topic 6 Flashcards
Alkanes
General formula - CnH2n+2
Saturated: Every C atom had 4 single covalent bonds around it
Non polar: No distinct dipole moments present. This makes them unreactive and insoluble as strong, covalent non polar bonds make them resistant to attack by other reactive species and polar water
Varied melting and boiling points: Increases with greater Mr, as stronger London forces as more electrons are involved, so more energy is required to separate molecules. Branching generally lowers melting/ boiling points when compared to straight chain isomers
Structural isomers
Molecules with same molecular formula but a different structural formula
Cyclical alkanes are not structural isomers of straight chained alkanes
Crude oil
It’s unrefined
Mixture of hydrocarbons
Mainly saturated (alkanes)
Cracking
The process of breaking up larger, less useful hydrocarbons into smaller, more useful ones.
Thermal cracking
High temperatures - 900C
High pressure - 7000Kpa
Free radicals formed by homolytic fission
These free radicals go on to form:
- mostly alkenes - used to make polymers
- smaller alkanes - used as fuels
- Hydrogen gas - useful in industry/ as fuels
Thermal cracking
High temperatures - 900C
High pressure - 7000Kpa
Free radicals formed by homolytic fission
These free radicals go on to form:
- mostly alkenes - used to make polymers
- smaller alkanes - used as fuels
- Hydrogen gas - useful in industry/ as fuels
Catalytic cracking
Lower temperatures - 500C
Catalyst- silicon dioxide and aluminium oxide (zeolites)
Heterolytic fission (both electrons go to same carbon) forms carbocations
These carbocations go on to form:
- Mostly smaller alkanes - fuels
- Some reforming of alkanes
Chain lengths produced are random. Fractional distillation required.
Catalytic cracking is a more greener process as lower temps and less energy needed
Reforming
Straight chain alkanes can form
Branched chain alkanes - fuels
Benzenes
Cyclical alkanes (cycloalkanes)
Saturated ring structure alkanes
General formula CnH2n - isomeric with straight chain alkenes
Same physical/chemical properties as alkanes
Complete combustion of alkanes
Products are carbon dioxide and water
Reaction of alkane with oxygen
Is exothermic
The greater the chain length, the greater the energy released when the products are formed
However the greater the chain length, the more energy needed to react (harder to burn)
Incomplete combustion of alkanes
The products form carbon monoxide (instead of dioxide) and water.
carbon monoxide is highly toxic as it binds to red blood cells
More likely in longer chain alkanes
Some pure carbon (soot) can also be produced
Impurities such as sulphur and nitrogen may also be present in the fuel producing sulphur dioxide and nitrogen dioxide respectively
Environmental impact of combustion:
Carbon dioxide
Impact: Global warming
Solution : Use of carbon neutral fuel sources, e.g. biofuels
Environmental impact of combustion:
Sulphur dioxide
Impact: Acid rain.
Sulphur dioxide further reacts with oxygen producing sulphur trioxide, which reacts with water to produce sulfuric acid (acid rain)
Solution: desulphurisation: using calcium oxide or calcium carbonate
Environmental impact of combustion:
Nitrogen oxide
Carbon monoxide
Unburned alkanes
Nitrogen oxide - Acid rain (nitric acid)
Carbon monoxide - Health issues/smog
Unburned alkanes - Global warming
Solution - Catalytic converters
2CO + 2NO ———-> N2 + 2CO2
C8H18 + 25NO ———> 12.5N2 + 8CO2 + 9H2O
Environmental impact of combustion:
Carbon particulates
Impact: Smog, health issues such as cancer
Solution: Use fuels that produce fewer particulates, e.g. petrol produces less than diesel
Free radical substitution
Substitution of alkane hydrogen atoms with halogen free radicals
Process of free radical substitution:
Initiation
Formation of free radicals
Homolytic fission produces free radicals in the presence of UV light in chlorine
Cl2 ———> Cl• + Cl•
• represents an unpaired electron. Highly reactive species
Process of free radical substitution:
Propagation
CH4 + Cl• ————> •CH3 (methyl radical) + HCl
•CH3 + Cl2 ———–> CH3Cl (product) + Cl•
Process of free radical substitution:
Termination
Free radicals combine
Cl• + Cl• ———-> Cl2
- CH3 + Cl• ——–> CH3Cl (which is the product)
- CH3 + •CH3 ———> C2H6 ( alkane twice the size of original produced)
Process of free radical substitution:
Overall reaction
CH4 + Cl2 ———-> CH3Cl + HCl
Process of free radical substitution:
Problems
- Will not occur in the dark (UV needed)
- Substitution is random. No control over which hydrogen substituted in larger alkanes
