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Organic Chemistry

Alkenes, pi bonds and electrophilic addition

  • 3 things to know
  • A2 2

Overview

Alkenes and the bromine water test, naming alkenes, sigma and pi bonds, hydrogenation and margarine, trans fats, electrophilic addition of hydrogen bromide and bromine, and hydration.

What you need to know

Answer each one before you reveal it.

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Process

Describe the electrophilic addition mechanism for the reaction of ethene with hydrogen bromide.

Key term

What is the general formula of the alkenes?

Key term

Why are alkenes described as unsaturated?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What is the general formula of the alkenes?

    Model answer

    The alkenes have the general formula CₙH₂ₙ. Each alkene molecule contains one carbon–carbon double bond.

  2. 2

    Why are alkenes described as unsaturated?

    Model answer

    Alkenes contain a C=C double bond, so more atoms can be added across it. Cyclohexene is also unsaturated because its ring contains a double bond.

  3. 3

    Describe the test for unsaturation using bromine water.

    Model answer

    Bromine water is shaken with the sample. An alkene decolourises the orange bromine water, while an alkane leaves it orange.

  4. 4

    Give the names and structural formulae of the unbranched alkenes with two to four carbon atoms.

    Model answer

    Ethene is CH₂=CH₂ and propene is CH₂=CHCH₃. The four-carbon alkenes are but-1-ene, CH₂=CHCH₂CH₃, and but-2-ene, CH₃CH=CHCH₃.

  5. 5

    Name the alkene CH₃CH₂CH=CHCH₂CH₃.

    Model answer

    CH₃CH₂CH=CHCH₂CH₃ is hex-3-ene. The double bond starts at carbon 3 from whichever end the six-carbon chain is numbered.

  6. 6

    Name CH₂=C(CH₃)CH₂CH₃.

    Model answer

    CH₂=C(CH₃)CH₂CH₃ is 2-methylbut-1-ene. The chain is numbered from the end nearer the double bond, which puts the methyl branch on carbon 2.

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