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3.4.2Physical Chemistry

Isomerism in Alkenes — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 77 of 102

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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Structural Isomers of Alkenes

Alkenes can show chain and position isomerism as well as stereoisomerism.

Position isomers
  • Position isomers But-1-ene and but-2-ene have the same carbon chain with the double bond in a different position.
Chain isomers
  • Chain isomers Methylpropene has a branched carbon skeleton, and it is a chain isomer of the butenes.
C₄H₈
  • C₄H₈ The alkenes with formula C₄H₈ are but-1-ene, but-2-ene (two stereoisomers) and 2-methylpropene.
Naming
  • Naming The position of the double bond is given by the lower numbered carbon of the C=C, as in pent-2-ene.

Part 2 of 5E/Z Isomerism and Priority Rules

E/Z isomerism requires two different groups on each carbon of the double bond.

1
For each carbon of the double bond, look at the two atoms directly attached to it.
2
Assign priority to the atom with the higher atomic number, so for example Br > Cl > C > H.
3
If the two atoms are the same, move outwards along each chain and compare the next atoms until a difference is found.
4
If the two higher-priority groups are on the same side of the double bond, the isomer is Z.
5
If the two higher-priority groups are on opposite sides, the isomer is E.

Part 3 of 5Counting Isomers

Counting Isomers
Check each double bond
  • Check each double bond Examine every double bond in an isomer to see whether each carbon has two different groups.
Not E/Z
  • Not E/Z Compounds such as but-1-ene have two hydrogens on one carbon, so there is no E/Z isomerism.
Cyclic isomers
  • Cyclic isomers Cycloalkanes also have the formula CₙH₂ₙ, but they are not alkenes and are not counted here.
FormulaAlkene isomers (including E/Z)
C₄H₈4 (but-1-ene, E-but-2-ene, Z-but-2-ene, 2-methylpropene)
C₅H₁₀6 (pent-1-ene, E and Z pent-2-ene, 3 methylbutenes)

Part 4 of 5Worked Example

Question Assign E or Z to the isomer of CHBr=CHCl in which bromine and chlorine are on the same side of the double bond.

  1. On the first carbon, Br (atomic number 35) has higher priority than H.
  2. On the second carbon, Cl (17) has higher priority than H.
  3. The higher-priority atoms, Br and Cl, are on the same side.

Answer Z isomer, (Z)-1-bromo-2-chloroethene

Part 5 of 5Exam Checklist & Tips

3.4.2 – Key Points for the Exam

  • E/Z isomerism Stereoisomerism in which groups on a C=C bond lie on the same (Z) or opposite (E) sides.
  • Cahn–Ingold–Prelog rules The system that assigns priority to groups by atomic number so that E/Z can be assigned.
  • Chain isomer A structural isomer with a different carbon skeleton.
  • Position isomer A structural isomer in which the double bond occupies a different position.

Common Mistakes

  • Assigning Z or E using the largest group rather than the highest-priority group.
  • Claiming but-1-ene has E/Z isomers.
  • Counting the same E/Z isomer twice by drawing it from a different angle.
  • Using cis/trans names when the two carbons carry different groups.

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