- If left to run, multiple substitutions can occur
Multiple products made! Not precise process
Alkenes
General formula: CnH2n
Unsaturated: they contain double bonds between carbon atoms so tend to undergo addition reactions
Reactive: the double carbon bond is an area of high electron density so it’s open to electrophilic attack
Can show geometrical isomerism as the carbon to carbon double bond is non rotational
Non polar: No distinct dipole moments present. This makes them insoluble as strong, covalent non polar bonds make them resistant to attack by polar water
Varied melting and boiling points: Increases with greater Mr, as stronger London forces as more electrons are involved, so more energy is required to separate molecules. Branching generally lowers melting/ boiling points when compared to straight chain isomers
The C=C bond
Functional group
Area of high electron density, ie very negative
Open to electrophilic attack
Types of C=C bond:
Bond 1
Very strong sigma Bond
Formed between overlap of two s orbitals
Types of C=C bond:
Bond 2
Weaker Pi Bond
Formed between overlap of two p orbitals
Highly negative area in middle so are open to electrophilic attack
Pi bonds break during reactions
They are weaker than sigma bonds as they are above and below the nuclei so have a weaker attraction
testing for alkenes:
bromine water
Test: add bromine dropwise to the Organic sample at room temperature
Observation: positive = Brown to colourless negative = stage Brown
electrophilic addition reaction
Secondary test for alkenes:
Acidified potassium manganate solution
Test: add acidified potassium manganate solution dropwise to the Organic sample at room temperature
Observation: positive = purple to colourless negative = stays purple
Redox reaction
Test may also give a positive result for alcohols and aldehydes
Geometrical isomers
Form of stereoisomerism
Geometric isomers have groups that occupy different relative positions in space
Specific to alkenes as the carbon to carbon double bond is non rotational
Criteria molecules must have in order to show geometrical isomerism
- Must have a C = C bond
2. Two different groups bonded to each carbon in the c = c Bond
How to know if something shows geometrical isomerism
Focus on c = c
Summarise groups
Focus on heavier groups “priority”
Naming geometrical isomers (E/Z):
Cann - ingold prelog system
- look up atomic numbers of all the atoms that are bonded to each carbon in the c = c Bond
- highest atomic number takes priority
- If two atomic numbers are the same on one carbon in the c = c Bond, we move to the next atom in the chain and compare
- Where are the priority groups relative to each other?
Ze zame Zide = Z
Opposite sides = E
Naming geometrical isomers (E/Z):
Cis/ tran system
If the two priority groups in the molecule are the same then the cys/tran notation can be used
Same = sis
opposite = tran
What is electrophilic addition
electrophilic addition is when an alkene is converted into a halogenoalkane
Electrolphilic addition
Draw step 1 and explain
HX has a permanent dipole
H (&+) is the electrophile
Pair of electrons are retracted from the pi Bond to the H
Heterolytic fission of the HX bond occurs
Electrophilic addition
Draw step 2 and explain
Carbocation is formed
X(..-) is now a nucleophile
electrophilic addition step 3
Single halogen atom added to the molecule
Difference when a halogen is used in electrophilic addition instead of a hydrogen halide
in step one the dipole is induced by the pi Bond
markovnikov’s rule
One major and one minor product is made during the addition of an asymmetrical alkene
Carbocation
3° most stable
2°
1° Least stable
Asymmetrical alkene
different number of hydrogen atoms on each carbon in the c = c Bond
Hydrogenation
Alkene is turned into an alkane
Reagent is hydrogen gas, needs nickel or platinum catalyst and a temperature of 150°C
Very important reaction in food industry. Unsaturated vegetable oils are converted by this process into saturated ones
Hydration
Alkene is turned into an alcohol
Reagent is water
Needs a concentrated phosphoric acid catalyst
Temperatures above 100° ( steam)
Addition polymerisation
Alkenes can be made into longer saturated hydrocarbons
Atom economy = 100%
method of showing repeating unit
Break double bonds (pi)
Extend bonds out
Add square brackets and the letter n
Method of finding the monomer
Look for the simplest repeating unit
Ignore side groups above and below
Take away the square brackets and add the double bond back
Polymers
polymers are unreactive as they are saturated molecules
