2.5 hydrocarbons Flashcards
what are the 3 reactions of alkanes?
- combustion
- cracking
- free radical substitution (MECHANISM REQUIRED)
what is a combustion reaction?
one where the substance of interest is burned in oxygen
what are the 2 types of combustion reaction?
- full
- partial
what does full combustion lead to the formation of?
(alkane + oxygen) —> carbon dioxide and water
what does partial/incomplete combustion lead to the formation of?
carbon monoxide and water
what does full combustion mean?
the reaction has access to plenty of oxygen
what does partial combustion mean?
a reaction where the oxygen supply is limited
e.g propane C3H8
full combustion equation?
partial combustion equation?
full = C3H8 + 5O2 -> 3CO2 + 4H2O
partial = C3H8 + 3 1/2 O2 -> 3CO + 4H2O
in what order do you balance combustion equations?
C
H
O
what are some advantages of using fossil fuels (alkanes)?
- their capacity to generate huge amounts of electricity in just a single location
- fossil fuels are very easy to find
- more energy is generated per gram of fuel compared to many renewable and ‘green’ sources of energy such as ethanol
- when coal is used in power plants, they are very cost effective. coal is also in abundant supply
- transporting oil and gas to the power stations can be made through the use of pipes making it an easy task
- power plants that utilise gas are very efficient
- power stations that make use of fossil fuels can be constructed in almost any location. this is possible as long as large quantities of fuel can be easily brought to the power plant
ONLY LEARN 2
what are some disadvantages of using fossil fuel combustion?
- non-renewable resources (their supply is limited and they will eventually run out) while e.g wood can be renewed endlessly
- release co2 when they burn, which adds to the greenhouse effect and increases global warming
- coal and oil release sulfur dioxide gas when they burn, which causes breathing problems for living creatures and contributes to acid rain
- sulfur dioxide and carbon dioxide are known as acidic gases as they react with water to produce acidic solutions
ONLY LEARN 2
out of the three fossil fuels (oil, coal and natural gas), for a given amount of energy released, what produces the most carbon dioxide and what produces the least?
most = coal
least = natural gas
what affect does carbon dioxide have on the environment?
- it adds to the greenhouse effect
- and increases global warming
what affect does sulfur dioxide have on the environment?
- causes breathing problems for living creatures
- and contributes to acid rain
what is cracking?
the process of breaking down longer alkanes into smaller, more useful alkanes and alkenes
what are the conditions for cracking?
- 1-2 atm pressure
- zeolite catalyst
what is a zeolite catalyst?
aluminium and silicon based catalyst
(aluminosilicates)
do cycloalkanes have the same formula as their normal alkane?
yes
what is a free radical?
an atom/molecule/species with an unpaired electron
are free radicals reactive or non-reactive?
they are highly reactive
- because of their unpaired electron
how are free radicals formed?
by homolytic bond fission
how are free radicals represented?
by putting a dot after the atom/molecule/species
e.g a chlorine free radicals is represented by Cl•
what is heterolytic fission? using e.g Cl2
the covalent bond breaks with one atom retaining both electrons
- this forms positive and negative ions
- Cl- and Cl+
what does homo mean in homolytic bond fission?
what does fission mean?
homo = the same
fission = to break
what is homolytic fission?
the covalent bond breaks with both atoms receiving one electron. this forms free radicals
e.g Cl2 gets split so that they each have 7 electrons
this type of fission creates radicals as the result is that each atom now has one unpaired electron - making them radicals
Cl2 -> Cl• + Cl•
are alkanes reactive or unreactive?
unreactive
why can we get alkanes to react with free radicals?
because the free radicals are highly reactive and force the alkanes to react
(can also get them to react with a catalyst)
what are the 3 steps of the free radical substitution (FRS) mechanism?
- initiation
- propogation
- termination
what is the initiation stage of FRS?
THE CREATION OF FREE RADICALS BY HOMOLYTIC FISSION OF THE CHLORINE BOND
in the upper atmosphere, the UV has enough energy to cause homolytic bond fission of a covalent Cl2 bond
Cl2 (g) —(UV light)—> 2Cl•
what are the 2 conditions of the initiation stage of FRS?
- UV light must be present
- the process undergoing homolytic bond fission must be in the gas phase
what is the equation for the initiation stage of FRS?
Cl2 (g) —(UV light)—> 2Cl•
what is the propogation stage of FRS?
