A-level Chemistry: 3.3.4 Alkenes

∵ they’re 2 pairs of electrons in the C=C double bond

(High conc. of electrons )

1. C-C single bond & p-orbital (contains single electron) on each carbon

2. These 2 orbitals overlap & form an orbital with a cloud of electron density above and below the single bond

3. Called π orbital & its presence = bond can’t rotate

Van der Waals forces

insoluble

more

∵ C=C forms electron-rich area = can be easily attacked by electrophiles

∵ double bond has lots of electrons = easily attacked by electrophiles

electron-pair acceptors

Attracted to areas where there’s lots of electrons

e.g. Positively charged ions (H+ NO2+) and polar molecules (δ+)

∵ Bromine is added across C=C double bond to form colourless dibromoalkane

(happens via electrophilic addition)

• Halogen
  

  • X—X = dihalogenoalkane

• Hydrogen Halides
  

  • H—X = halogenoalkane

• Sulfuric Acid
  

  • H2SO<span>4</span> = alcohol

You get 2 possible products

How stable carbocation formed is = known as carbocation intermediate

∵ alkyl groups feed electrons towards positive charge

Alkyl hydrogen sulfates

By adding water & warming reaction mixture

Long chain molecules formed when monomers join together

alkenes and substituted alkenes

Can act as monomers and form polymers ∵ double bond can open up & join together to make long chains

• Polyalkene chains = saturated molecules

• Main carbon chain of polyalkene = non-polar

Van der Waals forces

| (∵ polyalkene chains = non-polar)

stronger Van der Waals forces between chains will be

strong & rigid

weaker & more flexible

1. ∵ Chain = saturated alkane molecule

   • have non-polar C-C bonds

2. Means they cannot be attacked by biological agents (e.g. enzymes)/nucleophiles

By adding plasticiser to polymer = more them more flexible

1. ∵ plasticiser molecules get between polymer chains & push them apart

2. = reduces strength of intermolecular forces between chains

3. ∴ chains slide around more = making polymer easy to bend

An intermediate chemical from which many useful products can be made

1. Made by partially oxidising ethene

2. Mixture of ethene and oxygen is passed over catalyst = silver

1. Epoxyethane = poisonous and carcinogenic

2. It's highly flammable/explosive when in contact with air

1. Bonding pairs in ring of atoms in molecule = forced very close together

2. ∴ bond angles = 60° rather than 109.5° i.e. C–O–C bond angle much smaller than 109.5°

3. Bonding pairs repel each other

4. = ring tends to spring open (molecule has 'ring strain' due to the repulsion between bonding pairs - atomic orbitals overlap) as system becomes more stable if ring is broken

Plasticisers or detergents

ethane-1,2-diol

antifreeze

C2H4O + H2O → C2H6O2

Rigid PVC:

• Drainpipes

• Window frames

Plasticised PVC:

• Electrical cable insulation

• Flooring tiles

• Clothing

• Reaction goes via intermediate carboncations

• Formed via tertiary carboncation which is more stable (than secondary carboncation)

• Due to more alkyl groups release electron density towards the positive carbon

2 CH2CH2 + O2 → 2 CH2CH2O

• 250-300°C

• 15 atm

• Silver catalyst

• C=C is area of high electron density

• (this) induces a dipole in the bromine molecule