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Alkenes and the C=c Bond
Alkenes: C=C, unsaturated, CnH2n (one double bond, open). The extra pair sits in a pi bond (sideways overlap); the sigma is the end-on bond. Pi is the reactive bit. Geometric isomers if each double-bond carbon has two different groups. Meaning: alkenes add; alkanes do not (school test with Br2 water). C=C pi bond weaker than sigma — site of addition; planarity around double bond. Show curly arrows
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Alkenes and the C=c Bond
What is it?
Alkenes: C=C, unsaturated, CnH2n (one double bond, open). The extra pair sits in a pi bond (sideways overlap); the sigma is the end-on bond. Pi is the reactive bit. Geometric isomers if each double-bond carbon has two different groups. Meaning: alkenes add; alkanes do not (school test with Br2 water). C=C pi bond weaker than sigma — site of addition; planarity around double bond. Show curly arrows on addition mechanisms; Markovnikov reasoning must precede the product structure.
In simple words
- Alkenes: C=C, unsaturated, CnH2n (one double bond, open).
- 1. SIGMA AND PI: Sigma: overlap along the line.
- 2. ISOMERS: But-1-ene vs but-2-ene (position).
- Exam reminder: Exam: why no rotation about C=C?
Important terms
ALKENES AND THE C=C BOND: Alkenes: C=C, unsaturated, CnH2n (one double bond, open).
1. SIGMA AND PI: Sigma: overlap along the line.
2. ISOMERS: But-1-ene vs but-2-ene (position).
What you should learn
- Explain alkenes and the c=c bond in simple chemical language.
- Connect 1. sigma and pi with 2. isomers using equations, observations, or trends where needed.
- Answer a short Federal Board question with the correct terms and a clear reason.
Examples
- Ethene planar
- Cannot spin the double bond
- sp2 named
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.
ISOMERS
But-1-ene vs but-2-ene (position). Cis-but-2-ene vs trans. Functional? Not yet. Count H: C4H8 several isomers including cycloalkane. Geometric isomerism possible when each C has two different groups (later cis-trans page).
Recommended for Class 11
Full school-level explanation
Alkenes: C=C, unsaturated, CnH2n (one double bond, open). The extra pair sits in a pi bond (sideways overlap); the sigma is the end-on bond. Pi is the reactive bit. Geometric isomers if each double-bond carbon has two different groups. Meaning: alkenes add; alkanes do not (school test with Br2 water). C=C pi bond weaker than sigma — site of addition; planarity around double bond. Show curly arrows on addition mechanisms; Markovnikov reasoning must precede the product structure.
Bond, then shape:
WHAT — ALKENES AND THE C=C BOND
Alkenes: C=C, unsaturated, CnH2n (one double bond, open). The extra pair sits in a pi bond (sideways overlap); the sigma is the end-on bond. Pi is the reactive bit. Geometric isomers if each double-bond carbon has two different groups. Meaning: alkenes add; alkanes do not (school test with Br2 water). C=C pi bond weaker than sigma — site of addition; planarity around double bond. Show curly arrows on addition mechanisms; Markovnikov reasoning must precede the product structure.
WHY — Why this topic matters
ALKENES AND THE C=C BOND 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 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. Examples: Ethene planar; Cannot spin the double bond; sp2 named. ISOMERS: But-1-ene vs but-2-ene (position). Cis-but-2-ene vs trans. Functional? Not yet. Count H: C4H8 several isomers including cycloalkane. Geometric isomerism possible when each C has two different groups (later cis-trans page). Examples: Position of C=C; Cis same side; Trans opposite. 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.
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.
Remember
- pi is reactive
Examples
- Ethene planar
- Cannot spin the double bond
- sp2 named
ISOMERS
But-1-ene vs but-2-ene (position). Cis-but-2-ene vs trans. Functional? Not yet. Count H: C4H8 several isomers including cycloalkane. Geometric isomerism possible when each C has two different groups (later cis-trans page).
Remember
- lock the double bond
Examples
- Position of C=C
- Cis same side
- Trans opposite
Remember
- C=C = sigma + pi
- unsaturated, addition
- pi locks
- bonding only
- pi is reactive
- lock the double bond
- C=C = sigma + pi
- unsaturated, addition
How the ideas connect
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. Connect the definition to reasons, relationships, exceptions, and exam wording.
How the ideas connect
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
1. SIGMA AND PI
2. ISOMERS
Exam Example
Exam focus
Quick revision
- C=C = sigma + pi
- unsaturated, addition
- pi locks
- bonding only
- ALKENES AND THE C=C BOND: Alkenes: C=C, unsaturated, CnH2n (one double bond, open).
- 1. SIGMA AND PI: Sigma: overlap along the line.
Bonding idea. No polymer-plant design yet.
For long answers on Alkenes and the C=c Bond: define clearly → explain the main parts → give one example → state one exam caution.
Exam Focus
Exam Focus
Important points
- C=C = sigma + pi
- unsaturated, addition
- pi locks
- bonding only
Important formulas
- C=C = sigma + pi
- unsaturated, addition
- pi locks
Common exam traps
- Do not confuse 1. SIGMA AND PI with 2. ISOMERS; state the difference before giving an example.
- Writing only “C=C = sigma + pi” is not enough. Explain what it means in a complete sentence.
Long question structure
Definition → key parts → example → exam caution
Common Mistakes
Common Mistakes
- Mistake
Do not confuse 1. SIGMA AND PI with 2. ISOMERS; state the difference before giving an example.
- Mistake
Writing only “C=C = sigma + pi” is not enough. Explain what it means in a complete sentence.
- Mistake
Bonding idea. No polymer-plant design yet.
- Mistake
Bonding idea. No polymer-plant design yet.
Revision Zone
60-Second Revision
Quick cues from this topic — use them for last-minute revision.
C=C = sigma + pi
unsaturated, addition
pi locks
bonding only
Alkenes: C=C, unsaturated, CnH2n (one double bond, open). The extra pair sits in a pi bond (sideways overlap); the sigma is the end-on bond. Pi is the reactive
C=C = sigma + pi
- Definition
- Alkenes: C=C, unsaturated, CnH2n (one double bond, open). The extra pair sits in a pi bond (sideways overlap); the sigma is the end-on bond. Pi is the reactive
- Formula
- C=C = sigma + pi
- Example
- Ethene planar
- Critical warning
- Do not confuse 1. SIGMA AND PI with 2. ISOMERS; state the difference before giving an example.
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Exam Practice
Define
Q1. Define Alkenes and the C=c Bond.
Explain
Q2. Explain: SIGMA AND PI.
Example
Q3. Give one clear example of Alkenes and the C=c Bond and say why it fits.
Mistake
Q4. State one common mistake students make about Alkenes and the C=c Bond.
Exam note
Q5. Write a short exam note on Alkenes and the C=c Bond: definition, two points, one example.
Sources & References
School-standard facts for Matric and Intermediate. Original explanations — not copied from textbooks.