Chapter 13: Alkenes (13.1-13.5) Flashcards
general formula for alkenes
CnH2n
properties of the C=C double bond
consists of a π-bond (pi-bond) and a σ-bond (sigma bond)
the π-bond prevents rotation around the C=C bond
trigonal planar shape around it, bond angle 120
properties of the π-bond
pi-bond is the sideways overlap of adjacent P orbitals above and below the bonding atoms
In the middle of the overlapping orbitals is the sigma bond (overlap of orbitals directly between the bonding atoms)
why are alkenes more reactive than alkanes
because alkenes have a π-bond
The pi-bond induces a region of high electron density above and below the plane of atoms
Alkenes are therefore more susceptible to attack from electrophiles causing the alkene to react.
what are the two types of stereoisomerism
- E/Z isomerism (only occurs in some alkenes)
- optical isomerism (occurs in a range of different compounds)
what must a molecule have in order for it to show E/Z isomerism?
- must have a C=C double bond
- must have 2 different groups attached to each carbon atom of the C=C bond
What is the difference between cis-trans isomerism
cis: the repeated group is on the same side of both C atoms
trans: the repeated group is on the opposite side of both C atoms
what gives an atom high priority (Cahn-Ingold-Prelog priority rules)
the atom with the greater atomic number has the higher priority
what makes an isomer a Z isomer
a Z isomer has both of the higher priority groups on the same side pf the C=C bond
(Zame Zide)
what makes an isomer an E isomer
an E isomer has the higher priority groups on the opposite side of the C=C bond
what happens if the the first atoms attached to a C in a C=C double bond have the same priority?
keep going along the chains until you find a difference
what happens when alkenes undergo addition reactions
a small molecule adds across the double bond, causing the π-bond to break and new σ-bonds to form
there is only 1 product
the double bond is lost, product is saturated
alkene + hydrogen –>
alkane
addition reaction, needs a nickel catalyst
alkene + halogen –>
dihaloalkane
addition reaction, no catalyst
test for unsaturation (shows that an addition reaction has occurred)
unsaturated hydrocarbons decolourise bromine water:
- orange bromine water is added to an excess of aqueous organic compound (mixture is shaken)
- in the presence of a double bond, the mixture becomes colourless (bromine adds across the double bond)
- if the organic compound is saturated, no reaction will occur and bromine water will not decolourise
alkene + hydrogen-halide –>
haloalkane
(addition reaction, no catalyst)
if the alkene has a carbon chain of 3 or more, there are 2 possible products, depending on which carbon the halogen adds to (e.g. 1-bromo vs 2-bromo)
alkene + steam –>
alcohol
(addition reaction, acid catalyst e.g. H3PO4)
if the alkene has a carbon chain of 3 or more, there are 2 possible products, depending on which carbon the -OH adds to
mechanism for the reaction of an alkene with an electrophile
electrophilic addition
steps of the electrophilic addition mechanism (polar molecule)
step 1:
- a molecule (e.g. H-Br) acts as an electrophile, accepting the pair of electrons from the pi-bond of C=C and double bond breaks
- the electrophile bond (e.g. H-Br) breaks by heterolytic fission
step 2:
- a carbocation and negative ion (e.g. Br-) are formed
step 3:
- the negative ion (Br-) is attracted to the carbocation, forming a bond (C-Br). The addition product is formed
how electrophilic addiction works in non-polar molecules
the high electron density of the pi-bond induces a dipole in the non-polar molecule so it can now act as an electrophile
how to predict which halo-alkane will be the major product when electrophilic addition happens (unsymmetrical alkenes)
Markownikoff’s rule states that when an electrophile H-X reacts with an unsymmetrical alkene, the H atom is more likely to attach to the carbon atom with the greatest number of H atoms attached to it
i.e. secondary carbocation will form the major product
why do alkene monomers polymerise?
they are much more stable without the C=C double bond
environmental issues with putting waste polymers into landfill
addition polymers are unreactive and take a long time to biodegrade
how can waste polymers be processed in a non-evironmentally damaging way?
- recycling
- combustion: polymers are burnt and the heat used to produce electricity
- used for chemical feedstocks
problems with halogenated plastics
combustion of halogenated plastics can from toxic waste products (e.g. HCl gas) which must be removed to prevent discharge to the environment
biodegradable polymers
polymers that are plant-based and break down naturally to form CO2, H2O, and other biodegradable compounds (non-toxic therefore doesn’t harm environment)
photodegradable polymers
polymers that contain bonds that weaken in the presence of light, initiating the breakdown of polymers
Electrophile
Electron pair acceptor
E/Z isomerism
A type of stereoisomerism in which different groups attached to each carbon of a C=C of a double bond may be arranged differently in space because of the restricted rotation of the C=C bond.
Cis- trans isomerism
- A type of E/Z isomerism
- One of the groups attached to each carbon atom is the same
- If the group is on the same side = cis and opposite side = trans
Stereoisomers
Compounds with the same structural formula but a different arrangement of atoms in space
How can H-Br act as an electrophile?
The H accepts a pair of electrons
