3.3.4 Alkenes

Look at C=C: split the double bond into one σ bond and one π bond. The π bond comes from sideways overlap of p orbitals, with electron density above and below the internuclear axis.

The π bond is weaker and more exposed than the σ bond, so it is the part of C=C that is broken when an alkene undergoes electrophilic addition.

Point to the π electrons when you explain why alkenes attract electrophiles. They make C=C electron-rich, so an electrophile is attracted to that region and accepts an electron pair in the first step of addition.

This electron-rich π bond can even polarise an approaching non-polar Br₂ molecule, creating the electrophilic end needed for attack. That is why bromine reacts readily with an alkene without the molecule having a permanent dipole beforehand.

Know the required addition products across C=C. HBr gives a bromoalkane; Br₂ gives a dibromoalkane; concentrated H₂SO₄ first gives an alkyl hydrogensulfate, which can be hydrolysed to an alcohol.

For an unsymmetrical alkene, more than one structural product may be possible. Keep product prediction separate from the detailed mechanism until you have identified what is adding across the double bond.

For the bromine test, tie the observation directly to addition across C=C: orange/brown bromine is decolourised as Br₂ adds to the alkene.

If the mechanism is requested, show the π electrons polarising/attacking Br₂ and form the dibromo product. Do not turn the simple test observation into an oxidation-state colour change.

An unsymmetrical reagent can add to an unsymmetrical alkene in two orientations, so two structural products may form. Compare the possible carbocation intermediates to decide which pathway is favoured when a major product is required.

Draw both plausible first-step carbocations before choosing. That makes the product ratio a consequence of intermediate stability. You do not need a separate orientation slogan if you can compare the intermediates.

In electrophilic addition, the alkene π pair attacks the electrophile and the π bond opens. If a carbocation forms, the nucleophile then donates a lone pair to that positively charged carbon.

Before I draw an electrophilic-addition mechanism, I mark the electron-rich π bond and any δ charges that matter. That usually makes the first curly arrow obvious; then you only have to track the intermediate and the nucleophile in the second step.

With an unsymmetrical alkene, draw or imagine the two carbocations you could make after the first electrophilic step. The route through the more stable carbocation is favoured: tertiary > secondary > primary.

The explanation is about the intermediate, not the final product being “more stable”. Draw the two possible carbocations if the orientation is not immediately obvious.

For addition polymerisation, picture the alkene π bonds opening and being replaced by σ bonds along a long carbon chain. You join many monomers without eliminating a small molecule.

In addition polymerisation, the C=C π bond opens and the carbon atoms join into a C–C backbone. Draw the repeating unit with continuation bonds through the brackets and keep all substituents attached to the correct carbons.

When you draw a repeating unit, find the smallest section that reproduces the polymer chain. Put the brackets through the two backbone continuation bonds and keep each substituent on the correct backbone carbon.

Do not include an extra repeat just to make the drawing look symmetrical. The bracketed unit should be the minimum complete repeat.

For naming an addition polymer, identify the alkene monomer first and then use poly(monomer name). Chloroethene, for example, gives poly(chloroethene).

This naming line is simpler than the structural one: identify the monomer name accurately first, then place it after “poly”.

Look at what the polymer backbone actually contains when you explain why addition polymers are usually unreactive. You have strong C–C and C–H bonds and you have lost the reactive C=C bond from the monomer.

Polyalkenes mainly contain strong non-polar C–C and C–H bonds and lack easily hydrolysed functional groups, so many common reagents and enzymes have no easy point of attack.

Polymer properties can be changed by molecular structure, chain packing, additives and processing. Materials science has developed by connecting those structural variables with measurable properties and useful applications.

Treat this as an evidence-and-design idea: chemists can change a material formulation or processing method, measure the resulting property and use that relationship to design a better polymer product.

If you add a plasticiser to PVC, it sits between the polymer chains and weakens the effective attractions between them. The chains can then move past one another more easily, so the material becomes more flexible.

The plasticiser does not break the PVC covalent backbone. It changes intermolecular interactions and chain mobility, which changes the bulk material property.

To turn an alkene monomer into its addition-polymer repeat, replace C=C with a C–C link in the backbone and extend a bond from each of those two carbons through the brackets. Keep every substituent on the carbon where it started.

The monomer double bond disappears because its π bond is used to make new σ bonds to neighbouring monomer units.

Given a section of polymer chain, find the smallest backbone segment that repeats and bracket just that segment. Put the bracket boundaries through C–C continuation bonds, not through atoms or substituents.

Check by mentally copying the chosen unit once to the left and right. If the original chain is not reconstructed exactly, the repeat unit is wrong.

To recover the alkene monomer, take the two backbone carbons in one repeat, remove the external continuation bonds and restore a C=C between those carbons. Leave their substituents unchanged.

Do not add or remove substituents when you restore the double bond. Addition polymerisation changes the π bond connectivity between monomer units, not the identity of the groups on those carbons.

The bonding in an addition polymer explains its inertness: polymerisation removes the reactive alkene π bond and leaves a saturated carbon backbone dominated by strong C–C and C–H σ bonds. There is little chemically reactive functionality for common reagents to attack.

If you are explaining poor biodegradability, the same structure matters biologically: common enzymes have no convenient hydrolysable functional group in a simple polyalkene backbone, so chain cleavage is difficult.

For polyalkenes, London forces are the main attractions between the largely non-polar chains. A larger contact area or longer chain can strengthen those attractions and affect properties such as softening/melting behaviour and mechanical strength.

Relate the property to interchain attraction and chain movement. Avoid saying the covalent bonds inside the chain become stronger when you are explaining a change in a physical property.