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Organic Chemistry

Structure and formulae of alkenes

  • 3 things to know
  • Paper 2

Overview

Alkenes as hydrocarbons with a carbon–carbon double bond, their general formula CₙH₂ₙ, unsaturated alkenes and saturated alkanes, the first four alkenes and their formulae, why ethene is the smallest alkene, and telling an alkene from an alkane by its formula.

What you need to know

Answer each one before you reveal it.

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Key term

Name the smallest alkene, explain why no smaller one exists, and how an alkene's formula compares with the matching alkane.

Key term

State the general formula of the alkenes.

Key term

Propane is C₃H₈. Compare the formula of propene with it.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What feature of its structure makes a hydrocarbon an alkene?

    Model answer

    It contains a carbon–carbon double bond.

  2. 2

    State the general formula of the alkenes.

    Model answer

    The general formula of the alkenes is CₙH₂ₙ.

  3. 3

    Use the general formula of the alkenes to give the molecular formula of the alkene with four carbon atoms.

    Model answer

    C₄H₈.

  4. 4

    An alkene molecule contains 10 hydrogen atoms. How many carbon atoms does it contain?

    Model answer

    Five.

  5. 5

    What does it mean to say that an alkene is unsaturated?

    Model answer

    It contains a carbon–carbon double bond, so it does not hold the maximum possible number of hydrogen atoms.

  6. 6

    How many fewer hydrogen atoms does an alkene have than the alkane with the same number of carbon atoms?

    Model answer

    Two fewer.

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