Why are alkenes more reactive than alkanes?

Alkenes have a carbon-carbon double bond, which is very reactive.

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What is a functional group?

A functional group is a group of atoms in a molecule that determine how the molecule reacts.

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What chemical test can we use to distinguish alkenes from alkanes?

  1. Add orange bromine water to the unknown alkene or alkane and shake.
  2. If it's added to a saturated compound like alkane, it will remain orange.
  3. If it's added to an unsaturated compound like alkenes it will turn colourless.

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What type of reaction can an alkene be part of, because of the carbon-carbon double bond?

Alkenes can be part of addition reactions, due to their carbon-carbon double bond.

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How do addition reactions work?

An addition reaction is when two molecules combine to form a single molecule.


The carbon-carbon double carbon bond breaks and opens up, then a new atom is added to each carbon.

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What is hydrogenation?

Hydrogenation is the addition of hydrogen to an alkene in the presence of a catalyst.


Hydrogenation forms an alkane.

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How does hydrogenation work?

  1. An alkene reacts with hydrogen in the presence of a catalyst.
  2. This gets rid of the double bond, and adds a hydrogen to each carbon.

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How do alkenes form alcohols?

  1. They react with steam (gaseous water at a high temperature), in the presence of a catalyst.
  2. This breaks the double bond, hydrogen goes on one side, and OH goes on the other.
  3. We then have to separate out the alcohol.

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What happens when alkenes react with halogens?

The double bond will break, and each carbon will bond to a halogen atom.

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Do alkanes tend to undergo complete or incomplete combustion?


Do alkenes tend to undergo complete or incomplete combustion?

Alkanes usually combust completely.


Alkenes often undergo incomplete combustion.

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What is incomplete combustion?

Combustion without sufficient oxygen to react completely.

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What are the drawbacks of incomplete combustion?

  1. A smoky yellow flame.
  2. Less energy is released than from complete combustion.
  3. Produces pollutants.

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What harmful products can be produced by the incomplete combustion of hydrocarbons?

  1. Carbon.
  2. Carbon monoxide.
  3. Unburnt hydrocarbons.

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