HALOGENATION AND REGENERATION OF RADICALS
- always occur in pairs
- e.g a small alkane molecule of methane
the alkane molecule reacts with the free radical to form an alkyl radical and HCl
CH4 + Cl• —> •CH3 + HCl
the alkyl radical reacts with a molecule of Cl2 to form a new halogenoalkane and also regenerates the Cl•
•CH3 + Cl2 —> CH3Cl + Cl•
using a small alkane molecule of methane as an example, what are the two steps in the propagation stage
the alkane molecule reacts with the free radical to form an alkyl radical and HCl
CH4 + Cl• —> •CH3 + HCl
the alkyl radical reacts with a molecule of Cl2 to form a new halogenoalkane and also regenerates the Cl•
•CH3 + Cl2 —> CH3Cl + Cl•
if they ask you in an exam for the overall equation when dealing with a free radical substitution question, what should you do?
write out the two propagation steps and cancel out the radicals
e.g
CH4 + Cl• —> •CH3 + HCl
•CH3 + Cl2 —> CH3Cl + Cl•
CH4 + Cl• + •CH3 + Cl2 —> •CH3 + HCl + CH3Cl + Cl•
= CH3 + Cl2 —> CH3Cl + HCl
what is the termination stage in FRS?
combination of two free radicals in a termination reaction to form a non-radical (stable) species
e.g Cl• + Cl• —> Cl2
•CH3 + •CH3 —> C2H6
Cl• + •CH3 —> CH3Cl
does free radical substitution have a high or low yield?
low yield
why are free radicals dangerous?
if left to go into the upper atmosphere, they will start a chain reaction by reacting with ozone molecules
this destroys the ozone layer which filters out harmful UV rays
a decrease in the amount of ozone will leave more harmful UV light getting through the ozone layer, leading to an increased cause of mutations in skin cells (cancer)
are alkanes or alkenes more reactive? why?
alkenes are more reactive compared to alkanes
because they have an area of high electron density around the C=C
why do alkanes have no area of high electron density?
as they only contain sigma bonds
therefore they are unreactive and used mainly as fuels
why are alkanes unreactive so mainly used as fuels?
they have no area of high electron density as they only contain sigma bonds
therefore they are unreactive
what does having a high electron density around the C=C in alkenes cause?
this causes alkenes to attack molecules with a low amount of electron density (electrophiles)
how many electrons are in between the area between a double bond in an alkene?
4e- (electron dense)
what is the general formula for an alkene?
CnH2n
what 2 bonds make up a C=C bond?
1 pi (𝛑) bond and 1 sigma (σ) bond
what is the pi bond created due to?
the side-on overlap of 2 p-orbitals
this creates a region of electron density above and below the plane of the double bond. two electrons occupy the sigma bond and make up the pi bond
what shape is every double bond? what angle is between each bond?
every double bond is trigonal planar (flat) with a 120°C bond angle between each bond
what type of isomer is geometric isomerism a type of?
stereoisomerism
what is the definition of a geometric isomerism?
isomers that have the same structure formula but atom orientation is different in space (due to the limited rotation around the C=C)
geometric isomers can exhibit what we call E or Z isomers
why are C=C bond stuck in position/have restricted rotation?
because of the pi bond between overlapping p-orbitas
- a pi bond is formed as the p orbitals are aligned to overlap with each other
- this occurs due to side on overlap of p orbitals
- this essentially ‘locks’ the molecule in shape
- any rotation will cause disalignment with the p orbitals and so breaks the pi bond
- bond breaking costs energy so we say that double bonds do not rotate
what are the rules for deciding if a double bond is E or Z?
- check it has a double bond
- check each C in the C=C that it has two different groups attached to it
- look at the two different groups on each C in the double bond and rank the two groups in terms of mass (a larger mass takes priority)
- if the higher priority groups are on the same side of the double bond, then it is the Z isomer. if on opposite sides then it is the E isomer
how do you name E/Z isomers?
e.g (Z)-but-2-ene
are sigma bonds (single bonds) allowed to rotate?
yes
how do alkenes react?
by using their double bond to attack molecules that come into proximity with them
what is a mechanism?
a description of steps / a ‘story’ of what happens to the molecule that is reacting
(an organic mechanism is a way to understand how molecules interact and react with each other a step by step basis)
(a lot is based on charges attracting and dipoles that occur due to differences in electronegativity)
what are some tips when drawing mechanisms?
