3.2 Hydrocarbons Flashcards
Alkanes
Alkanes are saturated hydrocarbons with single covalent bonds between atoms. Alkanes have the general formula CnH2n+2.
sigma bonds σ
Alkenes
Alkenes are unsaturated hydrocarbons with a double covalent bond between two carbon atoms. Alkenes have the general formula CnH2n.
fractional distillation
separates hydrocarbons within crude oil
separates the hydrocarbons into simpler mixtures depending upon their boiling points.
hydrocarbon fractions from the primary distillation of crude oil are of limited use without further processes
cracking
Cracking produces smaller alkanes and unsaturated alkenes, such as ethene and propene, which are the basis of the manufacture of many polymers.
large molecules to smaller more useful molecules
conbustion of Alkanes
good fuels
release lots of energy when they burn
complete or incomplete combustion
complete - produces carbon dioxide and water
Incomplete combustion
reaction where the oxygen supply is limited.
As a result, the products contain less oxygen atoms than they might have had with complete combustion.
This means carbon monoxide or even just carbon can be a product rather than carbon dioxide.
drawback of fossil fuel combustion
CO2 production
incomplete combustion - formation of soot (carbon) and toxic carbon monoxide
combustion of impurities in the fuels - Acidic gases, such as sulfur dioxide (SO2) and toxic nitrogen oxides (NOx)
fission
breaking of a covalent bond
homolytic fission
The covalent bond breaks with both atoms receiving one electron.
forms free radicals - species with an unpaired electron, which makes it very reactive.
For example, Chlorine Cl2 breaks to form two chlorine radicals, shown as Cl●.
hetrolytic fission
The covalent bond breaks with one atom retaining both electrons.
This forms positive and negative ions.
For example, HCl breaks to form H+ and Cl- ions.
photochloronation reactions of alkanes
chain reactions
producing chloroalkanes
reaction mechanism is a free radical substitution
Reagent: Cl2
Conditions: UV light
The reactions involve the substitution of hydrogen atoms in the alkanes for chlorine atoms.
free radical substitution
Homolytic fission of halogen bond in presence of UV light
produce halogenalkanes
What happens to the bonds in free radical substitution
UV light breaks halogen bonds
Producing intermediates (molecules formed during a reaction)
called free radicals
what do free radicals do to alkanes
attack alkanes
lead to reactions:
initiation
propagation
termination
initiation
halogen broken down with UV light
the formation of radicals by homolytic fission of the chlorine bond. UV light is necessary for this stage.
propagation
hydrogen replaced with halogen radical formed
radical acts as a catalyst
reactions take place to form further radicals and some stable products, such as chloromethane and dichloromethane
termination
two radicals joing to end chain reaction
form one stable product
propagation extra
can result in multiple substitutions
chain reaction
conditions altered to favour terimnation step
alkenes
unsaturated hydrocarbons
carbon carbon double bond
area of high electron density - susceptible to attack from electrophiles
double bond has σ and π
bonds in alkenes
tests for alkenes
bromine water - orange/brown to colourless forms dibromo-alkane, double bond opens up forms bonds with bromine atoms
potassium maganate (VII) - under acidic conditions purple to colourless, alkaline conditions purple to dark green
Stereoisomers
same molecular formula
different spatial arrangement
types of stereoisomers
E-isomer - high priority group diagonally across
Z-isomer - high priority group same side
carbocations
primary carbocation
secondary carbocation
tertiary cation
reaction proceeds by most stable intermediate
in addition reactions multiple products can form
Sigma bond σ
S orbitals overlap
can rotate
no change in molecule
alkanes form only sigma bonds
Pie bond π
alkenes
between two atoms formed by side to side overlap of p orbitals.
can’t rotate, leads to goemetric isomers
gives a region of high electron density
This is susceptible to electrophilic attack/ attack by an electron
deficient species
* (This attack) leads to addition reactions
just hydrocarbons
Hydrogen groups on same side = CIS isomer
Hydrogen groups on opposite sides = TRANS isomer
EG bonds can be broken hetrolytically
hetrolytic fission
electrophile
species attracted to an area of high electron density
can accept a pair of e-
may or may not be charged
electrophilic addition
happens to alkenes due to electron dense double bond
electrophiles attack double bond in alkenes
lead to formation of positive carbocation intermediate
electrophilic addition of bromine
ethene
asymetric alkene
groups or atoms attached to either end of the carbon-carbon double bond are different.
