These are detailed Revision Notes for Topic 3.4 of AQA A-Level Chemistry (Alkenes). They were written by me using a combination of the textbook and class notes. I will also be uploading the other topics and creating bundles.
Topics Included:
- Alkenes
- Reactions of Alkenes
- Addition polymers
Alkenes
14.1 Alkenes
- Alkenes are unsaturated hydrocarbons, CnH2n 14.1 Alkenes
- The angles between each bond in ethene is 120o, it is a planar flat molecule.
14.2 reaction of alkenes
- There is no rotation on the double bond. There is a p-orbital containing a single
electron on each carbon which overlap forming an orbital with a cloud of electron 14.3 addition polymers
density above and below the single bond. This is called a 𝜋-orbital/bond.
- E/Z isomerism notation is based on atomic numbers, when two atoms of higher atomic number are on
the same side of the C=C bond the isomer is Z. If not, it is E.
- Van de Waals forces are the only forces between molecules, they have very similar properties to
alkanes.
- Melting and boiling points increase with the number of carbon atoms.
- Alkenes are not soluble in water.
- Single bond C–C has an enthalpy of 347kJmol-1 and double C=C bond has an enthalpy of 612kLmol-1
- They are more reactive than alkanes because of the high electron density between the two carbon
atoms, this can be attacked by positively charged reagents (electrophiles).
- As they are unsaturated, they can undergo addition reactions.
14.2 Reactions of Alkenes
- Alkene + Oxygen à Carbon Dioxide + Water
- Electrophilic addition reactions:
o The electrophile is attracted to the double bond
o Electrophiles are positively charged and accept a pair of electrons from the double bond. The
electrophile may be a positively charge ion or have a positively charged area.
o A positive ion (carbocation) is formed
o A negatively charged ion forms a bond with the carbocation.
Reaction with hydrogen halides:
- Bromine is more
electronegative, so the
hydrogen bromide
molecule is polar. The
bond breaks
heterolytically.
- Forms a halogenoalkane
- The electrophile is
equally attracted
towards either of the carbon atoms around the double bond so you can get different isomer products.
The product that is most likely to be formed is the major product.
- When a hydrogen halide reacts with an unsymmetrical alkene the hydrogen attaches itself to the
carbon atoms with the greater number of hydrogen atoms and the smaller number
of carbon atoms.
- Markonikov’s rule – the major product is the most stable (the hydrogen will add on to
the carbon with the most hydrogens.
- Positive inductive effect
o Alkyl groups have a tendency to release electrons. This is called a positive
inductive effect and is represented by an arrow along their bonds showing
the direction of release.
o This effect stabilises the charge of the intermediate carbocation.
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