ChemistryChapter 16: Hydrocarbons
Alkenes and the C=c Bond
Alkenes: C=C, unsaturated, CnH2n (one double bond, open).
Learning objectives
- 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.
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 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.
Formula
C=C pi bond weaker than sigma
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).
Relationship
- C=C = sigma + pi
Overview
Bond, then shape:
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
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.
In simple words
Bonding idea. No polymer-plant design yet.
Key points
- C=C = sigma + pi
- unsaturated, addition
- pi is reactive
- lock the double bond
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Alkenes and the C=c Bond
Sticky Notes · Quick Revision
Core Idea
- 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.
1. 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.
- Ethene planar
- Cannot spin the double bond
- sp2 named
2. 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).
- Position of C=C
- Cis same side
- Trans opposite
Exam Example
- 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 Misconceptions
- 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.
FBISE Exam Check
- Explain alkenes and the c=c bond, compare its two main parts, and include the correct chemical evidence.
- Bonding idea. No polymer-plant design yet.
60-second revision
- 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?
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