reaction of asymetric alkenes with H-BR
reaction of an asymmetric alkene with H-Br
overall reaction free radical substitution
overall reaction free radical substitution
single covalent bonds rotation
can freely rotate
atoms can move around the molecule
Therefore, the positioning of the chlorine atoms around the carbon atoms in the diagram is not important.
double carbon carbon bond isomérisation
no free rotation
This causes E -Z isomerism (also known as geometric isomerism) in this molecule.
therefore the positioning of the chlorine atoms does matter.
E -Z isomerism (also known as geometric isomerism)
E - opposite sides
Z - same side
same structural formula but different spatial arrangement of groups around the carbon double bond.
For a molecule to show E-Z isomerism, both carbon atoms in the double bond must have two different groups bonded to them.
atom priority E Z isomerisation
The atom bonded to the carbon with the higher atomic number (smaller) is given the higher priority.
2 geometric isomers of 1,2-dichloroethene
single covalent bond formed by
the overlap of s orbitals or an s orbital with a p orbital, or the end to end overlap of two p orbitals.
These are all forms of sigma σ bond and the electrons are between the two atoms.
double covalent bond formed by
sigma bond and a pi bond.
The pi bond is the vertical overlap of two p orbitals, with the electrons above and below the plane of the two carbon atoms.
ethene pi and sigma bonds
reactivity of alkenes
double bond - region of high electron density
susceptible to electrophilic attack
double bond is weaker than two single bond strengths
therefore it breaks quite easily to form a single bond.
This makes alkenes very reactive compared to alkanes and
thus they easily undergo addition reactions such as hydrogenation, hydration and bromination.
hydrogenation of alkenes
Reagent conditions and eg
addition of hydrogen across the alkene double bond,
for example, in turning polyunsaturated fats into saturated fats (liquid vegetable oils into solid edible fats or margarines).
Reagent: Hydrogen H2(g)
Conditions: Heat
Catalyst: Nickel metal (in the form of fine grains known as Raney nickel)
Type of reaction: Addition / hydrogenation
For example, hydrogen can be added to either but-1-ene or but-2-ene to form butane.
eg turning unsaturated oils into saturated fats
electrophilic addition reactions of alkenes
- addition of hydrogen bromide to alkenes
Reagent reaction eg
- Addition of hydrogen bromide to alkenes
Reagent: Hydrogen bromide HBr
Reaction mechanism: Electrophilic addition
The electron-rich double bond is attracted to the delta positive hydrogen – this is called an electrophile.
The double bond breaks to form the intermediate, known as a carbocation.
The HBr bond also breaks, allowing the hydrogen to bond to the carbon and also forming a bromide ion.
The bromide ion then joins onto the carbocation.
Curly arrows show the movement of electron pairs.
This reaction produces mostly 2-bromopropane (as shown above) rather than 1-bromopropane.
This is due to the greater stability of the secondary carbocation compared to the primary carbocation.
electrophilic addition reactions of alkenes
2. Addition of bromine to alkenes
Bromine, Br2, can also be added across an alkene double bond in a similar electrophilic addition reaction.
For example, but-1-ene reacts with bromine to form 1,2-dibrombutane.
C4H8 + Br2 –> C4H8Br2
The mechanism is similar, although the bromine bond is only polar once it is in the region of the carbon double bond.
Alkenes can be joined together in long chains of thousands of carbon atoms.
polymer
Polymerisation of ethene
Conditions and catalyst
The polymerisation of ethene needs heat (200˚C), pressure (2000 atm) and an oxygen initiator.
Ziegler catalysts can be used to create different polymer structures.
The process is called addition polymerisation – this is because the carbon double bond is broken and the monomer units are added onto one another.
polymerisation of propene will produce the repeating unit:
