and trans can exist as different isomers.
Recommended for Class 11
Full school-level explanation
Sigma: overlap along the line. Pi: above and below. No free rotation about C=C (cis-trans). Hybridisation sp2 named (trigonal, 120 school). Ethene C2H4 simplest; test: decolourises bromine water. This page separates the idea from the wider topic “ALKENES AND THE C=C BOND” so it can be revised and assessed on its own.
Study this learning unit in three steps:
WHAT — ALKENES AND THE C=C BOND — SIGMA AND PI
Sigma: overlap along the line. Pi: above and below. No free rotation about C=C (cis-trans). Hybridisation sp2 named (trigonal, 120 school). Ethene C2H4 simplest; test: decolourises bromine water. This page separates the idea from the wider topic “ALKENES AND THE C=C BOND” so it can be revised and assessed on its own.
WHY — Why this topic matters
ALKENES AND THE C=C BOND — SIGMA AND PI is an independent FBISE learning unit because students must use its definitions, conditions, equations, or trends separately in explanations and calculations.
HOW — How it works
Sigma: overlap along the line. Pi: above and below. No free rotation about C=C (cis-trans). Hybridisation sp2 named (trigonal, 120 school). Ethene C2H4 simplest; test: decolourises bromine water. Examples: Ethene planar; Cannot spin the double bond; sp2 named. Exam use: Exam: why no rotation about C=C? The pi bond would have to break. That costs energy at room T. So cis and trans can exist as different isomers.
KEY IDEA
Sigma: overlap along the line. Pi: above and below. No free rotation about C=C (cis-trans). Hybridisation sp2 named (trigonal, 120 school). Ethene C2H4 simplest; test: decolourises bromine water.
Remember
- pi is reactive
Examples
- Ethene planar
- Cannot spin the double bond
- sp2 named
EXAM APPLICATION
Exam: why no rotation about C=C? The pi bond would have to break. That costs energy at room T. So cis and trans can exist as different isomers.
Remember
- pi locks
Examples
- Ethene planar
- Cannot spin the double bond
- sp2 named
Remember
- pi is reactive
- pi locks
- Federal Board only
- pi is reactive
- pi locks
- pi is reactive
- independent learning unit
- pi is reactive
How the ideas connect
Sigma: overlap along the line. Pi: above and below. No free rotation about C=C (cis-trans). Hybridisation sp2 named (trigonal, 120 school). Ethene C2H4 simplest; test: decolourises bromine water. Examples: Ethene planar; Cannot spin the double bond; sp2 named. Exam use: Exam: why no rotation about C=C? The pi bond would have to break. That costs energy at room T. So cis and trans can exist as different isomers.
Deeper understanding
Go beyond memorising Alkenes and the C=c Bond — Sigma and Pi. Connect the definition to reasons, relationships, exceptions, and exam wording.
How the ideas connect
Sigma: overlap along the line. Pi: above and below. No free rotation about C=C (cis-trans). Hybridisation sp2 named (trigonal, 120 school). Ethene C2H4 simplest; test: decolourises bromine water. Examples: Ethene planar; Cannot spin the double bond; sp2 named. Exam use: Exam: why no rotation about C=C? The pi bond would have to break. That costs energy at room T. So cis and trans can exist as different isomers.
Common mistakes
Exam focus
Core idea
WHAT
WHY
HOW
Exam focus
Quick revision
- pi is reactive
- pi locks
- Federal Board only
- SIGMA AND PI: Sigma: overlap along the line.
- ALKENES AND THE C=C BOND: Alkenes: C=C, unsaturated, CnH2n (one double bond, open).
- WHAT — ALKENES AND THE C=C BOND — SIGMA AND PI: Sigma: overlap along the line. Pi: above and below. No free rotation about C=C (cis-trans). Hybridis
Federal Board Class 11 Chemistry learning unit. Easy wording; no provincial-board material added.
For long answers on Alkenes and the C=c Bond — Sigma and Pi: define clearly → explain the main parts → give one example → state one exam caution.
Exam Focus
Exam Focus
Important points
- pi is reactive
- pi locks
- Federal Board only
- SIGMA AND PI: Sigma: overlap along the line.
Important formulas
- pi is reactive
- pi locks
- Federal Board only
Common exam traps
- Do not treat sigma and pi as the same as every other part of alkenes and the c=c bond; define this unit and use its own evidence, steps, or conditions.
- Do not memorise a label alone; connect the definition to a process, structure, condition, equation, or example.
Long question structure
Definition → key parts → example → exam caution
Common Mistakes
Common Mistakes
- Mistake
Do not treat sigma and pi as the same as every other part of alkenes and the c=c bond; define this unit and use its own evidence, steps, or conditions.
- Mistake
Do not memorise a label alone; connect the definition to a process, structure, condition, equation, or example.
- Mistake
Do not use provincial-board wording or add material outside the Federal Board Class 11 scope.
- Mistake
Federal Board Class 11 Chemistry learning unit. Easy wording; no provincial-board material added.
Revision Zone
60-Second Revision
Quick cues from this topic — use them for last-minute revision.
pi is reactive
pi locks
Federal Board only
SIGMA AND PI: Sigma: overlap along the line.
Sigma: overlap along the line. Pi: above and below. No free rotation about C=C (cis-trans). Hybridisation sp2 named (trigonal, 120 school). Ethene C2H4 simplest
- Definition
- Sigma: overlap along the line. Pi: above and below. No free rotation about C=C (cis-trans). Hybridisation sp2 named (trigonal, 120 school). Ethene C2H4 simplest
- Example
- Ethene planar
- Critical warning
- Do not treat sigma and pi as the same as every other part of alkenes and the c=c bond; define this unit and use its own
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Exam Practice
Define
Q1. Define Alkenes and the C=c Bond — Sigma and Pi.
Explain
Q2. Explain: KEY IDEA.
Example
Q3. Give one clear example of Alkenes and the C=c Bond — Sigma and Pi and say why it fits.
Mistake
Q4. State one common mistake students make about Alkenes and the C=c Bond — Sigma and Pi.
Exam note
Q5. Write a short exam note on Alkenes and the C=c Bond — Sigma and Pi: definition, two points, one example.
Sources & References
School-standard facts for Matric and Intermediate. Original explanations — not copied from textbooks.