- curly arrows are used to show the movement of an electron pair
- curly arrows should start from a lone pair or the centre of a bond
- curly arrows should go from an area of high electron density to an area of low electron density
- draw out all of the bonds - they need to be clear for the examiner to see
what are curly arrows used to show?
the movement of an electron pair
what is electrophilic addition?
a reaction of alkenes
- alkenes have an area of high electron density around the C=C
- therefore, they will attack species with areas of low electron density - electrophiles
- this mechanism is called electrophilic addition
the pair of electrons in the 𝛑 orbital means that alkenes have a region of high electron density and are susceptible to attack by electrophile. this mechanism involves heterolytic bond fission and leads overall to electrophilic addition
why do we call electrophilic addition an addition reaction?
there is one produce even though we start with 2 reactants
electrophile definition
a species with a low electron density
electrophilic addition:
H2C=CH2 + Br2 —> Br Br
H-C-C-H
H H
curly arrow from double bond to δ+ Br
curly arrow from single bond to δ- Br
goes to:
1 Br attached to C
Br with full outer shell (but - charge)
curly arrow from electron pair of Br to + C
LOOK IN NATHAN BOOK
what is markovnikov’s rule?
out of the two carbons in the double bond, the one bonded to more carbons will get the halogen atom
(this rule applies to unsymmetrical alkenes)
the reason behind this is due to carbocation stability
what is a carbocation?
a species that has a full positive charge on a carbon atom
are carbocations stable or unstable?
highly unstable
how can you make carbocations more stable?
by surrounding them with alkyl groups (these are usually carbon chains)
- these push electron density towards the positive charge, partially ‘neutralising it’. you might hear alkyl groups being describes as ‘electron releasing’ groups
- this makes it less positive and therefore more stable
- therefore when given a choice in a mechanism- look to make the more stable carbocation. this will make your major product
what are the 3 types of carbocation?
- primary carbocation
- secondary carbocation
- tertiary carbocation
look for how many carbons the carbon with the positive charge is bonded to work out what type of carbocation it is
tertiary > secondary > primary
e.g C3H6 + HBr electrophilic addition
which carbon in the double bond should the H bond to first?
left as it’s more stable so more likely to proceed by the mechanism
what are the 2 tests for a double bond/alkene?
- add bromine (Br2) water
- add potassium manganate (VIII)
what do you do in the test for a double bond using bromine water? what is the result?
add bromine water
colour change from orange/brown to colourless is a C=C is present
the alkene is converted to a halogenoalkane
what happens in the test for a double bond using potassium manganate (VIII)?
- add potassium manganate
the colour change is from purple to green/brown precipitate
the alkene is converted to a diol
what are the 3 reactions of alkenes?
- electrophilic addition (MECHANISM REQUIRED)
- hydrogenation of alkenes
- addition polymerisation
what does hydrogenation of alkenes mean?
- this means to add hydrogen across the alkene double bond
(sometimes referred to as addition reaction)
what happens in the hydrogenation of alkenes?
- a hydrogen atom gets added on each C in the C=C
alkene —> alkane
conditions:
- nickel, palladium or platinum catalyst - finely divided
what is the major reaction for the production of margarine?
hydrogenation of alkenes
(turning polyunsaturated fats into saturated fats) (liquid vegetable oil into solid edible fats or margarines)
what is the reagent used in the hydrogenation of alkenes?
hydrogen
what are the conditions for the hydrogenation of alkenes?
- nickel catalyst
- heat
what is addition polymerisation?
the joining of smaller monomer units (with C=C) into a large overall molecule (polymer)
why are addition polymers chemically inert (unreactive)?
as the main backbone contains strong C-C bonds
what happens in addition polymerisation (of alkenes)?
- alkenes react with other alkenes to form one overall product
- there are no by-products
- the C=C bond breaks to form a single C-C and each C that was part of the double bond attaches to the next monomer
(the 𝛑 bond breaks and the electrons are used to form σ bonds with other monomers either side of it)
- you need to be able to identify and draw a repeat unit or draw a monomer from its repeat unit
- the n represents how many you start with - if you have 100 monomers which all join together, there will be 100 repeat units in the polymer repeating unit
what are some common addition polymers?
- PTFE = polytetrafluoroethene = used as the non-stick part of a frying pan
- PVC = polyvinylchloride = used as material for plastic doors and window frames
- polystyrene = used as a packaging material
what is bond fission?
the breaking of a covalent bond
what are the 2 ways covalent bonds can break?
- heterolytic fission
- homolytic fission
the photochlorination reactions of alkanes are chain reactions that produce chloroalkanes
the reaction mechanism is a free radical substitution
in alkanes, is the positioning of the (chlorine) atoms around the carbon atoms important?
no
becUse alkanes have only single covalent bonds which can freely rotate, meaning the atoms can move around the molecule
there is no ______ ________ about a double bond?
no free rotation
what can a single covalent bond be formed from?
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
look at blended learning
what makes alkenes susceptible to electrophilic attack
the double bond creates a region of high electron density
what makes alkenes very reactive compared to alkanes?
alkenes have a high electron density in the C=C. this makes them more susceptible to attacks from electrophiles, compared to alkanes, making alkenes more reactive
the double bond is weaker than two single bond strengths, therefore it breaks quite easily to form a single bond
ethene has ___ sigma bonds between the carbon and hydrogen atoms?
