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Chemistry · Grades 10–12

Constitutional isomers and stereoisomers

Tell changed connectivity from changed spatial arrangement.

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Constitutional

Constitutional isomers have the same molecular formula but different connectivity.

Stereo

Stereoisomers have the same connectivity but differ in spatial arrangement.

Conformations

Rotations around ordinary single bonds may give conformations rather than new constitutional compounds.

Evidence

A flat formula can omit needed three-dimensional information.

Worked examples

Butane and 2-methylpropane share C₄H₁₀ but connect differently. They are what?

The atom connections differ.

Constitutional isomers.

Two structures have identical connectivity but different fixed spatial arrangements. They may be what?

Stereoisomerism concerns spatial arrangement with retained connectivity.

Stereoisomers.

Check your understanding

  1. How many constitutional isomers are there for acyclic alkane C₄H₁₀?

  2. Why does turning an entire molecular drawing on the page not necessarily produce a new isomer?

  3. Which information must constitutional isomers share?

    1. Molecular formula.
    2. Every bond connection.
    3. Every physical property.

Answers and marking notes

Answers and marking notes
  1. 2

    The unbranched and singly branched four-carbon skeletons are the two possibilities.

  2. The same connectivity and three-dimensional arrangement can be retained under overall rotation.

    A drawing orientation is not itself a new compound.

  3. Molecular formula.

    Identical atom counts are required, but connectivity differs.

Further reading and review references

These are reference links, not copied exercises, institutional endorsement or proof of independent review.

Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.