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Chemistry · Grades 11–12

Hydrocarbon unsaturation index

For a neutral closed-shell hydrocarbon, each ring or double bond adds one DBE; a triple bond adds two.

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Rule and assumptions

For a neutral closed-shell hydrocarbon, each ring or double bond adds one DBE; a triple bond adds two.

DBE = (2C + 2 − H)/2 for a hydrocarbon

Worked examples

For hydrocarbon formula C13H20, calculate DBE = (2C + 2 − H)/2. The index alone does not identify the structure. Round to 3 decimal places if needed.

DBE = (2C + 2 − H)/2 for a hydrocarbon (2 × 13 + 2 − 20)/2 = 4

4

For hydrocarbon formula C8H10, calculate DBE = (2C + 2 − H)/2. The index alone does not identify the structure. Round to 3 decimal places if needed.

DBE = (2C + 2 − H)/2 for a hydrocarbon (2 × 8 + 2 − 10)/2 = 4

4

Try two questions

  1. For hydrocarbon formula C4H6, calculate DBE = (2C + 2 − H)/2. The index alone does not identify the structure. Round to 3 decimal places if needed.

  2. For hydrocarbon formula C12H26, calculate DBE = (2C + 2 − H)/2. The index alone does not identify the structure. Round to 3 decimal places if needed.

Answers and reasoning

Answers and marking notes
  1. 2

    DBE = (2C + 2 − H)/2 for a hydrocarbon (2 × 4 + 2 − 6)/2 = 2

  2. 0

    DBE = (2C + 2 − H)/2 for a hydrocarbon (2 × 12 + 2 − 26)/2 = 0

Further reading and review references

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