4
a ___ bond can be formed from the overlap of s orbitals or an s orbital with a p orbital?
sigma
a ____ bond consists of a sigma bond and a pi bond?
double
the bond angle between the carbon and hydrogen atoms in ethene is ____?
120°
alkenes easily undergo _____ reactions?
addition
what is the difference between the electrophilic addition of hydrogen bromide to alkenes compared to the electrophilic addition of bromine to alkenes?
the mechanism is similar, but the bromine bond is only polar once it is in the region of the carbon double bond
(the electron dense C=C induces a dipole in Br2)
give an example of a type of reaction that uses a radical substitution mechanism
photochlorination of alkanes
why can alkenes undergo electrophilic addition?
alkenes are susceptible to attack from electrophiles because of the high electron density in the C=C double bond
describe the electrophilic addition of bromine to ethene
- the electron dense C=C induces a dipole in Br2
- this causes the 𝛑 bond to break and form a bond with the slightly positive bromine, breaking the Br2 molecule
- this forms a positive carbocation intermediate
- which forms a bond with the negative bromide ion
explain why the electrophilic addition of bromine to propene can produce a major and a minor product?
- when propene undergoes electrophilic addition, two possible products can be formed: the major and minor product
- this is because two different carbocation intermediates can be formed- the primary and the secondary carbocation
- the secondary carbocation is more stable so this produces the major product 2-bromopropene
- the primary carbocation forms the minor product 1-bromopropane
what reaction does an alkene undergo to form an alkane? name the reagents and conditions
reaction type : hydrogenation
reagents : hydrogen gas
conditions : nickel catalyst, 150°
what is the atom economy of addition polymerisation?
100%
in the addition of potassium manganate (VIII), what does the alkene get converted into?
a diol
what does free radical substitution form?
a halogenoalkane
alkanes are (polar/nonpolar)?
non polar
so are generally unreactive
why is incomplete combustion bad?
- produces carbon monoxide
- toxic/poisonous
- produces less energy than complete combustion
- poly(ethene) is unreactive and flexible so it is used to make plastic bags etc
- when poly(ethene) was first made, the polymer chains had side branches coming from the main chain and these prevented the polymer chains packing together
- this meant that the density of the polymer was low and there were few points of contact for VdW forces so that the melting temperature was also low
- catalysts can be used to make poly(ethene) with straight chains
- this means that the chains can pack together so that the polymer has a high density and higher melting temperature
- these properties mean that it is used where more rigidity is needed and/or the temperature is comparatively high
- the properties of polymers can also be altered by using substituted alkenes as the monomer. this means that the polymers have a huge variety of uses
NOTE:
you dont need to be able to quote specific uses for specific polymers but you should be aware of the principles involved in the polymerisation and how the different physical properties make different polymers suitable for different functions
what is the use of poly(ethene)? why?
- making plastic bags
- because its unreactive and flexible
what is the use of poly(propene)? why?
- used in food containers and kitchen equipment
- because rigid
what is the use of poly(chloroethene)? why?
(PVC)
- electrical cable insulating covering
- as well as pipes etc
- property can be modified to make it a flexible covering
describe the electron density in ionic bonding? [1]
high electron density centred round ions
describe the electron density in covalent bonding? [1]
high electron density between nuclei/atoms
describe the electron density in intermediate bonding [1]
high electron density between nuclei/atoms but higher nearer one of them / ions with electron distortion of negative ion
students were discussing the reactivity of organic compounds. one said that the reactivity was due to dipoles produces by differences in electronegativity.
show that this is not correct by discussing the difference in reactivity between alkanes and alkenes. you should include reference to the bonding in both series. [6 QER]
- alkanes and alkenes both contain C and H
- C and H have similar electronegativities
- there are no dipoles in either alkanes or alkenes
- alkenes have double bonds and are much more reactive
- due to high electron density of 𝛑 bond
- caused by p-p sideways overlap
- alkanes are saturated whilst alkenes are unsaturated
- this makes alkenes susceptible to electrophilic addition
- alkanes need light to react
- by radical substitution
the different fractions are separated by fractional distillation. explain why the different fractions have different boiling temperatures [2]
- boiling temperatures increase with increasing chain length/number of carbon atoms
- more carbon atoms leads to greater number of van der Waals’ forces between molecules
briefly describe how fractional distillation can be carried out [2]
- substance heated/evaporated
- fractions condense at different temperatures/separated into fractions with different boiling temperatures
state what is meant by the term propagation stage [1] in free radical substitution
a step during which a radical reacts and another one is formed
