Home Science Additional Science AS Units A2 Units

AQA Unit 1 - Alkenes

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Alkenes - Nomenclature

(1) Names :

Alkenes are named from their saturated alkane counterparts.

A prefix derived from the number of carbon atoms present (see naming alkanes in the alkanes page) is capped with the suffix _ene.

With four or more carbon atoms present the position of the double bond becomes important. The position number after double bond is found by counting from the nearest chain end to the start of the double bond. his number is inserted between the prefix and suffix with hyphens.

Examples -

ethene - but-1-ene -
(2) Cis/trans Isomerism :

The carbon-carbon double bond has the effect of stopping rotation of the groups attached to the carbon atoms (see bonding section later on).

This means that the two molecules shown below are not the same -

The left-hand molecule has both methyl groups on the same side of the double bond and is called cis-. The right-hand one has the methyl groups opposite one-another and is called trans-.

Therefore, the two molecules above are properly named -

cis-but-2-ene trans-but-2-ene
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Alkenes - Reactions of Ethene

(1) Hydrogenation :

This reaction turns alkenes into alkanes by the addition of a molecule of hydrogen gas. The reaction requires high temperatures and pressures and a nickel catalyst.

e.g. ethene can be turned into ethane,

(2) Hydrolysis :

This process is the addition of water (in the form of steam) to an alkene to give an alcohol. The reaction requires a high temperature and pressure, which turns the water into steam, as well as a (acidic) catalyst.

e.g. ethene can be turned into ethanol,

(3) Halogenation :

This reaction involves the addition of a halogen (i.e. fluorine, chlorine, bromine or iodine) to an alkene to give a dihaloalkane. e.g. ethene can be turned into 1,2-dibromoethane using bromine water,

The reaction with bromine water, Br2(aq), is commonly used as a test for a carbon-carbon double bond. With the positive result being the decolourisation of the orange bromine water.

(4) With Hydrogen halides :

This reaction follows the same pattern as the previous three, with the hydrogen halide being added across the carbon-carbon double bond to turn an alkene into a haloalkane.

e.g. ethene can be turned into bromoethane when reacted with hydrogen bromide,

(5) Oxidation :

(i) With cold dilute acidified manganate(VII)(aq) ions -

With this set of reaction conditions an alkene is oxidised (i.e. has oxygen added) to form a diol -

This provides a second convenient test for a carbon-carbon double bond; with a positive result being the decolourisation of the deep purple manganate(VII) solution.

(ii) With hot concentrated acidified manganate(VII)(aq) ions -

This set of conditions causes the carbon-carbon double bond to break and leads to the alkene forming two carbonyl compounds (see aldehydes and ketones).

e.g. ethene can be turned into two molecules of methanal,

(6) Polymerisation :

The carbon-carbon double bond is able to undergo free-radical polymerisation when a suitable initiator is added (e.g. UV light, an organic peroxide). The individual alkene units, monomers, join together to form long continuous chains, polymers.


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Alkenes - Reaction Mechanism

At AS level alkenes undergo only one type of reaction - addition reactions. Since the carbon-carbon double bond is a great source of electrons the overall reaction of an alkene is electrophilic addition.

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Alkenes - p-bonds

The electron configuration of carbon is 1s2s22p2. This shows that there are four electrons in the outer shell, and they are unevenly distributed. In the bonding state for carbon the distribution is 2s12p3, with one electron in each of the p orbitals.

When the carbon atoms in ethene form s-bonds they use the 2s electron and two of the 2p electrons. These combine to form three sp2 hybrid orbitals, leaving a spare p-orbital on each of the carbon atoms. These p-orbtals overlap causing areas of electron density spanning across the top and the bottom of the s-bond joining the two carbon atoms. This overlap is the p-bond.

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written by Dr Richard Clarkson : © Saturday, 1 November 1997

updated : Tuesday, 11th August, 2009

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