ChemistryChapter 16: Hydrocarbons
Cis-Trans and Markovnikov (exam)
Cis-trans: two different groups on each C of C=C.
Learning objectives
- Explain cis-trans and markovnikov (exam) in simple chemical language.
- Connect 1. cis-trans with 2. major product using equations, observations, or trends where needed.
- Answer a short Federal Board question with the correct terms and a clear reason.
Definition
Cis-trans: two different groups on each C of C=C. But-2-ene yes; 2-methylbut-2-ene maybe not if one carbon has two methyls. Markovnikov: in HX addition to an unsymmetric alkene, H goes to the carbon already holding more hydrogens (the other carbon gets X). School reason: more stable carbocation if a two-step picture is taught. Meaning: name the major product in an exam. Cis same side on C=C; trans opposite — needs two different groups on each C. Show curly arrows on addition mechanisms; Markovnikov reasoning must precede the product structure.
Formula
C=C. But-2-ene yes
Important terms
- CIS-TRANS AND MARKOVNIKOV (EXAM)
- Cis-trans: two different groups on each C of C=C.
- 1. CIS-TRANS
- Ethene no (two H on each C).
- 2. MAJOR PRODUCT
- Propene + HBr -> 2-bromopropane major.
Overview
Who can be cis, then who gets H:
Cis-Trans
Ethene no (two H on each C). Propene no (CH2 end has two H). But-2-ene yes. EZ named if the book uses CIP, else cis-trans is enough. Markovnikov: H adds to C with more H already (rich get richer) for HX addition.
Examples But-2-ene · Maleic/fumaric named if asked · No rotation
Major Product
Propene + HBr -> 2-bromopropane major. 1-bromopropane minor. Draw both if the paper asks. Not a synthesis of a solvent stock. Anti-Markovnikov with peroxide for HBr only — school exception named.
Examples H to CH2 · Br to middle · Carbocation reason if taught
WHAT — CIS-TRANS AND MARKOVNIKOV (EXAM)
Cis-trans: two different groups on each C of C=C. But-2-ene yes; 2-methylbut-2-ene maybe not if one carbon has two methyls. Markovnikov: in HX addition to an unsymmetric alkene, H goes to the carbon already holding more hydrogens (the other carbon gets X). School reason: more stable carbocation if a two-step picture is taught. Meaning: name the major product in an exam. Cis same side on C=C; trans opposite — needs two different groups on each C. Show curly arrows on addition mechanisms; Markovnikov reasoning must precede the product structure.
WHY — Why this topic matters
CIS-TRANS AND MARKOVNIKOV (EXAM) 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
CIS-TRANS: Ethene no (two H on each C). Propene no (CH2 end has two H). But-2-ene yes. EZ named if the book uses CIP, else cis-trans is enough. Markovnikov: H adds to C with more H already (rich get richer) for HX addition. Examples: But-2-ene; Maleic/fumaric named if asked; No rotation. MAJOR PRODUCT: Propene + HBr -> 2-bromopropane major. 1-bromopropane minor. Draw both if the paper asks. Not a synthesis of a solvent stock. Anti-Markovnikov with peroxide for HBr only — school exception named. Examples: H to CH2; Br to middle; Carbocation reason if taught. Worked propene + HBr: H to the CH2 end (already two H) Br to the middle carbon major name: 2-bromopropane
Worked example
- Worked propene + HBr:
- H to the CH2 end (already two H)
- Br to the middle carbon
- major name: 2-bromopropane
In simple words
Product names. No bulk HBr addition plant.
Key points
- cis-trans needs two different groups each C
- H to richer-H carbon
- check both carbons
- Markovnikov named
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Cis-Trans and Markovnikov (exam)
Sticky Notes · Quick Revision
Core Idea
- Cis-trans: two different groups on each C of C=C. But-2-ene yes; 2-methylbut-2-ene maybe not if one carbon has two methyls. Markovnikov: in HX addition to an unsymmetric alkene, H goes to the carbon already holding more hydrogens (the other carbon gets X). School reason: more stable carbocation if a two-step picture is taught. Meaning: name the major product in an exam. Cis same side on C=C; trans opposite — needs two different groups on each C. Show curly arrows on addition mechanisms; Markovnikov reasoning must precede the product structure.
1. CIS-TRANS
- Ethene no (two H on each C). Propene no (CH2 end has two H). But-2-ene yes. EZ named if the book uses CIP, else cis-trans is enough. Markovnikov: H adds to C with more H already (rich get richer) for HX addition.
- But-2-ene
- Maleic/fumaric named if asked
- No rotation
2. MAJOR PRODUCT
- Propene + HBr -> 2-bromopropane major. 1-bromopropane minor. Draw both if the paper asks. Not a synthesis of a solvent stock. Anti-Markovnikov with peroxide for HBr only — school exception named.
- H to CH2
- Br to middle
- Carbocation reason if taught
Exam Example
- Worked propene + HBr:
- H to the CH2 end (already two H)
- Br to the middle carbon
- major name: 2-bromopropane
Common Misconceptions
- Do not confuse 1. CIS-TRANS with 2. MAJOR PRODUCT; state the difference before giving an example.
- Writing only “cis-trans needs two different groups each C” is not enough. Explain what it means in a complete sentence.
FBISE Exam Check
- Explain cis-trans and markovnikov (exam), compare its two main parts, and include the correct chemical evidence.
- Product names. No bulk HBr addition plant.
60-second revision
- Cis-trans: two different groups on each C of C=C.
- 1. CIS-TRANS: Ethene no (two H on each C).
- 2. MAJOR PRODUCT: Propene + HBr -> 2-bromopropane major.
- Exam reminder: Worked propene + HBr: H to the CH2 end (already two H) Br to the middle carbon major name: 2-bromopropane
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