Alkenes and the C=c Bond — Sigma and Pi
Sigma: overlap along the line.
Learning objectives
- 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.
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; 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.
Mathematical form
C=C (cis-trans). Hybridisation sp2 named (
C=C BOND
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).
Overview
Study this learning unit in three steps:
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.
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.
Examples Ethene planar · Cannot spin the double bond · sp2 named
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.
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. 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.
In simple words
Federal Board Class 11 Chemistry learning unit. Easy wording; no provincial-board material added.
Key points
- pi is reactive
- pi locks
Exam preparation
Exam Focus
Quick revision
Revision Zone
Alkenes and the C=c Bond — Sigma and Pi
Sticky Notes · Quick Revision
Core 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. 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.
WHAT
- Sigma: overlap along the line.
WHY
- 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
- 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 misconception
- 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.
FBISE exam check
- Define alkenes and the c=c bond — sigma and pi, explain how it works, and support the answer with one relevant example.
60-second revision
- Sigma: overlap along the line.
- Ethene planar
- Exam link: pi is reactive.
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