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Alkenes (Topic 6C)
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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.
An electrophile is a species able to accept a pair of electrons.
Electrophiles can be:
- Positively charged ions, such as
- Polar molecules, such as
- Non-polar compounds with empty p orbitals, such as or , which only contain six electrons in their outer shells.
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.
Alkenes react spontaneously with halogens to form dihaloalkanes in an electrophilic addition reaction.
One bromine adds to each carbon in the double bond.
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, via electrophilic addition, with steam in the presence of an acid catalyst to form alcohols.
The acid catalyst forms a hydronium ion with water, making it a better electrophile. The reaction completes with the release of a ion, meaning that the total amount of in the system does not change. The acid is reformed and so is catalytic.
A common catalyst is phosphoric acid, .
Note that halide, sulfur, and nitrogen based acids are not suitable as they provide competing nucleophiles.
Where the alkene is not symmetrical the reaction can result in a mixture of two structural isomers.
or
The manufacture of margarine involves catalytic hydrogenation of vegetable oils, which are naturally unsaturated fats.
In this process, hydrogen gas, , is added across the carbon-carbon double bonds () to make the oil more saturated and increase its melting point. The resulting fat is solid.
A nickel catalyst is used to speed up the reaction, typically at 150–200 °C under moderate pressure.
Partial hydrogenation is preferred to achieve the right consistency without making the fat too hard. This process can, however, create trans fats, which have health risks, so their formation is minimised.
The result is a spreadable margarine with a texture similar to butter.
Manganate (VII) ions are a strong oxidising agent.
Alkenes are oxidised into diols in a redox reaction.
The double bonds in the alkenes are areas of high electron density and react with the .
The alkene undergoes oxidation and two hydroxyl groups are added to adjacent carbon atoms. This is a diol.
The pale purple solution becomes colourless as the permanganate ions, , are reduced to manganese(II) ions, .
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.
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.
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.
Question walkthrough
Electrophilic addition
Drawing the mechanism and major product when an alkene reacts with HX.
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.
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.
Combustion of waste polymers for energy production reduces the volume of landfill and contributes to the energy demand.
The heat produced is used in the production of electricity.
Halogenated plastics, such as polyvinyl chloride (PVC), produce toxic hydrogen halides during combustion, which must not be released to the environment.
Sodium hydrogen carbonate in the processor neutralises the hydrogen halides in flue gas.
Waste polymers can be used as chemical feedstocks.
Waste polymers are physically and chemically broken down before being separated into a combination of monomers, gases and oils. These can be used in production of new polymers and organic materials.
Use of waste polymers as chemical feedstocks has an advantage over traditional recycling as mixed and unwashed materials can be used. It also reduces the reliance on crude oil for the supply of monomers.
Reusing and recycling of waste polymers has many advantages;
- Using a polymer based product multiple times reduces the number of products that need to be made and disposed of. For example, refillable water bottles and ‘bags for life’.
- Once their useful life is over, polymers can often be reprocessed and the material used again in the manufacture of new products.
- This reduces the demand on raw materials as well as reducing the amount of polymer sent to landfill.
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.