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3.2.5Additional lessons

Structural Isomers — IB Chemistry SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 47 of 98

📚 IB Chemistry SL Hub / Topical Revision
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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 6What Makes Isomers

Same molecular formula
  • Same molecular formula Isomers contain exactly the same numbers of each type of atom, so they have the same relative molecular mass.
Different structure
  • Different structure Structural isomers differ in the order in which atoms are bonded together, not just in how they are drawn on paper.
Different properties
  • Different properties Because the bonding differs, isomers often show different boiling points, reactivity and sometimes different functional groups.
Same drawing trap
  • Same drawing trap Rotating a molecule or bending a chain on the page gives the same compound, so bond connectivity is what must be compared.

Part 2 of 6Three Types

Three Types
Chain isomers
  • Chain isomers The carbon skeleton is arranged differently, for example butane has a straight chain whereas methylpropane has a branch.
Position isomers
  • Position isomers The carbon skeleton and functional group are the same, but the group sits on a different carbon, as with propan-1-ol and propan-2-ol.
Functional group isomers
  • Functional group isomers The atoms form different functional groups, such as ethanol CH₃CH₂OH and methoxymethane CH₃OCH₃, both C₂H₆O.
Why they matter
  • Why they matter Each type shows how a single formula can describe several compounds,
    • which is why a molecular formula alone cannot identify a substance.
TypeExample pairFormula
Chainbutane and methylpropaneC₄H₁₀
Positionpropan-1-ol and propan-2-olC₃H₈O
Functional grouppropanal and propanoneC₃H₆O

Part 3 of 6Drawing Every Isomer

A methodical search prevents missed or repeated isomers.

1
Write the molecular formula and calculate how many carbons, hydrogens and other atoms must appear in every isomer.
2
Draw the longest straight carbon chain first, then shorten it by one carbon at a time and place the spare carbon as a branch at each different position.
3
Avoid duplicates by checking that a branch on the second carbon from one end is the same as one on the second carbon from the other end.
4
Repeat with each functional group placed in every non-equivalent position, then look for different functional groups that fit the same formula.
5
Name each structure systematically and confirm that no two names are the same.

Part 4 of 6The Five Isomers of C₆H₁₄

The Five Isomers of C₆H₁₄
Hexane
  • Hexane The unbranched chain of six carbons, with the highest boiling point of the five because its molecules pack closely together.
2-methylpentane
  • 2-methylpentane A five-carbon chain with a methyl group on carbon 2, giving a slightly lower boiling point than hexane.
3-methylpentane
  • 3-methylpentane A five-carbon chain with a methyl group on carbon 3, which is different from the 2-methyl form.
2,2-dimethylbutane
  • 2,2-dimethylbutane A four-carbon chain with two methyl groups on carbon 2, forming a compact, nearly spherical molecule.
2,3-dimethylbutane
  • 2,3-dimethylbutane A four-carbon chain with one methyl group on each of carbons 2 and 3.

Part 5 of 6Worked Example

Question Draw and name the structural isomers of C₄H₁₀O that are alcohols.

  1. Alcohols contain –OH, so the carbon skeleton is C₄H₉ with one OH replacing one hydrogen.
  2. Straight chain
    • OH on carbon 1 gives butan-1-ol;
      • OH on carbon 2 gives butan-2-ol (carbons 3 and 4 repeat these by symmetry).
  3. Branched chain (methylpropane)
    • OH on an end carbon gives 2-methylpropan-1-ol;
      • OH on the central carbon gives 2-methylpropan-2-ol.
  4. Check that each has formula C₄H₁₀O and that no two are identical.

Answer Four alcohol isomers: butan-1-ol, butan-2-ol, 2-methylpropan-1-ol and 2-methylpropan-2-ol.

Part 6 of 6Exam Checklist & Tips

3.2.5 – Key Points for the Exam

  • Structural isomers Compounds with the same molecular formula but different arrangements of atoms bonded together.
  • Chain isomerism Isomerism caused by different branching of the carbon skeleton.
  • Position isomerism Isomerism caused by a functional group or substituent attached at different positions on the same skeleton.
  • Functional group isomerism Isomerism in which the isomers contain different functional groups.
  • Molecular formula The actual number of atoms of each element in one molecule.

Common Mistakes

  • Counting a rotated or flipped drawing as a new isomer when the connectivity of atoms is identical.
  • Changing the molecular formula by accident when adding a branch, for example leaving a carbon with only three bonds.
  • Forgetting functional group isomers such as ethers for alcohols or ketones for aldehydes.
  • Claiming that isomers must have the same boiling point because they share the same relative molecular mass.

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