Term Definition
Alkenes Unsaturated hydrocarbons with a C=C double bond
Stereoisomer Same structural formulae but take up different arrangements in space
Geometric Isomers Occur in molecules with restricted rotation around a double bond, locking groups
either side of the C=C bond in position
Z Isomer Largest groups are locked on the same side of the molecule (zusammen)
E Isomer Largest groups are locked on opposite sides of the molecule (entgegen)
Electrophile Species that is attracted to electron-rich regions in other molecules/ions. Either
carries a full or partial positive charge
Carbocation Positive charge is carried by a carbon atom
Positive Inductive Effect Electron-releasing methyl groups push electrons towards +ve charge
• In addition to a normal σ C-C single bond, in C=C bonds there is a cloud of high electron density
above and below the single bond called a π bond. Its present prevents rotation of the bond.
• Geometric isomerism is caused by this restricted rotation; if there are two different groups on the
LHS of the double bond and two different groups on the RHS then there will be geometric
isomerism.
• The E/Z system uses the Cahn-Ingold-Prelog Rules to assign relative priority to the two groups on
each carbon of the double bond. Rank the atoms attached directly to each carbon atom in the
double bond based on largest atomic number. If the first atoms are identical, rank until a difference
is found. Look at where the different groups are arranged. If the highest groups are on the same
side of the molecule it is a Z isomer (zusammen); opposite sides indicates an E isomer (entgegen).
Electrophilic Addition
An electrophile is a species that is attracted to electron-rich regions in other molecules/ions, carrying
either a full or partial positive charge. The double bond in alkenes is an area of high electron density,
so will attract electrophiles. In the addition reaction, the π bond breaks, with the pair of electrons being
used to join other atoms or groups onto the alkene, creating a saturated compound.
1. H
δ+
is strongly attracted to the electrons in the π bond
2. Dlectrons from the π bond move towards the area of positive charge, forming a covalent bond,
which repels the electrons in the H-X bond even further towards the X atom until the bond breaks,
forming a carbocation intermediate
3. The lone pair on the anion formed will be strongly attracted to the carbocation, forming a new
covalent bond. The overall result is the addition of hydrogen and X across the double bond.
Electrophile Description Mechanism
HBr Formation of a bromoalkane
from an alkene under cold
conditions
H2SO4 Adding cold concentrated
sulfuric acid leads to the
formation of
alkylhydrogensulfate. This is
readily hydrolysed when
warmed with water, forming
an alcohol. In this process,
sulfuric acid acts as a catalyst:
CH3CH2OSO2OH + H2O →
CH3CH2OH + H2SO4
Br2 When approaching the C=C
bond, the high electron
density induces a temporary
dipole, leading to electrophilic
addition, forming a
dibromoalkane
If an alkene is unsymmetrical about its double bond, two different products could result, depending on
which end of the double bond reacts with the electrophile. The more energetically stable the
intermediate is (requiring a lower EA), the more likely it is to exist for longer and in greater quantities,
meaning it is more likely to then react with a nucleophile.
Carbocation stability increases 1° < 2° < 3°. The build-up of charge is countered by methyl groups, which
push electrons towards the area of positive charge density – this will afford stability to the ion by positive
inductive effect. This effect is in the ratio 1:2:3 for carbocations, with 1° being the least stable, as only
one methyl group surrounds the +ve charge. Since 2° is twice as stable as 1°, ions exist twice as long
and so are more likely to react with an anion.
Addition Polymers
Addition polymers are formed when the π electrons in substituted alkenes monomers form C-C bonds
between molecules. Since polymers are saturated, they are relatively unreactive and their main
reaction is combustion. They are non-biodegradable – cannot be broken down by microorganisms.
The reaction on the left shows the equation of producing
poly(chloroethene). The naming system for polymers uses
poly(alkene monomer), following normal IUPAC rules.
Depending on the conditions used for polymerisation, two variants are possible:
• Low density poly(ethene) has branched chains and softens at low temperatures
• High density poly(ethene) has straighter chains and softens at higher temperatures. Since the
chains are able to pack closer together, the larger surface area causes stronger van der Waals
between polymer molecules, hence HDPE has a higher melting point
Mechanical recycling involves separating different types of plastics, which are washed then grounded
into pellets which are melted and remoulded. Feedstock recycling, however, heats plastics to break
bonds to reform monomers, which are then used to make new plastics. The issue with this is heating
causes shortening chains, degrading polymer properties.
Plasticisers are used to tailor polymer properties; they are small molecules which get between polymer
chains to allow sliding, increasing flexibility. This is how poly(chloroethene) (PVC) can be made rigid
enough to use as drainpipes and flexible enough for plastic raincoats and bags. PVC is specified as an
example on the specification so these details should be learnt.
