Structure & Formulae of Alkenes (GCSE Chemistry)
Structure & Formulae of Alkenes
Alkenes
Structure and Formula
- Alkenes contain a double bond. As mentioned earlier, alkenes are hydrocarbons. They contain a double bond between two of their carbon atoms, represented as C=C.
- CnH2n represents the alkanes general formula. Like alkanes, alkenes are also part of a homologous series, which has a general formula.The general formula of alkanes can be represented by CnH2n.
Unsaturation
- Alkenes are unsaturated. Alkanes are known as saturated compounds, and alkenes are unsaturated. This is due to the double bond that they contain.
- Alkenes are more reactive than alkanes. Alkenes are more reactive than alkanes because of their double bond. The bond can be opened up, meaning that atoms can bond onto the carbon.
- Alkenes can have specific names. As they are part of a homologous series, alkenes can be named. The first four members of the alkenes are ethene, propene, butene and pentene.
Representing alkenes
- Alkenes can be represented. We can represent alkenes very easily, by using the number of carbons each molecule contains. Ethene can be represented as C2H4.
- Alkenes can be drawn out. We can also represent alkenes using their displayed formulae. For example, the displayed formulae of ethene is shown below.
Worked example: Draw the structure of propene.
Answer:
1. Work out the number of C atoms.
The prefix is “prop” which means there are 3 carbon atoms
2. Draw one double bond. This is between two of the carbon atoms and a single bond between the other two C atoms.
3. Check the number of bonds. Make sure there are four bonds from each carbon atom. Add hydrogen atoms to all other bonds.
4. Use the general formula CnH2n. This will help you to check you have the correct number of hydrogen atoms.
Alkenes are a group of unsaturated hydrocarbons that contain at least one carbon-carbon double bond.
Alkenes have at least one carbon-carbon double bond, while alkanes only have carbon-carbon single bonds.
The general formula for alkenes is CnH2n, where n is the number of carbon atoms in the molecule.
The structure of an alkene consists of carbon atoms bonded to each other by a double bond, with the remaining valence electrons bonded to hydrogen atoms.
To identify an alkene, look for a carbon-carbon double bond in the molecular formula or the structural formula.
Alkenes are named based on the number of carbon atoms in the molecule, using the prefix “ene”. For example, a three-carbon alkene is called “propene”.
Cis and trans isomers refer to the arrangement of atoms around a carbon-carbon double bond. Cis isomers have the same functional groups on the same side of the double bond, while trans isomers have the functional groups on opposite sides of the double bond.
Hydration is the addition of water to an alkene, which results in the formation of an alcohol. This process is catalyzed by an acid, such as sulfuric acid.
Hydrogenation is the addition of hydrogen gas to an alkene, which results in the formation of an alkane. This process is catalyzed by a metal catalyst, such as platinum or nickel.
Halogenation is the addition of a halogen (such as chlorine or bromine) to an alkene, which results in the formation of a dihaloalkane.
Alkenes are important in chemistry because they can be used to synthesize a variety of important compounds, including plastics, solvents, and pharmaceuticals. They are also important in the petroleum industry, as they are a major component of crude oil.
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