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Alkenes (4.1.3)
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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.
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.
Stereoisomers are molecules with the same structural formula but a different arrangement in space.
The two relevant types of stereoisomerism in organic chemistry are optical and E/Z.
E/Z isomerism is applicable to alkenes whereas optical isomerism applies where four different groups are bonded around a carbon.
In an alkene, restricted rotation around the carbon-carbon double bond gives two discrete sides to the bond. Where the bond is drawn horizontally, the sides are above and below the double bond.
When both carbons involved in the double bond are bonded to two different groups then E/Z stereoisomers are possible.
This alkene will have E/Z isomers since both the left and the right carbon are bonded to two different groups.
cis/trans isomerism is special case of E/Z isomerism. This is only used when at least one group on carbon 1 matches a group on carbon 2.
Where the matching groups are on opposite sides then ‘trans’ is used in the naming of the compound.
Similarly ‘cis’ is used where the matching groups are on the same side.
In pent-2-ene, each carbon in the is bonded to a single hydrogen.
The hydrogen groups can both sit on the same side of the double bond or on opposite sides; cis/trans isomerism exists in pent-2-ene.
E and Z descriptors are used to differentiate between stereoisomers and are based on the position of the two highest priority groups.
Group priority is assigned using the Cahn–Ingold–Prelog (CIP) rules:
- Groups are assessed based on the first point of difference, starting with the atom directly bonded to the carbon.
- The atom with the highest atomic number is given the highest priority.
- For isotopes, the atom with the highest atomic mass is given the highest priority.
- Where no difference is present in the atoms directly bonded to the carbons, consider the next connected atoms using the same rules.
If the highest priority groups are on opposite sides of the double bond, it is the E stereoisomer, from the German for opposite ‘entgegen’.
If the highest priority groups are on the same side of the double bond, it is the Z stereoisomer, from the German for together ‘zusammen’.
The configuration of E and Z stereoisomers is the opposite to the visual shape of the letter.
To recall which configuration is linked to which descriptor, remember ‘ it’s not what it looks like! ’.
Question walkthrough
E/Z isomerism
Assigning E/Z to an alkene using CIP
Question walkthrough
E/Z isomerism
Drawing and naming the E/Z isomers from a chemical formula
Question walkthrough
E/Z isomerism
Drawing the E/Z isomers from a name
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 .
In an addition reaction, two or more reactants combine to make a single product.
Catalytic hydrogenation of alkenes is an addition reaction.
Alkenes react with hydrogen in the presence of a suitable catalyst to form alkanes.
The most commonly cited catalysts for this reaction are nickel, palladium, and platinum.
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.
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.
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 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.
Question walkthrough
Electrophilic addition
Drawing the mechanism and major product when an alkene reacts with HX.
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.
In 1865, Vladimir Markownikoff recorded the observation:
When a protic acid () is added to an asymmetric alkene, the acidic hydrogen attaches itself to the carbon having a greater number of hydrogen substituents whereas the halide group attaches itself to the carbon atom which has a greater number of alkyl substituents.
This was later linked to an increased stability of carbocations formed on more heavily substituted carbons.
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.
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.
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.