Intermolecular forces and therefore physical properties do vary depending on site groups
Waste polymers:
Recycling
Some polymers can be remoulded into new products (thermosoftening)
Some must be chipped and reformed into new products (thermosetting)
Waste polymers:
Incineration
Plastics can be burnt for energy production
Halogen containing plastics eg PVC release toxic gases e.g. HCL
These must be removed during the process e.g. by neutralization
Waste polymers:
Feedstock
May be cracked to produce smaller more useful alkanes and alkenes
These could be used for fuels or production of other organic chemicals
Degradable plastics
Biodegradable: broken down by enzymes
Photodegradable: broken down by UV light
Halogenoalkanes
General formula CnH2n+1X
Saturated: every carbon atom has four single covalent bonds present
Polar: permanent dipole present on the C-X bonds as the X (halogen) is highly electronegative
Undergo nucleophilic substitution and elimination reactions
Not water-soluble: C-X Bond is not polar enough to interact with the waters intermolecular forces
Varied melting and boiling points: dipole dipole intermolecular forces present due to the polar C-X bonds. This results in a higher melting and boiling points than the corresponding alkanes. However the melting and boiling points increases with increased Mr as there are stronger induced dipole forces between nonpolar sections
Primary secondary and tertiary halogenoalkanes
Can be primary secondary or tertiary
1° Primary: 2 hydrogens bonded to the same carbon atom as the halogen
2° Secondary: 1 hydrogen bonded to the same carbon as the halogen
3° Tertiary: 0 hydrogen-bonded on the same carbon as the halogen
Testing for halogenoalkanes
Unable to test for presence of halogen when bonded to a carbon
Nucleophilic substitution releases x - (halide ion) which can be tested for
- Reflux with aqueous sodium hydroxide - releases X- (halide ion)
R-X (aq) + NaOH (aq) ———> R-OH + Na^+ + X^- - Add excess nitric acid - neutralise excess OH- (hydroxide ions)
HNO3 + OH- ——> NO3- + H2O - Add silver nitrate - identifies halide ions (can follow up using conc/dilute ammonia)
When testing for halogenoalkanes why do we add excess nitric acid?
The next step is to add silver nitrate test for halide ions
If OH- (hydroxide ions) are present, then the silver ions and hydroxide ions will form silver hydroxide which is a brown precipitate. This interferes with the test
Must be nitric acid as hydrochloric acid and sulfuric acid form a white precipitate with silver ions
What is nucleophilic substitution
Nucleophilic substitution is when a halogen atom is substituted for nucleophilic group in a halogenoalkane
Nucleophilic substitution:
forming alcohols (with hydroxide ions aka hydrolysis)
Haloalkene is converted into an alcohol
- Reagent is aqueous sodium hydroxide + heat
- haloalkane and sodium hydroxide must be mixed in ethanol to make them miscible
Draw and explain the first step of nucleophilic substitution when forming alcohols
Lone pair on the OH - is attracted to the Delta positive carbon
C-x Bond Breaks by heterolytic fission
Overall: R-X + NaOH ———> R-OH + NaX
Nucleophilic substitution:
forming a nitrile group (with cyanide ion)
Halogenoalkane is converted into a nitrile
Reagent is potassium cyanide dissolved in ethanol + heat
Carbon chain length increased by addition of Cn
Draw and explain the first step of nucleophilic substitution:
forming nitrile with cyanide ion
Lone pair on Cn- is attracted to the Delta positive carbon
The c-x bond breaks by heteroytic fission
Overall: R-X + KCn ————-> R-Cn=N HXn
Nucleophilic substitution:
forming an amine group with NH3
Haloalkane is converted into an amine
Two step mechanism
Reagant is excess concentrated ammonia dissolved in ethanol + high pressure
Overall: R-X + 2NH3 ——-> R-NH2 + NH4X
Draw and explain the first step of nucleophilic substitution forming an amine with nh3
Lone pair of electrons on the nh3 is attracted to the Delta positive carbon
C-x Bond Breaks by heterolytic fission
NH bonds Breaks by heteroytic fission
H+ reacts with more NH3 as a base
Relative ease of substitution
The ease of substitution depends on the bond enthalpy of the c-x bond
The lower the c-x bond enthalpy, the weaker the bond, the more reactive it is, the easier it is to substitute
Rates of substitution practical
Halogenoalkane + water + AgNO3 (heat) ———–> alcohol + H+ + x -
As substitution occurs with water, halide ions are released and immediately form a precipitate
The time taken for precipitate form can be measured to work out the rate of substitution
