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Alkenes (3.3.4)
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Alkenes are a type of unsaturated hydrocarbon.
They contain at least one double bond, which are centres of high electron density.
The general formula for a straight chain alkene with one double bond is .
Note that cyclic alkanes have the same general formula as straight chain alkenes.
A carbon-carbon double bond contains one bond and one bond.
A bond forms when two orbitals overlap directly along the axis between the two atoms.
The sideways overlap of the two p orbitals above and below the bonding atoms forms a bond.
There is no free rotation around bonds.
The bond within in alkenes has a much lower enthalpy than that of the bonds present in both alkanes and alkenes.
As a result the energetic barrier to break the bond within is lower and reactions occur more readily in alkenes than in alkanes.
Typical bond enthalpies for , , and are given in the table below.
The increase in the bond enthalpy compared to the bond enthalpy is due to the bond; this is weak at only .
Electron pairs in bonds repel each other. Molecules adopt an orientation to maximise the distance between electron dense areas, and therefore reduce repulsion.
In an alkene, the carbon within a double bond has three areas of electron density, all of which are bonding. It will have trigonal planar geometry, with bond angles based around 120°.
Increased repulsion from the , which contains two electron pairs, results in the angle between the double bond and the single bonds slightly exceeding the angle between two single bonds.
All four groups attached to the must be on the same plane to allow overlap of the p orbitals within the bond.
The mechanism for any electrophilic addition to an alkene features heterolytic fission of the bond.
Electrons from the bond attack an electrophile, forming a carbocation intermediate.
In the mechanism, the curly arrow should be drawn from the centre of the bond to the atom on the electrophile.
The newly formed carbocation is a lone pair acceptor (an electrophile) and is therefore susceptible to nucleophilic attack.
A curly arrow is drawn from the electron source in the nucleophile towards the positive charge on the carbocation.
The electron source on the nucleophile could be a lone pair, if in a compound, or a negative charge on an ion.
As the number of residual groups (R groups) on a carbocation decreases, the stability of the carbocation decreases.
The ability of alkyl groups to stabilise carbocations is NOT an inductive effects. This has been disproved following 75 years of acceptance. In reality, electron donation is caused by a combination of polarisability and hyperconjugation. Exam boards are currently awarding marks for inductive donation, but this is likely to change in due course.
Where an asymmetrical alkene reacts with a haloalkane or steam, a mixture of structural isomers is formed.
Propene is an asymmetrical alkene.
Two different carbocations are possible after the initial addition of an electrophile to propene.
This leads to the formation of two different products.
The major product in an electrophilic addition reaction of an alkene is the one formed via the most stable carbocation.
The impact of carbocation stability on electrophilic addition of an alkene with a hydrogen halide or steam is a major product, with the or group on the most substituted carbon.
Alkenes react spontaneously with hydrogen halides to form haloalkanes in an electrophilic addition reaction.
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Where the alkene is not symmetrical, the reaction can result in a mixture of two structural isomers.
or
Alkenes react spontaneously with halogens to form dihaloalkanes in an electrophilic addition reaction.
One bromine adds to each carbon in the double bond.
The electrophilic addition of bromine to alkenes is used as a test for unsaturation.
is orange and bromoalkanes are colourless. When the substances are mixed they react and as the reaction progresses the orange colour fades.
To complete the test, bromine water is shaken with a test sample, and the results are observed.
Cold, concentrated, sulfuric acid, () can react with alkenes in an electrophilic addition to form alkyl hydrogen sulfates.
The in the bond in the sulfuric acid has a partial positive charge and acts as an electrophile.
The electrons in the double bond attack the leading to the formation of a carbocation and breaking the bond in sulfuric acid.
The nucleophile then attacks the carbocation to form an alkyl hydrogensulfate.
The alkyl hydrogen sulfate product is susceptible to nucleophilic substitution reactions and will form an alcohol in aqueous conditions.
Question walkthrough
Electrophilic addition
Drawing the mechanism and major product when an alkene reacts with HX.
Addition polymerisation is the joining of a large number of alkene monomers to form an addition polymer.
The -bond of the of the alkene breaks and new -bonds are formed between monomer units.
To show one repeating unit of an addition polymer from a given monomer:
- break the -bond from the
- replace it with two trailing bonds going through brackets.
To identify the monomer from a single repeating unit:
- remove the trailing bonds and brackets
- add a -bond between the carbons.
In addition polymerisation, repeating units will always have two carbons – there are two carbons in the alkene functional group.
Additional carbons may feature as side chains.
IUPAC states that addition polymers are named after the monomers that they are made from.
- To name an addition polymer, you add the prefix ‘poly’ to the bracketed name of the monomer.
- In the case of the monomer being an alkene, the suffix will still be -ene despite the pi bond having broken during polymerisation.
For example, the addition polymer of ethene would have the IUPAC name poly(ethene), although alternatives such as polythene and polyethylene are frequently used.
The general formula of addition polymers is shown by identifying the repeating unit and putting brackets around it followed by the subscript ‘n’ indicating the number of repeat units in the chain.
Polymers are commonly viscous liquids or solids and are never gases.
The intermolecular forces between molecules of polyalkenes are primarily London dispersion forces due to the non-polar nature of the polymer chains. These forces arise due to the temporary dipoles induced in the electron cloud of the molecules.
Compared to simple covalent molecules, polymers are big and London dispersion forces can be significantly stronger. Longer polymer chains lead to more electrons, and therefore stronger dispersion forces.
Branched polymer chains are less able to make surface contact than straight chains, reducing the strength of the London dispersion forces. This also reduces the rigidity of the polymer and its melting point.
Addition polymers are unreactive.
During polymerisation the double bonds are broken, resulting in a saturated carbon chain.
Sigma covalent bonds are very stable and not easily broken, resulting in the polymer backbone being highly inert. The main polymer backbone is non-polar, further reducing the reactivity.
The side chains on addition polymers can influence reactivity but these are often hydrocarbon based and therefore possess the same resistance to reaction as the polymer backbone.
Since the first simple addition polymers technology has significantly expanded the functionality and properties of polymers.
Biodegradable and photodegradable polymers are being developed to reduce the environmental impact of persistent plastic waste.
Biodegradable polymers are designed to decompose in the presence of bacteria or other living organisms.
Photodegradable polymers are designed to decompose in the presence of UV light.
Starting materials for the synthesis of these novel polymers are increasingly biobased, reducing the dependency on the finite resource of crude oil and natural gases.