Alkenes

 
 

Properties of alkenes

  • Alkenes have the general formula CnH2n
  • They contain at least one double carbon bond - they are unsaturated molecules.
  • The double bond is formed from one sigma bond and one pi bond.
  • Sigma bonds are formed from the head-on overlap of s orbitals while pi bonds are formed from the sideways overlap of p orbitals.
  • P-orbital overlap results in two regions of electron density above and below the plane of the carbon atoms, forming a pi bond with restricted rotation.

Since each carbon (of the C=C bond) is only bonded to three other atoms, it has a trigonal planar geometry and bond angles of 120o. The three bonding pairs arrange themselves a maximum distance apart to minimise repulsion between them.

Stereoisomerism

Stereoisomers are compounds with the same structural formula but different arrangements of atoms in space. Alkenes can form cis/trans or E/Z isomers due to the restricted rotation around the double carbon bond.

Alkenes can be labelled as cis/trans if both atoms of the double carbon bond have at least one group in common.

  • Cis isomers have the group on the same side of the double carbon bond (i.e. both groups are above the C=C bond).

  • Trans isomers have the matching group on different sides of the double carbon bond (i.e. one group is above and the other is below the C=C bond).

 
 

Alkenes that don’t have two groups in common can be labelled as E/Z isomers. This time we have assign priority values to each of the groups attached to each carbon of the C=C bond. All we need to do is add up the molecular masses (Mr) for each group. The larger the Mr, the higher the priority.

  • Z isomers have the two highest priority groups on the same side of the double carbon bond. Z stands for the word for together in German, zusammen.

  • E isomers have the two highest priority groups on opposite sides of the double carbon bond. E stands for the word for opposite in German, entgegen.

 
 

For molecules to display E/Z isomerism, they need to fit two criteria:

  1. Contain a double carbon bond – this means that they have restricted rotation about the C=C bond and the groups attached to the double bond are fixed in a certain position.

  2. Each carbon of the C=C bond needs to be attached to two different groups.

Addition reactions

Alkenes are more reactive than alkanes, because the pi bond is a region of high electron density. This region of concentrated negative charge is able to attract polar molecules. The double carbon bond is much weaker than single carbon bonds, since it has a lower bond enthalpy, making it much easier to break.

Alkenes can take part in addition reactions, in which the double carbon bond breaks and other atoms attach themselves to the carbon atoms. Addition reactions form just one product so they have 100% atom economy.

Hydrogen can react with alkenes to form alkanes in hydrogenation reactions. The reaction requires a nickel catalyst and temperatures of 150o.

 
 

Halogens can react with alkenes in the same way to form halogenoalkanes. This reaction is referred to as halogenation and takes place at room temperature and pressure. It involves an electrophilic addition mechanism.

 
 

Here’s a step by step breakdown of what’s happening here:

  1. The extra pi electrons in the C=C bond makes the bond particularly negative. When a halogen molecule approaches, this region of negativity repels the bonding electrons in the halogen-halogen bond.

  2. This creates an induced dipole, where the halogen furthest from the alkene is slightly negative and the one closest to the alkene is slightly positive.

  3. The pi electrons are attracted to the slightly positive region on the halogen and attack it, forming a bond between the carbon atom and the slightly positive halogen atom.

  4. The electrons in the halogen-halogen bond jump onto the slightly negative halogen atom, causing the bond to break by heterolytic fission and creating a negative halide ion.

  5. Since the C=C bond has broken, one of the carbon atoms is only forming 3 bonds. This gives it a positive charge. This compound is referred to as a carbocation intermediate.

  6. The negative halide ion will donate its electrons to the positive carbon, forming a covalent bond between itself and the carbon atom. The final product is a dihalogenoalkane.

 
 

An electrophile is something that likes (-phile) negatively charged things (-electro). The proper definition is a species which will accept a pair of electrons. The electrophile in this reaction is the halogen (Br2) because it is accepting the pair of pi electrons in the double carbon bond.

This is the reaction that takes place when we use bromine water as a test for unsaturated compounds. Unreacted bromine is an orange colour, but as soon as it reacts with the alkene to form a halogenoalkane, it turns colourless.

Hydrogen halides react with alkenes in just the same way, except this time only one halogen atom will be present in our final product.

 
 

Two products are formed in this reaction as the bromine can attach to either carbon of the double carbon bond. If one of the carbons is attached to more alkyl groups (e.g. –CH3) than another, the bromine will prefer to attach to this carbon and that will be the major product. This is because alkyl groups are electron-pushing and stabilise the positive charge on the intermediate, creating a more stable carbocation.

 
minor major product.jpg
 

The addition reaction between alkenes and water forms alcohols. This requires steam and the presence of an acid catalyst, such as phosphoric acid (H3PO4). The C=C bond breaks, with one of the carbon atoms attaching to a hydrogen atom and the other bonds to a hydroxyl group.

 
 

Polymerisation

Addition polymerisation involves the breaking of double carbon bonds within alkene molecules and stitching lots them together to form a long polymer. In the polymerisation of ethene for example, the double bond between the two carbon atoms breaks and many ethene molecules bond together to form a long chain of ethene molecules known as poly(ethene) or ‘polythene’. Polythene is used to make thin plastics and can be found in plastic bags and shampoo bottles.

Rather than drawing out hundreds of ethene molecules connected together in one long chain, it’s far easier to draw a single monomer with brackets around it. We write an ‘n’ in the bottom right corner to show that there are lots of the same molecule joined together in a chain.

 
 

Let’s have a look at drawing a more complicated polymer. Have a look at the molecule below, 2-methylprop-2-ene. To draw the repeat unit, we need to ensure that only the carbons that make up the double carbon bond form the backbone of the polymer with everything else coming off as a side-chain. This means that you can only ever have a two-carbon chain in a single repeat unit.

 
 

Waste polymers and alternatives

After plastics are discarded, they may be:

  • Recycled by melting and remoulding into other items

  • Used as an organic feedstock – the polymer is broken apart into monomers and those units are used to make other chemicals

  • Burned to release energy for electricity generation – this can produce toxic gases such as HCl which needs to be removed during the burning process by reacting with a base

  • Buried in landfill – this leads to pollution and habitat destruction

To minimise the impact of plastics on the environment, scientists are trying to develop biodegradable polymers which naturally decompose. These are made from biological material such as starch or oil (e.g. isoprene). Decomposing microorganisms such as fungi and bacteria are able to break the bonds in these compounds, breaking them down into their monomers when they are in an environment where there is plenty of oxygen and moisture.

Scientists are also developing photodegradable polymers, which break down into their monomers in the presence of UV light.